engleski

Non-GSO satellite navigation system

The explosive growth in the terrestrial mobile communications has evoked an interest in providing an extension of this growth in the field of multimedia satellite communications for mobile users [1, 11]. In the ground segment, usage of the personal handsets with nondirectional antennas without tracking is preferred, using same protocols as in terrestrial part, integrating the terrestrial and satellite mobile networks. In the satellite segment, technological advances have accelerated a renaissance in usage of non-geostationary (non-GSO) low orbit communication satellites in provision of various services domestically and globally. Early ventures into communication satellites have used satellites in low Earth orbits. The main reasons for this mode of operation were that the booster technology was not available to lift satellites into a higher orbit, the satellite technology was not sufficiently mature to achieve higher performance such as power generation and signal bandwidth. Also, there was a problem with reliability due to operation in hostile space environment. After that early stage, more than 30 years the most dominant domain of communication using satellites has been in circular geostationary equatorial Earth synchronous orbit [9-12]. As the satellite system technology developed, it became possible to provide services by the use of satellites in various orbital regimes like low Earth orbit (LEO at about 1000 km altitude), medium Earth orbit (MEO at about 10000 km altitude), geostationary orbit (GSO) and the highly elliptical orbit satellites (HEO). Radio navigation satellite systems (RNSS) fall into one of the three categories: Doppler navigation (e.g. NNSS Transit), range navigation using non-GEO and GEO satellites (e.g. Euteltracs) and range difference navigation (e.g. Navstar GPS) [1,5,6]. Historically, the navigation concept using one way signal transmissions evolved from single satellite two-dimensional service using Doppler frequency shift to multiple satellite three-dimensional range difference navigation. Doppler navigation systems like Transit provided two dimensional positioning service and they acquired substantial time (10-15 minutes) for position fix due to principle of integration of frequency shift to determine the change of distance between satellite and terminal. Today, range difference RNNS are predominantly used for position determining services and they showed reliability, flexibility and precision, but they need multiple satellites above horizon (four) for three-dimensional position determination. Detailed explanation of history and performance of range based RNSS are given in [5-8]. This paper gives mathematical derivation of the trajectory coordinates of non-GSO satellites using inclined circular orbits. Based on this, LOS relative velocity component is deduced giving explicit equations for the frequency shift observed by a transceiver situated anywhere on the Earth. Furthermore S-shaped Doppler frequency shift – time curves are derived (with maximum elevation angle as parameter) [2]. One of the problems ground transceivers are facing when communicating with non-GSO satellites is significant Doppler frequency shift. This time varying phenomenon is caused by the line of sight (LOS) component of the relative velocity vector evolving from the rapid movement of the satellite in its orbit relative to the ground transceiver including satellite velocity and the relative velocity due to the Earth’s rotation [2,4]. Previous researches in this area mainly focused on efficient methods for compensation of frequency shift. In this paper, it is proposed to use the combination of Doppler shift and range difference measurements for navigation purposes using good features of each concept. This gives reliable and precise three dimensional position determining service which has simple system architecture, usesreduced required number of satellites above the horizon for position determination, and have reduced time for position fix processing.

satellite navigations systems; non-gso satellite

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