Precise Orbit Determination of Global Navigation Satellite System of ...

Chapter 8 Geostationary Orbit Determination And Prediction During Satellite Maneuvers
To counteract these drifts due to perturbations, sufficient fuel is loaded into the maneuver rockets onboard of all
geostationary satellites to periodically correct any changes over the planned lifetime of the satellite. These
periodic corrections are known as stationkeeping or satellite maneuvers. North-south stationkeeping corrects the
slowly increasing inclination back to zero and east-west stationkeeping keeps the satellite at its assigned position
within the geostationary belt. These maneuvers are planned to maintain the geostationary satellite within a small
distance of its ideal location (both north-south and east-west). This tolerance is normally designed to ensure the
satellite remains within the ground antenna beamwidth without tracking.
In Germany, Deutsche Telekom owns and operates three geostationary telecommunication and direct
broadcasting satellites. The Telekom DFS-Kopernikus satellites are maneuvered in longitude (EW) on a weekly
and in inclination (NS) on a two week basis. TVSAT-2 is operated in EW and NS on a two week cycle. It results
that the Flight Dynamics System has to cope with 11 maneuvers on a fortnight basis (Bisten, M.& Damiano, A.
1996)
8.2 Orbit Determination During Satellite Maneuvers
If a geostationary satellite is used for navigation purpose, the orbit information (ephemeris) should be
continually broadcasted to the users for their navigation and positioning computation. Due to the large orbit
errors during satellite maneuvers, the orbit information at that time can not be used. During this period, users
have to stop their navigation and positioning applications. Normally, the geostationary satellite maneuver is
operated weekly, which makes a big problem for navigation users to get correct orbit information in real-time. In
order to overcome this problem, the precise orbit determination and prediction methods during satellite
maneuver should be considered.
The situation of satellite orbit changes during the satellite maneuver can be shown in Figure 8-4.
5: Orbit After
Maneuver
2: Predict Orbit
before Maneuver
4: Predict Orbit
during Maneuver
T2: Maneuver Stop
Figure 8-4 Three Phases of GEO Satellite Tracks during Maneuver
Figure 8-4 shows five tracks of GEO satellite orbit. Track 1 is an orbit track before maneuver; Track 2 is the
predicted orbit before maneuver; Track 3 is the track of GEO satellite during maneuver; Track 4 is the predicted
orbit during maneuver; Track 5 is a satellite orbit after maneuver. Tracks 1, 3 and 5 are actually GEO satellite
orbits. Without maneuver operation, actual GEO satellite tracks should be Track 1 and Track 2. Normally GEO
satellite orbit is determined and predicted using precise satellite dynamic models which include geopotential,
solar and lunar attractions, solar radiation pressure etc., but no satellite maneuver force model is included due to
difficulties to model the maneuver force. Currently, the most of the GEO satellites are used for communication
purposes, people are more concerned about GEO satellite drift, they are not so interested in the actual satellite
orbit. But for navigation application, the actual orbit of GEO satellite is very important. Users need the actual
position and velocity of GEO satellite as references to compute their own positions and velocities.
There are two phases that are big problems for satellite navigation application during satellite maneuver. The
first phase: satellite maneuver begins at T1(see Figure 8-4); navigation users use the predicted orbit Track 2
(predicted before maneuver), but the actual orbit is Track 3. This will introduce a big orbit error for the
navigation users in their navigation computation. The second phase is that the satellite maneuver stops at T2, the
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3: Orbit During Maneuver
1: Orbit Before Maneuver
T1: Maneuver Start