TOPEX Data Reprocessing using a Numerical Retracking Approach

Jean-Damien DESJONQUERES (NASA Jet Propulsion Laboratory, United States)

CoAuthors

Matthieu TALPE (NASA Jet Propulsion Laboratory, United States); Philip CALLAHAN (NASA Jet Propulsion Laboratory, United States); Shailen DESAI (NASA Jet Propulsion Laboratory, United States); Joshua WILLIS (NASA Jet Propulsion Laboratory, United States)

Event: 2019 Ocean Surface Topography Science Team Meeting

Session: Instrument Processing: Measurement and Retracking

Presentation type: Type Oral

The historical TOPEX data products contain well-documented drifts which are particularly obvious in the time series of Significant Wave Height (SWH) from the side-A altimeter. As this degradation became severe, the redundant side-B chain was activated in February 1999.
We have developed a numerical approach to process - or “retrack” - the altimeter echoes from the TOPEX side-A and B altimeters using instrument calibrations, as measured in-flight, to mitigate the altimeters’ evolution.

We present results from our investigation of a numerical approach where the echoes are corrected from the instrument equivalent filter and then the Point Target Responses (PTR) are used to generate models of the echoes [Thibaut et al., 2012]. The two echoes are then compared to compute estimates of the range, the significant wave height (SWH) and the backscatter coefficient (sigma0) of the ocean surface.

The TOPEX altimeter was calibrated in-flight measuring the PTR and the instrument equivalent filter, respectively, from cal-1 and cal-2 modes. The PTRs as measured daily in routine cal-1 mode are not oversampled. As the side-A altimeter degradation became obvious, new onboard software was developed and uploaded to allow the measurement of more accurate oversampled PTRs using the so-called sweep calibration mode. Due to the late patch of the onboard software, only a few sweep calibrations are available at the end of side-A, while for side-B, these calibrations have been performed more regularly throughout its lifetime.

We have extracted the cal-1, cal-2 and sweep calibration data from the TOPEX Science Data Record (SDR) products and have reprocessed those data. Our retracking algorithm is based on the numerical convolution of a theoretical echo model (derived from Brown’s echo model) with an oversampled PTR. For side-B, we used the sweep calibrations. For side-A, there are only a limited number of available sweep calibrations. We have therefore developed a technique to generate a homogenous time series of oversampled PTRs using both measured sweep and routine PTRs.

In addition, we have found the root cause of the hemispheric bias that has been observed in all the altimeter geophysical parameters provided on the TOPEX GDR products, and have applied an approach to significantly mitigate those errors. We present evidence to support our explanation and results from our approach to correct for these errors.

[Thibaut et al., 2012]: P. Thibaut, J.C.Poisson, F. Boy And N. Picot, “Numerical Solution For the Retracking Algorithm: Performances On Conventional Altimeter Waveforms”, Presented at the 2012 OSTST Meeting,Venise.

 

Oral presentation show times:

Room Start Date End Date
The Forum Tue, Oct 22 2019,09:00 Tue, Oct 22 2019,09:15
Jean-Damien DESJONQUERES
NASA Jet Propulsion Laboratory
United States
jean-damien.m.desjonqueres@jpl.nasa.gov