Understanding The Indoor GPS Signal

T. Haddrell and A. R. Pratt

Abstract:	This paper describes the evolution of the Parthus Technologies GPS 3000 Receiver in order to track GPS signals inside buildings and provide location solutions. GPS300 is available as Intellectual Property (IP), which is integratable into cellular telephones and PDA's. It is necessary to be able to measure the GPS signal indoors in order to understand its structure and characteristics. The development of the Indoor Signal Sensor (ISS) GPS receiver was completed for this purpose. This receiver uses two antenna's and two RF front ends. One antenna was placed outside or in a reference position allowing GPS signals to be acquired by conventional techniques. The other antenna was placed indoors at the location where the GPS signal was to be characterized. Using the reference code and carrier from the tracked (outside) signal, the code and carrier for the indoor signal could be generated, such that long integration periods could be achieved, with coherent signal integration, relative to the outside antenna. Twelve physical channels were assigned to the indoor antenna, with variable and selectable code delays relative to the outdoor signal. This technique provided real-time measurements of indoor GPS signals, for multiple code delays, and for various coherent integration times. The fundamental "Integrate and Dump" correlation time was set to be 125 microseconds. Each code delay correlator sample of the indoor signal was normalized against the outside signal to remove the effects of the 50 bits per second GPS data message. Each channel thus produced a relative code magnitude and carrier phase measurement. Internally, samples were accumulated over periods ranging from 1 millisecond to 4 seconds. Longer integration times are simply a convenient method of measuring the indoor GPS signal relative to the outside signal. The receiver was enabled to output the results both in real-time and in a format suitable for post processing. The real-time output consisted of D/A outputs to drive a standard oscilloscope. The Y-axis was used to display signal magnitude, relative carrier phase and raw I and Q correlator counts, verses the code delay on the X-axis. Actual video clips of the oscillator display are included in the accompanying presentation of the paper. The video clips show the constitution of the GPS signal under several conditions including stationary and slow moving antenna positions. The effects of multiple signal paths, standing waves, and the corresponding carrier phase relationships can be clearly seen. For more formal data collection, suitable for postprocessing, the receiver outputs text messages containing raw correlator counts, or relative signal strength, for all code delays. This data is captured on a PC and processed by a variety of means including Matlab and Excel to produce two and three dimensional plots. To conclude, the results of several environments are illustrated and discussed. The most important characteristics of indoor GPS signals are identified. Examples of standing wave patterns, multiple signal pathways and general signal attenuation are given, and the optimum coherent and incoherent integrations times estimated.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	1487 - 1499
Cite this article:	Haddrell, T., Pratt, A. R., "Understanding The Indoor GPS Signal," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 1487-1499.
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