Session B2a: Advancements in Navigation Algorithms

An Efficient Method for Acquiring Doppler Frequencies of Satellite Signals Based on Assisted Global Positioning System(A-GPS)
Qiongqiong Jia, Yuchen Zhao, Tianjin Key Lab for Advanced Signal Processing, Civil Aviation University of China
Location: Beacon B

The Global Navigation Satellite System (GNSS) provides users worldwide with all-weather, high-precision, and real-time information on location, velocity, and time, with extensive applications in both military and civilian fields. A conventional GNSS receiver typically comprises three main modules: acquisition, tracking, and positioning, where acquisition serves as the prerequisite for satellite signal tracking and positioning. The objective of acquisition is to obtain the spreading codes of currently visible satellites, along with rough estimates of their corresponding Doppler frequencies and code phases, thereby providing initialization parameters for the tracking loop [1]. Traditional acquisition methods require separately searches for the Doppler frequency and code phase of each potential satellite signal, indicating that conventional acquisition necessitates exploration in the three-dimensional space of unknown spreading codes, Doppler frequencies, and code phases [2].
Traditional serial searching requires performing a correlation operation for each search cell in the three-dimensional space [3]. Although parallel code phase acquisition based on the Fast Fourier Transform (FFT) enables for simultaneous processing of code phases through a single correlation operation, it still necessitates a serial search for the signal's Doppler frequency [4]. While parallel frequency acquisition can determine the signal's Doppler frequency using a single FFT, it cannot compute the Doppler frequencies of all visible satellites simultaneously [5].
Traditional acquisition requires an independent correlation operation at each Doppler frequency band within the search space; therefore, reducing the search range for Doppler frequencies can effectively decrease the computational load. Gao et al. proposed a method to compress the Doppler search range by jointly processing signals based on spreading code periods [6]. This method first selects a set of discrete frequency points within the 0–1 kHz range, generates corresponding phase compensation sequences for each selected frequency point, and multiplies these sequences with the jointly processed satellite signals before applying the FFT. The maximum amplitude identified in the resulting spectrum indicates the Doppler frequency. Yang et al. suggested that by combining local reference signals from different frequency bands, acquisition can be achieved through frequency-domain correlation with the received signal, thus avoiding per-frequency searches; however, this method does not yield precise Doppler frequency values [7]. Additionally, the introduction of assisted information can reduce the parameter search range and enable the receiver to perform a warm start, thereby lowering the Time to First Fix (TTFF). Information from Inertial Navigation Systems (INS) can serve as a reference for the Doppler frequency of satellite signals, compressing the search range to a specified area and avoiding serial searches across all frequency bands [8]. Assisted Global Positioning System (A-GPS) can provide approximate receiver locations, current visible satellite spreading codes, and satellite ephemeris data, significantly reducing the search range for the unknown Doppler frequencies of visible satellite signals and thus improving acquisition efficiency [9].
The aforementioned methods primarily enhance acquisition efficiency by reducing the search units for the Doppler frequencies; however, they still require separate acquisition for the Doppler frequency of each satellite signal. To further decrease the computational burden of acquisition, we propose an efficient method for acquiring the Doppler frequencies of satellite signals based on A-GPS, which does not rely on the spreading codes of visible satellite signals and can estimate the Doppler frequencies of all visible satellite signals through a single Fast Fourier Transform (FFT) operation when the spreading codes are unknown. The proposed method first squares the received signal to eliminate the influence of spreading codes and then estimates the Doppler spectrum of all visible satellite signals using a single FFT operation. The estimated Doppler frequencies for each visible satellite are then determined based on the spectral peaks. To further validate these estimated values against the corresponding satellite signals, assisted information from A-GPS is utilized to estimate the range of Doppler frequency values for each visible satellite signal. The estimated values that fall within the Doppler frequency range of the corresponding satellite signal are taken as the final values for that signal's Doppler frequency.
The experiment evaluates the Doppler frequency acquisition performance of the proposed method using simulated signals. In the experiments, ephemeris files and base station locations are input into a GNSS signal simulator to generate the satellite signals received at the base station, while the same ephemeris files and the user receiver's position are used to generate the satellite signals for the user receiver. A software receiver processes the satellite signals received by both the base station and the user receiver. The Doppler frequency obtained from the signal acquisition at the base station, along with the approximate distance between the base station and the user receiver, is used as assisted data to estimate the range of visible satellite signals and their corresponding Doppler frequencies at the user receiver. Subsequently, the proposed method is employed to process the satellite signals at the user receiver, estimating all the Doppler frequencies.
Experimental results demonstrate that the computational efficiency of the proposed method for acquiring Doppler frequencies of satellite signals based on A-GPS is significantly higher than that of traditional acquisition methods. Assuming that the code phase is known and only the Doppler frequency is being acquired, the computational load of the proposed method is only 1/N of that required for parallel code phase acquisition, where N represents the number of visible satellites.If the complete acquisition of satellites is carried out under the condition of unknown code phase, when the number of satellites is 1 and the sampling frequency is 10MHz, the computational cost of this method is 4.87% of that of parallel code phase acquisition. When the number of satellites is 10, the computational cost of this method is 2.93% of that of parallel code phase acquisition.
Reference
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