# 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.
- Date
- Ascii date string (ie 97JUN30).
- Time
- 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
- Azimuth of satellite (radians).
- Elevation
- 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.
Return to the Station Software Guide.
The National Radio Astronomy Observatory
(NRAO)
is a facillity of the
National Science Foundation
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Associated Universities, Inc.

glangsto@nrao.edu Last update: 97 July 1