ARTEMIS, S-band and Ka-band inter orbit link

Abstract

The European Space Agency's satellite ARTEMIS will support data links between spacecraft in low earth orbit, ARTEMIS in geostationary orbit and ground stations in Europe. The links between spacecraft in low earth orbit and ARTEMIS will operate at S-band (2 GHz), Ka-band (25 GHz) or optical wavelength (800 nm). This paper addresses the S-band and Ka-band links, which both operate via a steerable 2.85 m diameter reflector antenna on ARTEMIS. The link design has been performed to minimize the signal degradations, and has led to rather demanding requirements on the flatness of the amplitude and group delay distortion versus frequency, on the phase noise and on the linearity of the payload. The measured values for these parameters are presented. In S-band, the antenna pointing is performed in open loop according to a preloaded table, while in Ka-band a closed loop pointing system is implemented and open loop pointing is back-up. The design, test and operation of the antenna pointing system are presented. The European Space Agency (ESA) participates in the Satellite Network Interoperability Panel (SNTP) together with NASA and NASD A. SNTP has established a set of interoperability parameters making it possible for low earth spacecraft from the three agencies to operate through the relay satellites of NASA, NASDA and ESA. ARTEMIS will be the first ESA satellite to offer data relay services, and its performances are compared to the interoperability parameters established by SNIP. Inter Orbit Links Spacecraft in low earth orbit (LEO) are restricted in their ability to communicate with the ground, because of the limited time a spacecraft in LEO is visible from a single ground station. ARTEMIS, located in the geostationary orbit, will significantly increase the communication time between the spacecraft in LEO and a single ground station. This is illustrated in Fig. 1. When a spacecraft is flying over the cross hatched areas in Fig. 1 at 300km altitude, no contact is possible via ARTEMIS. This area is slightly increased for a spacecraft at 1000km altitude as marked by the contour. Spacecraft can maintain contact to ground via ARTEMIS, when they are over SouthAmerica, eastern part of North-America, the Atlantic, Europe, Africa, most of Asia, the Indian Ocean and the polar regions The data relay function is provided by communication links between the ground and ARTEMIS named feeder links and links between ARTEMIS and the LEO spacecraft named inter orbit links (IOL). The feeder link operates at 20 GHz from ARTEMIS to ground (i.e. return feeder link) and at 30 GHz from ground to ARTEMIS (i.e. forward feeder link). The IOL can operate in three different bands namely S-Band (i.e 2.025 to 2.110 GHz for the forward IOL from ARTEMIS to LEO spacecraft and Copyright © 1997 by J. Sandberg. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission.