Don’t be misled by the title. This is not another round of speculation about who should be proclaimed the rightful “Father of GPS.”

Rather my comments have to do with the ancestry of GNSS signals and the increasingly crowded path ahead. The subject comes come to mind now for several reasons: the imminent arrival of a spate of new signal designs from all four GNSSs and several regional systems, problems with the navigation payload on the GPS III, wide acceptance of the original GPS C/A-code in consumer equipment, and the general question of whether “modern” is necessarily better.

Every generation seems obliged to tell those coming after it that things were simpler in the old days. In the case of GNSS, this is demonstrably true: one system, GPS; two transmit frequencies, L1 and L2; and three signals, P-code (on L1 and L2) and C/A-code (L1 only).

Those days are gone.

The company building the navigation payload for the GPS Block III satellites has eight signals that must be broadcast on three frequencies. That probably contributes to all of the satellite production delays and also fuels suggestions that the hoary C/A-code should be phased out to free up space and power for more modern signal designs.

Meanwhile, receiver manufacturers are trying to figure out which of the new Galileo, BeiDou, GLONASS, and regional systems’ signals they should adopt — as well as when and how they should incorporate them into their products. In its many variations, we are frequently reminded these days that “the best is the enemy of the good,” a sentiment traced back to a poem by Voltaire.

In the title of his article in this issue about the robust presence of the original civil GPS C/A-code in the midst of latter-day entrants, Frank van Diggelen asks, “Who’s your Daddy?” (For those unfamiliar with this bit of American slang, perhaps Wikipedia says it best: it is an expression “commonly used as a boastful claim of dominance over the intended listener.”)

In fact, Frank is only pointing out the reality of — and debt owed to — the C/A-code’s historical precedence, its persistent utility, and its incorporation into more than one billion GNSS receivers in use today.

The original designers of the first GPS civil signal got a lot right, which subsequent signal designs have built on. But innovations have also been introduced into more recent GNSS signal designs.

These are not simply attempts to improve on perfection. The C/A-code hails from a technological era in which electronics, computing power, battery designs, applications, and many other factors were less advanced than they are today. Or than they will be tomorrow.

Every parent hopes that his or her offspring will go farther and accomplish more while still recognizing the assets and values that they inherited. So it should be with technologies and their progenitors, too.

Tradeoffs inevitably occur in changing or adding features to GNSS signals. Ultimately, the marketplace — including the installed base of receivers and already satisfied users — will decide which of these have more utility and, therefore, are worth paying for. Regulatory mandates may influence the outcome, but end users and equipment manufacturers will have the most to say about it.

And that is also as it should be.

*******************

One final note: Dean Bruckner, who wrote a letter to the editor that we published in our last issue, asks that I mention that his remarks reflected his personal opinion, not any institution with which he is affiliated. And while we’re at it, I should state that the sentiments expressed in these “Thinking Aloud” commentaries are mine alone, and should not be taken to reflect the opinions of the magazine’s editorial advisors, authors, and advertisers — or my parents or children, for that matter.

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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. WHAT’S LOVE GOT TO DO WITH IT? 
Detroit, Michigan USA 

1. WHAT’S LOVE GOT TO DO WITH IT? 
Detroit, Michigan USA 
√ The Federal Aviation Administration nixed a Michigan florist’s plan to deliver Valentine bouquets via drone in February. Flower Delivery Express was beta testing their single drone on a small group of Bloomfield Hills Loved Ones when the FAA asked them to stop. On March 6, a National Transportation Safety Board judge said flying commercial UAVs below 400 feet was fine. Now the FAA is appealing, and the florist says he’ll continue to test the drone delivery he believes is the wave of the future.

2. HELLO, SVN64 
Cape Canaveral, Florida USA 
√ On February 20, IIF-5 took a ride on a United Launch Alliance Delta IV rocket to take its place in slot 3, plane A of the GPS constellation. SVN64 will replace a IIA satellite that is joining the reserves after 16 years of active duty.

3. BLACK SWAN EVENT? 
Teddington (London), United Kingdom 
√ In a Royal Institute of Navigation conference keynote on resilient PNT, Brad Parkinson, the pioneer director of the Navstar GPS program, said both GPS and Galileo signals are more vulnerable to sabotage and disruption than ever before and Western governments are ignoring the risk. A UK report said conditions were ripe for an unpredictable “Black Swan” event that could knock out critical GPS systems.

4. Glonass-M #54
Plesetsk Space Center, Russia 
√ Glonass-M #54 is at Plesetsk Cosmodrome, being prepared for launch on March 24. It carries a high-accuracy thermal stabilization unit that will be tested for use on nextgeneration Glonass satellites. Russia cancelled planned launches in September and October 2013. Despite the diplomatic showdown between Russia and the U.S. over Ukraine, NASA chief Charles Bolden said in a March 4 press briefing that space relations between the two countries are normal and expected to remain so. (UPDATED APRIL 2) Well, that didn’t last long….

5. The Second Geosynchronous Satellite
Satish Dhawan Space Center and  Byalalu (Bangalore), India 
√ The second geosynchronous satellite in India’s regional satellite navigation system will head into space on Polar Satellite Launch Vehicle C24 on March 31. India’s goal is to have space, ground and user segments manufactured in-country. The space agency, ISRO, opened a dedicated IRNSS navigation center (INC) in Byalalu in May of last year, shortly before their first satellite launched on July 1. (UPDATED APRIL 4) IRNSS 1B launches successfully.

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With the optimism of college-bound seniors touring the Ivy League, GPS managers have been weighing options to dramatically change the GPS constellation. Now, after studying the costs, considering the benefits, and assessing the funding climate, officials have made the starkly fiscal decision to stick close to home and take a few extra years to finish. 

Although the final decisions will not be made until sometime this spring, proposals for a distinctly different type of GPS constellation appear to be off the table, sources tell Inside GNSS. The plan now appears to forego any major shift in the design of the satellites such as those proposed in Lower Cost Solutions for Providing Global Positioning System (GPS) Capability, an Air Force report delivered to Congress last April. 

The established course of modernization will proceed largely unchanged, say sources, but it will take longer to build and launch the GPS III satellites and add the new signals. Full implementation of the new military M-code, for example, will be pushed back roughly four years at least, noted one source. Those waiting for the new civil signals will also have to be patient. 

The current course of action will likely raise the total cost of the modernized system although the higher costs will be spread out over time in a way that fits more appropriately into budgets constrained by sequestration and the overall post-war downsizing taking hold at the Pentagon. 

“We’ll pay a unit price that’s a little higher, but just like when you buy your car on payments, you pay more for the car but you have cash flow management,” Maj. Gen. Robert McMurry, director for Air Force space acquisition told reporters during a briefing on the military space budget. 

The fiscal year 2015 (FY15) budget McMurry was describing reflects the decisions so far. The FY15 request for money to procure GPS III satellites is just $292.397 million — down dramatically from the White House’s request for $477.598 million in FY14 and roughly half of the $531 million the White House projected it would need for the program just last year. It is less even than the $450.598 million allocated by Congress. 

The administration is also asking $212.571 million for GPS III development, somewhat less than the $221.276 million requested last year and only slightly smaller than the $215 million that was projected to be needed last year. The request is a bit more than the $201.276 million Congress appropriated for FY14. 

The request to procure Block IIF satellites is about the same as last year — $52.09 million — but the new ground control segment faces al reduction. Whether it is a big or small cut depends on how you look at it. The White House is asking for $299.76 million in FY15 for the Next Generation Operational Control System or OCX. That is down sharply from its $383.5 million request last year and the congressionally approved FY14 amount of $373.5 million. 

In the projections that accompany each budget, however, the White House last year only anticipated asking for $303.5 million for FY15. In fact, both the FY14 and FY15 budgets project falling GPS allocations for the next several years. This is despite the fact that the program has been experiencing delays and challenges — the sort of things that normally add to the cost. How these two trends will mesh in the end is unclear. 

Implications 
Where it had planned to buy two GPS IIIs this year, the Air Forces now plans to buy only the ninth in the series and put down money for long lead items on Space Vehicle 10 (SV10). It will then buy just one new satellite next year and three each year for the next three years, according to McMurry. 

The launches will be stretched out a bit as well. Five GPS missions will see their booster procurement moved past 2017, said McMurry. That has implications for Department of Defense (DoD) plans to introduce more competition into the launch procurements. 

“Those five will still be available for competition,” he said. “It’s to be determined how we’ll do that competition. It will be part of the phase two, and they’re working on that strategy.” 

The long life of the existing satellites is making it possible to find savings without undermining the quality of the GPS system, officials said. 

“(The) satellites are living longer than we predicted so we didn’t need to replenish those as fast as we originally planned. And it made no sense to spend that money if we didn’t need the satellites,” said Troy Meink, deputy under secretary of the Air Force for space. 

The Constellation 
Less clear is how many satellites the Air Force plans to have in the constellation over the long run. 

The FY15 budget “reprofiles Global Positioning System, GPS III, to meet constellation sustainment demands,” said Under Secretary of the Air Force Eric Fanning. 

“Sustainment,” however, does not necessarily mean supporting the constellation in its current configuration, which stands at 31 satellites plus spares. While having more satellites is advantageous to those on the ground, it is not strictly necessary according to Air Force mandates. 

“The requirement is 27 satellites to maintain 24,” McMurry told the audience at a March 7 breakfast on Capitol Hill sponsored by the Mitchell Institute. “I think our approach will be to absolutely assure that requirement, which meets that mandated performance, but in doing so I will expect that we’ll, in reality, maintain a slight surplus to that as we move on.” 

“It is clear that they are going to stay with the ‘enhanced 27,’ which is 27+3 for as long as they can,” said a source familiar with the issue. “And even if for some reason those (satellites) that are turned on now drop out, they’ve got as many as five that they can turn back on.” 

Those backup satellites just have “to last until the next launch,” said the source, suggesting it is realistic that the Air Force will be able to keep the number of satellites up even though the spacecraft have been operating far past their design lives. 

The question is whether the next launch, or launches, can happen fast enough. The existing satellites were launched in clusters and are therefore at risk of failing in groups as they age out. Will the Air Force have the satellites it needs and the lift capacity required to deal with a sudden, rapid loss of the older spacecraft? 

It appears that the answer may be “Yes.” 

Dual Launch After All 
Inside GNSS has learned that, even though the Pentagon has slashed the funding for dual launch, plans are in the works to enable the lighter payloads and dual-launch capability needed to rapidly refresh the constellation. 

The United Launch Alliance, a 50/50 joint venture between Lockheed and Boeing, is stepping in to finish the work. “ULA is funding the launch vehicle development work that will enable dual launches of GPS III and other potential satellites, with a planned first launch capability in 2017,” the company said in a written response to a question from Inside GNSS. 

And the Air Force will continue to fund the technology needed on the satellites to make dual launch possible. 

“We have maintained the development of dual-launch capability within the satellite line,” McMurry said. “The satellite itself will have the dual transponders and radio frequencies in place so that, if you launch two of them together, you could communicate with both of them independently.”

Slimmer Sats in a Pinch
The satellites might still be too heavy to launch two at a time unless some other changes are made. 

To address that, said a source, military managers are planning to build flexibility into the GPS III satellites that will enable the service to drop the Nuclear Detonation Detection System or NDS payload from the GPS III satellite if necessary. 

“They are going to ensure that they have the option to fly GPS III without the NDS,” said the source, adding that, without NDS, “should there be the requirement . . . to rapidly replace the constellation, they will actually be able to launch two.” 

The built-in flexibility also means the GPS program will not have to plan around any delays in the new NDS payload — which is on schedule but still needs a good deal of testing, the source said. 

Dual-launch capability and potentially lighter satellites are a variation on a far more ambitious proposal to develop a stripped-down version of the GPS III spacecraft that would not carry the NDS and provide fewer signals. These smaller satellites would likely have been used in combination with some of the larger, regular GPS spacecraft. 

The austere NavSats or “NibbleSats” were cheaper to build and could have been launched at least two at a time — perhaps even in clutches of three or four — dramatically reducing the cost of maintaining the constellation. But the proposal was set aside after hitting a number of stumbling blocks involving funding and contract management, sources told Inside GNSS. 

“I think the pressure to reduce the cost of the satellite is very much there,” said an expert. “Obviously anything the Air Force can do to drive the cost of the satellite down — I think they’re still alert to those opportunities. But I think the Congress has made it very difficult for them too create a ‘new start.’”

It is not only hard to get money for new starts (new programs), the rules governing the seemingly endless rounds of Congressional budget extensions mean the program could have become tangled in delays, said Stan Collender, national director of financial communications for Qorvis Communications and an expert on the federal budget. 

“Under a continuing resolution,” said Collender, “new starts are prohibited.” 

Moving to small satellites would also have been such a big change that it likely would have required the Air Force to recompete the GPS III contract. New procurements face a long process fraught with complexities and the uncertainties created whenever new contractors enter a program where legacy satellites have been in place so long that they are frequently referred to jokingly as being “old enough to vote” — that is, have reached 18 years of age. 

The prospect of reopening the contract is why “NavSats, small sats, little sats — that stuff is off the table,” said a source, who has been following the issue. 

Thrifty Innovation
That does not mean the GPS III program will proceed without a few enhancements. 

Col. Bill Cooley, director of the GPS Directorate, said the Air Force is “looking at a design turn” and is examining using better solar panels and traveling wave tube amplifiers or TWTAs (TWEEtas) which could reduce power requirements. 

Changing the batteries is also under consideration, according to sources. One expert pointed out that some of older battery technology is not even available anymore. Another source said that lithium-ion batteries were being considered. 

“The decisions are in the process of being made,” Cooley told Inside GNSS. That decision process may be playing a role in the delay of the first GPS III satellite, which is now not expected to be ready until FY16. 

The design of the first satellite is supposed to be a template of sorts for the rest, and Cooley made it clear to the Mitchell Institute audience that he wanted the design to be “repeatable.” 

Right now, however, interference problems within the satellite’s navigation payload have to be resolved. The problems in the payload, which is being build by Virginia-based Exelis, have already been cited as a reason for delays in the program. 

The GPS IIIs must generate and transmit eight signals — legacy military P-code on L1 and L2 frequencies and civil C/A-code on L1, as well as the new dual-frequency M-code, and civil L1C, L2C, and L5 signals. In order to keep the timing and everything synchronized, there is “one critical box” involved in their generation, said Cooley. 

“There’s a whole bunch of techniques you can use,” Cooley told Mitchell Institute attendees. You can use absorptive material, he said, redesign some of the boards or separate the signals by putting them in separate boxes. “All options are on the table.” 

Sources confirm that the Air Force seems to have workable solutions in hand. Although those solutions may need extensive testing, the potentially two-year delay could also give the program time to work in some of the innovations mentioned previously. Program managers may also just be working hard to get the most out of the new satellites. 

“The GPS IIIs have a design life of 15 years,” said Cooley. “That’s a real challenge — to get 15 years in that harsh environment. We hope that they will last much longer. We hope that we can get a satellite that can vote and drink and all those kinds of things.” 

What the changes, delays, and development problems mean for GPS III prime contractor Lockheed Martin is unclear. 

“Over the next few weeks we will review the budget in detail to understand the specific impacts to our business,” the company said in a written response to Inside GNSS. “We look forward to working with the administration and Congress over the coming months as budget discussions continue.” 

OCX and Civil Money
What the delay in the first GPS III satellite means to Raytheon is that the OCX prime will have more time to work on ground segment modernization that already slipped behind schedule, said McMurry, who attributed slippage in the initial GPS III delivery as the biggest reason for a new delay in the OCX program. 

OCX program managers may get more time still if all the money from the civil side of the GPS program does not come through. 

As noted earlier, the White House scaled back their FY15 defense budget request for OCX to $300 million This is just under the $303.5 million that was projected to be needed this year though the program has been experiencing difficulties. This is the request for the Defense Department. OCX also gets part of its money from the U.S. Department of Transportation (DoT), and a long string of underpayments from DoT has put GPS program managers in a position where they will be forced to reprogram OCX, adding some six months to the schedule and tens of millions to DoT’s bill, according to an expert with knowledge of the issue. 

The White House gave DoT the responsibility for funding those parts of the GPS program needed by civil users, and DoT handed the Federal Aviation Administration (FAA) the actual funding task. 

The FAA has largely failed, however, to persuade Congress to allocate the money for the civil funding. This has forced the agency, which has cost overruns on other programs, to short its payments to DoD for the last several years. 

The FAA is now trying to make up for those too-small payments but “it’s not pretty,” said the source, who spoke on condition of anonymity. 

The Administration’s FY15 budget request for civil funding actually jumped from the $20 million requested last year to $27 million — better, but still a far cry from the $40 to $50 million that was supposed to be allocated each year for five years. Even so, if FAA convinces Congress to approve the whole request it will be a dramatic improvement over the scant $6 million it got for FY14. 

Failure to win over lawmakers could have significant consequences. 

Up to now the Air Force has been able to manage around the budget shortfalls and keep things more or less on track. With sequestration and other cuts coming out of DoD space programs, GPS program managers are no longer in a position to finagle funding for civil capabilities. 

The source told Inside GNSS that, should the FAA fail to secure adequate monies, the OCX program to will have to be stretched out some six months at considerable expense — money that FAA will also be expected to make up. The total bill for what FAA owes plus the added cost of delay would top $100 million said the source — nearly four times the current budget request. 

DoD and FAA will have to work together pretty closely to manage during these austere times, said the source. “When you get cut to the bone you have to work pretty closely just to survive.” 

Sequestration Looms 
The budget crunch could get even worse if sequestration is applied without changes in 2016. 

Military officials made it clear that they are assuming sequestration will not resume in full in 2016, as is now the law. If it should reemerge, said Fanning, “we would be unable to procure one of the three GPS III satellites planned in FY17.” 

Still unclear is how the launch rate or other aspects of the program would be affected, but it is clear, said Collender, that budget politics mean sequestration is likely to remain a factor. He estimated a 3-out-of-4 chance that the cuts will return in full force in the 2016 budget. 

“It will be a presidential year,” he said. “You’ll have Republicans that don’t want to be blamed for increasing spending; so, spending cuts might be in place. There’s probably a 75 percent chance that the 2016 sequester stays in place as is.” 

That could force still more changes to the GPS program and perhaps even reopen consideration of a GPS III redesign,experts hinted. Asked what options the Air Force was looking at for the long term, Cooley left the door open. 

“What options are we not looking at?” he said.

The post GPS Modernization Stalls appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

The world’s GNSS systems are entering a phase of transformation — modernization of existing systems (the U.S. Global Positioning System and Russia’s GLONASS) and development of new systems (China’s BeiDou and Europe’s Galileo) that benefit from the lessons learned from the original GNSSs.

Notable among the modernization initiatives is an interest in implementing new satellite signal designs. These include the GPS L5, L2C, and L1C signals as well as those signals designed for Galileo and BeiDou. GLONASS designers are also working on modernized signals.

The world’s GNSS systems are entering a phase of transformation — modernization of existing systems (the U.S. Global Positioning System and Russia’s GLONASS) and development of new systems (China’s BeiDou and Europe’s Galileo) that benefit from the lessons learned from the original GNSSs.

Notable among the modernization initiatives is an interest in implementing new satellite signal designs. These include the GPS L5, L2C, and L1C signals as well as those signals designed for Galileo and BeiDou. GLONASS designers are also working on modernized signals.

In many cases, these new signals adopt or build on features designed and being introduced as part of the modernization of GPS, the first and still most widely used GNSS. Among others, these include such innovations as higher transmit power to improve reception under challenging conditions, longer codes for a better cross-correlation between satellites signals, data-less pilot channels that facilitate long integrations and improve the sensitivity threshold, and secondary codes — short pseudorandom noise (PRN) codes to simplify the data synchronization.

A unique cooperative agreement signed in 2004 between the United States and the European Union calls for common use of a binary offset carrier (BOC) modulation at 1575.42 MHz. Under this agreement, Galileo and GPS system operators (the European Space Agency and the U.S. Air Force, respectively) are implementing two different versions of a multiplexed BOC(6,1,1/11) signal.

Although currently operating as a regional system, the Phase III plan for the BeiDou B1 civil signal also calls for shifting to the L1 frequency centered at 1575.42 MHz and transmitting a multiplex binary offset carrier (MBOC 6,1,1/11) modulation similar to the modernized GPS civil signal (L1C) and the Galileo L1 Open Service signal.

GLONASS says it will introduce CDMA signals at 1575.42 MHz, which has emerged as the common frequency for current and future civil signals, in place of the frequency division multiple access signals currently transmitted at higher frequencies. But the new signals’ specifications, including such parameters as data rate and signal structure, are still under development.

We asked A. J. Van Dierendonck, one of the pioneers in GPS system development with 40 years in the satellite navigation field, to comment on some of the innovations seen in these new signals. Dr. Van Dierendonck is a codeveloper of the L5 signal structure that will be carried by the GPS spacecraft beginning with the Block IIFs now being launched. He also participates in the US/ EU bilateral discussions that take place under the auspices of the 2004 agreement.

IGM: More recent GNSS signals have adopted a variety of features not used in earlier signal designs — such as longer codes, higher data rates, message error detection and control methods, use of pilot channels, multiplexing, and so forth. What do you think have been the most important improvements and what benefits have they brought?

VAN DIERENDONCK: Some of these improvements were introduced first in the L5 signal design, although L2C was the first modernized signal to be implemented. The longer codes reduce the amount of self-code interference and cross-correlation, and thus, increase tracking margin. Acquisition margin is also increased, but with the penalty of the time it takes to search the longer code. Modern receivers will overcome that with the implementation of more correlators or channels.

Higher data rates were not introduced on L2C or L5. In fact, the data rate on L2C is cut in half in order to make up for implementing the pilot channel. (More on that later.) Galileo has increased the data rate on their E1 and E5b channels, but only to provide integrity data (which, so far hasn’t been implemented, and may never be). Higher data rates reduce tracking margin; so, if the higher rates are not needed, it is better that they are not implemented.

GPS has always had message error detection (called message parity), although it is not very strong. Aviation has made up for it by requiring that the message be collected twice (and be in agreement) before it can be used. The error detection (forward error correction or FEC) on the L5 and L2C signals is strong enough without the extra message collection. This error correction only improves the data collection margin, but does not improve tracking margin. FEC was implemented on satellite-based augmentation system (SBAS) signals before being added to GPS.

A pilot channel is a common practice in communication systems to improve signal tracking, but still retain a good data capability. It was introduced into the L5 signal design by Tom Stansell and Charlie Cahn as a quadrature channel. However, pilot channels have limitations. The extra channel steals power from the data channel. In the case of L5, we had the possibility to define signal power requirements, although these were not fully implemented of the Block IIF satellites (–154.9 dBW min instead of –154 dBW min). The desired power will be implemented on the Block III satellites.

The pilot channel does not have the same PRN code as the data channel. Thus, tracking can be completely independent. However, normally pilot-channel tracking is more robust (because of the possible narrower pre-detection bandwidth); so, it can be used to aid data channel tracking.

In the case of L2C, the PRN code on the data channel is a much shorter code to provide faster acquisition. On L1C, the power split is 3/4 on the Pilot and 1/4 on the data channel, for more robust tracking in challenged environments. These challenge environments usually get the data messages via another means, such as via the internet or Bluetooth, and thus, the motivation for more power on the pilot channel. Obviously, this is not possible in aviation applications and via SBAS.

The multiplexed BOC(6,1) signal added at the Galileo and GPS L1 frequency also steals power from those signals. Its purpose was to provide some signal in an additional GPS military Mcode null. GPS L1C does the same thing, providing the same spectrum but implemented differently. For the Galileo design, the (6,1) signal is added to the (BOC1,1) signal and can only be deleted by filtering. In the GPS design, higher rate code bits are multiplexed into the BOC(1,1) code to provide a TDMA version (TMBOC). These extra bits can be blanked with less loss.

Receivers can be designed to ignore the multiplexed signal, but in the case of Galileo, the power in that part of the signal spectrum is also lost. So, I don’t think that the benefit justifies the loss of power and the extra complexity.

IGM: What features in new GNSS signals have helped improve receivers’ performance in the presence of multipath? Of RF interference?

VAN DIERENDONCK: Higher chipping rates are usually associated with longer codes. However, that doesn’t necessarily increase performance in the presence of multipath unless there is also an increase in transmitted (and received) bandwidth. The big advantage of higher chipping rates is that the receivers can be implemented with narrower correlators (in terms of seconds, not chips).

The first receiver implemented that way used the C/A code. That worked so well because the signal transmit bandwidth was 20 megahertz or more, at least 20 times the C/A code chipping rate, allowing a correlator spacing as narrow as 0.05 chips. But that is only half a P-code chip or an L5 chip, Thus, the multipath performance of the P code or the L5 code tracking would be just as good using 1/2 chip spacing.

However, code-tracking performance would be better by the square-root of 2 with the same spacing in terms of seconds. For example, in the presence of noise or interference, code-tracking accuracy would be better using the higher chipping rate, but multipath performance would be about the same given the same receiver bandwidth. This is because multipath performance is based upon chip edge sharpness, whereas noise performance is based more on the square root of chip width.

In other words, not much in the new signals has or will improve multipath performance.

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Return to main article: "Who’s Your Daddy?"

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