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COMPASS POINTS

Engineering specialties

GNSS receiver data collection systems, GNSS receiver signal processing, GNSS for remote sensing of atmosphere and ionosphere, new navigation applications and technologies.

GNSS event that most signified to you that GNSS had ‘arrived’

Return to main article: "Jade Morton: The Long and Scintillating Road"

COMPASS POINTS

Engineering specialties

GNSS receiver data collection systems, GNSS receiver signal processing, GNSS for remote sensing of atmosphere and ionosphere, new navigation applications and technologies.

GNSS event that most signified to you that GNSS had ‘arrived’

When President Barack Obama, in his in his 2011 State of the Union speech compared GPS to computer chips and the Internet.

Role Model

Her grandmother, a “very hands-on capable woman who was not afraid to try new things.”

Engineering mentor

Jade Morton acknowledges several mentors, without whom, she says, she would not be where she is today, including James B. Y. Tsui, Frank van Graas, Mikel Miller, A.J. van Dierendonck, John Betz, Per Enge, and James Spilker Jr.

What popular notions about GNSS most annoy you?

“I often hear people blaming GPS when their navigation devices, such as the navigation app in their cell phones, are not working correctly. People should realize that it is often not the GPS that caused the problem, but the guidance algorithms or the maps that are incorrect.”

Favorite equation

Morton’s favorite equation is the wave propagation equation. “GPS signals are radio waves,” Morton explains, “and this equation governs how the signals travel from satellites to our receivers and all of the troubles it encounters along the way. In fact, it describes all sorts of waves. It is an equation that connects time and space.”

(see inset photo, above right)

u represents a wave field, t is time, c is the speed of the wave (for radio waves and light traveling in vacuum, c is the speed of light), and the upside-down triangle is the spatial Laplacian operator.

The post Jade Morton’s Compass Points appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

1996 soccer game in the Midwest, (Rick Dikeman image)

Nouméa ground station after the flood

A pencil and a coffee cup show the size of NASA’s teeny tiny PhoneSat

Bonus Hotspot: Naro Tartaruga AUV

Pacific lamprey spawning (photo by Jeremy Monroe, Fresh Waters Illustrated)

“Return of the Bucentaurn to the Molo on Ascension Day”, by (Giovanni Antonio Canal) Canaletto

The U.S. Naval Observatory Alternate Master Clock at 2nd Space Operations Squadron, Schriever AFB in Colorado. This photo was taken in January, 2006 during the addition of a leap second. The USNO master clocks control GPS timing. They are accurate to within one second every 20 million years (Satellites are so picky! Humans, on the other hand, just want to know if we’re too late for lunch) USAF photo by A1C Jason Ridder.

Detail of Compass/ BeiDou2 system diagram

Hotspot 6: Beluga A300 600ST

1. CARTOON FRENZY
Absolutely Everywhere, The World

1. CARTOON FRENZY
Absolutely Everywhere, The World
√ Everyone’s talking about Pokémon Go; so, we will, too. The location-based augmented reality game fits so well into neighborhood landscapes that it prods millions of people to go outdoors with their smartphones to find cartoon creatures in the shrubbery. It avoids the scariness of virtual reality — think The Matrix — “because humans still get to touch. . . with their real time physical world,” said IEEE member and expert on new immersive technologies Todd Richmond. The nextgen iteration of the 1990s video game franchise uses a mobile phone’s camera and navigation functions to overlay its “Pokestops” on public landmarks, historical buildings and unique local businesses geotagged on Google. Niantic Inc. designers also used five million locations contributed over several years by players of Ingress, a previous real world game they designed.

2. GNSS PHILATELY
Moscow, Russia
√ On July 5, the Russian Federation released a commemorative stamp worth about 33¢ honoring their third-generation GLONASS-K satellite. (Lighter, stronger, with longer life and greater accuracy, it broadcasts both FDMA and CDMA signals.) Two flight test models are in orbit, one launched in 2011 and the other in 2014. The second satellite went into service in February.

3. NULL ISLAND
Prime Meridian and the Equator
√ If Null Island didn’t exist, someone would have to invent it, right? Well, they did. It cannot be found at 0 ° N, 0 °E west of Cameroon, and it’s too small to be shown on maps, but we’ve all been there. Introduced by digital map developers, the coordinates are a programming default where coder and user errors go to die. About seven years ago, a cartographer for the open-source Natural Earth data set, in true human fashion, put a name to that non-place and pretty soon it had a history, population, language, quirks, and a T-shirt. If you happen to be sailing by, you’ll see a weather observation buoy collecting data on behalf of the Prediction and Monitoring Array in the Atlantic (PIRATA), and you might hear faint music and laughter from the tropical paradise where only bad data can be found.

4. HIDDEN EMPIRE
Angkor Wat, Cambodia
√ A huge archaeological project covering more than 734 square miles took three years instead of a lifetime of hacking through tropical jungle because of Light Detection and Ranging (LIDAR) scanning technology and high-precision GNSS mounted on helicopters. Between 2012 and 2015, researchers uncovered a 12th century megacity much larger than expected at the remarkable ruins of the Kmer Empire near Angkor Wat. The empire was the largest in the world at that time, with one individual city larger than Phnom Penh today. Penetrating the thick jungle and modern farmlands with 16 data points per square meter, the technology allowed Australian Damian Evans and his team to piece together a 3D point cloud model showing complex water and highway systems, quarries, stone structures, and geometric earthworks.

BONUS HOTSPOT: The Great Geomagnetic Storm of 1859
The Sun and the Earth
Good thing the global navigation satellite systems hadn’t been invented yet in 1859. From August 27 to September 7 the first observation of a solar flare and the most powerful geomagnetic storm in history happened, and it shook everybody up. The solar coronal mass ejection was so powerful, particles traveled from Sun to Earth in 18 hours. the Northern Lights were visible in Cuba and Hawaii, and the Sourthern Lights in Brisbane, Australia.(more)

The post GNSS Hotspots | July 2016 appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

The most public of these actions occurred June 30 when Secretary of the Air Force Deborah Lee James announced OCX would surpass by at least 25 percent the program’s estimated cost. She declared a critical Nunn-McCurdy breach, putting the program on a path to automatic cancellation.

The Air Force is defending the new GPS ground system, taking a stand against naysayers in Congress and declaring through its actions an intent to stick with the Next Generation Operational Control System program (OCX) — at least for now.

The most public of these actions occurred June 30 when Secretary of the Air Force Deborah Lee James announced OCX would surpass by at least 25 percent the program’s estimated cost. She declared a critical Nunn-McCurdy breach, putting the program on a path to automatic cancellation.

Secretary of Defense Ashton Carter has until no later than October 13 — that is 60 days from when the DoD submitted the Selected Annual Report on the OCX program — to recertify the importance of OCX and explain how the Department of Defense (DoD) is going to fix the program’s underlying budget control problems or the program will be terminated.

Although dire on its face, the declaration appears to be less about dumping OCX than dealing head on with the political hurdles facing an over-budget program in the congressional crosshairs. Senate appropriators, for example, moved earlier this year to kill OCX Blocks 1 and 2, truncating the program at a point where its ability to support the new GPS III satellites is questionable.

“It’s a critical system. It would be very disruptive to stop where we are and start all over,” Frank Kendall, undersecretary of defense for acquisition, technology and logistics said July 10 ahead of the Farnborough International Airshow in Hampshire, England, according to Defense News.

Evidence for DoD’s support for OCX came the same day as the declaration of the Nunn-McCurdy breach when defense officials filed a request with Congress to add $39.3 million to the OCX budget to keep it from running out of money.

The request is part of a $2.6 billion package of changes that would move monies between a 61-page list of over- and under-funded federal programs — changes that must be approved in advance by lawmakers. In addition to boosting funding for OCX, the request seeks to cut $31.7 million from the GPS III program, funding that DoD said was not needed this fiscal year because of program delays.

If approved, the money would increase fiscal year 2016 (FY16) spending on OCX by more than 11 percent, a significant vote in favor of continuing with OCX and its prime contractor Raytheon Company.

“They [DoD] requested the reprogramming because they want the work to continue,” asserted an industry watcher, who spoke on condition of anonymity. “If they did not want the program to continue, they would not have asked to move the money.”

Sources familiar with both the GPS program specifically and with defense spending in general, say Pentagon officials must get the reprogrammed money moved quickly to avoid a temporary shutdown.

Failure to win approval for the transfer will, by DoD’s estimate, cost the Air Force an extra $90 million to reopen the program and an additional four-month delay. The question is, can they get approval in time?

Money Crunch
Although the FY17 funding and authorizing bills for the Department of Defense have yet to be approved, there is no reason to believe, at this point in an election year, that DoD will not get new monies on October 1 — even if the funding comes through a continuing resolution or another omnibus spending bill. So, the money would be needed to cover the end of this fiscal year on September 30.

Sources indicated that the OCX program was still operating as of mid-July.

One knowledgeable source told Inside GNSS that spending on OCX has averaged $1 million a day. If the average daily spending level holds true, then the $39 million would cover all of September and some of August — suggesting the monies would be needed as soon as the fourth week of August.

It is not clear, however, that the spending levels will hold steady. In fact, DoD says program costs are rising due to a need for more staffing.

In the its request, the Pentagon told Congress that the reprogrammed “funds are required to complete a 24-month re-plan and Block 1 test to address deficiencies uncovered during Configuration Item Qualification Testing of the Next Generation GPS Operational Control Segment (OCX).

The 24-month re-plan requires the contractor to increase staffing by 25 percent, resulting in additional cost to the Federal government.” The reprogramming request, which is unclassified and will eventually be made fully public, was posted by Inside Defense.

The $39 million won’t go as far if daily spending jumps — suggesting a shutdown in September. Although it seems counter-intuitive, that actually could be a blessing for OCX. As of press time Congress was headed out of town until after Labor Day (September 5) and appeared in no hurry to deal with the reprogramming plan while they were gone.

“The committee is in the process of analyzing the request,” said a spokesman for the House Armed Services Committee. There was no specific timeline for its response, he said.

If OCX can continue working until Congress returns, it may stand a better chance of uninterrupted operations.

Getting a Thumbs Up
Congress as a whole need not be on hand for approval. The go-ahead comes from just eight key players — the chairs of the House and Senate Armed Services Committees and the Appropriations Committees as well as each committee’s top Democrats, that is, their minority members.

“This does not go to a full committee vote, but you have to get a communication back from the chair of Armed Services and the chair of Appropriations, in both houses, saying “Yep. You can go ahead,” said Gordon Adams, the founder of the Center for Strategic and Budgetary Assessments (CSBA) and a defense budget expert who spent five years as associate director for national security programs at the Office of Management and Budget.

Unlike legislation, the reprogramming package also need not be approved in its entirety. This reduces the chances that the OCX funding will be held up by an objection to one of the many other reprogramming requests. On the other hand it lowers the political cost of someone objecting to the new OCX monies.

“It’s like a line-item veto,” said Todd Harrison, a senior fellow and the director of defense budget analysis at the Center for Strategic and International Studies. “They can object to a single (funding) movement within the reprogramming request and allow all the rest to go through. That’s fairly often that you’ll see that happening.”

An objection would come in the form of a letter and, in the case of Sen. John McCain, R-Arizona, who has been quite vocal about his unhappiness with OCX, possibly a press release as well.

“In the grand scheme of things $39 million is not that much money in the defense budget. It really isn’t,” said Harrison. “But that $39 million in this particular fiscal year could be really important to a program like OCX. It could also be really important to members of Congress who are not happy with progress on that program.”

Into the Breach
The way DoD handled announcement of the breach underscores that OCX — and by extension the funding boost — is indeed important to DoD.

Whereas June 30 was, by law, the last day DOD could submit its reprogramming request, announcing the breach that day — the Thursday evening before the July 4th holiday — appears to be more of a strategic choice.

Notice of the breach was sent out after people had left for the day, and many were on the road to area beaches, a classic Washington maneuver all but guaranteed to blunt the reporting of bad news. That choice also may have limited the opportunity for congressional opponents, including McCain, to use the announcement as political ammunition. Many lawmakers were on their way home to campaign for re-election, and McCain was getting ready to greet troops on the ground in Afghanistan during the Monday holiday.

As it happens, declaring the breach on June 30 also means Carter’s certification will be submitted while Congress is still in recess.

“They definitely buried the news. There’s no question. This stuff does not have to be released on schedule,” said Adams. “They’ve got a program to defend, but they don’t necessarily want high visibility for it.”

Acknowledging the breach when they did also avoided a potentially drawn-out battle between DoD and Congress. When the Senate approved the Defense Authorization bill, it voted to withhold OCX funds unless the magnitude of the budget overruns was formally recognized with a Nunn-McCurdy declaration. The June 30 move, before the House and Senate had time to agree to final language for the legislation, took the issue off the table.

James took the opportunity to begin making the case for the program, pointing out that Raytheon has not received any fees on the OCX contract since August 2013.

“About $64.8 million in fee remains available on contract,” she said in the statement. “All remaining fee opportunity is being restructured to be earned only upon delivery of Block 0 and Block 1.”

She also began laying out the reasons for the cost over runs.

“Factors that led to the critical Nunn-McCurdy breach include inadequate systems engineering at program inception, Block 0 software with high defect rates and Block 1 designs requiring significant rework,” she said in her statement. “Additionally, the complexity of cybersecurity requirements on OCX and impact of those requirements on the development caused multiple delays.

“The corrective actions to resolve these problems took much longer than anticipated to implement.”

Diving Deep
Fortunately, those reviewing the program for Carter will have a lot of raw material from which to develop the certification and the required plan to bring costs in line. The program has been under detailed quarterly review since December when Kendall held a critical “Deep Dive” analysis and approved a two-year extension.

The most recent Deep Dive review took place July 7 before Kendall and James with the support of Lt. Gen. Samuel Greaves, Space and Missile Systems Center commander and Air Force program executive officer for Space. They concluded, the Air Force said in a statement, that Raytheon had “made progress” in implementing critical changes including increasing automation in software development, platform deployment, and improving their software approach.

Kendall told reporters at Farnborough, however, that the results of the review were, in fact, a bit more muddled.

“To be blunt, it’s a mixed bag. I’m seeing some evidence of progress, but I’m still seeing some problems,” he said, according to Defense News. “I think Raytheon is putting additional resources into the program and I do see some signs of improvement. We also have had a couple of hiccups, I’ll say.”

The cryptic reprogramming request also raises questions

Although sources agreed that any shutdown would cost the Pentagon extra money and time, it is not clear from the request why what would likely be no more than a six-week break would result in up to four months’ delay and a cost increase of some $90 million dollars — an amount roughly equal to a quarter of the entire FY16 funding package for the program.

The nature of the 24-month re-plan referred in the request is also unclear. In December, Kendall decided to expend the program by two years. It is not clear if the 24-month re-plan is part of that two years or, as the name suggests, a preparation period for the two years.

Raytheon referred questions to the Air Force, which declined to respond saying officials are “not in a position to discuss specifics in the omnibus, or any other pending reprogrammings, until we have responses from all four defense committees.”

Decisions
Perhaps what DOD is really be trying to do is buy enough time to make a fully informed decision on its next step with OCX—something it is not prepared to do this fiscal year.

The June 30 breach announcement said the Nunn-McCurdy process “is expected to conclude by October of this year.” Defense News reported Kendall as saying DoD planned to announce in October whether the Air Force would rework the program and apply a new cost baseline or terminate it.

The biggest immediate challenge to that decision making process may come from Capitol Hill whose actions, including proposed funding cuts, Kendall said, could seriously damage any hopes of getting OCX back on track.

“My biggest concern right now is some of the things Congress is doing to the program would make it impossible to execute successfully; so, we’re going to be having conversations with the relevant committees,” he said, according to Defense News.

The risk of Congress curtailing DoD’s options, however, dropped significantly July 14 when Senate Democrats blocked a procedural vote and effectively shut down the defense appropriations process until after Labor Day when Congress returns to Washington.

That greatly increases the chances lawmakers will have to pass a short-term funding bill, called a continuing resolution or CR. CRs generally continue funding across the board at the same level as the previous year or the current President’s Budget as marked, whichever is lower.

"The result is OCX faces an enormous challenge under a CR of being limited by both the SASC [Senate Armed Services Committee] and SAC-D [Senate Appropriations Commitee – Defense] marks to the program," according to a spokesman at the U.S. Air Force Space & Missile Systems Center.

With the Nunn-McCurdy issue now also off the appropriators’ table, Kendall, James, and Carter will have more time to decide how to proceed. The Air Force has said it believes OCX is its best option, and Kendall has made clear he wants a shot at making the program work.

“I believe it is still possible for Raytheon to deliver this product, and I want them to do that,” he said at Farnborough. “If we take steps that would preclude that possibility, that’s not helpful.”

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>>Jade Morton’s Compass Points

Yu — or Jade, in English — Morton is an electrical engineer, a professor at Colorado State University (bound for the University of Colorado Boulder in 2017), and a shining star in the world of GNSS. She left work for eight years to be a full-time mother, then returned to a university professorship and high-level research, where she has been recognized for her work on ionospheric effects on global navigation satellite systems.

>>Jade Morton’s Compass Points

Yu — or Jade, in English — Morton is an electrical engineer, a professor at Colorado State University (bound for the University of Colorado Boulder in 2017), and a shining star in the world of GNSS. She left work for eight years to be a full-time mother, then returned to a university professorship and high-level research, where she has been recognized for her work on ionospheric effects on global navigation satellite systems.

When her father gave nine-year-old Jade an AM radio, she became the only kid in her small Yangzi River delta town to have one, and she listened to its one station from the moment the broadcast started until it went off the air, every day. “That may be when I first became fascinated by radio waves,” she says.

Born to educated parents in Hubei province, she and her three sisters grew up during China’s population boom and political turmoil. Like many children, they were sent to the countryside to live with their grandparents because of their parents’ heavy workload. Her father was a geologist and her mother, a surveyor,who worked for the Chinese Geological Bureau in central China.“They spent a lot of time doing fieldwork, away from home for months at a time,” Morton says. “So, when we were very young, my sisters and I were sent to live with my grandparents in distant Jiangsu Province, on the east coast.”

A Rural Life
The family lived in a small town surrounded by farming communities, about three hours northwest of Shanghai. There was no electricity, no running water, no library, and no TV.

“My sisters and I were very close, and with two aging grandparents, we all had to work very hard,” Morton recalls. Starting at age six, together with her four-year-old sister, they were responsible for the family water supplies and many other household chores.

“I bossed my little sisters around a lot and I was also very protective of them. I think they looked up to me,” she said.

Morton’s parents were able to visit only about once every three years because of the long and difficult journey and the lack of extended vacation time. Morton met her parents only a handful of times before she was grown and left home.

“We did not get to know my parents well,” she remembers. “My grandparents were more like our parents.”

The grandparents encouraged the girls to excel in academics while forbidding them to do anything related to sports and restricting their social interactions, especially with boys, Morton remembers.

“My grandmother was a very hands-on, capable woman. I remember watching her build our kitchen. She was not afraid of trying new things. She cooked every day, sewed all of our clothing, hand-made all of our shoes, and was a great storyteller. She was my role model.”

Morton learned to cook, sew, knit, clean, and take care of her younger sisters at a very young age. But, she says, her grandmother didn’t know much about science and mathematics or engineering.

Voices from a Distance
It was during one of her parents’ rare visits that Morton’s father presented her with the crucial AM radio.

There was only one station, which repeated the same two-hour broadcast three times each day. In addition to an appreciation for radio waves, the radio is how Morton, who then knew only the local Wu dialect (a variation of Shanghainese), learned how to speak the national language, Mandarin.

It was also her father who, from a distance, encouraged her to go into engineering. She communicated with her parents only through letters. They wrote to each other every month, and she remembers a letter from her father around the time she was filling out college applications.

“He had made two lists for me,” she says. One was called “fields you should go into,” and it included topics like electrical engineering, computer science, lasers, infrared, wireless communication, and remote sensing..

The other list was called “fields you should avoid”. Its first item was geology, his own area of work, and the second item was surveying, her mother’s.

“I think my parents regretted being in work that prevented them from taking care of their kids,” Morton says, “and they didn’t want us to ever have to face that.”

Morton followed her father’s advice and didn’t study geology or surveying, but rather physics in which she earned a degree, with distinction, at Nanjing University, and served as a member of the physics faculty. While at Nanjing, she won a scholarship to pursue graduate studies at Case Western Reserve University in Cleveland, Ohio, arriving there in 1985.

It was a big move requiring some big adjustments, but Morton managed. In fact, she got more than she had bargained for: her husband, John Morton. Her roommate at Case was a doctoral student in geology, and through her Jade met John, a geology post-doc at the time.

John’s family approved, Morton believes, because “The first time I met my future in-laws, I won them over immediately by fixing a few appliances that were not working properly.”

Despite the distractions of love, or perhaps because of them, Jade promptly earned her masters degree in electrical engineering in 1987, followed by a doctorate from Pennsylvania State University in 1991.

After she completed her degree at Penn State, her husband took a lab position in the geology department at Miami University, in Oxford, Ohio. Miami was a liberal arts school with no engineering programs. The closest institution Morton could find with programs in her field was the University of Michigan; so, she joined the Michigan Space Physics Research Laboratory as a post doctoral research fellow and commuted four hours between Ann Arbor and Oxford.

By the time her son was born in 1992, the commute had taken its toll. “I ended up quitting my work, moved to Oxford, where my husband worked, and became a full-time mom.” Four years later, her daughter was born.

In 2000, Miami University built an engineering school, and she was hired to start the electrical and computer engineering department. Jade Morton was back in business.

Then Came GNSS
“When I resumed my professional career in 2000, I decided that I wanted to work in an application field that had a direct, visible impact on everyday lives,” she said. She spent two years searching for the ”right” field, and found it.

“The pivotal moment came when I read about work done on software-defined GPS receivers by Dr. James Tsui at Wright Patterson Air Force Base,” she says. “The possibility of replacing hardware functions with mathematics written into software fascinated me,” and that was when she decided to put positioning, navigation and timing (PNT) at the center of her new electrical and computer engineering program at Miami.

Her efforts were encouraged and supported by the administration although she had no prior experience in GPS research. In May 2002, she got her chance to work with Tsui himself at Wright Patterson as part of the U.S Air Force Summer Faculty.

WPAFB was within driving distance of her home and had resources to support a wide range of research projects. Her first project involved mitigation of GPS self-interference for navigation in urban environments.

Morton quickly forged collaborative alliances with outstanding GNSS and PNT researchers at other universities in Ohio, including Ohio University, the Ohio State University, and the Air Force Institute of Technology.

Some of her early projects focused on developing algorithms to improve accuracy and robustness of GNSS receivers and integrated navigation systems. This work ranged from detecting and mitigating multipath, ultrawide band (UWB) interference, and jamming to assessing higher-order ionospheric errors.

It was in the latter field that Morton began to show her particular talents. She went on to develop a unique multi-GNSS data collection system for ionosphere and space weather monitoring. It addressed a problem with GNSS receiver designs employed for that purpose at the time.

“Existing GNSS receivers used to monitor ionosphere and space weather activities are very much based on the same algorithms developed for navigation applications,” Morton says. “They are not optimized for ionospheric effects studies, and measurements generated by these receivers are either inaccurate or not available, especially during strong space weather events.”

In order to get accurate measurements of ionospheric effects on GNSS signals, high quality, raw IF signals need to be collected during the ionospheric events. But, Morton says, it is not feasible to collect data at such a high rate continuously, due to the demand on storage capacities. So, she developed an event-driven, software-defined, multi-GNSS system that records raw IF data only when the events are occurring.

Her solution was built using low-cost, commercial, off-the-shelf, general purpose radios and hardware, with custom-designed control software and post-processing algorithms.

The system has gone through extensive validation and performance evaluations and has been deployed at remote locations for a number of years, including major observatories in Chile, Puerto Rico, Ascension Island, Peru, Hawaii, Singapore, and the United States.

Morton and colleagues have collected nearly 200 terabytes of high-quality GNSS observation data during ionosphere and space weather disturbances, and this year or next they hope to add 13 more stations in Canada, Chile, Hawaii, Puerto Rico, and Ethiopia and are seeking grant approval for three stations at the High Frequency Active Auroral Research Program (HAARP) in Alaska.

In other work, Morton has developed robust GNSS receiver processing algorithms that can “untangle” ionospheric and tropospheric effects from other error sources. And, most recently, Morton has developed new software-defined UWB systems for simultaneous high-resolution imaging, wideband communication, and indoor navigation, much of this work in collaboration with Miami University’s Dmitriy Garmatyuk, an associate professor of electrical and computer engineering.

Morton joins the CU-Boulder Aerospace Engineering Science Department as a full professor beginning in August 20917. Moving to CU-Boulder will allow her more opportunities to collaborate with a larger group of faculty and students with expertise in GNSS and remote sensing and with interests in both engineering and science.

Jade Morton’s star status is confirmed by her awards and appointments, among them the Institute of Navigation (ION) 2014 Thurlow Award and a 2014 fellowship in the Institute of Electrical and Electronics Engineers (IEEE), and service on the editorial boards of a number of technical publications, including some produced by IEEE and ION.

Through it all, Morton remains humble, crediting by name many supporters, partners, and collaborators from throughout her career, way too many to list here. And she seems to have enjoyed every minute of it.

“I have fallen in love with GNSS over and over again,” she said. “The first time I was really thrilled with GNSS was when I worked on an autonomous lawnmower project. We did not have the funds to purchase a differential system; so, instead we bought two low-cost Garmin receivers for $145 and wrote our own differential software. We achieved an average of two centimeters horizontal position accuracy.

“The second time was during an experiment in Alaska. HAARP Scientists had been routinely sending high frequency radio waves into the sky to modify ionosphere plasma distributions. The first time we witnessed the artificially generated disturbance of our GNSS signals was unforgettable”.

Today, Morton says the powerful applications that use GNSS —such as detecting, monitoring, or even forecasting earthquakes, tsunami, nuclear explosions, space and meteorological weather, and global climate — keep her more excited than ever.

Home Matters
Jade Morton also has many years of full-time parenting on her resumé, a clear indication that home life is also a high priority.

“I simply enjoy spending time with my kids, and I enjoy my work,” she says. “If you love what you are doing, then you can find time to enjoy both aspects of life. Work and family do not have to be in conflict all the time.”

Morton says she has always been very close to her kids. She did not travel much when they were young. She packed their lunches, cooked, and they had sit-down dinner every evening.

“We had a very large kitchen with a wide counter space, and when I was cooking dinner, the kids would do their homework on the counter. They loved discussing their school work with me, and not just the math and science problems.”

For the children, the attention seems to have paid off. Her 24-year-old son simultaneously completed four majors — in computer science, electrical engineering, engineering physics, and mathematics and statistics — at Miami University. He is now a doctoral student studying computer science at the University of California, San Diego. He is a National Science Foundation graduate fellow, a Barry Goldwater scholar, and is conducting research in microbial computational genomics.

Her daughter just turned 20 and is a sophomore at the California Institute of Technology in Pasadena, majoring in computer science.

She is proud of them and the time she was able to spend on them. “Thanks to my children,” she says. “I had the opportunity to relive the childhood that I missed.”

Yet she holds no regrets.

“Looking back, I am grateful for having my childhood experiences. They made me stronger. They add rich color to our lives. Personally, I believe that a ‘humble’ background makes one appreciate opportunities more,” she says.

“The harsh conditions of our childhood also helped me and my sisters to build strong work ethics. We learned from a very young age that perseverance leads to significant outcomes. By taking small steps along the way and not giving up, you can achieve what you set out to accomplish even if the task appears so daunting at first.”

Morton and her sisters are the living proof of what she preaches. Two of her sisters now live in the United States. One has a PhD in pharmaceutical science and is working for the Food and Drug Administration in Washington D.C. and the other is a network administrator at the University of Illinois, with master’s degrees in operations research and business. Her third sister, who remained in China, is chief executive officer of her own company.

“Another important lesson we know by heart through my grandmother’s teaching and through our own experiences,” Morton says, “is that trust and respect are not given to you freely. You have to earn them. We do not expect freebees. In fact, we expect to work extra hard to get anywhere. Fortunately, we learned to really like hard work.”

Morton says her children will never go through what she went through, but she has found her own way to steer them in the right direction.

“Despite my career ambition,” she says, “I took off eight years to be a full time mother. I only realized recently that the decision to be home with my children was deeply rooted in my fear of not being able to spend enough time with them.”

We have no fear for Jade Morton. With her children now grown and out of the house, she has time to enjoy cooking, classical music, and the great outdoors, and Colorado is a great place to be.

“My heart sings when I am out in the wilderness,” she says.

And the singing seems to agree with her.

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The GPS Directorate is requesting public comments on proposed changes to several interface specification (IS) and Interface Control documents (ICDs) for GPS signals in space. GPS receiver designers will be particularly interested in the changes as they affect how GPS signals should be processed in user equipment.

The GPS Directorate is requesting public comments on proposed changes to several interface specification (IS) and Interface Control documents (ICDs) for GPS signals in space. GPS receiver designers will be particularly interested in the changes as they affect how GPS signals should be processed in user equipment.

Several developments have prompted the changes, including work on the Next-Generation Operational Control Segment (OCX), design of the GPS Block III satellites, and a new memorandum of agreement among several federal agencies that would introduce new satellite outage file (SOF) to Notice Advisory to Navstar Users (NANU) messages.

Affected documents are IS-GPS-200H, IS-GPS-705D, IS-GPS-800D, ICD-GPS-240A, and ICD-GPS-870B. The comment period will close on August 19, 2016. The comments are in preparation for the next meeting of the Interface Control Working Group (ICWG) on September 21–22.

Downloadable comment forms are available on the gps.gov website maintained by the National Coordination Office for Space-Based Positioning, Navigation, and Timing. The GPS Directorate contact is Capt. Robyn Anderson, email <ro**************@***af.mil>, telephone 310-653-3064.

The ICWG is a specialized technical working group that serves as a forum to develop and provide interface requirements, as well as focus on interface detail definition and issues. Members include representatives from the GPS Directorate in the Space and Missile Systems Center at Los Angeles Air Force Base, California, other government agencies and offices, contractors, and other industry organizations.

The SOF updates are included in ICD-GPS-240A, which defines the functional data transfer interface between the GPS Control Segment and the GPS user and user-support communities during the Operational Control System (OCS)/Architecture Evolution Plan (AEP) systems era, and ICD-GPS-870, which defines the functional data transfer interface between the OCX and the GPS user and user-support communities.

The PIRN reflects a June 2014 Interagency Memorandum of Agreement Department of Defense (DoD) Joint Functional Component Command for Space (JFCC SPACE), the Department of Homeland Security (DHS), the U.S. Coast Guard Navigation Center (NAVCEN), and the Department of Transportation (DOT) Federal Aviation Administration (FAA) National Operations Control Center (NOCC).

The proposed interface revision notice (PIRN) 240A-002 describes new OCX-NGA and OCX-USCG interfaces between the control segment and the NAVCEN, as well as the SOF, a machine readable format of GPS satellite outage information that will be included in NANU operational advisories. PIRN-240A-002 will also address numerous formatting errors in the publicly released version of ICD-GPS-870.

ICD-GPS-870 also introduces a new Transition & Support Exchange Matrix and new language, such as turning NANUs into GPS advisories (published periodically as needed) and operation advisories into Ops Status (published daily). A new Public Common Almanac will provide orbital state and health status of the GPS constellation previously included in GPS almanacs (SEM, YUMA), anti-spoof status, and Extended Signals Health Status (ESHS) information

IS-GPS-200 defines the requirements related to the interface between the GPS space and user segments for the L1 and L2 civil signals. Proposed revisions are intended to remove ambiguity in contractor interpretation of the specification by clarifying the definition of the parameter Time of Predict (T_op) and other timing parameters.

PIRN-IS-200H-004 also introduces an Integrity/Clock/Ephemeris (ICE) data set, the collection of satellite-specific user range accuracy parameters, clock correction polynomial parameters, ephemeris parameters, and related parameters (health flags, time tags, etc.) needed to use the space vehicle (SV) broadcast signal(s)in the positioning service. As noted in the PIRN, “ICE data is sometimes also known as the user’s ‘hot start’ data for the SV. Before modernization, an ICE data set was sometimes called a “Subframe 1-2-3 data set.”

PIRN-IS-705D-003 outlines proposed revisions regarding timing parameters and the ICE data set similar to those in PIRN-IS-200H-004, only for the L5 signal. A related PIRN-IS-705D-004 describes modifications “to clarify extraneous, ambiguous, redundant, or missing editorial and/or administrative information to enhance the public document [IS-GPS-705D] quality (clear and concise communication)” as suggested by ICWG participants, stakeholders, and key members.

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A Federal Register notice published on July 7, 2016 announced Department of Transportation (DoT) plans to conduct additional testing of GPS/GNSS receivers this month as part of  their Adjacent Band Compatibility (ABC) Study. The notice was issued by DoT Assistant Secretary for Research and Technology Gregory Winfree.

A Federal Register notice published on July 7, 2016 announced Department of Transportation (DoT) plans to conduct additional testing of GPS/GNSS receivers this month as part of  their Adjacent Band Compatibility (ABC) Study. The notice was issued by DoT Assistant Secretary for Research and Technology Gregory Winfree.

The additional lab testing will be conducted at Zeta Associates in Fairfax, Virginia, and MITRE Corporation in Bedford, Massachusetts. It takes place in the context of a renewed push by Ligado (formerly Lightsquared Inc.) to obtain Federal Communications Commission approval to establish a terrestrial wireless broadband network in the United States in radio frequency spectrum near the band occupied by GPS L1 signals and those the other GNSS systems.

The goal of the ABC Study, overseen through the DoT’s Office of the Assistant Secretary for Research and Technology (OST-R), is to evaluate the adjacent radio frequency band power levels that can be tolerated by GPS/GNSS receivers. It is also designed to advance DoT’s understanding of the extent to which such power levels impact devices used for transportation safety purposes, among other GPS/GNSS applications.

In April 2016, radiated testing of GNSS devices took place in an anechoic chamber at the U.S. Army Research Laboratory at the White Sands Missile Range (WSMR) facility in New Mexico.
The ABC Study provides for testing various categories of receivers, include aviation (non-certified), cellular, general location/navigation, high precision and networks, timing, and space-based receivers. The notice indicated that approximately 12 receivers, representing each of these receiver categories, will be selected for additional testing from among those receivers tested in April.

Over the past year DoT obtained comments from public outreach that included four public meetings with stakeholders on September 18 and December 4, 2014, and March 12 and October 2, 2015. OST-R issued a draft test plan on September 9, 2015, and received comments on it, which led to publication of a final test plan on March 9, 2016, and requested voluntary participation in this Study by any interested GPS/GNSS device manufacturers or other parties whose products incorporate GPS/GNSS devices.

According to OST-R, discussion at the public meetings highlighted the importance of conducting tests of the ability of GPS/GNSS receivers to acquire signals in the presence of interference from nearby spectrum, which had always been planned as part of the DoT GPS ABC Assessment, but was not feasible due to time constraints during the radiated test conducted at WSMR. The goal of the additional lab testing is fourfold:

(1) receiver characterization for comparison with results obtained in April at the anechoic chamber at the U.S. Army Research Laboratory
(2) evaluation of out-of-band-emission (OOBE) interference at prescribed and proposed levels with long term evolution (LTE) wireless uplink and downlink signals
(3) GPS/GNSS signal acquisition characterization. (Zeta Associates will employ the same instrumentation for these three tests as was used in the radiated test at the U.S. Army Research Laboratory at WSMR, employing the same GNSS playback system and interference generation equipment with modifications to support OOBE and acquisition test requirements.)
(4) antenna characterizations.

The acquisition test will be conducted using 10 megahertz LTE signals at four frequencies: base station frequencies of 1525 MHz and 1550 MHz; hand-set frequencies of 1620 MHz and 1645 MHz.

The Federal Register notice can be found here. Information referenced in the notice and further background can be found online at on the National Coordination Office for Space-Based Positioning, Navigation, and Timing website. For ruther information, contact Stephen Mackey at the DOT/OST-R Volpe National Transportation Systems Center: email <st************@*ot.gov> or telephone 617-494-2753.

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Four global navigation satellite systems are scheduled to be fully operational orbiting Earth in the coming years: the NAVSTAR Global Positioning System (GPS) from the United States, the GLObal NAvigation Satellite System (GLONASS) from Russia, the Compass/BeiDou-2 System (BDS) from China, and Galileo from Europe. A considerably high number of signals, coming from the satellites of those constellations, will share the radio electric spectrum.

Four global navigation satellite systems are scheduled to be fully operational orbiting Earth in the coming years: the NAVSTAR Global Positioning System (GPS) from the United States, the GLObal NAvigation Satellite System (GLONASS) from Russia, the Compass/BeiDou-2 System (BDS) from China, and Galileo from Europe. A considerably high number of signals, coming from the satellites of those constellations, will share the radio electric spectrum.

Moreover, some aeronautical radio navigation systems (ARNS) operate in the E5 Galileo band. For example, distance measuring equipment (DME) and tactical air navigation (TACAN) systems (both in the ARNS category) broadcast strong pulsed ranging signals that interfere with Galileo E5a and GPS L5 signals. As analyzed in the work by F. Bastide et alia, listed in the Additional Resources section near the end of this article, DME/TACAN interferences can severely degrade the receiver performance if left unmitigated.

Galileo receiver simulators are a powerful way to investigate the initial performance of Galileo receivers without the need of heavy measurement campaigns. Applications of open-source Galileo simulators, especially regarding the E5 band, are still hard to find in the current literature. This article presents the development of an open-source 64-bit Galileo simulator, including the acquisition and tracking parts and the interference mitigation blocks for continuous wave interference (CWI) and DME. The simulator is available on demand and upon agreeing to its open-source conditions (Details listed in the Manufacturers section at the end of this article).

This article thoroughly analyzes three narrowband interference mitigation methods explained in the next sections (notch filtering, zeroing, and pulse blanking) with Galileo E5a signals based on the open-source simulator created in our group (Signal Processing for wireless positioning group at Tampere University of Technology). The performance studies are done with both the benchmark CWI and the DME interferences.

The novelty of our work comes from analyzing jointly these three techniques with a practical Galileo simulator and from selecting the best method according to the interference type. We show that zeroing methods are best used for robustness and with strong narrowband CWI while pulse blanking methods are better than notch filtering methods for strong DME interferers. We also show that interferers with up to 10–15 decibels stronger power than the E5a signal power can be tolerated relatively well and that all considered approaches have relatively similar performance for medium strength interferers.

GNSS Interferences
Very simply put, GNSS interference can be defined as any signal, from any service, working in the same frequency as the satellite receiver. Wideband interference refers to interference with bandwidth comparable to or higher than the GNSS signal bandwidth, e.g., ultra-wideband (UWB) technology that transmits a huge amount of information with a very low power using a large bandwidth, inter-system interferences between satellites from different GNSSs, or intra-system interferences between satellites from the same GNSS (here Galileo). The spectrum is becoming overwhelmed by all the satellite systems deployed.

Some interference can be mitigated well using time or frequency processing methods. However, when dealing with wideband interference, the performance of these methods degrades, and additional processing has to be carried out, such as space-based processing methods (i.e., antenna array–based methods). Minimum variance distortionless response (MVDR) and minimum power distortionless response (MPDR) beamformers are some examples.

These spatial approaches are not assessed in this article, however, and we focused our research on narrowband interference — those whose bandwidth is much lower than the bandwidth of the GNSS signal of interest. Narrowband interference can be created, for example, by TV harmonics, inter-modulation products or signals from very high frequency (VHF) and ultra high frequency (UHF) stations, or signals generated by systems such as DME or TACAN. Figure 1 illustrates the different types of interference in Galileo bands.

Another criterion can be the intentionality. Within the unintentional interference group, we can emphasize: DME/TACAN, amateur radio, TV, surveillance radars, or wind profiler radars. Under the name of intentional interference (see Figure 2 and Figure 3), three different interference signals can be distinguished: jamming signals, which deliberately block or interfere with authorized wireless communications through illegal devices decreasing the signal-to-interference-plus-noise ratio (SINR); spoofing signals, which falsely imitate the signal-in-space (SIS) and may hack a targeted GNSS receiver; and meaconing signals, which are the interception and delayed-rebroadcast of actual GNSS signals.

In this article, we have simulated and studied two interference signals: CWI and pulsed signals such as those generated by the DME or TACAN systems. A CWI signal can be modelled as

Equation (1) (see inset photo, above right, for all equations)

where Δf_cwi is the frequency offset with respect to the GNSS carrier, A is the CWI amplitude, and ϕ₀ is the CWI signal initial phase.

Signals from air radionavigation systems, such as DME or TACAN, consist of Gaussian RF paired pulses. Pulse separation is 12 microseconds with each pulse lasting 3.5 microseconds. The maximum repetition rate is about 3,000 pulse pairs per second (pps).

DME systems are designed to provide service for 100 planes simultaneously and the transmitted power may vary from 50 watts to 2 kilowatts. A DME signal is typically modeled as:

Equation (2)

where α = 4.5 • 10¹¹ s^–2 is a parameter controlling the pulse width and Δt = 12 • 10^–16 s is a parameter controlling the gap between paired pulses. The DME system operates between 960 and 1215 MHz; hence, it overlaps the Galileo E5 band.

Figure 4 shows an example of a DME signal in the time domain, its envelope, and its frequency spectrum.

State-of-the-Art Narrowband Interference Mitigation
Mitigation approaches can be categorized into two groups: time-domain and frequency-domain techniques. Time-domain mitigation techniques are those that make use of only mathematical calculation without any operation in the frequency domain. Heavy computational loads are avoided and complexity is lower. Non-linear methods, filtering methods based on convolution operations, or blanking methods are some of the proposed approaches in the literature.

Frequency-domain approaches are those based on signal alterations in the frequency domain. The article by A. Rusu and E. S. Lohan listed in the Additional Resources section near the end of this article presents a filtering method that exploits the cyclostationarity property using the spectral correlation function (SCF) and, therefore, can suppress additive white Gaussian noise (AWGN). Another, even simpler method is called zeroing, which is an excision-based method that we will explain in the next section.

The literature also presents various transformed domain mitigations that are worth mentioning briefly. One is the wavelet transform which is a time-scale representation technique that overcomes the common limit of fast Fourier transform (FFT) transformations using the short time Fourier transform (STFT), and another is the Gabor transform. Both of these methods separate useful signal and interference, removing the coefficients with high energy before the inverse transform. (These methods are described in articles by E. Anyaegbu et alia and K. Ohno and T. Ikegami, respectively, cited in the Additional Resources section.

Studied Mitigations. We selected the methods explained in this section based on the tradeoff between performance in acquisition and tracking and the method’s complexity, which in turn is directly proportional to the amount of computational load. The pulse blanking and notch filtering methods are time-based approaches, while the zeroing method is a frequency-based one in which the simulated signal is grouped into blocks that become suitable for FFT processing.

Pulse Blanking. This method is simple to implement: it blanks incoming signals that exceed a certain threshold, as illustrated in Figure 5.

The threshold can be chosen, for example, as a factor of the mean value of the absolute value of the received signal, i.e., γ = k • E(|s(t)|) with k optimized according to the interference. In our simulations, we used, for example, k = 3.5, chosen empirically. Figure 6 shows an example of pulse blanking performance in the frequency domain in the presence of a DME interferer.

Notch Filtering. Another time-domain method is notch filtering. A second order infinite impulse response (IIR) notch filter to mitigate the narrowband interference has been proposed, for example, by C. Ying-Ren et alia (see Additional Resources), based on the following transfer function:

Equation (3)

is the 3 dB filter bandwidth, and f_I is the frequency of the interferer that must be canceled.

The interfering frequencies are searched in a recursive manner, based on a threshold, as illustrated in Figure 7. As an example, Figure 8 shows the spectrum of a GNSS signal affected by DME interference, with and without notch filtering-based mitigation.

Zeroing. The discrete Fourier transform of a sample GNSS signal s(n) can be written as:

Equation (4)

Narrowband interferences can be rejected just by zeroing the spectral samples above a certain threshold. This time, the threshold γFFT is obtained according to the mean and the variance of the absolute value:

Equation (5)

where ε is a parameter adjusting the threshold (in our simulations ε = 0.5). Figure 9 presents an example of the zeroing method (in the frequency domain).

Qualitative Comparison Among Narrowband Mitigation Techniques
Table 1 shows the strengths and weaknesses of each solution. Unlike the blanking approach, the zeroing and notch methods can be used for both CWI and DME interference. However, zeroing is much less effective than blanking against DME interference, and therefore it is not suitable for pulsed interference. The spread of the spectrum due to the steep variation in the time domain makes it more difficult to separate the useful signal from the DME signal. Some energy from the DME pulses remains after processing the signals employing the zeroing method.

Open-Source Simulator
The E5 Galileo band comprises two bands, an E5a band centered at 1176.45 MHz and an E5b band centered at 1207.140 MHz. The Galileo E5 signal is an AltBOC(15,10) modulated signal with a chipping rate of 10.23 Mcps. Figure 10 illustrates simulated and theoretical power spectral densities (PSDs) of an AltBOC(15,10).

Our team at Tampere University of Technology developed a simulator with which to analyze Galileo signals; Figure 11 illustrates an overview of this development. The simulator was initially started within the European Union’s Galileo Ready Advanced Mass MArket Receiver (GRAMMAR) project and is now offered via free licensing for research purposes.

The simulator implements the transmitted signal based on an AltBOC(15,10) modulation with a constant envelope signal, according to the Galileo Open Service SIS Interface Control Document (SIS-ICD). The signal is sent over a multipath channel with up to five Rayleigh fading paths; noise and interference are added inside the channel block.

Due to computing capacity, the signal is transmitted at an intermediate frequency (IF) of 20 megahertz. The down-sampling factor is applied before the channel is employed to reduce the simulation time. Because the processing of the E5a band is only carried out at the receiver, a lower bandwidth is needed. The E5a sampling rate in our simulator is 31.5 megahertz, while the transmitter sampling rate is four times higher. The interference generation block, included inside the channel simulation, is detailed in Figure 12.

The receiver includes the interference mitigation block, the acquisition, and the tracking unit. Figure 13 illustrates the interference mitigation block.

The acquisition block estimates the time and frequency initial values that are then fed into a tracking block. Figure 14 and Figure 15 show, respectively, examples of the time-frequency acquisition mesh without and with interference mitigation, in the case of a CWI interferer at 1176.45 MHz, i.e., an E5a carrier frequency.

The acquired signal is passed through a narrow correlator tracking block, including a delay lock loop (DLL) and a joint frequency lock Loop (FLL) – phase lock loop (PLL). Figure 16 presents the tracking unit block diagram.

Performance Comparison
We compared the performance of the mitigation techniques and present the results here in terms of detection probability at various carrier-to-noise (C/N0) levels. Figure 17 shows the acquisition performance in the presence of CWI for the zeroing and notch filtering methods (as the pulse blanking does not work for CWI cases). Figure 18 shows the acquisition performance in the presence of DME interference for the pulse blanking and notch filtering methods (as the zeroing method is not so suitable for DME interference). Both figures also show the situation without interference mitigation.

In order to achieve a high detection rate, the blanking method for DME pulses and zeroing method for CWI are the most effective techniques among those studied.

Regarding the tracking results, it is worth mentioning how large the tracking error can become if no mitigation is taken into account to deal with the interference. The acquisition threshold is selected based on the highest peak of the time-frequency mesh. (For further discussion of this point, see the article by E. Pajala et alia in Additional Resources.)

Due to some type of interference, for instance DME pulses, large fluctuations can appear at some point along this mesh, and as a result the initial values can be extremely large as Figure 19 shows. The computed position error could even be on the order of kilometers, due to the fact that the acquisition stage would feed an erroneous estimate into the tracking. However, as might be expected, the studied mitigations are able to keep this error within reasonable values as shown in Figure 20.

Conclusions
The main objective of our work has been to analyze the impact of CWI and DME narrowband interference on the performance of the E5 Galileo signal, and more specifically, E5a band when processed independently of the E5b band. We have implemented and evaluated three types of narrowband interference rejections, namely pulse blanking, zeroing, and notch methods. We have shown that the notch filtering has the worst performance among the three of them, while pulse blanking and zeroing methods are the best for DME and CWI, respectively (but none of them works for both interference types). We have also demonstrated that interferers with up to 10–15 decibels stronger power than the E5a signal power can be tolerated relatively well and that all considered approaches have relatively similar performance for medium strength interferers.

Acknowledgments
The authors express their warm thanks to the Academy of Finland (project 250266) and to the EU FP7 Marie Curie Initial Training Network MULTI-POS (Multi-technology Positioning Professionals) Grant No. 316528 for their financial support for this research work.

Additional Resources
[1] Abdizadeh, M., GNSS Signal Acquisition in the Presence of Narrowband Interference, Ph.D. thesis, Calgary University/PLANS Group, 2013
[2] Alonso de Diego, D., and N. G. Ferrara, J. Nurmi, and E. S. Lohan, “Simulink-Based Open-Source Simulator For the Narrowband Interference Mitigation in E5a Galileo Band,” Proceedings of the 5th International Galileo Science Colloquium, Braunschweig, Germany, October 2015
[3] Anyaegbu, E., and G. Brodin, J. Cooper, E. Aguado, and S. Boussakta, “An Integrated Pulsed Interference Mitigation for GNSS Receivers,” The Journal of Navigation, Volume: 61, pp. 239-255, 2008
[4] Appel, M., and A. Hornbostel, and C. Haettich, Impact of Meaconing and Spoofing on Galileo Receiver Performance, Institute of Communications and Navigation, German Aerospace Center (DLR), Oberpfaffenhofen, Germany, 2014
[5] Balaei, A., and B. Motella, and A. Dempster, “GPS Interference Detected in Sydney-Australia,” Proceedings of the IGNSS Conference, Sydney, Australia, 2007
[6] Bastide, F., and E. Chatre, C., Macabiau, and B. Roturier, “GPS L5 and GALILEO E5a/E5b Signal-to-Noise Density Ratio Degradation due to DME/TACAN Signals: Simulations and Theoretical Derivations,” Proceedings of the 2004 National Technical Meeting of The Institute of Navigation, San Diego, California, pp. 1049-1062, 2004
[7] Capon, J., “High-Resolution Frequency-Wavenumber Spectrum Analysis,” Proceedings of the IEEE, Volume: 57, pp. 1408–1419, 1969
[8] Dovis, F., “Recent Trends in Interference Mitigation and Spoofing,” Proceedings of the ICL-GNSS, Finland, 2011
[9] Gao, G. X., “DME/TACAN Interference and its Mitigation in L5/E5 Bands,” Proceedings of ION GNSS, Fort Worth, Texas, 2007
[10] Kang, C. H., and S. Y. Kim, and C. G. Park, “A GNSS Interference Identification using an Adaptive Cascading IIR Notch Filter,” GPS Solutions, Volume: 18, Issue: 4, pp.605-613, 2014
[11] Motella, B., and S. Savasta and F. Dovis, ”A Method to Assess Robustness of GPS C/A in Presence of CW Interferences,” International Journal of Navigation and Observation, Volume: 2010, Article ID 294525, 2010
[12] Musumeci, L., and F. Dovis, “Use of the Wavelet Transform for Interference Detection and Mitigation in Global Navigation Satellite Systems,” International Journal of Navigation and Observation, Volume: 2014, Article ID 262186
[13] Musumeci, L., and J. Samson and F. Dovis, “Performance Assessment of Pulse Blanking Mitigation in Presence of Multiple Distance Measuring Equipment/Tactical Air Navigation Interference on Global Navigation Satellite Systems Signals,” Radar, Sonar & Navigation, IET, Volume: 8, Issue: 6, pp. 647-657, 2014
[14] Ohno, K. and T. Ikegami, “Interference Mitigation Study for UWB Radio Using Template Waveform Processing,” IEEE Transactions on MTT, Volume: 54, Issue: 4, pp. 1782-1792, April 2006
[15] Pajala, E., and E. S. Lohan and M. Renfors, ”CFAR Detectors for Hybrid-Search Acquisition of Galileo Signals,” CDROM Proceedings of ENC-GNSS, 2005
[16] Rusch, L. A., and H. V. Poor, “Narrowband Interference Suppression in CDMA Spread Spectrum Communications,” IEEE Transactions on Communications, Volume: 42, Issue: 234, pp. 1969-1979, 1994
[17] Rusu, A., and E. S. Lohan, “Investigation of Narrowband Interference Filtering Algorithms for Galileo CBOC Signals,” Proceedings of the European Conference of Communications (ECCOM), Paris, France, 2012
[18] Rusu-Casandra, A., and E. Lohan and G. Seco-Granados, “Contributions to the Filtering of Narrowband Interferences in GNSS,” International Multidisciplinary Scientific GeoConference (SGEM), Albena, Bulgaria, 2013
[19] Rusu-Casandra, A., and I. Marghescu and E. S. Lohan, “Impact of Narrowband Interference on Unambiguous Acquisition Approaches in Galileo,” Proceedings of the International Conference on Localization and GNSS, pp. 127-132, Tampere, Finland, 2011
[20] Van Trees, H. L., Optimum Array Processing, Detection, Estimation, and Modulation Theory, Part IV, John Wiley & Sons; New York, New York USA, pp. 428–699, 2002
[21] Ying-Ren, C., and H. Yi-Cheng, Y. De-Nian, and T. Hen-Wai, “A Novel Continuous Wave Interference Detectable Adaptive Notch Filter for GPS Receivers,” Proceedings of the IEEE Global Telecommunications Conference, pp. 1-6, Miami, Florida USA, 2010
[22] Zhang, J., and E. S. Lohan, Effects and Mitigation of Narrowband Interference on Galileo E1 signal Acquisition and Tracking Accuracy, Proceedings of the ICL-GNSS, Tampere, Finland, 2011
[23] Zoltowski, M. D.. and A. S. Gecan “Advanced Adaptive Null Steering Concepts for GPS,” Proceedings of the 1995 IEEE Conference Record, Military Communications Conference (MILCOM’95), San Diego, California USA, pp. 1214–1218, 1995

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Japan’s regional and augmentation positioning system, the Quasi-Zenith Satellite System (QZSS), is a project yet to be developed. While it will become a constellation of seven satellites covering the western Pacific area, only “Michibiki,” the first satellite launched in 2010 for technological validation , is now in orbit.

Still, a reading of the government’s most recent Basic Plan on Space Policy reveals that it is one of the principal space projects of Japan. According to that plan, a constellation of four satellites will be in place by the end of fiscal year 2017, and three more satellites will be added to the constellation by 2023.

The operator of the satellites is, and will remain, the Japanese government. Since a Cabinet decision on September 30, 2011, the Cabinet Administration Office (CAO) has been responsible for the development, maintenance and operation of the QZSS. The National Space Policy Secretariat of the CAO is the section responsible for space policy, including the operation of the QZSS. The CAO has already procured three satellites to be launched by 2017 through a public bid.

The ground segment of the system, including the daily command and control of the satellites, was distinguished from the satellites and procured from the private sector by a private finance initiative (PFI) scheme. (The PFI Act of 1999 allowed public/private partnership arrangements in which private companies construct, renovate, and sometimes manage public infrastructure.)

The CAO invited bids for the ground segment in December 2012. NEC Corporation, a Japanese multinational information technology company and satellite manufacturer, was selected as the provider of the service.

The NEC group then incorporated Quasi-Zenith Satellite System Services Inc. (QSS), which became the contractor. The scope of business of QSS Inc. is promoting, improving and managing the ground system through 2032.

The scheme specified in the procurement conditions was “build, own and operate (BOO).” Therefore, QSS as contractor owns the facilities and is responsible for their renewal and maintenance, when necessary. Although the PFI Act allows contractors to charge a fee for services if it is allowed under the procurement conditions, no clause on such service is included in the contract for QZSS.

QZSS Concept: Its Origin and Development
The QZSS is a regional navigation satellite system that will augment and complement the U.S. Global Positioning System because QZSS signals are compatible with those of GPS. So in this sense, QZSS adds “extra satellites” to the NAVSTAR GPS constellation. At the same time, QZSS will send augmentation signals that improve the precision of GPS positioning. In this latter sense, it will serve as an augmentation system.

The project is based on the idea that three satellites forming a constellation in geosynchronous orbit on different planes will enable at least one of them to be observed from Japan near zenith all the time. The near-zenith position is emphatically suited to Japan, a mountainous country with cities crowded with tall buildings. A satellite on the low elevation angle would not be observable either in urban or rural areas. Once the constellation is in full operation seven years from now, four of them will always be observable from Japan. Positioning then will become possible solely by QZSS, without relying on GPS (or any other GNSS).

The project was initially proposed by the Keidanren, the Japanese Business Federation of national companies and associations, in 2001 The Cabinet-level Council for Science and Technology (now Council for Science, Technology and Innovation) endorsed the proposal in a report by a specialized committee in 2004.

In the beginning, the cost of the project was to be shared 50/50 by the government and private sector, which was loosely called a public-private partnership (PPP). The business sector expected that QZSS satellites would at the same time be available for telecommunication and broadcasting services. Consequently, the Advanced Space Business Corporation (ASBC) was formed with investment from across the whole industry. However, it soon turned out that telecommunication and broadcasting by satellites would probably not be competitive against equivalent services and the enthusiasm of the business sector for QZSS quickly waned.

Nonetheless, the project survived. The ASBC was dissolved, but the Space Positioning Research and Application Center (SPAC) was formed as a kind of its successor, focusing exclusively on positioning services. At the same time, the government started to take on larger responsibility. The Cabinet decision of 2011 confirmed it, and the project regained its momentum.

With this background, Japan has participated with all of the world’s other GNSS providers in the United Nations-sponsored International Committee on Global Navigation Satellite Systems (ICG). Japan hosted the ICG’s sixth annual meeting in Tokyo in 2011.

Legal Framework for QZSS
Under Japanese law, both space-specific laws and general laws are applicable to the QZSS.

The first to mention is the Basic Space Law of 2008, which sets forth a number of overarching state policies and stipulates six “basic principles” of Japanese space policy, namely, peaceful use of outer space, improvement of the lives of the citizenry, advancement of industries, development of human society, international cooperation, and consideration for the environment.

Under “improvement in the lives of the citizenry” the 2008 law mentions the “promotion of information systems on positioning,” in addition to satellite-based telecommunication and observation systems, and mandates the government to take measures necessary to achieve them.

As of April 2016, the bill concerning launch and control of satellites is tabled before the Diet, Japan’s bicameral legislature. The bill, sometimes called the Japanese “Space Activities Law,” provides a regulatory regime for space activities by private entities.

However, even after the bill is approved by the Diet and is enforced as the law, it will not be applicable to QZSS, because it does not regulate space activities (control of satellites) by the government. The ground segment alone will not qualify as a space activity as defined in the bill.

Among the general laws applicable to the QZSS is the Basic Act on the Advancement of Utilizing Geospatial Information (the so-called National Spatial Data Infrastructure (NSDI) Law of 2007). It provides a general framework for the use of geospatial information, again with the nature of declaring a policy program.

Two provisions of the NSDI Law specifically refer to satellite positioning. One of them mandates the government to take necessary measures to advance use of geospatial information through highly reliable positioning satellite services, while the other requires the government to proceed with technological research and development feasibility studies concerning satellite positioning, as well as to promote its application.

Finally, the signals emitted from the QZSS satellites are governed by the Radio Act. This protects QZSS signals from unlawful interference, in particular jamming or spoofing. To be more specific, any user of a device that transmits radio waves may not disturb the function of other radio equipment. Otherwise a license to operate a radio station will not granted. As a result, any person spoofing or jamming the QZSS signals, whether either a radio operator without a license or in breach of the conditions of the license, shall be considered in violation of the law. The penal sanction includes imprisonment of up to one year and/or a fine of up to one million yen (less than US$10,000.)

International Framework
The interference of radio waves is also a matter of international concern. Prior to the development of QZSS, Japan and the United States collaborated on development of Japan’s GPS augmentation system, the Multi-functional Transport Satellite-based Satellite Augmentation System (MSAS). The Ministry of Land, Infrastructure, Transport and Tourism (MLIT) administers MSAS. The two governments issued a Joint Statement on Cooperation in the Use of the Global Positioning System in 1998.

Regular consultation meetings have been held almost annually since then, and the collaboration has proven useful to the development of QZSS. The U.S. government has supported the development of QZSS by Japan through these consultation meetings. Under the framework of consultation meetings working groups have been established to ensure compatibility and interoperability of GPS and QZSS. After MSAS terminates its service, which is anticipated to be around 2020, QZSS will replace it as Japan’s satellite based augmentation system (SBAS) for GPS.

Privacy Issues and Potential Concerns of Users
From the users’ side, positioning by satellites is convenient, but also raises concerns about the privacy, which is actually already a problem because all the smart phones and mobile phones sold in Japan are equipped with a GPS signal receiver.

Legally, the privacy issue must be considered on several levels under Japanese law. The basis of the privacy is Japan’s Constitution. Although the Constitution which has not been amended since its adoption in 1947, does not mention privacy explicitly, the courts have held that the right to privacy is included in “the right to pursue personal happiness,” which is one of the basic human rights declared in Article 13 of the Constitution.

When a person alleges infringement of his or her privacy and claims damages, however, the Constitution cannot be the basis of the claim as such. The alleged victim must raise either a tort claim under the Civil Code (when the claim is against a private party) or a claim under the State Compensation Act (when the claim is against the national or local government). The Constitution can support such a claim as embodying the underlying value.

Furthermore, in the context of satellite positioning, two statutes are relevant. One is the Act on Personal Data Protection, which imposes some specific duties on those who collect information that can identify individuals (“personal information”). Whether that personal information falls under the privacy concept protected by the Constitution or not, the stipulated duties to protect personal data must be complied with.

The other privacy-related statute is the Act on Telecommunication Services, which regulates telecommunication service providers. One of the providers’ duties is to respect the secrecy of communication — another fundamental human right protected under Article 21 (2) of the Constitution. Although the Constitution is addressed to the government, the Act on Telecommunication Services extends the duty to private parties, namely service providers.

Against these backgrounds, the Ministry of Internal Affairs and Communications (MIC) issued Guidelines on the Protection of Personal Information in the Telecommunications Business, most recently amended in June 2015. The guidelines are accompanied by explanatory notes.

As regards the dissemination to a third party of the personal positioning information obtained from a user of a mobile device, the guidelines allow this only if the user has given consent or the judge has issued a warrant. Because positioning information from mobile devices is constantly obtained and recorded by the telecommunication service provider, even when the user does not make a call, it may not (necessarily) fall under the “secrecy of communication.” Therefore, the guidelines require providers to protect user privacy in general as a human right.

When a telecommunication service provider does share positioning information with a third party (recipient), it must “take necessary measures” to prevent infringement on the user’s right. The explanatory notes clarify that the necessary measures include (i) the user’s consent, (ii) alert for the users, by indication on the screen or otherwise, (iii) security against unauthorized access to the information and (iv) ensuring respect for the users’ privacy by, for example, appropriate arrangements with the recipient.

Further, the guidelines provide that the telecommunication service provider is permitted to obtain personal positioning information either upon a request of the police and in accordance with a warrant issued by a judge or upon request of the rescuing agency if the person is in serious and imminent danger.

The positioning information obtained from a user making a call is considered “sender’s information” and is treated differently. Such information is covered by the rules governing secrecy of communication. In principle, the telecommunication service provider shall not disclose the sender’s information except when necessary for its service (such as when the receiver requests its disclosure). Still, cases may arise in which the disclosure of a sender’s information will be justified, such as when (i) the user (sender) has given consent, (ii) the judge has issued a warrant, (iii) the police request the location of the sender in a case of criminal blackmailing by telephone (such as a call from a kidnapper) or (iv) a person makes an emergency call notifying authorities or service providers about an imminent threat to someone’s safety.

These guidelines are relevant to signals from any satellite system, whether GPS or QZSS. However, the owner or operator of the QZSS satellites is not a telecommunication service provider, nor is QSS Inc., the operator of the ground facility. Therefore, neither are subject to the guidelines.

Police Use of GPS Receivers
Law enforcement officers engaged in the investigation of a crime may prefer to place a GPS receiver on a suspect’s car to track its movements rather than acquire positioning information from a telecommunication service provider. Recently, it has been disputed whether such an action by the police requires a warrant by a judge. The legal issue is whether the placement of a GPS receiver without consent of the vehicle’s owner is “compulsory disposition” for which the police must comply with the procedure specified in the Criminal Procedure Law.

The decisions of the lower court are divided. In one case (Osaka District Court, 5 June 2015, unpublished), the court excluded the positioning track record obtained by the GPS receiver placed on the car without a warrant, by holding that the record was “evidence obtained through an unreasonable investigation.”

The same court, however, later held that the accused was found guilty by other evidence than the excluded track record (Osaka District Court, 10 July 2015, unpublished). Before that decision, another judge at the same court (Osaka District Court, 27 January 2015, unpublished) held that acquiring GPS information in a similar way is not unreasonable. The facts of the two cases are different, not least the precision level of the device used. Therefore, how the case law will develop is yet to be seen.

Developing Applications: Key to QZSS Success
The importance of developing applications for QZSS signals is so well recognized that it was written into the conditions of the PFI procurement for ground facilities, which required the contractor to explore potential application services.

The operator of the system (whether the government or QSS) is not expected to enter into an agreement with a potential application service provider. The operator will unilaterally send out signals from the satellites, and anyone can use them to develop applications. Technically, the application service provider must accept the performance standard and interface specifications for the QZSS signals. These documents are distributed only to the members of QZS System User Society (QSUS). The membership of QSUS is open to any individual free of charge.

With regards the civil liability that could arise in case of errors in signals, an issue sometimes discussed in Europe, no specific arguments have been made. A general understanding seems to be that the operator can be immune from such liability, if the performance standard and interface specifications include a disclaimer that mentions the need for incorporating redundancies, where necessary.

The apparent absence of concerns about liability may partly be due to the fact that QZSS has developed as a complement and augmentation to GPS. The signals, like GPS, are sent out without charge and this may give the impression that the user makes use of the signals at their “own risk” without liability to the system operator.

This commonly held belief may not be entirely correct, however, as levying a charge is only one of the factors that determine an operator liability and not a conclusive one. Further, unlike in the United States, sovereign immunity has been abandoned in Japan and the government can incur liability based on negligence under the State Compensation Act. These differences, however, have not attracted much attention yet.

Conclusions
Japan’s QZSS is similar to the European Galileo in that its use is limited to civil and commercial purposes, with no military use being intended. As such, not only the space-specific laws but also general laws such as the Civil Code, Radio Act and Telecommunication Services Act will be relevant.

Until now, only the privacy issues have been much debated, because they are common to GPS and, therefore, are already real problems. Other issues such as the contractual framework with application service providers or the tort liability for erroneous signals have not yet discussed. Still, they might gain larger importance once the system is in full operation.

Additional Resources
[1] Aoki, Setsuko (2009), Current Status and Recent Developments of Japan’s National Space Law and its Relevance to Pacific Rim Space Law and Activities, Journal of Space Law 35: 363
[2] Kitamura, Naohiro (2015), The Impact of Japan’s New Space Policy on Business, Space Law Newsletter of the International Bar Association Legal Practice Division, 2015: 3
[3] Kozuka, Souichirou (forthcoming), Law and Navigational Satellite Systems in Japan, in: Ram Jakhu (ed.), Routledge Handbook of Space Law, Routledge
[4] Murakami, Hiroshi (2008), New Legislation on NSDI in Japan: “Basic Act on the Advancement of Utilizing Geospatial Information”, Bulletin of the Geographical Survey Institute 55: 1
[5] Pekkanen, Saadia M., and Paul Kallender-Umezu (2010), In Defense of Japan: From the Market to the Military in Space Policy, Stanford University Press.
[6] Tsujino, Teruhisa (2005), Effectiveness of the Quasi-Zenith Satellite System in Ubiquitous Positioning, Science & Technology Trends – Quarterly Review (NISTEP) 16: 88.

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Q: How does one compute the noise power to simulate real and complex GNSS signals?

Q: How does one compute the noise power to simulate real and complex GNSS signals?

A: Simulation of GNSS signals is very important to test and validate algorithms. With some algorithms, such as acquisition or tracking, we do not need to simulate a realistic constellation with actual ranges, Doppler or navigation data. We can simply simulate the signal after reception at a receiver’s front-end and only take into account and fix to desired values certain parameters, such as the intermediate and the sampling frequencies, the Doppler and Doppler rate, or the signal and the noise power. This last parameter requires some caution, because the noise power depends on the type of sampling (real or complex).

In this article we show how to simulate noisy GNSS signals after a front-end. We will first present the model considered for the received signal and review the constraints on the intermediate and sampling frequencies for real and complex sampling. Then, we will introduce the noise, review the properties of a white noise, and discuss the sampling of a band-limited white noise to determine the expression of the noise power.

Signal model
The GNSS signal received at the antenna can be modeled as

S_r(t) = a_Ix_I(t) cos(2πf_rt + ϕ_r) + a_Qx_Q(t) sin(2πf_rt + ϕ_r),    (1)

where t is the time; a_I and a_Q are amplitudes of the in-phase (I) and quadrature-phase (Q) components; x_I(t) and x_Q(t) are the baseband signals of the I and Q components consisting of a spreading code and possibly a secondary code, a sub-carrier or data; f_r includes the carrier and Doppler frequencies; and φ_r is the carrier phase.

It is also possible to have only one component, e.g., a_Q = 0 for the GPS L1 C/A signal. The model could be more complete and include the Doppler effect on the code or the Doppler rate, but that would not change the following discussion and is thus omitted. An example of the spectrum for s_r(t) — denoted s_r(f)— is given in Figure 1 (see photo at the top of this article for Figures 1, 2 and 3), where B is the bandpass bandwidth of the signal.

Let us first consider the front-end depicted in Figure 2 (top), which performs real sampling. This is a very simplified representation, but enough for our discussion. This front-end is composed of:

• a bandpass filter (BPF) for image-rejection
• a mixer to bring the signal to a lower frequency
• a low pass filter (LPF) for anti-aliasing (aliasing is explained next); this filter can also be a bandpass filter
• an analog-to-digital converter (ADC)

Of course, there are also a few amplifiers in a front-end, not shown in Figure 2. See the Additional Resources section for details about real and complex front-end designs.

After the mixer and the low pass filter, the signal is

Equation (2) (see inset photo, above right, for equations)

where f_i = f_r – f_LO is the intermediate frequency and φ_i = φ_r – φ_LO, with f_LO and φ_LO the frequency and phase of the local oscillator. Figure 3 shows the spectrum of this baseband signal. To avoid aliasing, i.e., an overlapping of the positive and negative sides of the spectrum, we require f_min ≥ 0 or, equivalently, f_i ≥ B/2.

After the ADC, the signal is sampled and becomes

Equation (3)

where T_s is the sampling period equal to 1/f_s, with f_s being the sampling frequency. The spectrum of the sampled signal is the spectrum of the continuous-time signal duplicated around each multiple of f_s, as shown in Figure 3. To avoid aliasing, we require f_s ≥ 2f_max, or f_s ≥ 2f_i + B. This is the well-known Nyquist-Shannon sampling criterion.

Now, let us consider the front-end depicted in Figure 2 (bottom), which performs a complex sampling. After the mixer and the low pass filter, the baseband signals are

Equation (4)
Equation (5)
Equation (6)

(see Additional Resources section for a reference about this). In this case, even if there can be an aliasing in each branch, when considering the signal as complex, aliasing is not possible because the negative side of the spectrum is canceled as shown in Figure 4 (second from bottom). Therefore f_i can take any value, whatever the signal bandpass bandwidth B.

After sampling, the signal is

Equation (7)

The bottom portion of Figure 4 shows the signal spectrum at this point, and, to avoid aliasing, we should have

f_s + f_min ≥ f_max ⇔ f_s ≥ f_max − f_min ⇔ f_s ≥ B.    (8)

Therefore, the sampling frequency now depends only on the signal bandpass bandwidth.

Noise Power Computation
In reality, an element of noise is present in addition to the received signal. This noise, called thermal noise, is induced by the antenna and the front-end themselves and is assumed to be an additive white Gaussian noise (AWGN). So, we next explain how to compute the power of this noise at the output of the front-end.

A white noise is a noise whose power spectral density (PSD) is equal at all frequencies, as shown in Figure 5, top. As the PSD is the Fourier transform of the autocorrelation (Wiener-Khinchin theorem), the autocorrelation of a white noise is null everywhere except at zero, as shown in lower plot in Figure 5. This implies that the values of a white noise at different instants are uncorrelated, and, therefore, the mean of a white noise is zero. This is true whatever the probability density function of the noise, which can be Gaussian, Laplace, uniform, and so forth (as long as the mean is zero).

The two-sided PSD of the thermal noise is equal to N₀/2, where N₀ is the noise power density and depends on the effective temperature of the front-end. However, the front-end filters the incoming signal; therefore, the thermal noise is filtered, too, and the noise just before the ADC is not white. We will thus see the conditions to obtain a white noise after the ADC.

With a real sampling, assuming an ideal brick wall filter of baseband bandwidth B_F, the noise just before the ADC will be a band-limited white noise, as shown in Figure 6, and the noise power (which is equal to the integral of the noise PSD) will be σ₂ = N₀B_F.

The autocorrelation function of this band-limited white noise is

Equation (9)

This can also be seen using the PSD. After the sampling, the noise spectrum is duplicated around each multiple of f_s. Figure 7 (top) shows the spectrum of the band-limited white noise sampled with a sampling frequency higher than 2B_F, and the noise is clearly not white. Figure 7 (middle and bottom) shows the spectrum of the band-limited white noise sampled with sampling frequencies of 2B_F and B_F, respectively. In both cases, the noise is white and the noise power (which is equal to the integral of the noise PSD in the interval

Equation (a)

On the other side, to respect the Nyquist-Shannon theorem, we should have f_s ≥ 2B_F. Therefore, the only possibility to satisfy both conditions is f_s ≥ 2B_F, and thus the noise power must be σ₂ = N₀f_s/2.

Similar developments can be done for the case of complex sampling. The difference is that the sampling frequency must be equal to f_s = B_F/k to get a discrete white noise, and the Nyquist-Shannon theorem imposes f_s ≥ B_F. Therefore, the only possibility to satisfy both conditions is f_s = B_F, and thus the noise power on each branch must be σ₂ = N₀f_s, i.e., twice the noise power that exists with real sampling.

Application to GNSS signals
Using the previous models, at the output of the front-end, a real GNSS signal can be modeled as

s_b(nT_s) = a_Ix_I(nT_s) cos(2πf_inT_s + ϕ_i) + a_Qx_Q(nT_s) sin(2πf_inT_s + ϕ_i) + w(nT_s),    (10)

and a complex GNSS signal can be modeled as

s_b(nT_s) = (a_Ix_I(nT_s) − ja_Qx_Q(nT_s)) e^{j(2πf_inT_s+ϕ_i)} + w_I(nT_s) + jw_Q(nT_s),        (11)

where w, w_I and w_Q are white Gaussian noises of power σ2 (the value of σ2 for w_I and w_Q is twice the one for w, as mentioned previously). The values for a_I and a_Q can correspond to the amplitudes after the antenna, and σ can be computed using the expressions of the previous section. However, a normalization is usually performed because only the ratio between the signal power and the noise power matters, not their absolute value.

To describe the level of a received GNSS signal, two parameters can be used: the signal power, or the carrier power to noise power density ratio (or carrier-to-noise ratio). The power of the baseband signals x_I(t) and x_Q(t) is equal to 1 because they are based on binary waveforms of amplitude ±1. Therefore, the power of an I/Q component is

Equation (b)

The carrier-to-noise ratio is simply the ratio between the signal power and the noise power density, i.e., C/N₀ = P/N₀, thus

Equation (c)

Both signal amplitude and noise standard deviation (σ) have very low values. Consequently, in simulation, it is convenient to perform a normalization, usually to have a noise standard deviation (std) of 1. For that, it is sufficient to divide both signal amplitude and noise std by the noise std.

Previously we have seen that the noise power is not the same depending on the type of sampling. With the normalization, the noise std is the same for both real and complex sampling; so, it is the signal amplitude that will be different according to the type of sampling. Table 1 summarizes the expressions to compute the noise std and the component amplitude from the component power and noise power density, without and with normalization. Note that in the normalized case, the signal amplitude can be determined directly from the carrier-to-noise ratio and the sampling frequency, thus the computation of the noise power density N₀ is not required.

Let’s consider two examples to illustrate all this. First, we want to simulate a GPS L1 C/A signal of C/N₀ = 25 dBHz, sampled at f_s = 5 MHz with real sampling. With normalized values, the noise std will be 1, and the signal amplitude will be

Equation (d)

Second, we want to simulate a GPS L5 signal with a power of –174 dBW, sampled at f_s = 524 MHz with complex sampling, and N₀ = –204 dBW/Hz. Thus, the power of each I/Q component is –177 dBW since both components have the same power. With normalized values, the noise std will be 1, and the signal amplitude will be

Equation (e)

To conclude, Table 2 provides a concise summary with all the parameters discussed in the article.

Acknowledgments
The authors thank Myriam Foucras, Marc-Antoine Fortin, and Miguel Angel Ribot for their help, and Jia Tian whose question initiated this article.

Additional Resources
For more information about parallel acquisition architectures, refer to:
Leclère, J., “Resource-efficient parallel acquisition architectures for modernized GNSS signals,” Ph.D. thesis, EPFL, Switzerland, 2014.

For information about GNSS receivers, including real and complex front-end designs, refer to:
Borre, K., et al., A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach, Birkhäuser Boston, 2007

Chastellain, F., and C. Botteron, and P.-A. Farine, “Looking Inside Modern Receivers,” IEEE Microwave Magazine, vol. 12, no. 2, pp. 87–98, 2011

van Diggelen, F., A-GPS: Assisted GPS, GNSS, and SBAS, Artech House, 2009.

For details about quadrature signals, please see:
Lyons, R., Understanding Digital Signal Processing, Prentice Hall, 2010.

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In February 2011, Russia launched the first satellite of the GLONASS-K1 series, i.e., SVN (space vehicle number) 801 (R26), which in addition to the legacy frequency division multiple access (FDMA) signals, for the first time was enabled to transmit code division multiple access (CDMA) signals on the GLONASS L3 frequency (1202.025 MHz). Later in 2014, the GLONASS program added SVNs 802 (R17) of series K1 and 755 (R21) of series M, and in 2016, SVN 751 of series M, with the capability of transmitting CDMA L3 signals to the constellation.

In February 2011, Russia launched the first satellite of the GLONASS-K1 series, i.e., SVN (space vehicle number) 801 (R26), which in addition to the legacy frequency division multiple access (FDMA) signals, for the first time was enabled to transmit code division multiple access (CDMA) signals on the GLONASS L3 frequency (1202.025 MHz). Later in 2014, the GLONASS program added SVNs 802 (R17) of series K1 and 755 (R21) of series M, and in 2016, SVN 751 of series M, with the capability of transmitting CDMA L3 signals to the constellation.

The GLONASS FDMA double-differenced (DD) ambiguity resolution is known to be hampered by the inherent inter-frequency biases. Several calibration procedures have been proposed to deal with this impediment. With GLONASS-CDMA however, the standard methods of integer ambiguity resolution can be applied to resolve the integer DD ambiguities. The goal of this article is to provide a first assessment of this L3 ambiguity resolution performance.

Measurement Setup
Our analysis is based on the GLONASS L3 data of the satellite pair R21-R26, collected by two multi-frequency GPS/GLONASS receivers on an eight-meter baseline at Curtin University, Perth, Australia (Figure 1). We also compare these results with their GPS L5 counterparts for the satellite pair G10-G26. The rationale behind making this comparison is that both these modern signals have close frequencies (see Table 1) and the same BPSK(10) modulation.

Figure 2 shows their observed carrier-to-noise densities (C/N₀).As their C/N₀ graphs show a similar signature, their signals are expected to have similar noise characteristics. Figure 1 also shows the skyplot of the mentioned satellite pairs at Perth. For both the GLONASS and the GPS satellites, we used the broadcast ephemeris data. Table 2 provides further information on the data-set that we used.

Model of Observations
Because our analysis is based on satellite pairs, we first formulate the two-satellite observational model. With the expectation E{.} and dispersion D{.}, the corresponding double-differenced (DD) system of observation equations reads

Equation (1) (see inset photo, above right, for all equations)

in which p and φ are the DD code and phase observable, respectively, ρ the DD receiver-satellite range and a the DD integer ambiguity in cycles. The ambiguity a is linked to the DD phase observable through the signal wavelength λ. With the elevation-dependent weighting function wθ_s (s = 1, 2) for the s_th satellite with elevation angle θ_s, respectively, the final weight becomes

Equation (1a)

Here wθ_s is taken as

Equation (2)

where θ_s is in degrees. The zenith-referenced standard deviations of the undifferenced code and phase observables are denoted as σ_p and σ_φ. In our analysis we considered two different models. These are arranged in ascending order of strength as:

1. Geometry-free model (GFr): This is the model as formulated in (1). As it is parametrized in ρ, it is free from the receiver-satellite geometry. The single-epoch DD ambiguity is then estimated as

Equation (3)

2. Geometry-fixed model (GFi): In this model, the information on receiver position, from e.g. surveying, and satellite position, from navigation file, is available and thus ρ is assumed known. The single-epoch DD ambiguity is then estimated as

Equation (4)

Note that although the observations of only two satellites are used, both the geometry-free and geometry-fixed models are instantaneously solvable, i.e., based on data of only a single epoch. See the article by P. J. G. Teunissen (1997) listed in the Additional Resources section near the end of this article for a more detailed discussion of these models.

Ambiguity Resolution
The data used for our L3 and L5 ambiguity resolution performance analysis were one hertz sampled on DOY 21 of 2016 over the time period UTC [07:16:59-09:13:38]. As the observations of a satellite pair and a receiver pair result in only one unknown DD ambiguity, simple integer rounding can be used for integer ambiguity resolution. We denote the float ambiguity by â, the fixed (integer rounded) ambiguity by ă, and the reference ambiguity by a. The reference DD ambiguity a is computed based on the multi-epoch solution of the geometry-fixed model.

In Figure 3, the time series of â − a and ă − a are shown for the receiver pair CUT3-CUCC, for both the GLONASS satellite pair R21-R26 (left column) and the GPS satellite pair G10-G26 (right column).

While the geometry-fixed results of the two signals are comparable, the GPS L5 geometry-free ambiguity resolution outperforms that of the GLONASS L3, which can be explained by means of the satellites’ elevations: the higher the elevation, the lower the noise level, thus the better the ambiguity resolution performance (cf. 2, 3 and 4).

The bottom set of graphs in Figure 3 also illustrates the elevation time series of the chosen satellite pair (in blue) in addition to the geometry-fixed DD ambiguities. Here we can see that the elevations of the GPS satellite pair is higher than those of the GLONASS satellite pair. Also, the low elevation of R26 at the end of the period and the low elevation of G10 at the beginning of the period describe the larger fluctuations of, respectively, the GLONASS DD ambiguities and the GPS DD ambiguities at those time instants.

For both the geometry-free and the geometry-fixed scenario, we computed the formal and empirical ambiguity success-rates, defined as the probability of correct integer estimation. The formal ambiguity success-rate can be computed, as discussed in the article by P. J. G. Teunissen, (1998) cited in Additional Resources, as

Equation (5)

being the standard normal probability density function (PDF).

For the computation of the formal success-rate, the ambiguity standard deviation was taken as the square-root of an average of the formal variances, i.e., as

Equation (6)

Table 3 lists the empirical and formal success-rates for both GLONASS and GPS corresponding with Figure 3. Based on these results, the empirical values are consistent with their formal counterparts. Moreover, as the model gets stronger from one-epoch geometry-free to geometry-fixed, the ambiguity resolution success-rates experience a significant improvement. In case of the one-epoch geometry-free model, σ_â is governed by the code precision σ_p. Including the observations of k epochs, the corresponding σ_â of k-epoch geometry-free model is improved by almost √ k 
times.

Switching from geometry-free to geometry-fixed model, σ_â is then governed by the phase precision σ_φ which is much better than the code precision. For the geometry-free model to achieve a success-rate of more than 0.999, 40 epochs of observation in the case of GLONASS L3 and 10 epochs in the case of GPS L5 are required.

To further confirm the consistency between the data and models, we compare, for both the geometry-free and the geometry-fixed model, the formal PDF with the histogram of the estimated DD ambiguity. Normalizing the estimated DD ambiguity by means of the elevation weighting function results in a new quantity, i.e., √ w (â – a) which, assuming the data to be normally distributed, has a central normal distribution with the standard deviation of √ w σ_â. Depending on whether the underlying model is geometry-free or geometry-fixed, the value of √ w σ_â can be obtained from (3) or (4), respectively.

Figure 4 displays the histograms of the normalized DD ambiguity √ w (â – a), for geometry-free and geometry-fixed model. The corresponding formal distribution is also shown by the red curve. It demonstrates the consistency between the empirical and formal distributions.

Conclusion
We have presented a first assessment of GLONASS CDMA L3 double-differenced integer ambiguity resolution.

For our analyses, we made use of the GLONASS L3 signal transmitted by the satellite pair R21-R26 and of the GPS L5 signal from the satellite pair G10-G26. The carrier-to-noise densities of both signals were shown to have similar signatures.

The integer ambiguity resolution performance in the framework of geometry-free and geometry-fixed observational model was demonstrated. As the model gets stronger from geometry-free to geometry-fixed model, the ambiguity resolution improves significantly.

Our empirical results (in the form of success-rates and normalized ambiguity PDF) showed a good agreement with their formal counterparts, thereby showing the consistency between data and models. The ambiguity resolution of GPS L5 was better than that of the GLONASS L3, which was attributed to the higher elevation of the GPS satellites w.r.t the GLONASS satellites during the considered period.

Additional Resources
[1] Euler, H. J., and C. C. Goad, “On Optimal Filtering of GPS Dual Frequency Observations without Using Orbit Information,” Bulletin Geodesique 65(2):130-143, 1991
[2] Global Positioning Systems Directorate, NAVSTAR GPS space segment/navigation user segment Interface Specification, Revision F (IS-GPS-200H:24-Sep-2013)
[3] Hofmann-Wellenhof, B., and H. Lichtenegger and J. Collins, Global Positioning System: Theory and Practice, Springer Science & Business Media, 2013
[4] Information and Analytisis Center for Positioning, Navigation, and Timing, GLONASS constellation status, available here, accessed February 2, 2016
[5] Leick, A., GPS Satellite Surveying, John Wiley and Sons, 2003
[6] Oleynik, E., “GLONASS Status and Modernization,” United Nations/Latvia Workshop on the Applications of Global Navigation Satellite Systems, Riga, Latvia, 2012
[7] Reussner, N., and L. Wanninger, GLONASS Interfrequency Biases and Their Rffects on RTK and PPP Carrier-Phase Ambiguity Resolution,” Proceedings of ION GNSS 2011, Institute of Navigation, pp. 712-716, 2011
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