| Popis: |
The global navigation can be provided by GNSS satellite constellations such as GPS, GLONASS, and the recently emerging BEIDOU and GALILEO in medium and higher earth orbits. Generally speaking, those GNSS satellites, that are in the altitudes of about 20,000 km, move around the Earth with an orbital speed of approximately 3.88 km/s relative to the Earth. According to the Doppler Effect, the relative movement of GNSS satellites and GNSS receivers lead to a Doppler shift in the GNSS signals frequency on the receiver. It would be even worse if the GNSS receiver also has a high velocity relative to the GNSS satellites. A GNSS receiver in space, especially in Low Earth Orbit (LEO), suffers from the high dynamics. A satellite in the altitude of 550 km travels in an orbital speed of approximately 7.58 km/s which leads to a high Doppler frequency shift of about ±60 kHz. That is why tracking the GNSS signals for LEO satellites is always of a great challenge. In this paper, it is attempted to suggest and analyze a suitable method which can surmount the difficulty of high dynamic GNSS signal tracking in LEO. Vector tracking is the method which is selected for this purpose and its performance is analyzed regarding the requirements. It is shown that compared with FLL tracking loop, vector tracking increase the dynamics by at least 200%. |
