Using Almanac to Improve Efficiency of GPS Signal Acquisition
Date Issued
2008
Date
2008
Author(s)
Chiang, Kuang-Yuan
Abstract
When GPS technique is going to be integrated with cell phone or hand-held devices, it is expected that in most situations the receiver will be in a cold-start condition. In view of this, a signal acquisition method that can decrease TTFT (Time to First Fix) efficiently is well worth a further study. In this thesis, we propose a method that is able to efficiently reduce the time to acquire the GPS signal by utilizing the almanac data of the satellites. If a user''s rough location is known, the aforementioned information (satellite’s almanac data) can be used to determine the visible satellites and their corresponding Doppler frequency shifts. The almanac data can be either obtained from last received (complete) navigation data or transmitted from the cell phone wireless service provider. Another factor that may influence the frequency shift is caused by the drift rate of the receiver clock. In this thesis, we also propose a method to estimate the receiver’s clock drift rate to improve the overall performance of our algorithm, where the quantity of the drift rate can be obtained from the previous navigation records. These two procedures can be used to reduce the number of required searches in a GPS signal acquisition algorithm, and therefore the execution time can be decreased. Since the time needed for acquisition is saved, we can spend much more effort to acquire a relative weak signal. We also apply the proposed algorithm to acquire a weak signal for 10ms coherent integration time. In this case, the down-converted frequency will be processed with a bandwidth of ±50 Hz because 10 ms of data would have a corresponding frequency resolution of 100Hz (1/10 ms). Several experiments are conducted to prove the proposed method. In those experiments, the used data is actual raw GPS signal received by using an SE4110L ASIC-based Front-end. From the experimental results, the follow statements may be concluded. Firstly, the frequency search interval is reduced from ±5KHz to ±250Hz when the frequency shift prediction is applied. In other words, the number of frequency searches is reduced from 21 to 3. Secondly, for a static GPS receiver, the interval of frequency search in the frequency shift prediction algorithm is only ±150 Hz. In view of this, by utilizing the frequency shift prediction algorithm, weak signal acquisition may provide better performance and spend less execution time even in the case of a moving vehicle. Finally, the frequency shift caused by the drift rate is obtained from the previous navigation records by applying least-squares method and the effect of this quantity in weak signal acquisition is also discussed.
Subjects
GPS
Almanac
TTFF
Doppler Effect
Clock Bias
Acquisition
Type
thesis
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