CoAuthors

Marco Fornari (RHEA c/o ESA, The Netherlands); Jérôme Bouffard (RHEA c/o ESA, Italy); Tommaso Parrinello (ESA, Italy)

The main payload of CryoSat is a Ku-band pulsewidth limited radar altimeter, called SIRAL (Synthetic interferometric radar altimeter). When commanded in SARIn (synthetic aperture radar interferometry) mode, through coherent along-track processing of the returns received from two antennas, the interferometric phase related to the first arrival of the echo is used to retrieve the angle of arrival of the scattering in the across-track direction. In fact, the across-track echo direction can be derived by exploiting the precise knowledge of the baseline vector (i.e. the vector between the two antennas centers of phase) and simple geometry.
In order to monitor the performance of the CryoSat interferometer along the mission, in orbit interferometer calibration campaigns following the approach described in [1] have been periodically performed. During those campaigns, the satellite is rolled from side to side of ~0.4 deg as it flies over ocean and the SIRAL is commanded to acquire in SARIn mode. As discussed in [1], the end-to-end calibration strategy for the CryoSat interferometer using the roll manoeuvres has been verified to be effective and it has allowed assessing the performance of the CryoSat interferometer from 2010. However, the drawback of this approach is that the complete end-to-end calibration of the CryoSat interferometer can be done only suspending the science acquisitions in correspondence of the roll manoeuvres and the calibration is thus commanded about every 18 months.
In this abstract it is presented a complementary approach for the calibration of CryoSat interferometer based on the analysis of the operational SARin acquisitions for small ocean patches in order to increase the number of calibration opportunities. It is worth noticing that the advantage of this opportunistic end-to-end calibration of the CryoSat interferometer is that its impact on science acquisitions is minimized. By analysis of SARin acquisitions of an ocean patch for 18 months, the calibration parameters for the CryoSat interferometer have been computed and then compared with those obtained from the calibration campaigns in conjuntion with the roll manoeuvres. Recalling that the calibration parameters for the CryoSat interferometer are a linear coefficient (a) and a bias (χ), the comparison has shown that
• The bias χ retrieved by operational SARin acquisitions is comparable to the bias retrieved by calibration campaigns
• The linear coefficient retrieved by operational SARin acquisitions is much more noisy than the linear coefficient retrieved by calibration campaigns, as it was expected since this parameter is computed by a linear fitting as function of the roll angle which varies by few tenths of degrees in operational acqusitions
In conclusion, operational SARin acquisitions over open ocean can be used to increase the calibration opportunities for the CryoSat interferometer withouth any impact on the science acquisitions. The limitation of this approach is that it cannot allow to completely calibrate the interferometer because only the contribution addressed to the parameter χ in the calibration function can be measured with sufficient accuracy.

[1] Galin, N.; Wingham, D.J.; Cullen, R.; Fornari, M.; Smith, W.H.F.; Abdalla, S., "Calibration of the CryoSat-2 Interferometer and Measurement of Across-Track Ocean Slope," in Geoscience and Remote Sensing, IEEE Transactions on , vol.51, no.1, pp.57-72, Jan. 2013