Expert Advice: Who Won?

August 1, 2011 - By GPS World Staff

Logan Scott

By Logan Scott

Thousands of man hours and millions of dollars later, we finally have the 975-page GPS Technical Working Group (TWG) report, confirming what five minutes of back-of-the-envelope calculation predicted. Hooray for our side, good job GPS Industry Council; we’ve won the war and the foe is vanquished, never to brighten our skies again.

Well, maybe. LightSquared is now bypassing the Federal Communications Commission (FCC) and submitting technical papers directly to the United Nation’s International Telecommunications Union (ITU) Working Party that handles mobile satellite services (MSS) and radio determination satellite service (RDSS spectrum) and orbits (ITU-R WP 4C).

A few comments to all participants:

To the FCC: Quoting from the National Legal and Policy Center’s February 2, 2011, rather damning letter to U.S. House members Darrel Issa and Edolphus Towns: It is “the special responsibility of federal agencies to not only avoid conflicts of interest, but to avoid even the appearance of conflicts.” Integrity counts. It shouldn’t require congressional intervention for the FCC to do the right thing. An abbreviated, 10-day comment period ending the Monday after Thanksgiving on a ruling of this magnitude and one which would have severely damaged national infrastructure if left unopposed? What were you thinking?

After wiping the egg out of your eyes, you also might look around your organization and discover you have engineers. They’re the ones who use terms like bandwidth, compression point, and interference. They can tell you things about engineering issues. Your engineers are actually quite good and know what they are talking about. Use them. Listen to them. Maybe even put some on commissioners’ staff. A B.S. degree shouldn’t be a disqualification for helping to set national policy on technical matters.

To Department of Homeland Security (DHS): GPS is critical infrastructure and needs to be designated as such. If anything, this exercise has demonstrated how easily we could lose the benefits of GPS. LightSquared was not even targeting GPS, but if implemented as originally planned, its system would have damaged diverse areas of critical infrastructure; both civil and military. As a nation, we are entirely capable of shooting ourselves in the foot; no terrorists needed. We have no backup to GPS; protect it.

To LightSquared: You have a great system concept, but there are sound engineering reasons why the bands adjacent to GPS were designated for space-to-Earth mobile satellite services (MSS). Separation between GPS and high-power systems is essential, particularly with the current state of the art in GPS. Claims that you have been working with the GPS industry for the last eight years and that we gave “the green light to those plans” (June 30, 2011, Recommendation of LightSquared Subsidiary LLC, page 16) do not ring true. Even the most casual analysis of your plans shows significant harmful interference to GPS.

Some further observations on your recommendations: Trying to game the system and redefine what constitutes harmful interference (1dB versus 6 dB) is probably not a great idea given the GPS system navigates our airplanes and provides E911 capabilities. We routinely use up all of our margins and then some. A 6-dB hit is a big hit on position robustness.

Similarly, don’t play games with statistical propagation modeling. Your proposed Walfish-Ikegami line of sight (WI-LOS) models are wholly inappropriate for low-altitude aircraft using GPS for precision approach and landing. They are based on LOS street-canyon measurements made in the city of Stockholm and are not intended for handsets more than 10 feet off the ground. Two-ray models accounting for ground reflections show LightSquared signals at levels 6 dB above free space predictions several miles out (Figure 1). Live-sky testing at Holloman and Las Vegas showed “above free-space” levels even for some ground mobile users (June 15, 2011, National Public Safety Telecommunications Council [NPSTC] filing with the FCC, page 7, Item 3). Coverage models are not appropriate as safety-of-life models.

Upzoning the entire 1.6-GHz MSS band is not likely any time soon, at least in the United States. Figure out what you can do with less spectrum and less power in the low end of the S-E MSS allocation or find other spectrum; maybe pay for it like other cellular operators did. Don’t forget E-S interference, there are dragons there as well. Develop a transition plan and expect to pay for it.

Figure 1. LightSquared propagation models can underestimate interference by more than a factor of 100 (>20 dB). (Click to enlarge.)

To the GPS Industry: We have long lived in a world of clean, unobstructed spectrum — and it has been wonderful. At this June’s JNC2011 conference, Air Force General Kevin McLaughlin (U.S. Strategic Command) noted that space is increasingly “congested, competitive, and contested.” The same can be said for radio spectrum. LightSquared is trying to make good use of ~68 MHz of largely fallow spectrum straddling ours to provide a valuable and sorely needed wireless data service.

There is no successful business model in providing MSS services only. Motorola and Loral/Qualcomm proved that with their Iridium and Globalstar MSS systems. Both original ventures ended in Chapter 11 bankruptcy. LightSquared is the third or fourth incarnation of Mobile Satellite Ventures (MSV), which ended in bankruptcy. The core business problem is that MSS is expensive to deploy, provides very little capacity (bits/sec/Hz/km2), and these systems offer poor building penetration. Upzoned for terrestrial services (that is, cellular or ancillary terrestrial component, ATC), LightSquared’s spectrum allocation is worth about $15 billion for the license alone at current auction prices, and that’s for only the United States. With spectrum growing increasingly scarce and valuable, we in the GPS industry should expect, and be prepared for, the day when this spectrum is repurposed. It is not my intent to be an apologist for LightSquared, merely to illuminate the fact that this is potentially very valuable spectrum and it is not going to be MSS forever, especially if someone offers to pay for it.

LightSquared was stopped, at least temporarily, by regulatory constraints applicable only within the United States. In effect, the GPS industry lobbied for a 34-MHz guard band and won — maybe. This is not a sustainable position. LightSquared may yet prevail on the international stage and/or in a diminished capability. Also, be aware that in parts of the world interference now makes GPS unusable, for example, Balkan ports and parts of Africa.

We in the GPS industry can and must take steps to improve our ability to operate in congested spectrum. The TWG report showed enormous variations in receiver resistance to out-of-band LightSquared interference. Using a 1-dB C/No degradation criteria, in FAA-certified aviation receivers there was 26 dB of variation in LightSquared signal rejection (Table 3.1.1, page 42). In high-precision receivers, more than 30 dB of variation was seen (Table 10, page 243). Against LightSquared F5L (the lower frequency, 5-MHz-wide LightSquared signal, 1526.3–1531.3 MHz) modulation, high-precision receivers showed more than 70 dB variation in susceptibility to LightSquared interference (TWG Appendix H.1.1.10, Figure 38). Clearly, there are good ways to build a receiver, and bad ways. We need to do better.

Among the steps to consider:

Narrow front-end bandwidths. If you don’t want to be affected by out-of-band jamming, don’t let it in. This is antijamming (AJ) 101. The corollary of course is that most AJ techniques degrade position accuracy, and so it is with filtering. The C/A code is about 2-MHz wide but there are good anti-multipath motivations for using a wider bandwidth. GPS satellites have roughly a 28 to 32 MHz transmission bandwidth. Beyond that, there is nothing except interference. Filter accordingly and don’t forget: antenna selection plays an important role in determining overall receiver frequency selectivity.
Higher 1-dB compression point. Consider designing for a higher 1-dB compression point, particularly if you must use a wider bandwidth front end, say for phase linearity in precision survey receivers or for multipath rejection or for military signals. This also improves IP3 and mitigates intermodulation effects. IP3 is the third-order intercept point of an amplifier and is one of the more important parameters in describing the linear range of an amplifier. Low IP3 leads to higher intermodulation distortion, a process wherein two out-of-band signals can mix with each other in the GPS receiver to produce a third frequency within the GPS band. Yes, higher compression points lead to slightly higher power consumption, but out-of-band signal tolerance improves greatly when combined with downstream filtering. In the longer term, consider adaptive equalization methods.
Use L2C and L5 signals. Currently, nine satellites on orbit broadcast L2C and one broadcasts L5, with another IIF successfully launched July 16. One major reason precision receivers fare poorly against interference is that they require wide front ends to implement codeless and semi-codeless modes to measure L2 carrier phase for widelane ambiguity resolution. Wide bandwidths are also needed to precisely measure L1 pseudorange, again for ambiguity determination. Using L2C/L5 mitigates the need for wide-bandwidth front ends and at the same time creates signal diversity in carrier-phase tracking.
Report interference. One of the most stunning shortcomings in many GPS receivers, both civil and military, is their inability (or unwillingness) to report jamming and spoofing. In my work with DHS on the National Risk Estimate, one recurring theme across industry sectors is how confusing it is when GPS gets jammed. GPS is often deeply integrated with other systems to the point where it becomes invisible until it fails, and then its failure causes weird failure modes in dependant systems. For example, mobile satellite communication systems can fail if the GPS position is wrong; the antenna gets pointed the wrong way. A simple “I am jammed” alert would go a long ways towards diagnosing problems and taking corrective actions. This is as true for LightSquared signals as it is for personal privacy devices.
Integrity Monitoring. If you are lucky, interference causes a signals outage. Some interference types can capture receiver tracking loops and yield false positions. The effects of out-of-band interference on tracking are not well understood. Constantly checking for signal integrity and navigation integrity (for example, receiver-autonomous integrity monitoring) can detect many adverse results without imposing a significant burden on the GPS receiver. The algorithms are well documented. Use them.

Winston Churchill is famously quoted as saying: “Americans can always be counted on to do the right thing — after they have exhausted all other possibilities.” At this point, I think we are still looking at some of the other possibilities and I wouldn’t count the LightSquared situation as a victory for anyone just yet. There is still ample opportunity to snatch defeat from the jaws of victory, but by taking a proactive stance, both politically and technically, we can improve our chances.

Also, a nice pair of wellies might be a good investment; it’s a big barnyard.

Logan Scott has more than 32 years of military and civil GPS systems engineering experience. At Texas Instruments, he pioneered approaches for building high-performance, jamming-resistant digital receivers. While at Omnipoint, a cellular carrier, he developed cross-system interference mitigation strategies. He holds 33 U.S. patents.