Orbital-computation studies for future ESA geophysical satellites

The orbital computations undertaken by the Delft Technical University Section for Spacecraft Orbital Mechanics to predict orbital characteristics and plan ground tracking networks for the ESA satellites Popsat and ERS-1 are surveyed. Popsat is a precision orbital-positioning satellite of mass about 700 kg to be launched to a 7000-km 98.6-deg orbit in about 1992 and uses precision range and range-rate equipment in the 2, 7, and 8-GHz bands to provide earth-position determinations with absolute accuracy 10 cm and length-of-day variations to within 300 microsec; the ground segment includes a mission-control center, 16 mission-execution stations, groups of smaller stations for plate-tectonics studies, and numerous receive-only user stations. Calculations for Popsat include simulation of the forces affecting the orbit and investigation of the possibility of real-time position finding for the user stations. ERS-1 is a terrestrial-remote-sensing satellite to be launched in 1989 to a 700-km 98.5-deg orbit and uses an array of ranging and imaging instruments to provide data on ocean and coastal regions. The problem of precision tracking for ERS-1 is discussed, and the possibility of installing a Popsat transponder on future ERS spacecraft to permit satellite-to-satellite tracking is considered.