LightSquared, GPS Industry Square off on Interference Issue

LightSquared and its predecessors obtained mobile satellite service (MSS) spectrum from the FCC, paid Inmarsat half a billion dollars to allow it to consolidate many narrow frequency bands into bands large enough to support wireless broadband using LTE, then petitioned the FCC for approval to build a terrestrial network using these frequencies and spent another large amount of money designing it once it received FCC permission.

LightSquared designed its system to avoid creating interference to GPS and, as testing showed, limited out-of-band emissions in GPS spectrum to a level where by itself, it would not cause interference to GPS. Prior to LightSquared's plans, this spectrum had seen limited use for satellite communications. GPS receiver manufacturers, driven by the marketplace to produce products with the smallest size and lowest cost, did not include filtering to reject interference from terrestrial transmitters operating on adjacent spectrum licensed to LightSquared There are no FCC rules requiring them to meet interference rejection criteria—that's left up to the marketplace. High-precision GPS receivers were even designed to use augmentation signals transmitted in the same band now licensed to LightSquared.

Millions of GPS receivers without sufficient filtering (assuming that this is even practical) were sold and applications critical to public safety and commerce depend on these receivers. There is no feasible method of replacing all of these receivers. What a mess! Who is to blame?

Most of the media reports put the blame on LightSquared—after all, everything was fine until they moved into the spectrum. That's an easy argument. You can find examples in real estate. People buy homes near a nice vacant field which happens to be zoned commercial then complain when a shopping mall is built in that field. Perhaps the FCC, like the local government in my analogy, is the villain—it allowed LightSquared to build on vacant spectrum. Others would argue the people that bought the lots near the future mall should have known they'd have to deal with it and would have prepared for it. Of course, degradation of GPS service has a much greater impact on public safety and commerce.

This week I'll look at the two sides of the argument, from both the LightSquared and GPS manufacturers. Broadcasters should pay attention. We are likely to see similar issues arise as the FCC reallocates spectrum from broadcasting for use by wireless carriers. In my next RF Technology column I'll report on interference from LTE devices to reception of full power DTV stations operating near the top of the UHF band.

LightSquared Offers Temporary Solution for GPS Interference:

As described above, LightSquared appears to have played by the FCC's rules with regards to emissions in the GPS spectrum but that didn't help receiver overload issues caused by its terrestrial base stations generating much stronger signals on frequencies close to the extremely weak signals from GPS satellites. LightSquared acknowledged the problem in its June 30th Recommendation of LightSquared Subsidiary LLC filed with the FCC. One solution mentioned and rejected was that the FCC allow LightSquared to operate on its licensed frequencies and let the GPS manufacturers and public deal with replacing the majority of GPS receivers in use today. Another possible "solution,"—also rejected—was that LightSquared find frequencies elsewhere and the bands adjacent to the GPS signals be, in effect, given to the GPS industry as a buffer.

LightSquared's compromise solution is for it to temporarily avoid using the 10 MHz of its spectrum closest to the GPS band. LightSquared said that testing showed that operation on the lowest 10 MHz of its spectrum most removed from the GPS band "poses no risk to the users of 99 percent of GPS devices..." As noted in my introduction, some precision GPS receivers use frequencies in LightSquared's spectrum. LightSquared said it would "coordinate and share the cost of underwriting a workable solution for the small number of precision measurement and other devices that may be at risk." LightSquared would limit maximum base station EIRP per sector in the lower 10 MHz block to 32 dBW, significantly below the authorized EIRP.

LightSquared pointed out that this proposed solution "is identical to the primary mitigation measure suggested by the commercial GPS industry when it raised the receiver overload issue with the Commission in September 2010..."

LightSquared took note of the publicity surrounding the GPS interference issues, stating, "Despite this campaign of distortion and obfuscation, the record clearly establishes that the commercial GPS device industry must accept the fundamental responsibility of using the radio spectrum efficiently and not undermining the value of adjacent frequency bands."

LightSquared clearly intends to use its entire band at some point. There is agreement that this will require replacement of many if not most, of the GPS receivers currently in use. At least in some consumer applications, GPS receivers and devices are replaced fairly often as new features (bigger screens, better maps, etc.) so this may be possible if filters are feasible. Unfortunately, GPS receivers in other applications might be in use for as much as 15 years, perhaps more. LightSquared says the additional filters to protect the GPS receivers from interference could cost as little as $0.05, based on an estimate by Qualcomm for mobile phone handsets and general location and navigation devices. LightSquared said the cost may be higher for other GPS applications, including those requiring a higher degree of precision.

LightSquared devotes almost 20 pages of the 58-page report to a technical appendix with studies backing up its position. In spite of this strong technical showing, it seems unlikely LightSquared will be able to use all of its spectrum unless the FCC takes the unusual step of setting performance standards for new GPS receivers. Whatever happens, consumers are likely to end up the losers—either they eventually replace most if not all their existing devices using GPS or pay higher prices for wireless broadband due to less competition.

GPS Users Reject LightSquared Solutions:

The Coalition to Save our GPS wasted no time responding to LightSquared's proposal. On July 1, it posted LightSquared's "Recommendation" Document: A Review, which notes that LightSquared had said for months its operation would not interfere with GPS and that the new deployment scenario described above was not included in the initial scope of the working group analysis. The Review also said many of the mitigation measurements were never disclosed or discussed during the four-month working group study process. Coalition to Save Our GPS said the document claims, "without citation to the working group study or any other supporting data, that its proposal will solve interference for 99 percent of GPS receivers."

Coalition to Save Our GPS asks: "Did LightSquared not know that its proposed upper MSS band operations would cause interference? If it didn't know, that places its technical competence in severe doubt. Did LightSquared know, or have very strong reason to believe, before January 2011 that massive interference would result? If so, why did it not disclose this to the FCC?" The Coalition also questions whether LightSquared really needs to use the MSS band. "Since the RD clearly indicates that only 20 MHz of spectrum is needed for initial deployment, LightSquared already has ample spectrum to support deployment without using any of the MSS Upper or Lower band. In any case, to allow for a clear understanding of the impact of the present proceeding on its plans, LightSquared should provide a full and detailed accounting of its spectrum holdings instead of painting baseless 'doom and gloom' scenarios. "

The Coalition to Save Our GPS disputes LightSquared's argument that this is a fight between GPS manufacturers and LightSquared and that only the "commercial GPS industry" has raised interference objections to LightSquared's plans. The Coalition lists its members, many of which represent users of GPS, not manufacturers. It says, "LightSquared ignores the fact that the GPS constellation is a U.S. government-owned asset representing historical investment of over $35 billion of taxpayer money and annual investment of $1.7 billion to maintain the constellation. The U.S. government, including practically every major federal department and agency, is the biggest single user of GPS equipment and services, and has invested many more billions of dollars in GPS-based equipment and systems, such as the NextGen aviation guidance system. "

LightSquared's argument that inexpensive filters could solve the problem is rejected in the Review. It says, "GPS receivers incorporate filters that reject transmissions in adjacent bands that are hundreds of millions times more powerful than the signals that are transmitted within the band they are designed to receive. LightSquared, however, is proposing to transmit signals in the band adjacent to GPS that are billions of times more powerful than GPS satellite signals. In fact, the highest recorded power levels at the Las Vegas tests conducted in the study were a staggering 800 billion times the power of GPS signals. There has never been, nor will there ever be, a filter that can block out signals in an immediately adjacent frequency band that are so much more powerful, nor has LightSquared put forward any credible, independent expert opinion or other evidence that this is possible." Sounds like that "impossible" filter might cost more than $0.05!

Replacing GPS receivers isn't an option, as the Review explains: "Even real filters are not a solution for the 500 million receivers in use in the United States. The entire population of GPS users would require a minimum of 15 years to prepare for such a change. For example, factory GPS systems installed in automobiles are typically not replaced for the 10-15 years life of the vehicle and the same can be said for aircraft, trucks, ambulances, agricultural and construction machinery to name but a few. The idea of effecting such a transition in a matter of months is absurd."

Perhaps one of the strongest statements in the Coalition's Review is, "There is simply no escaping the fact that it is and was the FCC's responsibility to identify and proactively address GPS interference issues to protect the substantial investment the federal government has in GPS."

As I said at the beginning of this discussion, "What a mess!" I hope that in the FCC's reallocation of TV broadcast spectrum the expert and practical advice of the engineers at the FCC will carry as much weight as the opinions of policy makers and, as a result, broadcasters, wireless carriers and TV viewers will avoid the dilemma facing GPS manufacturers, LightSquared and GPS users.

Doug Lung

Doug Lung is one of America's foremost authorities on broadcast RF technology. As vice president of Broadcast Technology for NBCUniversal Local, H. Douglas Lung leads NBC and Telemundo-owned stations’ RF and transmission affairs, including microwave, radars, satellite uplinks, and FCC technical filings. Beginning his career in 1976 at KSCI in Los Angeles, Lung has nearly 50 years of experience in broadcast television engineering. Beginning in 1985, he led the engineering department for what was to become the Telemundo network and station group, assisting in the design, construction and installation of the company’s broadcast and cable facilities. Other projects include work on the launch of Hawaii’s first UHF TV station, the rollout and testing of the ATSC mobile-handheld standard, and software development related to the incentive auction TV spectrum repack.
A longtime columnist for TV Technology, Doug is also a regular contributor to IEEE Broadcast Technology. He is the recipient of the 2023 NAB Television Engineering Award. He also received a Tech Leadership Award from TV Tech publisher Future plc in 2021 and is a member of the IEEE Broadcast Technology Society and the Society of Broadcast Engineers.