PRECISION ORBIT DETERMINATION (POD)

Since the beginning of the space age the analysis of orbits of artificial satellites has been used as a means to improve or even determine geodetic models. Precision Orbit Determination (POD) requires the use of accurate models for gravity, tides and geodetic reference frames. POD's sensitivity to various types of geodetic information makes it a powerful tool in the refinement of geodetic models. POD is a core capability of the Space Geodesy Laboratory. The GEODYN orbit determination and geodetic parameter estimation software was developed by us for POD analysis and is used for geodetic model refinement (as with the EGM96 gravity field).

Although various research groups at NASA centers and at universities are active in POD analysis, the Space Geodesy Laboratory has retained unique capabilities in this field. These include the ability and expertise to exploit a wide variety of tracking data types and also the ability to combine multiple data types into a single POD solution. Of course, satellite laser ranging (SLR ), Global Positioning System (GPS) receiver data and various forms of 2-way and 1-way Doppler (including DORIS ) are a core part of the Space Geodesy Laboratory's POD analysis. However, in addition to these standard data types, the Space Geodesy Laboratory has pioneered the use of complex satellite-to-satellite tracking measurements taken through the Tracking Data and Relay Satellite System (TDRSS) for geodetic quality POD. The EGM96 gravity field greatly benefited from the inclusion of a wide range of data types (especially TDRSS) which in turn allowed a wide range of orbital inclinations to be sampled.

Another unique aspect of the Space Geodesy Laboratory's POD work is our treatment of satellite altimetry. The Space Geodesy Laboratory has always made use of satellite radar altimetry as a tracking data type in POD solutions. Radar altimetry used as direct ranges to the ocean surface or as crossover constraints is sensitive to the radial component of a satellite's orbit. When used in combination with other tracking data types such as SLR, it can add value to a POD solution. Over the last several years we have been the first to use laser altimetry in POD solutions. Laser altimeters generally have small footprints and can return data at significantly off nadir angles. These characteristics bring new possibilities to POD but they have also required significant upgrades to our altimetry measurement models in GEODYN. GEODYN POD solutions are capable of exploiting off nadir altimetric range observations to a complex surface. We have also developed and proven a "dynamic crossover" capability. This capability was designed to exploit the increased horizontal resolution of laser altimetry. As orbit and pointing parameters are iterated, change in crossover location is allowed and even exploited. Although this capability was designed for laser altimeters, it has proven useful for radar altimeters such as Geosat Follow On (GFO). The Space Geodesy Laboratory has been producing precision orbits for GFO. Because limited amount of tracking on GFO (the GPS receivers have not been returning useful data), dynamic crossovers have played a significant role in GFO POD.

The Space Geodesy Laboratory POD has played a significant role in the success of various missions and will continue to do so in the future. The Space Geodesy Laboratory produces the precise orbits which are used on the TOPEX mission geophysical data records (GDRs). Geolocation (including orbits and pointing refinement) for the Mars Observer Laser Altimeter (MOLA) Mars Global Surveyor (MGS) was (and continues to be) computed by the Space Geodesy Laboratory.  Similarly, we computed geolocation for the Shuttle Laser Altimeter ( SLA-01 on Shuttle mission STS-72 and SLA-02 on mission STS-85). That work resulted in Shuttle orbits of unprecedented precision. We have also produced precise GFO orbits for NOAA and the GFO calibration and validation team. In the future we will compute the geolocation (orbits and pointing refinement) for the Vegetation Canopy Lidar (VCL) mission, and verify the mission geolocation for the Ice Cloud and land Elevation Satellite (ICESat). A list of publications pertaining to POD in the Space Geodesy Laboratory follows.

POD Publications

David D. Rowlands, January 2001

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