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Surprise GPS Receiver Tests Involving Ligado Spectrum Effort

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July 27, 2016

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[Updated August 2, 2016] A government agency is poised to release a surprise test plan for GPS receivers that appears to incorporate a highly controversial yardstick for determining when a service in adjacent frequencies interferes with GPS signals.

Devised without public notice and with limited input from the GPS community, the “LTE Impacts on GPS Test and Metrology Plan” was developed under a cooperative agreement with Ligado, a company whose earlier plans to recategorize frequencies close to the satellite navigation band were shown to interfere with GPS receivers. A final version of the test plan released recently can be found here.

Inside GNSS has confirmed that the plan developed by the National Advanced Spectrum and Communications Test Network (NASCTN) was crafted under a Cooperative Research and Development Agreement (CRADA) with Ligado, the reconstituted and renamed LightSquared.

The plan, which was not announced in the Federal Register or posted online in draft form for review, apparently will be posted in its final form on the NASCTN website this week — in fact, the set-up for the tests has already started, Inside GNSS has learned from a source.

"It [NASCTN] is a fee for service organization," explained another source familiar with the issue. "An external client or customer comes in and asks for testing to be done, and then they pay the bill but they don't get to write the test plan at all. That's what the government does."

The sponsor, in this case Ligado, defines the scope of the project, they said.

The existence of a pending test came as a big surprise to some in the GPS community, which had been asking to be involved in NASCTN's work since the organization began coming together several years ago.

"All of a sudden, from out of nowhere, 'Oh here's the test plan for the GPS-Ligado testing going on at the NASCTN,'" said the source." We're like—what the hell?"

NASCTN reached out to a number of companies and federal agencies for feedback, although it is not at all clear how many organizations they contacted or who they were.  Some key players were clearly missed.

A source familiar with the issue said a briefing on the test was held June 10 in northern Virginia organized by the three agencies that agreed in March 2015 to support NASCTN — the National Telecommunications and Information Administration (NTIA), the National Institute of Standards and Technology (NIST), and the Department of Defense's Chief Information Officer.

Agencies were asked during that Friday meeting to submit written comments by the following Monday, June 13, even though, according to the source, a number of agencies only became aware of some key aspects of the test plan during that meeting. For example, said the source, a lot of agencies realized at that briefing that NASCTN was not planning to test the correct parameters for high precision receivers.

The received comments are to be posted along with the final test when it is released, according to NASCTN officials; however, a NASCTN document seen by Inside GNSS indicates those comments will only be posted in "aggregated form.

GPS receiver manufacturer Deere & Company expressed concern over the lack of transparency and stakeholder inclusion in comments it submitted to a Ligado-related docket managed by the Federal Communications Commission (FCC).

"Deere is unaware of how many GPS manufacturers and other interested parties NASCTN has contacted concerning the Test Plan," the company stated in its filing. "The universe of manufacturers, associations and standard setting bodies interested in evaluating compatibility between GPS and GNSS service and terrestrial spectrum uses is large, diverse and growing, extending far beyond the three manufacturers most heavily involved in the evaluation of the predecessor to Ligado’s network.

“The growth and diversity in interested parties can be seen in filings made in the FCC’s record concerning Ligado’s present network proposal. NASCTN cannot credibly argue that it has made an earnest effort to be inclusive without reaching out to these parties."

NASCTN officials declined to discuss the test with Inside GNSS until after the plan was released.

Ligado spokeswoman Ashley Durmer said in an emailed statement to Inside GNSS, “It’s our understanding that NASCTN is going to make public information about the testing and process very soon."  The firm also declined to field questions until after the test and comments were posted.

To What End?
The stated goal of the project "is to develop a rigorous testing methodology and collect supporting data to establish the impact of LTE (mobile communications signals) signals on GPS devices," NASCTN wrote in a May 2016 draft of its plan seen by Inside GNSS.

In its description of the scope of the GPS receiver test program, NASCTN’s draft document says, “A key aspect in the investigation is the quality and availability of measurands such as the signal-to-noise ratio (SNR), C/N0, position error, pseudo range error, etc. . . . The initial measurements will focus on stationary devices, with a moving satellite constellation.”

Even so, the proposed test is not as broad as this statement suggests, according to Deere.

"The Test Plan focuses solely on an LTE wave form occupying the specific frequencies presently proposed for Ligado’s terrestrial network," Deere wrote on June 16.

Indeed, Section 5.3.2 of the draft test plan, “GPS Position Testbed,” states, “The position tests investigate how the LTE waveform impacts the GPS receiver’s ability to provide accurate position.”

However, a later section — 6.2 “Relevant Experimental Variables and Sources of Uncertainty Response variables” — reads, “GPS lock (binary outcome), once GPS lock is achieved, available device outputs, such as position, time, pseudo-range values, C/N0, etc. will be recorded at regularly- spaced time intervals.”

According to the draft, an impetus for this new test effort — the fourth such undertaking including the original receiver tests conducted under FCC auspices several years ago, the Adjacent Band Compatibility (ABC) assessment being undertaken by the U.S. Department of Transportation, and Ligado’s own test program — is this impasse between Ligado (LightSquared) and the GPS community on what constitutes interference to a GPS receiver. In explaining what led up to the project, NASCTN described the testing that went on as the FCC and stakeholders worked through interference issues in 2011.

"However, even after the (GPS Technical Working Group) presented their results, consensus on the definition of what constitutes interference to a GPS receiver has yet to be achieved," the NASCTN wrote. "The GPS industry prefers to define interference as a 1 dB change in the carrier-to-noise-density ratio (C/N0) as reported by the receiver," wrote NASCTN in the draft plan. "This implies that a 1 dB increase to the noise floor, as measured by any receiver in a shielded or direct-wired environment, is considered interference. Potential users of the spectrum adjacent to the GPS bands have proposed a definition based on the end-user experience."

Ligado has continued to argue vehemently, in a variety of forums, that position accuracy as experienced by the user, should be the yardstick by which interference is measured. That yardstick, however, does not have support in the navigation community.

"In our experience, the most appropriate test criteria are those based on the reported carrier-to-noise ratio (C/N0),"
Rod Bryant, senior director of positioning technology at Swiss GNSS
manufacturer u-blox, wrote in an email response to Inside GNSS. "Any interference that affects the performance of
the receiver will show up in that way. Generally, a 1dB effect is used
as the criterion for jamming by most of the industry. However, another
threshold (eg 3dB) would be acceptable in this case and is easier to
detect."

"Despite assertions to the contrary, overwhelming consensus support exists within the GPS and GNSS communities for the 1 dB C/N0 standard, which offers the only universal and quantifiable metric for measuring harmful interference to GPS and GNSS service," Deere said in its comments.

"International regulatory bodies, United States federal agencies and affiliates, industry associations, standard setting bodies and GPS/GNSS manufacturers all committed their support to the 1 dB C/N0 standard during the evaluation of the predecessor to Ligado’s original terrestrial network proposal. For example, the NTIA clarified that 'both the International Telecommunications Union (“ITU”) and the NTIA use a 1-dB signal-to-noise degradation as a maximum tolerable GPS interference criterion.'”

There is a reason for that, according to GPS signal expert Logan Scott.

The signal-to-noise ratio is not meant to predict position but reliability, he explained in an earlier interview.

"It's pretty well-known that a GPS receiver, as long as it's tracking satellites, it's pretty accurate,” Scott said. “You have to throw a lot of interference at it before it actually loses lock — in other words, before you can't track the satellite."

Relying on position accuracy as a metric for interference means "you're not perceiving the risk," he said.  

Moreover, using position accuracy skews tests results in a way that favors mobile communications signals

"By substituting position error for C/N0 degradation," he said, "you're basically building in a strong case for LTE, in other words allowing very high levels of interference — damaging levels of interference."

NASCTN, however, intends to collect data on both position accuracy as well as the signal to noise ratio and other “measurands” — perhaps giving Ligado a new forum through which to make its argument. The GPS community is concerned, however, that the NASCTN project could be used to obscure, not clarify, the risks.

Trusted?
NASCTN describes itself as having been established to help "facilitate and coordinate spectrum sharing and engineering capabilities" and to "create a trusted capability for evaluating spectrum-sharing technologies."

NASCTN, however, is an adjunct of the Center for Advanced Communications (CAC) in Boulder, Colorado, according to a March 25, 2015, press release from the organization. Created in 2013 through a memorandum of understanding between NIST and NTIA, CAC was to implement a key provision of the 2013 presidential memorandum “Expanding America’s Leadership in Wireless Innovation.”

That memo directed the NTIA to collaborate with the Federal Communications Commission to 'make 500 megahertz of federal and nonfederal radio frequency spectrum available for wireless broadband use within 10 years.'"

The frequencies near GPS were seen from the beginning as being one way to help achieve the 500-megahertz goal.

Moreover, said a source who spoke on condition of anonymity, what NASCTN means by being a trusted agent "is that their data will be unassailable. That is, the data will have these methodologies that are very well . . . scientifically thought out and repeatable at other labs — and that the testing will just be all about collecting the data."

"But the interpretation of the data," said the source, "defining whether there is interference or not — which is the whole heart of the matter—they don't get involved in at all. They just leave that to other authorities to decide, which is highly problematic for a lot of folks in the GPS community."

The concern, said the expert, is that Ligado will try to use the data, with perhaps its air of having a "government stamp of approval" to suggest "it proves there's no interference," said the source. "It's lies, damned lies and statistics. You can take any data set and interpret it anyway you want."

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