Jason Series Research Articles

Precision Orbit Determination Standards for the Jason Series of Altimeter Missions

The Jason-1 altimeter satellite and its follow-on mission Jason-2/OSTM were launched in December 2001 and June 2008, respectively, to provide the scientific community with a high-accuracy continuous record of observations of the ocean surface topography. Both missions carry on board three state-of-the-art tracking systems (DORIS, GPS, SLR) to meet the requirement of better-than-1.5 cm radial accuracy for the operational orbit included in the geophysical data record (GDR) product. This article outlines the common set of models and processing techniques applied to both Jason reprocessed and operational orbits included in version C of the GDR, referred to as GDR-C standards for precision orbit determination (POD), and describes the systematic components of the radial error budget that are of most interest for the altimeter data analysts. The nonsystematic component of the error budget, quantified by intercomparison of orbits using similar models or with reduced dependency on the dynamic models, is generally at or below 7 mm RMS (root-mean-square). In particular, the average daily RMS of the radial difference between the JPL and CNES reduced-dynamic orbits on Jason-2 is below 6 mm. Concerning the dynamic models employed, the principal contributors to residual systematic differences appear to be the time varying gravity and solar radiation pressure, resulting in geographically correlated periodic signals that have amplitudes at the few-mm level. Concerning the drifts of the orbits along the North/South direction, all solutions agree to better than the 1 mm/year level.

Sea level anomaly signals in the Agulhas Current region

In the Agulhas Current and its sources and sinks, variability in steric height derived from the 0.1° POP model is 3–11 times greater than the mass‐load signal at periods shorter than 20 days, the approximate limit of temporal resolution for the Jason series of satellite altimeters. Model output is evaluated for realism against in situ SLA data from a suite of moorings deployed in the southeastern South Atlantic Ocean in 2003–2005. The effects of temporal filtering and alongtrack smoothing on output sampled at 10‐day intervals is examined. In approximately 14% of the region, including much of the Agulhas Retroflection, aliasing is ≤10% only at periods of ≥120 days. Smoothing reduces that proportion by 40% but also sharply reduces variability at periods of ≤73 days indicating that use of Jason‐style altimetry for time series analysis of this current system should be confined to seasonal and longer scales.