ION Technical Content
Indoor GPS Disciplined Clock
| Abstract: | Indoor GPS disciplined clock is the subject of this paper. It is a GPS timing receiver with enhanced sensitivity. It is capable of operating in an environment where GPS signal power is as low as –155 dBm while meeting all of the existing performance requirements. It is based on the current Trimble disciplined clocks with mostly software changes. This paper addresses most of new problems the indoor GPS disciplined clock faces and ways to solve them. The challenge includes weak signal acquisition and tracking, oscillator disciplining with noisier measurements, and meeting the holdover requirement. Trimble has developed a prototype indoor GPS disciplined clock. Test results are presented with discussion of the indoor GPS disciplined clock performance. A weak signal is defined as a signal below –143 dBm at the input of the antenna. Weak signal acquisition strategy and algorithm are discussed. The algorithm takes advantage of a stable Oven Controlled Crystal Oscillator (OCXO) clock. The frequency-offset uncertainty of the OCXO used in our prototype unit could be as much as +/- 75 ppb. Aging during the first 9 to 12 months primarily causes this frequency offset. A DFT search algorithm is used to search for multiple frequency bins simultaneously. Integrating I & Q samples over long period is used for additional processing gain. Additional processing gain is needed to acquire signal as low as –155 dBm. This paper discusses trade-off between coherent integration and incoherent integration of I & Q samples to achieve necessary processing gain. To discipline the oscillator, the weak signals must be tracked continuously. Carrier aiding is needed for weak signal tracking. A code loop with narrow bandwidth is used to maintain code lock of weak signal. Pseudo-range measurements are constructed from the code phase measurements and filtered with a code-carrier filter. The pseudo-range measurements are then used to compute the clock bias with an over-determined solution. Oscillator disciplining refers to the process of adjusting the frequency and phase of the timing outputs into alignment or synchronism with GPS or UTC time. Clock bias and bias-rate are used to discipline clock. The disciplining circuit time constant is chosen based mainly on oscillator performance instead of weak signal level. Holdover refers to the performance of a GPS disciplined clock when GPS signals drop out. During holdover, the clock relies on Kalman filter to correct for oscillator drift caused by temperature change and aging. Since our prototype unit uses a single ovenized oscillator, a requirement of less than 7 microseconds over 8 hours is used for the holdover performance testing. Preliminary test result has shown that this requirement has been met. Tests were conducted in two categories using the prototype unit. The first category includes tests conducted with antenna placed under a control environment. The second category includes tests conducted with antenna placed inside a building. These test results are analyzed and presented. |
| Published in: |
Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR |
| Pages: | 1195 - 1203 |
| Cite this article: | Yu, Jim Y., "Indoor GPS Disciplined Clock," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 1195-1203. |
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