Abstract
In order to track weak signals, GNSS receivers often use a frequency-locked loop (FLL) to track the carrier frequency. The performance of the frequency discriminator directly affects the receiver's signal tracking capability in the FLL. In weak signals, FFT is currently used to estimate the carrier frequency. However, the FFT has a fence effect. The signal length affects the accuracy of the frequency estimation. According to the problem of FFT frequency estimation with low accuracy, we introduce three frequency estimators, which have good frequency estimation performances under low signal-to-noise ratio for GNSS weak signal tracking and propose a carrier frequency tracking method. The article compares and analyzes the estimation error performance of different frequency discriminators in detail. The simulation experiments are used to verify and analyze the tracking sensitivity of the tracking loop under different carrier-to-noise ratios. Theoretical analysis and simulation results show that these estimators effectively improve the accuracy of frequency estimation compared to traditional FFT estimator. Using the open-loop tracking structure, the tracking sensitivity can reach 24 dB-Hz when the coherent integration time is 20 ms and the number of signal samples is 16. Adding a loop filter, the tracking threshold can be improved to 20 dB-Hz.
Highlights
The normal operation of the GNSS receiver is based on the stable tracking of the signal, including carrier tracking and pseudo-noise code (PN code) tracking
In order to improve the sensitivity of the tracking loop, the frequency-locked loop (FLL) is often
Since Candan’s algorithm reduces the bias of frequency estimation, the root mean square error (RMSE) performance of the frequency estimator has been significantly improved at high signal-to-noise ratio (SNR)
Summary
The normal operation of the GNSS receiver is based on the stable tracking of the signal, including carrier tracking and pseudo-noise code (PN code) tracking. Due to the short wavelength of the carrier, carrier tracking has become the most vulnerable link in the receiver [1]. Phaselocked loop (PLL) is often employed to track the carrier signal. The loop uses the local carrier to discriminate the phase of the received carrier. The phase delay of the local carrier can be detected. The local carrier aligns itself with the received signal by the feedback control of the local carrier generator. The phase-locked loop is the best tracking method when the signal environment is good. Under weak signal environment, large phase noise often causes the tracking loop to lose lock. In order to improve the sensitivity of the tracking loop, the frequency-locked loop (FLL) is often
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