Abstract
Almost all GPS signal clocks show a periodic fluctuation. Harmonics in GPS satellites are a well-known feature, such as 3-, 4-, 6- and 12-h terms, accurate extraction of these harmonics are helpful to improve the clock modeling. The inaction method (IM) originating from the concept of normal time–frequency transform (NTFT) can precisely extract the periodic signals which significantly present the NTFT spectrum. This method is essentially line-pass filtering, so it can avoid being polluted by noises to the maximum extent and then have the ability to extract the time-varying periodic signals with robustness. The simulation test demonstrates that IM is the best method for extracting the harmonics and time-varying harmonics from a time signal, compared to singular spectrum analysis and zero-phase digital filter. We focus on GPS satellite clock data, after removing 6- and 12-h terms extracted by IM, the oscillation phenomenon in Hadamard deviation virtually disappears; this demonstrates that these periodic signals have been extracted effectively by the IM.
Highlights
The onboard atomic frequency standard (AFS) is a fundamental element of a Global Navigation Satellite System (GNSS) satellite, it plays an important role in the determination of the user position and characterizing the timing performance of the GNSS satellites, the satellite clock stability directly affects the range measurement and limits the navigation accuracy (Senior et al 2008; Cernigliaro et al 2013a, b; Hauschild et al 2013)
We demonstrated the performance and robustness of the inaction method (IM), which essentially is line-pass filtering, on extracting the harmonics and time-varying harmonics from a noisy signal
The 12- and 6-h periodic terms in most GPS rubidium clocks have been detected in normal time–frequency transform (NTFT) spectrums, which can provide an unbiased measurement of the instantaneous frequency, phase and amplitude of the periodic signals
Summary
The onboard atomic frequency standard (AFS) is a fundamental element of a GNSS satellite, it plays an important role in the determination of the user position and characterizing the timing performance of the GNSS satellites, the satellite clock stability directly affects the range measurement and limits the navigation accuracy (Senior et al 2008; Cernigliaro et al 2013a, b; Hauschild et al 2013). In GNSS, each satellite of the constellation carries more than one atomic clock but one controls all the timing. Reservoir Area, Ministry of Education, China Three Gorges University, Yichang 443002, China operations, such as signal generation and broadcasting, GNSS satellites employ different clock technologies: the GPS uses Cesium clocks and Rubidium Atomic clocks, GLONASS uses Cesium clocks, whereas Galileo and the BeiDou use RAFS and passive hydrogen masers (PHM) (Cernigliaro et al 2013a, b; Hauschild et al 2013). The current GPS constellations are running four families of clocks as shown in Table 1: the Block IIR and Block IIR-M satellites carry rubidium clocks, exclusively (Hauschild et al 2013).
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