Dueling interference tests

Posted in LightSquared, Operators, Regulatory, Spectrum at 4:42 pm by timfarrar

The GPS interference testing conducted by Garmin and submitted to the FCC in January has received widespread publicity due to the “disastrous jamming” it predicted for aeronautical and in-car navigation devices. However, details have now emerged of testing conducted by LightSquared which showed much less of a problem for three smartphones.

The two studies have a number of similiarities (e.g. both conclude that the amount of interference that is needed to make the signal is unusable is about 10dB SNR degration) but come to quite different conclusions about how far from the LightSquared base stations this interference will be experienced. Assuming that the base station power levels at a given distance are equivalent to those given in Figure 1 of the Garmin test (which may be disputed by LightSquared, given that the LightSquared test uses a different setup with two 10MHz channels at 1526-1536MHz and 1545-1555MHz while the Garmin test uses a single 5MHz carrier at 1550-1555MHz), the cellphones would only lose the signal at 100-200 meters from the LightSquared base station, not 1100 meters as the Garmin in-car navigation device did. Obviously this could dramatically reduce the extent of the interference problem.

Indeed we’ve heard from third parties that their own tests showed interference would be experienced “at least several hundred meters away” from a base station, perhaps implying that the real world impact will be somewhere between the LightSquared and Garmin tests. However, it also seems plausible that different devices will have vastly disparate levels of susceptibility to interference, because they use different chipsets, filters, etc. For example, GPS World has highlighted that high-precision GPS users are expected to be disproportionately affected because their devices use a wideband front-end which allows more noise to be received. LightSquared filed a testplan last Friday which notes that one of the next areas to be decided is to “Select the categories of receivers and receivers to be tested” prior to the first progress report to the FCC on March 15 and so it will be interesting to see how wide a range of receivers are to be tested.

In the meantime, the GPS industry is mounting a concerted campaign to overturn the LightSquared waiver and a number of petitions for review have already been filed. One allegation that is common to most of these submissions is that the waiver went beyond the scope of the International Bureau’s delegated authority, and should only have been decided by the full Commission (which would potentially require further comment as well as open any ruling up to Congressional review). LightSquared’s reponse is expected on March 14, with reply comments due on March 29. As a result, there will be a great deal of activity on both fronts over the next four weeks.

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