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Depending on the environment, the received GPS signals can undergo attenuations in excess of 20 dB from nominal conditions MacGougan Accordingly, substantial signal processing gain is required to compensate for the massive signal attenuation to allow for successful GPS signal acquisition Shanmugam HSGPS receivers utilize large banks of correlators and digital signal processing for enhanced signal detection under adverse conditions.
The use of parallel correlation techniques significantly reduces acquisition time resulting in faster times to first fix TTFF under nominal signal power levels. Under weak signal conditions, the extra signal processing power is utilized to apply long coherently or noncoherently integration to detect the signals Shanmugam Several detection algorithms have been reported in the literature to address the problem of high Gebre-egziabher thesis acquisition.
These detection schemes can Gebre-egziabher thesis broadly categorized under coherent, noncoherent, differential detection or combinations of the three BorioShanmugamGernot To improve tracking performance, there are mainly three methods, namely extending the integration time, applying improved tracking loop and utilizing aiding sensor information.
Extending the coherent integration time is the most beneficial way to improve the signal to noise ratio SNR.
Unfortunately, the maximum coherent integration time in a GPS receiver can be limited by a variety of factors. For instance, the presence of navigation data modulation typically limits the coherent integration time to less than 20 ms. Navigation bit estimation algorithms can be used to predict and remove the data bits for long coherent integration Kazemi Another factor that limit the coherent integration time is the stability issue of a Phase Locked Loop PLLwhich require that the maximum achievable BLT product of loop bandwidth and integration time for a stable loop is limited to 0.
An alternative is to apply an open loop structure or batch processing, which remove the stability issue of a close loop and improve the signal observability. However, the computation load is much higher than conventional closed loop structure. Noncoherent integrations are commonly used for acquiring weak signals.
Unlike the coherent integration, the noncoherent approach relies on short coherent integration followed by a large number of noncoherent summations.
Noncoherent processing consists utilize a nonlinear transformation to the input signal, removing the effects of data transitions and reducing the impact of frequency errors Kaplan This nonlinear transformation generally amplifies the noise impact, incurring the so-called squaring loss.
Recent work from Borio et al applied noncoherent structure to extend to integration time for weak signal tracking with the sensitivity around 14 dB-Hz.
Instead of weighting all the measurements from the discriminator equally, Kalman filter could vary its gain based on the he changing measurement noise statistics and process noise statistics. The measurement noise statistics vary with SNR levels whereas the process noise statistics vary with user dynamics.
The vector based tracking loop which combines the tracking of the different satellite code and carrier signals into a single extended Kalman filter was originally introduced in Spilker In a vector based tracking loop, the individual tracking loops are eliminated and are replaced by the navigation filter.Demoz Gebre-Egziabher’s research focuses on the development of algorithms and systems for navigation and guidance of aerospace and ground vehicles.
He was a McKnight Land Grant professor from – and the recipient of the George W. Taylor Career Development Award for exceptional contributions to teaching. Sebhat Gebre-Egziabher. 2, likes. Sebhat GebreEgziabher. Sebhat Gebre-Egziabher is on Facebook.
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