The Calibration of ESA Altimeters from ERS-1 to CryoSat
Mònica Roca, et al.(1)
PiLDo Labs, Barcelona,
The first ESA altimeter on-board the first ESA Environmental satellite, ERS-1, was launched on 17 July 1991. Since ERS-1, ESA has launched another 2 altimeters, ERS-2 RA on 21st April 1995 and EnviSat RA-2 on 1st March 2002; and is about to launch the 4th ESA altimeter on-board CryoSat, in October 2005. In order for the scientists to achieve consistent and sensible results, the need of building a consistent time series (over these 15 years) is required. And to do that, all these altimeters need to be calibrated both in absolute terms, and among them.
Different calibrations have been performed over these years to these altimeters, driven by the scientific needs of the particular moment. The ERS-1 altimeter range was absolutely calibrated over the Venice tower (Francis, 1992). The ERS-2 altimeter was cross-calibrated against ERS-1 and TOPEX/Poseidon altimeters (Benveniste et al. 1997). The EnviSat RA-2 was calibrated in absolute terms for both: its range over the Mediterranean sea with a regional calibration (Roca et al. 2002) and, for the first time in altimetry, its backcatter using an ESA transponder (Roca et al. 2002).
The SIRAL altimeter, on-board CryoSat, calibration of range and sigma-0 will be based in the comparison of its measurements against other altimeters. The primary scientific objectives for the CryoSat mission (Wingham et al. 2004) are to improve the accuracy of measurements of ice sheet elevation and sea-ice thickness and thus enhance understanding of cryospheric dynamics. Over sea-ice this is to be achieved by the use of a radar altimeter with synthetic aperture forming capability to improve the along track resolution. In addition, over ice-sheet margins the direction (along and across track) of the leading edge of an echo is retrieved through the use of a second receiving antenna recording chain allowing interferometric capability. This new design of an altimeter also implies that new calibrations shall be performed. In particular the calibration of the interferometric baseline. For this particular purpose ESA has located a transponder in the Svalbard station. We will then use the transponder for the calibration of the angle of arrival, and also to estimate the datation error. Although the project will perform an indirect calibration of the range and sigma-0 by comparing its measurements with other altimeters, we will also use the ESA transponder for the calibration of range and sigma-0.
This paper will present the calibration performed over all ESA altimeters for 15 years, from ERS-1 to CryoSat.
Authors of this paper should include all the ERS-1 calibration team lead by R. Francis, all ERS-2 calibration team lead by J. Benveniste, all EnviSat RA-2 Absolute Calibration team lead by M. Roca, all EnviSat RA-2 Cross-calibration and Validation team lead by J. Benveniste and all CryoSat calibration team lead by R. Francis.
Francis, C.R., "The height calibration of the ERS 1 radar altimeter", in Proceedings of the First ERS 1 Symposium - Space at the Service of our Environment, Eur. Space Agency Spec. Publ., ESA SP-359 (I), 381-393, 1992.
Benveniste, J., "ERS-2 Altimetry Calibration", in Proceedings of the 3rd ERS Symposium, same issue, 1997.
M. Roca, H. Jackson, and C. Celani, "RA-2 Sigma-0 Absolute Calibration" EnviSat Validation Workshop Proceedings, Frascati, December 2002.
Roca M., et al., "RA-2 Absolute Range Calibration" EnviSat Validation Workshop Proceedings, Frascati, December 2002.
D.J. Wingham, C.R. Francis, S. Baker, C. Bouzinac, R. Cullen, P. de Chateau-Thierry, S.W. Laxon, U. Mallow, C. Mavrocordatos, L. Phalippou, Guy Ratier, L. Rey, F. Rostan, P. Viau & D. Wallis; "CryoSat: A Mission to Determine the Fluctuations in Earth's Landand Marine Ice Fields", Advances in Space Research, Submitted, 2004.