Station Realtime Orbit Interface

The JPL satellite orbit prediction is transformed into a GBES orbit file by the offline program ORBITCO. The output of ORBITCO is input to the realtime system.

The realtime task GEOMETRY reads the orbit file and maintains a circular buffer containing the orbit file entrys bracketing the current time. The realtime task POINT uses linear interpolation of the satellite azimuth and elevation to control antenna motion. The TWT task uses a cubic interpolation of the methode to calculate the predicted uplink and downlink frequency to the satellite, based on the range and range rate data.

Table Format

The orbit files are simple ascii tables time, tagged by date and time. The tables contain 8 columns.
Ascii date string (ie 97JUN30).
Ascii time string (ie 23h45m01s).
Uplink time
Time (in seconds) will take uplink signal sent now to arrive at satellite.
Uplink rate
Time derivative (in seconds/second) of uplink time.
Downlink time
Time (in seconds) it took downlink signal to arrive now from satellite.
Downlink rate
Time derivative (in seconds/second) of downlink time.
Azimuth of satellite (radians).
Elevation of satellite (radians).

The interval for update of the satellite position and range is variable, but for HALCA/VSOP updates at 5 second intervals appears to be sufficient.

The calculation of the satellite range, range rates, azimuth and elevation requires both accurate knowledge of the satellite and tracking station position. The tracking station position is determined from surveying relative to well known positions on the site and from precise calculations of the earth rotation model. The earth rotation model requires estimates of the difference UT1 and UTC and also the number of leap seconds. The earth rotation model data are provided by the Navy Earth rotation service.

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Last update: 97 July 1