Assessment Of Commercial Ka-Band Alternatives To TDRSS Services

摘要

This paper provides a technical assessment of the ability of the Teledesic user link and the IRIDIUM intersatellite link to provide support to NASA LEO spacecraft at Ka-band. The assessment was provided for two representative orbits: 300 km altitude, 28.5?inclination; 700 km altitude, 98.2° altitude. The assessment focuses on coverage considering antenna field-of-view for each system. The Teledesic user link is capable of providing 100% coverage to suborbital users; for LEO users, Teledesic provides decreasing coverage as altitude increases. The IRIDIUM ISL provides no coverage to users below 350 km because of the lower elevation angle limit of the ISL antenna. LEO users with polar orbit will experience a geometric scenario where the direction of travel of the user satellite and the service providing satellite cycles between both traveling in the same direction to both traveling in opposite direction. Coverage percent and coverage duration are found to be lower for the case were the user satellite is traveling in the opposite direction. The impact of this dramatic in the case of the IRIDIUM ISL since this link is also constrained by the ISL antenna angular rate capability. When the user satellite is traveling in the opposite direction the required angular rate is often beyond the capability of the ISL antenna. Both systems provide between 34 % and 98% coverage to NASA LEO users based on line-of-sight and antenna field-of-view constraints. These coverage percents may not be realized because of operational constraints pertaining to link acquisition and handovers. Both systems require a high gain antenna for the user satellite. In both systems the coverage is provided in periods from one to several minutes long. At the start of each coverage period the Ka-band link must be established with an antenna acquisition and signal acquisition process. To enable antenna acquisition and signal acquisition, the user satellite must have position information for the service providing satellites. The position information would be used to point a high gain antenna in the direction of the appropriate satellite and to perform Doppler compensation based on the computed range rate.