Impact of the antenna diagram approximation in conventional altimetry waveform processing: application to SARAL/AltiKa data

Sophie Le Gac (CNES, France)


Jean-Christophe Poisson (CLS, France); Fanny Piras (CLS, France); Pierre Thibaut (CLS, France); François Boy (CNES, France); Guillaume Valladeau (CLS, France); Amandine Guillot (CNES, France); Jean-Damien Desjonqueres (CNES, France); Nicolas Picot (CNES, France)

Event: 2015 Ocean Surface Topography Science Team Meeting

Session: Instrument Processing: Measurement and retracking (SAR and LRM)

Presentation type: Type Oral

In all ocean retrackers, the Brown model includes a Gaussian approximation of the antenna diagram. This approximation does not take into account possible distorsions from the theoretical antenna diagram. This effect appears non negligible for SARAL/AltiKa altimeter, whose antenna beam illuminates an 8.4 km-wide footprint only. Previous studies have shown a possible impact of AltiKa antenna Gaussian approximation on geophysical parameters, especially on the mispointing angle estimated using MLE4 retracking algorithm: a high dependency is observed in function of the wave height.
In the frame of the PEACHI project, this study aims at comparing the Gaussian approximation and the consideration of AltiKa’s real antenna patterns in the Brown model. For this purpose, actual ground measurement of AltiKa antenna diagram has been used. Differences between the Gaussian approximation and the real antenna diagram of up to 10% (0.3 dB) appear at the edge of the antenna footprint.
Brown models accounting for either the Gaussian approximation, or the real antenna diagram interpolated onto the waveform samples, have been simulated using analytical and convolved formulations. We show that the impact of the Gaussian approximation is not negligible on the retrieved parameters and is strongly dependent on wave height. These results suggest that for AltiKa, considering the antenna gain either in the retracker or via updated correction look-up tables is mandatory. In the latter case, we show that we correct for most of the dependency previously observed on the mispointing slope with respect to the wave height.
Further steps of this study include a comparison to Ku-band Jason-3 antenna gain patterns, whose impact is expected to be lower than AltiKa and an application to the numerical retracker, for a complete consideration of the altimeter instrumental features.

Oral presentation show times:

RoomStart DateEnd Date
Grand Ballroom 1 Tue, Oct 20 2015,16:54 Tue, Oct 20 2015,17:06
Sophie Le Gac