Watching and Waiting. And Questioning. GPS in the Balance

August 25, 2011  - By

The difference between navigation and communication signals — a key point not well or not at all understood in Washington — and an FCC rule that could cause LightSquared to foot substantial GPS refitting bills even if it prevails to interfere, were two of several subjects that came to light in last week’s “LightSquared Watch” webinar. As the Federal Communications Commission goes through its deliberations, two inside-the-Beltway experts joined me to speculate on what may happen, what we might do about it at that time, and the long, strange trip that brought us to this point. These matters, and your questions answered, in this month’s column.


To download the slides and one-hour audio recording of the “LightSquared Watch” webinar, click here.

Webinar speaker Scott Pace, director of the Space Policy Institute at George Washington University, included in his presentation a substantial chunk from an FCC filing by Glenn Borkenhagen of Cody, Wyoming. Here it is, verbatim.

Nav Signals Are Different from Com Signals

“The interference problems exhibited by precision GPS receivers can be fixed with filters.”  [according to LightSquared]

This sounds plausible, even to some engineers knowledgeable in radio-signal processing, until it is realized that the typical filtering concepts don’t really apply here because the critical data for accurate GPS position is the ranging information that is derived from the arrival times of the state transition in the code message modulated onto the GPS carrier frequency and the arrival times of the carrier waves.

Synchronized atomic clocks on each of the satellites tell us when the signals leave the satellites, and when the GPS receiver is tracing four or more satellites the receiver can measure with atomic-clock accuracy when the clean signals arrive at the receiver’s antenna. To oversimplify a bit, the important factor about a clean code-message signal is that it has a good sharp and square edge when the digital signal modulated onto the carrier frequency changes from a digital 0 to a digital 1 or vice-versa.  We know the signal traveled at the speed of light from the satellite to the receiver’s antenna and when we know how long it took to make the trip we know how far the receiver’s antenna is from each satellite and can determine the position of the receiver’s antenna.

Accurate edge/transition-time detection is necessary to determine when the signals arrive at the receiver’s antenna. When heavy filtering is applied to remove strong near-band interference, the signal edge transitions get rounded, blurred, and even time-displaced so determining an accurate arrival time becomes much more difficult if not impossible. It is easy in comparison to filter simple 0s and 1s to transmit a video file, for example – much more difficult to filter code and carrier without destroying the essential ranging information.  GPS is essentially determining position using a “measuring stick” that is moving at 3 x 10 **8 meter/second. 

[end of Glenn Borkenhagen’s comments, as excerpted in the webinar]

Thus, the fix proposed by LightSquared will not fix anything. It is broken to begin with.

Pace also alluded briefly to Section 25.255 of the FCC’s own rules. It states:

§ 25.255 Procedures for resolving harmful interference related to operation of ancillary terrestrial components operating in the 1.5./1.6 GHz, 1.6/2.4 GHz and 2 GHz bands.
If harmful interference is caused to other services by ancillary MSS ATC operations, either from ATC base stations or mobile terminals, the MSS ATC operator must resolve any such interference. If the MSS ATC operator claims to have resolved the interference and other operators claim that interference has not been resolved, then the parties to the dispute may petition the Commission for a resolution of their claims.
[68 FR 33653, June 5, 2003]

Note the date of enactment: 2003. This was at the time of, or immediately following, negotiations involving the FCC, a previous owner of the MSS band now held by LightSquared, and the U.S. GPS Industry Council. The regulation seems to imply that LightSquared could be held accountable for the costs associated with coping with the interference created by its signal, as incurred by the multitudinous arms of the GPS industry and user community, not to mention various arms of government such as the Federal Aviation Administration.  We’re talking many billions here.  Many billions.

Our other webinar speaker, Jules McNeff, vice president of strategy and programs for Overlook Systems Technologies, noted that this is a very political process that since the beginning has appeared heavily slanted to favor LightSquared entry. The political access of company executives and the owner to the White House has been well documented. Misinformation is rampant throughout the waiver petitioner’s arguments pre- and post-, and the pressure for action before analysis has been strong, surprisingly so. History has been reinterpreted — and McNeff should know, he was a key participant in those historical discussions of the late 90s and early 2000s — with facts twisted to fit the desired reality. The FCC’s  actions are inconsistent with what public should expect from an unbiased federal rulemaking agency: public statements by agency leaders and staffers undermine the GPS industry and its users, agency positions ignore the fundamental differences between GPS and comm., and its statements resonate with assertions from LightSquared about the GPS community.

Both speakers concurred that the safest and most fact-based course of action for the FCC to take — and the only approach fully consistent with the terms of both the National Space Policy and the Broadband Memorandum as well as the FCC’s own regulations — is for the agency to conclude that the terms of the LightSquared conditional waiver have not been met and withdraw LightSquared license to deploy a terrestrial network in the 1525-1559 MHz band.

And now, your questions:

Q: What GNSS frequencies will and will not be affected by Lightsquared?

Webinar speakers’ Answer: The entire Radionavigation Satellite Service (RNSS) band from 1559 to 1610 MHz will be affected by LSQ transmissions below the band (ground stations) and above (handsets).

Q: Will you discuss Doppler shift and how the GPS recieved frequencies may fall in the bandwidth being used by LightSquared?

A: GPS uses relativistic doppler shift corrections and the adjusted carrier frequency is in the navigation message. Doppler effects don’t shift the received frequencies out of the RNSS band.

Q: Are there other MSS service in the band that will be affected by the power levels of LS?

A: Inmarsat is the primary MSS service affected that I’m aware of. Omnistar and Starfire use MSS to provide DGPS serives

Q: Can we address the potential effects on GPS timed simulcast radio systems?

A: Any GPS-enabled systems, capabilities, or applications would be affected within the areas covered by LSQ ground transmitters

Q: Will this problem undermine the position of the FCC?

A: If, on investigation by competent oversight authorities, the FCC’s actions prior to and following the issuance of the LSQ waiver (including rulemaking in previous years) are found to violate accepted practices or be motivated by political bias counter to the public interest or adversely affecting public safety, then yes, it will undermine the position (perception?) of the FCC as an independent federal rul
emaking organization.

Q: Does anyone have a read of how the FCC will actually rule and when? If the FCC approves LightSquared deployment, is the "Save Our GPS" coalition prepared to go to court in order to stop LightSquared deployment?

A: No to both parts. Any further actions taken by the FCC are subject to unpredictable political considerations at present. The coalition itself likely does not have the legal standing necessary to bring a lawsuit.  Individual members and specific adversely affected parties would have to act

Q: This appears to me to be a factual and fair interpretation of the situation. Thank you. I assume that that since the slides provide credit to their authors and origin, I can share them with others without reproach.

A: Yes. To download the slides and full audio of the webinar, click here.

Q: How long do you think the FCC will take to review the docket before issuing a decision? Does anyone know when the FCC will render its final decision?

A: Any further actions taken by the FCC are subject to unpredictable political considerations at present.

Q: Given that the laws and regulations cited are settled law, is the GNSS industry prepared to go to The Court of Appeals for the District of Columbia to stop the harmful and illegal waiver process?

A: This matter is still being considered in the political arena for the moment, and so going to court is premature at this point.  If the FCC upholds the waiver it issued at the beginning of this year, then my personal opinion is that adversely affected parties would have to bring suit individually (at least at first) based on the specific damages they can attribute to the FCC’s decision.

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