Development of the Open Source GPS Software Receiver Emulator

Clifford W. Kelley, Frederick Niles, and Douglas Baker

Abstract:	Open Source GPS was originally developed to work with the GEC Plessy, now Zarlink GP2015/2021 chipset. When the SiGe Semiconductor Corporation offered us the use of their software receiver based on their SE4110L chip for open source development it became apparent that it would be possible to write an emulator that could take the output file from the SiGe receiver and emulate the operation of the Zarlink GP2021 and reuse the Open Source GPS code with little or no modification. The software developed for the SiGe by Stephan Esterhuizen of the University of Colorado (based on the GNU radio open source project) was modified to record 60 seconds worth of IF data at the 16.3676 MHz sample rate. An extension to OpenSource GPS called SoftOSGPS was written to read this file and then “interrupt” itself by calling the interrupt service routine every 512 microseconds of data to process the samples as defined by the OpenSource GPS rcvr_par.dat file. A full emulation of all of the features of the GP2021 was not attempted; rather only enough to allow the Open Source GPS software to work was done. This included setting up 256 16 bit “read” registers along with their corresponding “write” registers, the tic measurement sampling, the integrate and dump functions along with code slewing, and setting of the millisecond and data bit counters. Most of the complexity in the emulation turned out to be the implementation of the correlator dump timing to include code slewing. With the modularity of the open source GPS code it was found to be very easy to emulate the GP2021 even though there is a large difference in IF (4.1304 MHz for the SiGe vs. 1.405 MHz for the GP2015). The primary difficulty encountered with reuse of the OpenSource GPS software was due to the fact that the GP2015/2021 uses a high side down conversion while the SiGe uses a low side down conversion. This results in a sign change whenever the carrier information is used. To directly emulate the GP2021 a floating-point implementation was used. While this allows the use of OpenSource GPS code with no changes it is also very slow. To prepare for eventual real-time operation a fixedpoint integer GP2021 like emulation version was begun which should greatly improve the speed.
Published in:	Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007) September 25 - 28, 2007 Fort Worth Convention Center Fort Worth, TX
Pages:	2808 - 2812
Cite this article:	Kelley, Clifford W., Niles, Frederick, Baker, Douglas, "Development of the Open Source GPS Software Receiver Emulator," Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007), Fort Worth, TX, September 2007, pp. 2808-2812.
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