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Ocean Surface Topography from Space
Mitigation of Spatial and Temporal Orbit Errors in Satellite Altimeter Sea Surface Height Measurements


Shailen Desai - (Jet Propulsion Laboratory)

We propose to use advanced precise orbit determination (POD) techniques to mitigate spatial and temporal orbit errors in the Jason series of satellite altimeter sea surface height (SSH) observations. These observations are constructed from differences between the radial component of the satellite positions as computed from POD and the corrected (for environmental delays) range measurements from the altimeter. Errors in the radial component of the POD solutions, the radial orbit errors, therefore map directly into the SSH observations. The POD solutions also provide the coordinate frame for the SSH observations, which is especially important for the 20-year continuous record. Our primary objectives are to continue, and improve upon, our proven record of generating POD solutions for the Jason series of satellites with radial accuracies of better than 1 cm (RMS) and temporal stability of < 1 mm/year. We will also apply results from our investigation towards achieving short-latency (few-hour) POD solutions with radial accuracies of 1 cm (RMS). The proposed investigation will leverage our team’s demonstrated expertise with optimizing POD accuracy using tracking data from the Global Positioning System (GPS).

Our investigation directly contributes to the stated goals of the Ocean Surface Topography Science Team (OSTST) to facilitate the production of the best possible satellite-derived altimetry data sets for demonstration of Earth science and applications arising from analyses of ocean surface topography data. Radial orbit errors manifest in the satellite altimeter SSH observations as systematic long-wavelength spatial errors and periodic (e.g., 60-day and seasonal) or long-term (e.g., secular) temporal errors. Minimizing both the spatial and temporal radial orbit errors therefore benefits almost all applications of the satellite altimeter observations of ocean surface topography. These include a large fraction of the studies to be supported by or completed through the solicited OSTST investigations such as physical oceanography applications, high-resolution merged altimetric data sets, comparison studies between the Jason-series missions, and operational near-real-time applications of satellite altimetry. The latter will benefit from our capability to generate high-accuracy POD solutions with short latencies.

Our approach is aimed at investigating the root source of spatial and temporal errors in the POD solutions and mitigating them for the benefit of applications that utilize the Jason-series SSH observations. The need for accurate knowledge of the satellite positions for altimetry is underscored by the use of three tracking system instruments onboard the T/P and Jason-series: GPS, Satellite Laser Ranging (SLR), and Doppler Orbitography Radiopositioning Integrated by Satellite (DORIS). Errors in the underlying force models as well as the tracking data are both sources of errors in the POD solutions. Our investigation aims to optimize POD accuracy for T/P and the Jason-series by identifying and mitigating deficiencies in the background force models and systematic errors in the tracking data measurements. Following our previous investigations, we will emphasize techniques that take advantage of the GPS tracking data, while typically reserving the SLR tracking data for independent validation of the POD solutions. Including comparisons between DORIS-based and our GPS-based POD solutions will then enable assessment and cross-validation of data from all three tracking systems to expose respective errors. Our investigation of the various error sources leverages our dynamic and reduced-dynamic POD approaches. Dynamic POD solutions are useful for identifying and quantifying systematic errors in the tracking data measurements, while reduced-dynamic POD solutions are useful for identifying and overcoming deficiencies in the force models.

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