GPS World, January 2018

OUT IN FRONT A GRAVE THREAT TO GPS AND GNSS international 1 dB standard is equivalent to a 25 reduction in GPS radiated power TWO ADDITIONAL POINTS The 1 dB is not simply to protect signal lock it is to protect ranging accuracy Most GPS receivers will stay locked for higher levels of interference but lose high precision This is particularly a problem for highprecision receivers which need relative timing to sub nanosecond accuracies These measurements are equivalent to the time it takes light to travel inch Protecting such accuracies is of paramount importance to PNT users and applications Allowing such maximum degradation from a single source is not the whole picture There are many other potential sources of interference and attenuations of the GPS signal For example foliage may reduce the GPS signal A receiver must cope with all of these difficulties Allowing a single cause such as the Ligado repurposing the 258 equivalent reduction might be considered quite generous but it is the accepted International Standard Ligado has specifically rejected this criterion largely because testing has shown that the Ligado repurposing would then be unacceptable for many PNT user classes To support its rejection of the International Standard Ligado has repeatedly alleged that five of the major manufacturers are in complete agreement regarding its repurposing This is a substantial distortion The record was set straight by Brian Ramsay of MITRE at the November PNTAB meeting Four of the five parties that reached agreements with Ligado except for Topcon Positioning support the 1 dB Interference Protection Criterion IPC in comments filed in response to this Public Notice Further support was highlighted by Captain Robyn Anderson In June 2017 the Air Force produced a white paper on the 1 dB IPC that explained the relationship between harmful interference levels that affect GPS receiver performance and the 1 dB IPC keeps interference below a level that would cause harmful interference Lightsquareds motivation in 2011 was clear a 10 billion windfall profit estimated increased value of the spectrum on open market auction The FCC did not confirm Lightsquareds modified request and in 2012 the company went into bankruptcy Reorganizing as Ligado and emerging in December 2015 it continued to pursue repurposing of its spectrum sponsoring tests by Roberson and Associates and tests at National Institute of Standards and Technology NIST National Advanced Spectrum and Communications Test Network NASCTN to establish test procedures Both groups of tests were carefully reviewed by our PNTAB who found serious flaws In general Ligado rejected the 1 dB criterion and did not accept the need to protect all classes of users particularly high precision receivers In addition it did not consider the new GPS L1 signals L1C and L1M nor did it check the impacts on the international GNSS The PNTAB assembled a 14 point summary of deficiencies and 10 GPS WORLD WWW GPSWORLD COM JANUARY 2018 requested updates and corrections for the flaws NASCTNS response did not really address the points or claimed that there were no funds to correct the problems The PNTAB then developed a Six Point Criteria for acceptable interference testing see online version of this article gpsworld com gravethreat with notes and references summarized as Accept and strictly apply the 1 dB criterion Verify interference for all classes of receivers Test and verify for all operating modes Focus analysis on worst cases Include the new GNSS signals Include GNSS expertise and openly publish results We believe it is a very reasonable set that aims to protect PNT users and our economic benefits In its sponsored tests and in representations to the FCC Ligado has consistently overlooked a basic facet of radio ranging it is ranging accuracy not simply locking onto a signal that is the fundamental objective for PNT Both Ligado test sets clearly failed on all six points DOT ABC TESTS While the Ligadosponsored tests were neither independent nor adequate the Department of Transportation led Continued from page 6 Standoff Distance General Location Navigation High Performance Receivers Timing Receivers Cell Phones 10 Meters 08 milliwatts P2 is 197900 times too large 64 microwatts P2 is 24687500 times too large 87 milliwatts P2 is 181609 times too large 123 Watts P2 is 128 times too large 100 Meters 794 milliwatts P2 is 19899 times too large 65 milliwatts P2 is 243077 times too large 09 Watts P2 is 1756 times too large 12 kilowatts P2 is 125 times too large TABLE 1 DOT ABC test results Maximum tolerable effective radiated power EIRP for classes of the most susceptible GPS receivers for modified Ligado proposal P2 of 158 kilowatts In red are the factors that Ligado P2 exceeds the maximum tolerable radiated power See Grave Threat page 14