The Arctic houses an estimated 90 billion barrels of undiscovered, technically recoverable oil and 44 billion barrels of natural gas liquids according to the U.S. Geological Survey. These potential energy reserves represent 13 percent of the untapped oil in the world.

The Arctic houses an estimated 90 billion barrels of undiscovered, technically recoverable oil and 44 billion barrels of natural gas liquids according to the U.S. Geological Survey. These potential energy reserves represent 13 percent of the untapped oil in the world.

Russia, Canada, and the United States plan to explore the Arctic for extensive drilling soon. At the same time, the Arctic is becoming more accessible to normal shipping because of global climate change. New summer sea lanes have already opened up, and projections of sea ice loss suggest that the Arctic Ocean will likely be free of summer sea ice sometime between 2060 and 2080.

The combination of undiscovered oil and climate change are driving a dramatic increase in the demand for navigation in the Arctic. In this article, we examine different approaches to improve accuracy and enable integrity in the Arctic, including the addition of more satellite-based augmentation system (SBAS) reference stations in or near the Arctic, integration of Iridium satellites with GNSS, and use of multi-constellation GNSS.  

More SBAS Reference Stations
The Arctic is a sensitive environment, and thus navigation should have high integrity. For this reason, we are interested in extending SBAS coverage to serve this region.

At present, none of the three operational SBAS provide meaningful service in the far North. In fact, Figure 1 (see inset photo, above right) shows the current SBAS availability coverage with vertical alert limit (VAL) equal to 35 meters, and horizontal alert limit (HAL) equal to 40 meters.

Figure 1 is based on two of the currently operating SBASes: the U.S. Wide Area Augmentation System (WAAS) and the European Geostationary Navigation Overlay Service (EGNOS).

. . .

For the purposes of our analysis, we assume that all these references stations provide the same measurement quality as current WAAS reference stations. We also assume the availability of continuous user connectivity, that is, the user is always able to receive the SBAS corrections.

Although SBAS GEO coverage is limited in the Arctic, other ways exist with which to maintain the connectivity, such as using low earth orbit (LEO) satellites. We will address this topic in more detail in the next section.

. . .

Iridium for SBAS Messages
The second requirement for ensuring integrity in the Arctic is continuous connectivity — in other words, how the SBAS messages are delivered seamlessly to users. Currently, WAAS uses geosynchronous orbit (GEO) satellites to broadcast error corrections. Because the GEO satellites are located directly above the Earth’s equator, WAAS GEO coverage does not include the Arctic.

. . .

The over-the-pole design of Iridium orbits ensures very good high-elevation satellite visibility in the Arctic. Because Iridium satellites already provide voice and data services to satellite phones and integrated transceivers around the globe, Iridium is a strong candidate for enabling SBAS linkage to Arctic users.

. . .

As a bonus, Iridium satellites could improve the vertical dilution of precision (VDOP) if the Iridium satellites also broadcast ranging signals. VDOP is a measure of how well the positions of the satellites are arranged to generate the vertical component of the positioning solution. Higher VDOP values mean less certainty in the solutions and can be caused if the satellites have low elevation angles in relation to users.

. . .

With added Iridium satellites, the VDOP values increase to 1.6 from 2.1 for 24 GPS satellites, and to 1.3 from 1.8 for 31 GPS satellites. Moreover, the VDOP values are more even over the Earth’s surface. For both scenarios of 24 and 31 GPS satellites, adding Iridium satellites improves VDOPs in the Arctic.

Multiple Constellations for High Availability of Integrity
A third issue, although not as critical as the first two, is the VDOP degradation encountered in the Arctic. Because GPS satellites are in an orbital plane of 55 degree inclination, not enough satellites are visible at high elevation angles for users in the Arctic. For this reason, VDOPs in the Arctic are worse (i.e., higher) than those close to the equator.

. . .

If using only two constellations, adding GLONASS to GPS is the most beneficial combination. GLONASS satellites orbit at 19,100 kilometers (11,842 miles) altitude with a 64.8-degree inclination. Compared to the 55-degree inclination of the GPS orbital planes, the GLONASS constellation produces better coverage in high latitudes. The VDOP improvement in the Arctic is more dramatic using three or even all four constellations.

Conclusion
This article identified a need for high-integrity navigation in the Arctic and analyzed techniques to extend SBAS coverage to this important region. We show that the current network of reference stations can be augmented to provide Arctic integrity with high availability. Moreover, Iridium satellites could provide a broadcast channel to the SBAS users. Multiple GNSS constellations significantly improve VDOPs and thus reduce vertical positioning errors in the Arctic.

For the complete story, including figures, graphs, and images, please download the PDF of the article, above.

Acknowledgment
The authors would like to thank the Federal Aviation Administration’s Satellite Navigation Program Office and the Boeing Company for supporting this research.

Additional Resources
[1] Evans, J.V., “Satellite Systems for Personal Communications,” Proceedings of the IEEE, Volume: 86, Issue: 7, 1998
[2] United States Geological Survey, “90 Billion Barrels of Oil and 1,670 Trillion Cubic Feet of Natural Gas Assessed in the Arctic,” USGS, July 2008

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[updated April 13] Federal Aviation Administration (FAA) officials say that loss of control over an Intelsat geostationary (GEO) carrying a GPS Wide Area Augmentation System (WAAS) transponder could subject users in the National Air Space (NAS) to temporary outages for the rest of this year, beginning within the next two to four weeks as the GEO drifts out of a useable orbit.

Intelsat S.A. announced the anomaly in Galaxy 15 (G-15) on April 8. Although the communications services provided by G-15, located at 133 degrees west longitude (WL), have not been affected, according to Intelsat, the satellite apparently is not responding to commands by controllers. The anomalous condition began on April 3, according to the FAA.

The Luxembourg-based Intelsat is moving an older spacecraft (G-12) that serves as a backup for G-15 from its location at 123 degrees WL. However, G-12 does not have an L-band transponder, which is needed for WAAS transmissions.

[updated April 13] Federal Aviation Administration (FAA) officials say that loss of control over an Intelsat geostationary (GEO) carrying a GPS Wide Area Augmentation System (WAAS) transponder could subject users in the National Air Space (NAS) to temporary outages for the rest of this year, beginning within the next two to four weeks as the GEO drifts out of a useable orbit.

Intelsat S.A. announced the anomaly in Galaxy 15 (G-15) on April 8. Although the communications services provided by G-15, located at 133 degrees west longitude (WL), have not been affected, according to Intelsat, the satellite apparently is not responding to commands by controllers. The anomalous condition began on April 3, according to the FAA.

The Luxembourg-based Intelsat is moving an older spacecraft (G-12) that serves as a backup for G-15 from its location at 123 degrees WL. However, G-12 does not have an L-band transponder, which is needed for WAAS transmissions.

G-15 is one of three GEOs currently in orbit that provide WAAS services (see accompanying graphic) and provides the farthest coverage west across much of the Pacific Ocean. In terms of absolute U.S. service coverage, WAAS signal unavailability will only occur at 16 airports in northwestern Alaska.

The impending outage would have caused those sites to lose the capability to support aircraft approach and landing procedures based on localizer performance with vertical guidance (LPV, sometimes called “lateral precision with vertical guidance”). However, none of those airports has a published LPV approach yet. Aviation users in the NAS outside the affected area will continue to have LPV service, according to the FAA.

LPV is the highest precision WAAS-enabled aviation instrument approach procedures currently available without specialized aircrew training requirements, such as required navigation performance  (RNP). Landing minima are similar to the instrument landing system (ILS), that is, 200 feet decision altitude and half-mile visibility.

Pilots flying into the 16 Alaska airports will still be able to use GPS-based published lateral navigation (LNAV) approaches so long as they check on the availability of receiver autonomous integrity monitoring (RAIM) before they depart on a flight, Leo Eldredge, the FAA’s GNSS program manager, told Inside GNSS. RAIM requires a minimum number of GPS satellite signals to be available in order to work.

Due to a lack of redundant GEO coverage, however, WAAS users may experience temporary service interruptions —  for example, during WAAS GEO uplink station (GUS) switchovers — until a new GEO comes on line. These switchovers may occur three to five times per year, according to the FAA, and require up to five minutes to restore LPV service.

The risk of a single-point WAAS failure will exist in some areas until redundancy is restored to the system. The FAA only began tests a few weeks ago on a newly leased transponder on one of the other two spacecraft, Inmarsat 4F3 (broadcasting on PRN-133). Called the Gap Filler, the 4F3 GEO will ensure all of the continental United States and most of Canada and Alaska have redundant coverage.

During the test period, the 4F3 navigation signal will be unusable for navigation purposes, according to the FAA. The test will run from March to December 2010 at which time the broadcast signal is expected to be certified as operational and usable for navigation. “We are working several alternatives to accelerate the Gap filler GEO and also procure a replacement for the Intelsat GEO that has failed,” Eldredge told Inside GNSS. However, it typically takes four years to restore a failed GEO, which is one of the reasons WAAS must have a minimum of three GEOs.”

Among those alternatives is re-integration of the Inmarsat-III Pacific Ocean Region GEO that was formerly used by WAAS prior to switching to Intelsat. FAA is also considering accelerating acquisition of an additional GEO using funds saved from the loss of the Intelsat G-15.

The G-15 satellite primarily provides transmission capacity for cable programmers in North America. Based on current technical information, Intelsat says it expects no service interruption for the media customers on this satellite.

All media traffic on this satellite is planned to be transitioned to Intelsat’s Galaxy 12 satellite, when it arrives at 133 degrees WL around April 14. However, the company has not announced any plans regarding the L-band service.

Launched in 2005, G-15 is an Orbital Star model built by Orbital Sciences Corporation and designed to be operational through 2022.

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PCTEL, Inc., of Bloomingdale, Illinois USA, announced that its high-precision Wide Area Augmentation System (WAAS) ground station GPS antennas will be deployed in India’s GPS-Aided Geosynchronous Augmented Navigation (GAGAN) system.

PCTEL, Inc., of Bloomingdale, Illinois USA, announced that its high-precision Wide Area Augmentation System (WAAS) ground station GPS antennas will be deployed in India’s GPS-Aided Geosynchronous Augmented Navigation (GAGAN) system.

This summer the Indian Space Research Organization (ISRO), which is building GAGAN in collaboration with the Airports Authority of India, announced the award of an $82-million contract to Raytheon Company to build the GAGAN ground stations.

PCTEL’s WAAS antennas are currently deployed for the North American WAAS system and Japan’s MTSAT Satellite-based Augmentation System (MSAS). http://www.insidegnss.com/node/107

A product line within PCTEL’s military and government antenna product family, the land-based WAAS antennas enable highly precise navigation and tracking of aircraft. Part of PCTEL’s Maxrad-branded products, the model 2225NW WAAS antenna features high-rejection filters and covers L1, L2, and L5 GPS frequencies.

“PCTEL is proud to offer this advanced level of precision antenna technology for the aviation industry and expand our activities outside the U.S.,” said Jeff Miller, vice-resident and general manager of PCTEL`s Antenna Products Group. “We stay committed to the development of high performance GPS antenna solutions for commercial and defense markets,” added Miller.

PCTEL will be demonstrating its WAAS GPS antennas as well as its portfolio of antenna products at the ION GNSS 2009 conference, exhibit Booth #523, September 22-25 in Savannah, Georgia.

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GNSS manufacturer NovAtel Inc., of Calgary, Alberta, Canada, has received a new contract from the Federal Aviation Administration (FAA) to develop the next-generation GPS Wide Area Augmentation System (WAAS) reference receiver (the “GIII” receiver). The three-year contract is worth up to US$9.7million.

GNSS manufacturer NovAtel Inc., of Calgary, Alberta, Canada, has received a new contract from the Federal Aviation Administration (FAA) to develop the next-generation GPS Wide Area Augmentation System (WAAS) reference receiver (the “GIII” receiver). The three-year contract is worth up to US$9.7million.

WAAS is a space-based system that broadcasts corrections data and information to improve the overall accuracy and integrity of GPS satellite signals. NovAtel has worked with the FAA WAAS program since 1995, providing and supporting two previous generations of reference receivers for the WAAS ground network.

According to NovAtel, the “technology refresh” receiver contract will add support for the latest-generation L1C, L2C, and L5 signal capability on a qualified RTCA DO-178B software and DO-254 hardware platform.

The WAAS GIII development and qualification receiver program, under which NovAtel will build up to 14 receivers, includes a growth provision for additional signal capability such as the European Galileo satellite navigation satellite system.

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(Updated Jan.26) President Barack Obama’s nomination of William J. Lynn III, a senior vice-president at Raytheon Corporation, for deputy secretary of defense and his granting Lynn a waiver from the new administration’s own rules on former lobbyists has provoked considerable criticism from some quarters.

As the number two official in the Department of Defense (DoD), Lynn would report directly to Robert Gates, the current secretary of defense who has continued in that position in the new administration, the only holdover from ex-President Bush’s cabinet. Gates has come out strongly in support of Lynn, saying that he requested the waiver from the president.

Among other responsibilities, the deputy secretary serves as the co-chair of
the Space-Based Positioning, Navigation, and Timing (PNT) Executive
Committee (ExCom). Lynn would succeed Gordon England, who has paid a lot of attention to GPS during his term in office and enhanced the role of the PNT ExCom as an arbiter and advocate for the GPS program throughout the federal government.

(Updated Jan.26) President Barack Obama’s nomination of William J. Lynn III, a senior vice-president at Raytheon Corporation, for deputy secretary of defense and his granting Lynn a waiver from the new administration’s own rules on former lobbyists has provoked considerable criticism from some quarters.

As the number two official in the Department of Defense (DoD), Lynn would report directly to Robert Gates, the current secretary of defense who has continued in that position in the new administration, the only holdover from ex-President Bush’s cabinet. Gates has come out strongly in support of Lynn, saying that he requested the waiver from the president.

Among other responsibilities, the deputy secretary serves as the co-chair of
the Space-Based Positioning, Navigation, and Timing (PNT) Executive
Committee (ExCom). Lynn would succeed Gordon England, who has paid a lot of attention to GPS during his term in office and enhanced the role of the PNT ExCom as an arbiter and advocate for the GPS program throughout the federal government.

Although Lynn apparently hasn’t had a close association with the GPS program, in his role as Raytheon’s senior vice-president for government operations and strategy he is the chief liaison with federal executive and legislative branches for a company that does. Raytheon is the prime contractor for the Federal Aviation Administration’s GPS Wide Area Augmentation System (WAAS) and leads one of two competing teams seeking the contract to build the next-generation GPS operational control segment (OCX).

Overall, Raytheon reportedly did $18.3 billion worth of business with the U.S. government in 2007. Lynn has agreed to sell his stock in Raytheon and other defense contractors to avoid potential conflicts of interest in the future.

According to an Associated Press report, until early last year Lynn had lobbied Congress and federal agencies on a variety of programs including missiles, sensors and radar, advanced technology programs, and space and intelligence funding. That put Lynn inside the two-year ban on lobbing of federal agencies and Congress that Obama has set for new appointees — hence, the need for a waiver in the case of the Raytheon executive.

In a January 22 press conference, Gates defended that decision, noting, "People in the transition certainly recognized that it [Lynn’s Raytheon role] was an issue. And I interviewed Bill Lynn. I was very impressed with his credentials. He came with the highest recommendations of a number of people that I respect a lot. And I asked that an exception be made because I felt that he could play the role of a deputy — of the deputy — in a better manner than anybody else that I saw."

Gates has been “intimately involved” in the process of identifying and interviewing appropriate candidates for various vacancies throughout the department, according to Pentagon Press Secretary Geoff Morrell.

Morrell added that, before Obama made his announcement, Gates has been busy working with the president-elect’s transition team to identify appropriate candidates for various vacancies throughout the department and interview them personally.

“I think he feels as though . . . we’ve made some good progress toward identifying some very capable candidates to fill some very big jobs within the department,” Morrell said.

Among the people expressing concern about the nomination and waiver are members of the Senate Armed Services Committee, which conducted a hearing on Lynn’s nomination last week. Committee Chairman Carl Levin (D-Michigan) said that under congressional
ethics rules, Lynn would still have to recuse himself for one year from
matters related to Raytheon, but he says he will back the waiver.

Senator John McCain (R-Arizona) the ranking Republican on the Armed
Forces Committee and the Republican nominee in last November’s presidential election, said on Sunday, January 25: "I have asked to see which areas that Mr. Lynn will be recused from. But I think we need to probably move forward with his nomination."

Lynn returns to the capital as no stranger to either DoD or Congress. From 1997 to 2001, Lynn served as one of five under secretaries of defense, acting as the department’s comptroller and principal advisor to the Office of the Secretary of Defense (OSD) for all budgetary and fiscal matters.

Another significant DoD position held by Lynn was that of OSD director of program analysis and evaluation (PA&E) from 1993 to 1997, where he oversaw all aspects of the DoD’s strategic planning process.

Raytheon recently announced that its OCX team completed the segment design review and modernized capability engineering model demonstration on Dec. 13, 2008. The company is working under a $160 million Phase A system design and risk reduction contract awarded by the GPS Wing in November 2007. A team led by Northrop Grumman is the other contender for the OCX contract. A final decision on the OCX prime contract is expected later this year.

Before entering the DoD in 1993, Lynn served for six years on the staff of Senator Edward Kennedy as liaison to the Senate Armed Services Committee. He has also been a Senior Fellow at the National Defense University, on the professional staff at the Institute for Defense Analyses and served as the executive director of the Defense Organization Project at the Center for Strategic and International Studies.

A graduate of Dartmouth College, Lynn has a law degree from Cornell Law School and a Master’s in Public Affairs from the Woodrow Wilson School at Princeton University.

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Following in the footsteps of the recent update of the GPS SPS Performance Standard, the Federal Aviation Administration (FAA) has issued a performance standard for the Wide Area Augmentation System (WAAS).

Following in the footsteps of the recent update of the GPS SPS Performance Standard, the Federal Aviation Administration (FAA) has issued a performance standard for the Wide Area Augmentation System (WAAS).

On November 6, the agency reported that it has now published 1,333 localizer performance with vertical guidance (LPV) approach procedures based on WAAS. The LPVs cover runways at 833 airports.

WAAS is the multi-billion-dollar U.S. satellite-based augmentation system (SBAS) developed under an FAA contract by Raytheon Corporation and designed to provide real-time differential corrections, integrity messages (satellite signal “health”), and ranging signals that WAAS-capable equipment can use to improve navigation. It is used primarily by private general-aviation pilots, business and regional aircraft, and some cargo aircraft.

The system includes two geostationary satellites and a network of ground reference stations that monitor GPS satellite signals. WAAS corrects for the GPS satellite position errors, ionosphere delays, and other disturbances in the GPS signals and warns pilots when the satellites are not functioning correctly and should not be used for navigation.

The WAAS performance standard was released October 31 under the signature of Steve Zaidman, FAA vice-president for technical operations. Copies may be downloaded from the Public Materials section on the website of the National Coordination Office for Space-Based Positioning, Navigation, and Timing (PNT).

The document specifies the levels of navigation performance that will be available to suitably equipped users who use both the GPS standard positioning service (SPS) broadcast signals and the WAAS augmentation signal and commits the U.S. government to meeting the minimum levels identified in it.

Leo Eldredge, FAA’s GNSS program manager, characterizes the WAAS PS as “a composite of the current specifications and standards that WAAS already complies with.” According to Eldredge, the National Space-Based PNT Executive Committee (EXCOM) had issued an action item for FAA to publish a WAAS performance standard similar to the GPS SPS PS to formalize the commitment of the USG to provide PNT services based on GPS.

As the GPS system continues to modernize, FAA will update the WAAS PS to include, for instance, the L5 civil signal that will enable dual-frequency positioning in protected aeronautical radionavigation bands. As with the SPS PS, those updates will also follow on the Department of Defense (DoD) process for documenting GPS specifications and standards.

“For civil aviation purposes, we depend on the commitments contained in the SPS PS as the basis for our commitment to provide service through augmentation and the approvals for aviation use of standalone GPS,” Eldredge told Inside GNSS. “Before we could approve use of L5, for either standalone use or as part of an augmented service, the commitment to provide that service would first need to be provided by DoD in a PS.”

Historically, he added, the DoD includes the new signals in a performance standard after the full operational capability (FOC) has been achieved. The Air Force is adding the second civil signal (L2C) and L5 on the IIR-M and IIF space vehicles (SVs), respectively, and later L1C on the GPS III satellites.

“It is not yet known when the new signals will be included in a performance standard or if the current paradigm of two performance standards (SPS PS & PPS PS) will continue to describe the PNT services provided by GPS,” Eldredge said. “Under the current paradigm, we would expect all of the civil signals (L1-CA, L1C, L2C, and L5) to be included in updates to the SPS PS as each new signal achieves FOC (24 SVs operating).”

Eldredge added that the FAA plans to upgrade WAAS to use the L5 signal at the monitoring stations and in aircraft avionics, which will eventually require an update to the WAAS PS. The GPS master schedule currently shows L5 achieving FOC in 2018 and a recent Federal Register announcement regarding use of L2 semi-codeless GPS requires the agency to complete the transition of L2 to L5 by December 31, 2020. WAAS currently uses L2-semicodeless at the monitor stations only.

Another Milestone
The November 6 announcement marks a milestone for WAAS-supported LPV approach procedures, which now surpass the number of approach procedures based on its ground-based predecessor, the Category-I instrument landing system (ILS).
LPV enables pilots to use instrument flight rules for approach and landing operations down to a decision height of 200 feet. FAA is scheduled to declare the full LPV performance (FLP) phase of WAAS operational early next year.

WAAS-capable GPS receivers are specified under FAA Technical Standard Orders 145 and 146. More than 37,000 WAAS-capable units have been sold into the general aviation market and are currently increasing at the rate of about 1,000 units per month, according to Eldredge, and more than 500 business and regional aircraft have been equipped since 2007.

Like the GPS SPS standard, the WAAS PS defines the WAAS signal-in-space (SIS) characteristics, navigation message, and performance requirements. For instance, LPV is designed to provide 16-meter horizontal accuracy and 20-meter vertical accuracy 95 percent of the time. LPV status can also calls for a 6.2-second time to alert when the system is not meeting specified requirements.

Actual performance has exceeded these levels for WAAS, however. For example, vertical error has not been observed to exceed 12 meters in the history of WAAS operational service. Real-time and statistical performance results for WAAS and GPS are available at the FAA Technical Center WAAS Test Bed website.

The agency’s goal is to produce 500 new WAAS procedures each year until every qualified runway in the National Air Space has one.

In addition to the supporting IFR operations, WAAS provides for both more flexible approach and departure routings and more direct, fuel-efficient routings through the air traffic control system. It also provides an improved navigation source for other aviation innovations such as terrain avoidance warning systems and automatic dependent surveillance-broadcast (ADS-B).

Using GPS, an ADS-B–equipped aircraft determines its own position and periodically broadcasts this position and other relevant information to air traffic control stations and other aircraft with ADS-B-in equipment flying in the area.

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NOAA’s National Geodetic Survey (NGS) has recently added 43 GPS tracking sites into the Continuously Operating Reference Station (CORS) network overseen by the federal agency.

NOAA’s National Geodetic Survey (NGS) has recently added 43 GPS tracking sites into the Continuously Operating Reference Station (CORS) network overseen by the federal agency.

The network helps ensure the consistency and accuracy of the nation’s spatial reference system. Each CORS site provides GNSS — GPS and GLONASS — carrier phase and code range measurements, recorded on a 30-second or shorter interval (see figure). Data are free and accessible via the Internet.

NGS invites organizations and individuals to share data from their permanent GPS base stations by including these stations in the National CORS network, following a rigorous a set of criteria established by the agency.

The NOAA CORS network now contains more than 1,200 sites spanning the United States, its territories, and several foreign countries. Surveyors, GIS users, and others can combine their own GPS data with GPS data from the CORS network to determine three-dimensional positional coordinates that approach a few centimeters in accuracy. Alternatively, users can submit their GPS data to the web-based Online Positioning User Service utility to have NOAA compute such coordinates automatically.

Among the new sites are 13 established by the Federal Aviation Administration as part of its Wide Area Augmentation System (WAAS). Four of the new WAAS sites are located in Alaska, four in Canada, and five in Mexico.

WAAS provides differential GPS correctors for airline navigation across North America. These correctors help more precisely determine a position and enable pilots to determine the three-dimensional location of their aircraft with an accuracy of a few meters. The WAAS network now contains 38 GPS tracking sites.

According to NOAA officials, the addition of the new sites significantly improves both the geographic coverage of the CORS network as well as the accuracy with which CORS users can position things, including property boundaries, transportation arteries, buildings and other map-worthy objects. The expanded coverage will also benefit those organizations that apply CORS data to monitor “space weather,” including the distribution of water vapor in the atmosphere and the distribution of free electrons in the ionosphere.

Knowing the distribution of water vapor is critical for accurately forecasting severe weather such as hurricanes, tornadoes, and thunderstorms. An overabundance of free electrons in the ionosphere can disrupt those communications services that involve satellite links.

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Researchers at Cornell University recently released their analysis of the effects of solar radio bursts (SRBs) during a solar flare last December that produced “unprecedented” fades in received GPS signal power at L1 and L2 over an extended period.

Numerous reports appeared of high-quality, dual-frequency GPS receivers losing lock on satellite signals during the incident, although ironically some single-frequency C/A-code receivers appeared to do better during the solar event. Receiver “failures” included many at International GNSS Service (IGS) sites and a Federal Aviation Administration Wide Area Augmentation System (WAAS) receiver in Houston Texas, according to the Cornell report.

Coming as it did during the relatively quiet interval between sunspot peaks, the magnitude of the recent event was doubly surprising. It also underlines the question of whether the new GNSS system developments under way with GPS, GLONASS, Galileo, and Compass will occur in time to improve the situation for users. And whether the new signal designs and planned improvements in satellite operations will ameliorate the effects of SRBs.

The next cyclic peak in sunspots — the so-called “solar max” — will occur in 2011, but may affect space weather for 1-1/2 to 2-1/2 years before and after the peak. During the last solar max in 2000, numerous instances of GPS receiver malfunctions were reported. Some scientists believe that SRBs during the 2011 solar max will exceed the levels reached in the previous one.

According to Paul Kintner, Jr., a professor in Cornell’s School of Electrical and Computer Engineering, the sun’s magnetic field is destroyed in solar flares during the sunspot maximum. Some larger flares produce solar radio bursts of broadband noise in the RF spectrum from 10 MHz to10 GHz that may directly affect GPS receivers on the dayside of the earth facing the sun. GPS L1 is centered at 1,575.42 MHz and GPS L2 is at 1,2276 MHz.

A Record Burst.  The SRBs discussed by the Cornell scientists were associated with solar flares monitored on December 6 and December 13–14. In some cases the fades in received signal strength exceeded 20 decibels. One power burst recorded at the Owens Valley Solar Array (OVSA) in California’s high desert exceeded one million solar flux units (SFUs) in the 1,200–2,00 MHz band, a record at those frequencies, according to the Cornell researchers.

Kintner says the scaling factor for signal fading is that 10,000 SFU equals 3 dB reduction in GPS carrier-to-noise (C/No) ratio and each doubling of solar radio burst power produces another 3 dB loss.  So, for example, 20,000 SFUs equals a 6 dB reduction and 640,000 SFUs equals a 21 dB loss.

Sunspot activity and individual SRBs produce variable and largely unpredictable patterns within the affected portions of the RF spectrum. Data from a WAAS receiver in Honolulu, Hawaii, recorded December 14, for example, showed a “drastic” signal fade at L1 but almost no effect at L2, indicating that little SRB power entered the latter band.

Conceivably, receivers with a wider range of signal-reception and –processing capabilities might do better, as might receivers with flexible designs that adjusted their operation in response to electromagnetic conditions. Although Galileo has some frequency allocation in C-band (between 5,000 and 5,030 MHz), the upper range for currently transmitted GNSS signals is a GLONASS frequency around 1609 MHz.

The lower end of the GNSS spectrum will drop down from L2 by 50 Mhz with the GPS L5 signal, which will appear with the launch of Block IIF satellites in 2008 and lies in the band for aeronautical radionavigation at 1176.45 MHz. The planned Galileo E5a frequency will overlay L5.

Asked about potential benefits of modernized GNSS signals, Kintner told Inside GNSS, “The new L2C signals, especially the L2CL pilot code, may yield better C/No dynamic range and in some situations be more resistant to solar radio bursts. . . . I have not completely thought through the implications of a 10 MHz chipping rate compared to a 1 MHz chipping rate, but there is no a priori reason to think that L5 will not be affected by SRB.  The SRB spectral properties are arbitrary and will vary from event to event.”

Reducing SRB Effects.  The effects of SRBs on different types of antennas also vary. Omnidirectional antennas receive all signals in view indiscriminately. But controlled radiation pattern antennas can null out signals and interference arriving from particular directions and choke ring antennas alternately increase or decrease signal gain depending on angle of arrival.

“A controlled radiation pattern would in fact make a substantial difference either positive or negative,” says Kintner. “The choke ring antenna with reduced gain at low elevations and increase gain at higher elevations is more sensitive to solar radio bursts at high elevations compared to low elevations.” He also believes that the flex power and spot beam capabilities of new military GPS signals will “absolutely” improve the situation for suitably equipped users.

Kintner says that Cornell hopes to pursue further investigation of into the performance of various types of GNSS receivers in the face of SRBs, as well as whether various signals designs — for example, the binary offset carrier (BOC) waveform at L1 rather than bi-phase skip keying (BPSK) — will make a difference.  For more details on the Cornell research on-line, visit their website and follow the “Space Weather” link.

Copyright 2007 Gibbons Media and Research LLC

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