The independent blog on spectrum policy issues
that welcomes your input on the key policy issues of the day.

Our focus is the relationship between spectrum policy
and technical innnovation.

A net neutrality free zone: We pledge no mention of any net neutrality issues before 2018.

When they deserve it, we don't hesitate to criticize either NAB, CTIA or FCC.

GPS/LightSquared: Round 2 Begins

This week Round 2 of the GPS/LightSquared struggle opened with a change in LightSquared initial operating plans after yet unpublished, but widely leaked, test results indicated some interference to the incumbent population of GPS receivers.

LightSquared announced on Monday

After assessing this information, LightSquared immediately began developing an alternative deployment plan focused on the lower block of spectrum to launch its nationwide wireless broadband service. It also entered negotiations with Inmarsat, the satellite company that controls the alternative block of spectrum in the L Band, to accelerate the schedule for LightSquared to begin using the frequencies.

(Note that this change did not deter LightSquared from its regular full page ads in the Washington Post that deal with the benefits of their planned service but never mention the GPS controversy.)

Wireless Week reports that the actual spectrum involved in the new plan is 1526 MHz to 1536 MHz as shown above in red. Wireless Week add that “Even with the fixes, however, LightSquared’s network is still likely to affect precision GPS systems used by the military, aviation industry and agriculture.” Note that there are now 23 MHz between the upper end of the band LightSquared plans to use and the lower end of the GPS band.

The GPS community was not very conciliatory about the news. Jim Kirkland, Vice President and General Counsel of Trimble said on the Save Our GPS website:

“This latest gambit by LightSquared borders on the bizarre. Last week LightSquared unilaterally delayed filing of the study report that culminated months of intensive work to evaluate interference to GPS, because they purportedly needed two more weeks to analyze the results. Days later, well before the report is scheduled to be filed, LightSquared unilaterally announces that it has found a ‘solution.’

“LightSquared’s supposed solution is nothing but a ‘Hail Mary’ move. Confining its operation to the lower MSS band still interferes with many critical GPS receivers in addition to the precision receivers that even LightSquared concedes will be affected. The government results submitted to date already prove this, and the study group report will also confirm this. It is time for LightSquared to move out of the MSS band.

This is interesting because the GPS industry cheering group, PNT Advisory Board, heard on June 9 from FAA’s Deane Bunce, Co-chair National PNT Engineering Forum (NPEF) , the following mitigation options:

Thus the FAA is being more open minded than the GPS manufacturers. It will be interesting when the detailed test results of the joint testing are released. There are hints of wide variability in GPS receiver susceptibility, probably due to odd filter choices used in the receivers.

The public interest-based solution to this problem probably will involve 3 phases:

1. An initial phase where LightSquared must design their system to protect all safety-related GPS uses, e.g. military uses and aviation-related uses, and most nonsafety-related uses in reasonable circumstances.

2. A middle phase in which FCC in cooperation with other agencies creates an environment to improve the GPS receiver population on a slow and deliberate basis. This could include improved mandatory standards for safety-related uses such as aviation uses even though FAA to date has kowtowed to AOPA and have never fully implemented analogous ICAO Annex X standards for ILS receiver immunity. For nonsafety related systems either a regulation, which FCC does not have legal authority for at present, or a labeling requirement indicating the interference susceptibility of the receivers in certain scenarios.

3. A final phase in which greater use of the MSS band is allowed subject to power flux density (pfd) limits based on separation from the GPS band.

Note the inclusion of pfd limits here. FCC has historically limited transmitters based on effective radiated power (e.r.p.) or effective isotropically radiated power (e.i.r.p.), measures of power leaving the antenna. This is a historic relic of when frequencies were very low, wavelengths were very long, e.g. 200m, and antennas had little gain. Interference is actually related to power at the receiver’s antenna which is more closely related to pfd. At the frequencies involved here clever antenna designs can lower pfd in undesired locations, as alluded to in the first point in Mr. Bunce’s slide shown above. This lower pfd will lower interference potential although the current population of GPS receivers may include some models that are exceptionally vulnerable to almost any power in the adjacent band.

An example is shown in the following diagram used several times by Deere & Company at FCC:


Deere claims that “Wideband filters are required for higher rate, precision codes”. Well a review of this diagram indicates the wideband filter is used primarily to allow a common front end for the GPS, GLONASS, and Inmarsat receiver in their system. Since the GPS community, including Deere, has been on notice since 2003 that some terrestrial use was allowed in the MSS band, why would a manufacturer use such a filtering approach? Note that most of the MSS band has no GPS information, rather it contains narrowband Inmarsat signals with GPS augmentation information. A reliable source in the GPS community has told your blogger that the wideband filters used in precision (RTK - cm accuracy) GPS receivers that overlap the MSS band somewhat are there mainly to speed acquisition time and are not the determining factor in location accuracy. I hope the GPS community clarifies this issue.

The nation’s wheat growers apparently use such Deere equipment and have filed a bizarre letter at FCC claiming

“There are over 160,000 wheat farms in the U.S. and a sizable portion of them are now utilizing GPS technology for an estimated total net investment of about $3 billion from wheat farmers alone.”

It appears unlikely that it will take anywhere near “$3 billion” to update Deere’s design mistake, which they should admit, rather than hiring lawyers to obfuscate the issue.


On 6/23 there was a House subcommittee hearing on this topic. My hometown paper, The Washington Post, sent their transportation writer to cover it and came up with this headline: “Wireless expansion plan would eat GPS signals, critics say”.

Other good quotes:

Opponents of the plan by Reston-based LightSquared described in stark terms the impact it would have on GPS, endangering rescues at sea, adding risk to planes landing in bad weather and garbling dashboard-unit reception.

“LightSquared’s proposal is sort of like driving a lawn mower in a library,” Philip Straub, a vice president of GPS manufacturer Garmin.

“LightSquared has no intention of operating a system in any way that will compromise government or commercial aviation operations,” LightSquared Vice President Jeffrey J. Carlisle said.

“LightSquared’s proposed broadband terrestrial network will cause catastrophic and perhaps life-threatening harm to reliable GPS services,” Straub said.

“The impact of the LightSquared upper-channel spectrum deployment is expected to be complete loss of GPS receiver function,” Margaret T. Jenny, president of the Radio Technical Commission for Aeronautics told the committee.

It was unclear from the article whether the above sound bites applied to the original LigthtSquared band plan (in blue in the top diagram) or the band plan revised after the initial tests (in red).

Those of us who are veterans of past spectrum squabbles such as NorthPoint, UWB, AWS-3 are used to these types of sound bites, but they highlight the need to improve the institutions of spectrum management in the US, perhaps along the lines outlined in a previous IEEE-USA letter to FCC. Effective and efficient spectrum use is key to GDP growth and underutilized spectrum is thus a drag on the economy. Increased spectrum use often results in some impact on incumbents and an objective and timely mechanism is needed to resolve these controversies. Otherwise, no one in their right mind will invest in innovative wireless technology R&D other than marginal improvements to existing technologies.

Other coverage of hearing from more balanced sources:




The FierceWireless article includes the following reasoned statement from DOT:

"We have now tested one proposal here, and we found unfortunately that it did not work as originally hoped," said Roy Kienitz, the Department of Transportation's undersecretary for policy. "That does not mean the story is over. Our goal at DOT is to look for a win-win where we can have much better broadband service nationwide, but to do so without disrupting GPS and vital services it provides.


blog comments powered by Disqus