DOT GPS Adjacent Band Compatibility Assessment Test Results

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The goal of the U.S. Department of Transportation (DOT) Global Positioning System (GPS) Adjacent Band Compatibility Assessment is to evaluate the adjacent radiofrequency band power levels that can be tolerated by GPS and Global Navigation Satellite System (GNSS) receivers and to advance the Department’s understanding of the extent to which such power levels impact devices used for transportation safety purposes, among numerous other civil GPS/GNSS applications. There are two components of the DOT Study; - one component, led by the Federal Aviation Administration (FAA), is focused on certified GPS avionics, and is being conducted by analysis to determine the adjacent band power level that conforms to existing certified GPS aviation equipment standards; - the other component of the Study is focused on the interference susceptibility of GPS/GNSS receivers for all civil applications apart from certified GPS avionics, and that effort is led by the DOT Office of the Assistant Secretary for Research and Technology (OST-R) and is the topic of this paper. Through this component of the Study, DOT, with its interagency partners, has conducted testing of categories of receivers that include aviation (non-certified), cellular, general location/navigation, high precision, timing, and space-based. The GPS Adjacent Band Study has been the product of an extensive and transparent process to gather stakeholder views and input. DOT held a number of public workshops to discuss development of the test plan and review the draft test plan document. After the Test Plan was finalized, GPS/GNSS receiver testing was conducted at the U.S. Army Research Laboratory’s Electromagnetic Vulnerability Assessment Facility (EMVAF) at White Sands Missile Range (WSMR) in New Mexico in the spring of 2016. DOT and other participants tested 80 GPS/GNSS receivers during the WSMR anechoic chamber testing. Four types of testing were conducted which involved a linearity test, 1 MHz Bandpass Noise, 10 MHz Long Term Evolution (LTE), and effects of 3rd order intermodulation. Of the 80 receivers tested, 14 were subsequently subjected to additional conducted/wired testing at Zeta Associates in July 2016. The objectives of the wired testing included: (1)evaluation of the impact of adjacent-band interference on signal acquisition, (2) determination of receiver susceptibility to adjacent band interference with 1 MHz Bandpass noise and 10 MHz LTE (same signals as used in the anechoic chamber), and (3) an assessment of the impact of the presence of an adjacent band transmitter noise floor (out-of-band to the interference source, in-band to GPS/GNSS) in addition to the fundamental emission. Additionally, antenna characterization took place at MITRE from June through August 2016. This paper describes the testing approach and data analysis used to develop interference tolerance masks (ITMs) based on a 1 dB carrier-to-noise-ratio (CNR) degradation. This paper also presents the resulting ITMs and puts forward a recommendation for the bounding ITM for each GPS/GNSS receiver category. Given a particular use case scenario, the significance of these bounding ITMs is that they provide information that is necessary for the downstream analysis to determine the maximum Effective Isotropic Radiated Power (EIRP) that can be tolerated in the adjacent radiofrequency bands on a per category basis. Furthermore, this paper discusses acquisition results as they relate to the 1 dB CNR degradation limit, and a cross comparison for some of the receiver results between radiated and conducted tests incorporating the appropriate antenna characterization data.