A Calibrated Precision GNSS Simulator for Timing Applications

J. Fischer and L. Perdue

Abstract:	Testing GNSS receivers specifically for timing applications to determine the accuracy of the receiver’s 1PPS output presents challenges in measuring accuracy to sub nanosecond levels. Using the live sky signals as a test reference has many problems. It has been shown that receiver biases vary at different frequency bands because of varying group delay in the RF circuitry; biases vary across different GNSS signal types because of the different modulation, various decoding techniques and different correlator sizes; and biases even vary from satellite to satellite for the same signal type because of different transmitter characteristics. Moreover, changing atmospherics over time makes the live sky signals a poor reference for repeatable tests. For all of these reasons, it is desirable to use a GNSS simulator to generate the various constellation signals in a repeatable, reliable fashion. The paper will show the results of a project underway to take an affordable but fully capable precise GNSS simulator and calibrate its RF output so a receiver’s 1PPS can be measured. The goal is to calibrate the 1PPS output reference output of the simulator relative to the simulator’s RF output to a resolution of 0.1 ns for any particular constellation signal. The operating bands of interest for the four major GNSS systems are examined: GPS L1, L2 and L5; GLONASS L1 and L2; Galileo E1/E2, E5, E6; and Beidou B1, B2, and B3. The measurements are noted for each operating mode and filter selection possibilities. Also, variations due to temperature changes will be examined. The results should yield a calibrated simulator for precise timing measurements of GNSS receivers. Though the 1PPS reference pulse output delay will be different for each frequency band, the difference will be known and repeatable, so that a thorough evaluation of GNSS receivers operating with all the major constellations can be made.
Published in:	Proceedings of the 45th Annual Precise Time and Time Interval Systems and Applications Meeting December 2 - 5, 2013 Hyatt Regency Bellevue Bellevue, Washington
Pages:	107 - 111
Cite this article:	Fischer, J., Perdue, L., "A Calibrated Precision GNSS Simulator for Timing Applications," Proceedings of the 45th Annual Precise Time and Time Interval Systems and Applications Meeting, Bellevue, Washington, December 2013, pp. 107-111.
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