Just because the satellite is in the desired operational orbit doesn't mean it will stay there forever. "Maintaining the orbit" is still required so that its ground track won't deviate from the desired path by more than 1 km (0.6 miles) and that data can be collected over the same areas of the ocean repeatedly, which is not an easy task. The TOPEX/Poseidon mission requires 95 % of all nodal crossings to be maintained within the ±1 km control band. The TOPEX/Poseidon satellite orbits the Earth at an altitude of 1336 km (830 miles), which is low enough to operate sensors properly and high enough to reduce the influence of atmospheric drag and gravity of the Earth. The orbit is nearly circular and it has an inclination angle of 66 degrees (relative to the equator), which was chosen to achieve coverage of higher latitudes. The orbit period is 112 minutes, which means the satellite completes one revolution around the Earth every 112 minutes. Its ground track has 10 day period cycles, which means the ground track repeats every 10 days and the satellite fly over the same point every 10 days.
In addition to the atmospheric drag and Earth's gravity, there are some other forces that push the satellite up and down away from the desired path. ("up" and "down" cause the satellite to drift westward and eastward respectively on the ground track.) These forces are solar radiation, thermal emissions from the satellite itself, gravitational effects of the Sun, Moon, and mass concentrations in the Earth, and some anomalous forces.
According to Terrence P. Adamski, flight operations system manager, I quote, "Although TOPEX/Poseidon's orbit is high enough to minimize drag from the Earth's atmosphere, heat from the Sun causes the upper atmosphere to expand. This causes the spacecraft to run into more molecules, and that slows down the spacecraft. Once we detect it starting to sink, its rocket engines are fired to boost it back into the precise orbit we want to maintain. We expect to have to do this about once every 2 months."
This is what is called "Orbit Maintenance Maneuver (OMM)." In the long run, the atmospheric drag reduces the satellite altitude as described above; as a result, the ground track is drifted eastward. Therefore, a "Orbit Maintenance Maneuver" that will bring back the satellite right on the track westward must be performed. In fact, such maneuvers were performed 8 times during the first three years of the mission.
The orbit maintenance is the responsibility of the Navigation Team, which is supported by the Flight Dynamics Facility (FDF) at the NASA Goddard Space Flight Center (GSFC) and the Space Environment Services Center (SESC) in Boulder, Colorado. The FDF performs operational orbit determination for all mission phases using tracking data obtained through the TDRSS. The SESC provides solar activity observations and predictions for atmospheric drag calculations. OMMs is performed to maintain the ground track and altitude within the specified ranges by controlling the mean semi-major axis of the orbit. Such orbit maintenance maneuver are usually small, which only involve velocity changes on the order of about 3 mm/sec and 3 to 4 seconds of thruster firings.
The following table shows when they were performed. The intervals between OMMs shows the lower frequency of OMMs as time went by, which is the evidence of improvement of the anomalous force prediction models.
|Orbit Maintenance Maneuvers||Performed Dates||Intervals between OMMs|
|OMM 1||October 12, 1992||-|
|OMM 2||December 21, 1992||70 days|
|OMM 3||March 30, 1993||99 days|
|OMM 4||August 6, 1993||129 days|
|OMM 5||January 31, 1994||178 days|
|OMM 6||May 20, 1994||109 days|
|OMM 7||October 6, 1994||150 days|
|OMM 8||May 22, 1995||227 days|
Go to an article on orbit maintenance written by Terrence P. Adamski, flight operations system manager.
Right after the launch in August 1992, some unknown forces were found to be causing the orbit to deviate significantly from the desired path. This along-track forces dropped the mean semi-major axis much faster than drag (a primary cause of an orbital decay) would do and caused the orbital decay, which was found to decline exponentially during the assessment phase. After the first OMM on October 12, 1992, however, a significance boost force was observed contrary to the expected exponential decay.
In order to predict their effect on the satellite ground track, the TOPEX/Poseidon Navigation Team and the FDF developed an empirical model that combined all unmodeled along-track forces called "anomalous forces" that were known to affect the ground track. It took almost 10 months for the Navigation Team to characterize these forces and use them to advantage.
Refer to an article on anomalous forces for more details.
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This page is created by Masaharu Suzuki The University of Texas at Austin
Last Modified: Wed Feb 11, 1999