GPS World, January 2018

OUT IN FRONT A GRAVE THREAT TO GPS AND GNSS transmit power level consistent with typical 4G LTE networks The initial reduced power sounds like a major move in the right direction but further questioning revealed two major issues TOWER SPACING Green was very evasive on the spacing of transmitter towers Clearly at the reduced power level greater density would be needed to carry the original data bandwidth At about 1 100th the power density would have to increase by a factor of 100 and the spacing would have to decrease to 1 10th for the same data output rate Green referred us to an earlier filing which specified 025 mile but did not clearly state that this was the plan she claimed the details were proprietary If this fundamental parameter spacing is not specified it is hard to see the basis for the FCC evaluation of any new proposal If the transmitter spacing is reduced to less than 1 10th of a mile the sources of potential harm would be multiplied in a very worrisome way FUTURE POWER CONSTRAINT A public presentation does not ensure that Ligado will actually file and agree to abide by those power constraints indefinitely Board members pressed Green on the permanence of the power constraint She suggested it would be tied to the RTCA Minimum Operational Performance Standard Revising the MOPS takes many years if not decades both to formulate and to implement Retrofitting the commercial aircraft fleet is very expensive and time consuming Further her statement focuses only on commercial aircraft ignoring the highprecision classes as well as future signals A modified summary chart TABLE 2 for the lower power based on the DOT ABC test results shows that even at the lower power the threshold for highprecision receivers is exceeded by a factor of over 3000 at 100 meters In fact only cell phones which are relatively inaccurate could operate at 100 meters without exceeding the threshold With these expectations and uncertainties the PNTAB did not find the new revision acceptable to the PNT community THREE FUNDAMENTAL ISSUES Ligado has steadfastly not accepted the realities of non interference 1 dB Acceptance of the 1 dB 258 noise increase International Interference standard is fundamental to protecting GPS applications throughout the country ALL CURRENT AND FUTURE USES Users of great concern are emergency services helicopter and general aviation UAVs and precision survey and machine control For example many of the underground utilities in the U S have been mapped with precision GPSbased geographic information receivers This application requires sub meter accuracy and operates in both rural and urban environments Ligado has tended to simply focus on certified aviation claiming that protecting that class of user is enough The PNT community rejects that view All current and future PNT users must be protected WORST CASE INTERFERENCE The recent round of testing was largely in a laboratory Extrapolating to the real 14 GPS WORLD WWW GPSWORLD COM JANUARY 2018 world must examine the situations with greatest interference For example Number of simultaneous interfering transmitters A single transmitter situation is not typical three or more are apt to be in range The additive power must be considered Propagation Models Propagation models for communications differ from those for evaluating potential interference to a navigation signal For assured communication a typical model assumes transmitted signal fall off a little faster than 1 distance squared Ligado would naturally prefer to use this model which is far from worst case for interference The early round of tests in Las Vegas verified the communications model would vastly underestimate interference levels by factors of 10 or more A more realistic model must be used Degradation Radius This is the size of the circle within which the International Standard is violated for receivers in a specified class If the spacing of transmitters is 400 meters and the degradation radius is 200 meters virtually all receivers are in the degradation zone Ligado suggested an appropriate degradation radius is 250 feet for aviation approximately 100 meters Thus they claim the PNT community should tolerate violation of the standard when closer than 100 meters to their transmitters At 400 meters spacing 25 See Grave Threat page 36 Continued from page 12 Standoff Distance General Location Navigation High Performance Receivers Timing Receivers Cell Phones 10 Meters Too powerful by factor of 24938 Too powerful by factor of 311719 Too powerful by factor of 2293 Too powerful by factor of 2 100 Meters Too powerful by factor of 251 Too powerful by factor of 3069 Too powerful by factor of 22 Cell phone at 100 meters is OK 002 TABLE 2 Results of DOT ABC test with Ligado transmitters constrained to 1995 Watts 13 dBW This illustrates that the International Interference Limit is exceeded many times over at 100 meters for certain high precision receivers highlighted in red