--by Masayuki Itoh, ISAS
Telemetry data from ASCA are downlinked at ground stations including KSC (Kagoshima Space Center, Japan) and the three NASA DSN (Deep Space Network) Stations at Goldstone, Canberra, and Madrid. At KSC, there are usually five contacts per day between the satellite and the station. JPL (Jet Propulsion Laboratory) is responsible for the operation of the DSN stations. JPL supports multiple missions besides ASCA, and the usage of the stations is negotiated in advance. The final schedule is published the Friday before the actual operations. On average, there are about 6 contacts per day at the DSN stations. No data are recorded during the 10-minute data dump at DSN stations.
Because of the limited capacity of the data recorder, the time one can record with the high bit rate is limited to about 1 hour and 8 minutes. When the interval between two sequential ground contacts is long, one may have to use medium bit rate. There are no operations at KSC on Sunday. This reduces the data-recording coverage with the high bit rate on Sundays.
The data acquired at KSC are transmitted to ISAS in real time, while the data acquired at DNS stations usually are transmitted within 24 hours. In either case, the telemetry data first are stored in a database on a FUJITSU mainframe computer, and then reformatted to FRF format in a unix environment.
The orbit of the satellite is determined by NASDA (National Space Development Agency, Japan), and the orbital elements are provided to ISAS once every week. Based on the telemetry data and the orbital information, satellite attitude is determined at ISAS. The orbital data and the attitude determination results are reformatted into FRF orbit and attitude files.
One of the important tasks in FRF production is the time assignment to the data. At this stage, corrections are made for the radio propagation time between the satellite and the ground station, delays in the on-board telemetry data formatter and the ground system, and the temperature-dependent drift of the satellite clock frequency. Correction for the time lag in the on-board X-ray detectors and the front-end data processing are done in the downstream software because it is dependent on the detector and the data processing mode.
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