Exploiting the S6-MF effective number of looks (ENL) for sea state applications
This case study is aimed at verifying the existence of a discrepancy in S6-MF geophysical parameters obtained starting from low-resolution mode waveforms because of the higher pulse repetition frequency. While conventional low-resolution mode altimeters like J3 operates at PRF around 2 kHz, S6-MF operates at PRF around 9 kHz, so that, according to the results in (Egido and Smith, 2019), significant sea-state-dependent biases are expected introduced during the retracking.
The first studies on the correlation properties of consecutive pulses from nadir-looking pulse-limited radar altimeters where mainly focused on determining the maximum pulse repetition frequency at which statistical independence could be achieved (Walsh, 1982). Following these studies, the pulse repetition frequency (PRF) for the Jason series was selected to be approximately 2 kHz.
Copernicus Sentinel-6 Michael Freilich is the first altimeter operating in a continuous high-rate pulse mode, i.e. interleaved mode. This design allows the simultaneous production of low-resolution mode measurements with a pulse repetition frequency of ~9KHz for Ku-band. S6-MF thus allows to obtain an elevated number of highly correlated single looks with respect to the fewer number of Jason-3 slightly correlated altimeter pulses. Recent studies, (Scagliola, 2016) and (Egido and Smith, 2019), revealed that for LRM waveforms the averaging of higher number of correlated single looks (~9KHz) allows to obtain higher ENL with respect to the averaging of fewer almost uncorrelated single looks (~1.8KHz). Moreover, it was showed that despite the fact the at higher PRFs the noise in the estimation of geophysical parameters is reduced, the significant dependence of the statistical properties on the range gate also introduces significant biases in the retracked parameters. These biases have been found to be sea-state dependent and needs to be properly accounted for in order to integrate Sentinel-6 Michael Freilich measurements to Jason-3 data which is the current reference mission of the Ocean Surface Topography satellite series.
This activity aims to:
- Evaluate the autocorrelation properties of the echoes acquired by S6 Poseidon instrument by making use of a theoretical waveform model
- Compare the theoretical results with the ENL estimated from real S6-MF data
- Exploit the Aresys L2 geophysical parameters retrieval tool to verify the effect of the varying ENL on the precision of the retrieval of the geophysical parameters as a function of the multilooking posting rate
REFERENCES
- Walsh, E.J., 1982. Pulse-to-pulse correlation in satellite radar altimeters. Radio Sci. 17, 786–800.
- M. Scagliola, L. Recchia, D. Giudici, and M. Kuschnerus, “Pulse limited waveforms from interleaved mode: An analysis on the achievable speckle noise reduction,” presented at the OSTST Meeting, La Rochelle, France, Oct./Nov. 2016. [Online]. Available: https://meetings.aviso.altimetry.fr/fileadmin/user_upload/tx_ausyclsseminar/files/IPM_05_scagliola_OSTST_2016_v3_17h15.pdf
- Egido, A., Smith, W.H.F., 2019. Pulse-to-pulse correlation effects in high prf low-resolution mode altimeters. IEEE Trans. Geosci. Remote Sens. 57, 2610–2617.
- Egido, Alejandro & Dinardo, Salvatore & Ray, Christopher. (2020). The Case for Increasing the Posting Rate in Delay/Doppler Altimeters. Advances in Space Research. 10.1016/j.asr.2020.03.014.
- Wingham, D.J., Giles, K.A., Galin, N., Cullen, R., Armitage, T.W.K., Smith, W.H.F., 2018. A semianalytical model of the synthetic aperture, interferometric radar altimeter mean echo, and echo cross-product and its statistical fluctuations. IEEE Trans. Geosci. Remote Sens. 56, 2539–2553.
- Recchia, Lisa & Scagliola, M. & Giudici, Davide & Kuschnerus, Mieke. (2017). An Accurate Semianalytical Waveform Model for Mispointed SAR Interferometric Altimeters. IEEE Geoscience and Remote Sensing Letters. PP. 1-5. 10.1109/LGRS.2017.2720847.
- Amarouche, L., P. Thibaut, O. Z. Zanife, J.P. Dumont, N. Steunou, & P. Vincent, 2003: Improving the Jason1 ground retracking to cope with attitude effects. Marine Geodesy, Special Issue on Jason1 Calibration/Validation, Part 2. DOI:10.1080/01490410490465210.