The surface elevation is calculated from the corrected altimeter height measurement by subtracting the satellite orbital height above a reference ellipsoid. Therefore the precision of the elevation is directly related to the orbit precision, and any alterations in the reference ellipsoid or the station coordinates used in calculation of the orbit affect the referencing of the elevations to a common datum. The following table shows the availability and global ocean crossover RMS of the orbits for each mission:

Notes on table:

• NYA - Not Yet Available
• NA - Not Applicable
• GEM - Goddard Earth Model
• JGM - Joint Gravity Model
• NPO - Navy Precision Orbit
• D-PAF - German Processing and Archiving Facility

The RMS estimates of precision indicate the repeatability of the orbit calculations using the same models and gravity fields, but are not useful for assesing the absolute accuracies of the orbits. Regional differences between orbits calculated with different gravity models (e.g. GEM-T2, Navy gravity field, JGM-2) are typically .5 to 2m. Also, regional differences of that magnitude are found in comparing measurements from satellites in different inclinations (e.g. Geosat and ERS-1). The effect of the geographically-correlated differences can be minimized by using orbits calculated using the same gravity fields and reference system.

The RMS also varies with geographic location. The Geosat ERM JGM-2 variability as a function of location shows regions in the northern latitudes where the variability is high either due to orbit error or oceanography. Looking at the RMS variabiity over the oceans for the 3 different orbits that are available on the Seasat data give an indication of the improvement in the orbit calculation gained with the JGM2 and JGM3 orbits. The crossover residual variability for the Seasat GEM-T2 orbits is large due to the larger errors in the orbit calculation and has an overall rms of approximately 70cm. The ascending-descending bias plots show that there is still a timing error in the Seasat data. The Seasat JGM2 orbits show a marked improvement with a global rms of approximately 40 cm. The Seasat JGM3 orbits show a similar precision at the JGM2.

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Curator: Jairo Santana NASA Official: Dr. H. Jay Zwally Last Updated: 12/11/2006