DOT Says GPS Adjacent Band Testing Now Turning to Setting EIRP Levels

With GPS Adjacent-Band Compatibility (ABC) Assessment L-band testing done, the next step is coming up with tolerable equivalent isotropically radiated power (EIRP) levels to protect GPS from adjacent band interference, the Transportation Department told us Tuesday. Those tolerable EIRP levels will come from radiated test interference tolerance masks, use case scenarios representing relevant applications and services, and propagation modeling, Transportation said. The agency presented its ABC testing results Friday at a workshop and said there are no further plans for testing. That testing comes as Ligado has been trying to get FCC approval for its broadband terrestrial low-power service in spectrum adjacent to GPS and arguing that the proposed power and out-of-band emission levels protect GPS (see 1602250032). Ligado in a statement said it's "glad that DOT's nearly two years of testing is finally over. Now the government agencies with authority over our nation’s spectrum policy, the FCC and the NTIA, can take this information and all the other studies into account and make critical decisions. It is odd that the DOT study did not address at all the actual harm to consumers, as the other studies did. DOT also made clear that it recommends a mask based on the worst device it could find. Others will have to decide whether that is sound policy.” DOT said the reacquisition tests that were part of wired testing showed that for some receivers, there wasn't significant margin beyond the interference power causing 1 dB degradation. The degradation was particularly apparent for lower-power GPS signals, like those that would be associated with operating in challenged environments such as urban canyons or under foliage or with low-elevation satellites in open skies, DOT said. When asked about what its test findings might mean for actual device performance, the agency said the wired testing results "suggest exceeding the 1 dB interference level can adversely affect receiver performance by slowing satellite acquisition times." The department said radiated test results -- which show "the more comprehensive view of adjacent band interference causing a 1 dB degradation" in carrier to noise ratio -- were nonetheless "in good agreement ... when the performance of the active antennas were taken into consideration."