Geostationary orbit (GEO) communication satellites can be extended in lifetime by switching to inclined-orbit operations. In this mode, a small amount of propellant is reserved to maintain the assigned orbit longitude. Inclination is allowed to build up at a rate of approximately 0.8° per year. Developing these space resources can bring out a number of benefits. Besides communication application, these satellites can be used to construct navigation constellation of the Chinese Area Positioning

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... ystem (CAPS). In this present paper, the realization way of communication and navigation applications is studied and the benefits and problems are explained. Unauthenticated Download Date | 7/29/18 8:46 PM 2 The Chinese Area Positioning System (CAPS) is an area positioning system based on GEO communication satellites. Navigation and ranging signals are continuously generated at a ground station and retransmitted by the transponder on GEO communication satellite. Combining with barometric altimetry technique the user realizes navigation and positioning in the experimental verification phase of CAPS (Ai, et al., 2008(Ai, et al., , 2009. Several CAPS satellites have been executed the inclined-orbit operations and become SIGSO satellites. Besides navigation applications, signal transponders on these SIGSO satellites continue to perform communication function (Ai et al., 2008; Ai et al., 2009; Shi et al., 2009). In this paper we study the benefits of inclined-orbit operations for GEO communication satellites. Section 2 and section 3 describe some problems and give resolving ways during communication and navigation applications in CAPS respectively. The final section presents a summary of the findings of this paper. COMMUNICATION APPLICATION Satellite downlink beam coverage As mentioned above, when NSSK of a geostationary satellite is relaxed, the orbit of the satellite becomes inclined with an inclination that increases gradually with time. Downlink beam coverage of on-board antenna is subjected to the figure-8-like orbit place of the SIGSO satellite. Periodic change exists in the edge of antenna beam coverage even if the communication satellite adopts global beam antenna. Fig. 1 shows the increase in area of the earth's surface that will have direct line-of-sight to the APSTAR 1 satellite that is nominally located at (142°E, 0) if that satellite is drifted into an inclination with 5°. The shaded area represents the extra illumination of the earth. It is obvious that the majority of this new coverage falls in the polar region. Unauthenticated Download Date | 7/29/18 8:46 PM 6 Fig. 6 Daily PDOP change of constellation with n SIGSO satellites (n=1,2,3) without altimeter aiding When 4 SIGSO satellites are used to construct navigation constellation, PDOP value in above stations exceeds 1000. Fig. 6 doesn't show this case. It can be seen that the user can be realized to navigation with poor precision in about half a day. From the figure, navigation and positioning is impossible in whole day on above stations. Meanwhile daily PDOP change with 1 SIGSO satellites is close to that with 2 SIGSO satellites. When 3 SIGSO satellites are used daily PDOP change will become worse. CONCLUSIONS Inclined-orbit operations can obviously extend the operational lifetime of GEO communication satellites. The SIGSO satellites from GEO satellites are smoothly applied to communication functions after adjusting satellite antenna coverage. Different from the GEO communication satellites, tacking mode for ground antenna should be regulated in order to utilize these satellites for obvious variation scale of azimuth and elevation angles exists in SIGSO satellites. Additionally, polarization regulation should be performed if satellites adopt linear polarization antenna. For navigation applications, navigation constellation can be constructed with low cost in a short period. The Doppler frequency shift can be countered by the frequency pre-bias technique. After processing carrier phase measurements, CAPS user can make velocity measurements. Combining with altimeter technique, improvement and deterioration is found in daily PDOP change. Because the GEO satellites locate at the equatorial plane it is impossible that navigation is performed in constellation with only GEO satellites. However, with SIGSO satellites, better navigation performance can be realized in about half a day. It is important how to effectively use SIGSO satellites not only with altimeter aiding but also without altimeter aiding, and detailed analysis will be included in an extended work in future. Agrawal B. (1986) Design of geosynchronous spacecraft. Prentice-Hall, Englewood Cliffs, NJ.