Transponder calibration of the Envisat RA-2 altimeter Ku band sigma naught

Pierdicca, Nazzareno; Bignami, Christian; Roca, M.; Féménias, Pierre; Fascetti, M.; Mazzetta, M.; Loddo, Carolina Nogueira; Martini, Audrey; Pinori, S.

doi:10.1016/j.asr.2012.12.014

Cited by 8 publications

(4 citation statements)

References 27 publications

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“…We note that there is significant uncertainty about the absolute calibration of σ 0 for all past and present altimeters. Efforts to produce an absolute calibration for σ 0 from Envisat using calibrated on-ground transponders still had an uncertainty of 1 dB (Pierdicca et al, 2013), whereas the required tolerance for climate studies necessitates knowing the drift to better than 0.03 dB/decade. Thus, in practice, all current wind speed algorithms are empirical, based on matching up altimeter observations of σ 0 with a host of meteorological buoys and instruments on ships or other platforms of opportunity.…”

Section: Surface Roughness and Wind Speed From Altimetersmentioning

confidence: 99%

Observing Sea States

et al. 2019

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Sea state information is needed for many applications, ranging from safety at sea and on the coast, for which real time data are essential, to planning and design needs for infrastructure that require long time series. The definition of the wave climate and its possible evolution requires high resolution data, and knowledge on possible drift in the observing system. Sea state is also an important climate variable that enters in air-sea fluxes parameterizations. Finally, sea state patterns can reveal the intensity of storms and associated climate patterns at large scales, and the intensity of currents at small scales. A synthesis of user requirements leads to requests for spatial resolution at kilometer scales, and estimations of trends of a few centimeters per decade. Such requirements cannot be met by observations alone in the foreseeable future, and numerical wave models can be combined with in situ and remote sensing data to achieve the required resolution. As today's models are far from perfect, observations are critical in providing forcing data, namely winds, currents and ice, and validation data, in particular for frequency and direction information, and extreme wave heights. In situ and satellite observations are particularly critical for the correction and calibration of significant wave heights to ensure the stability of model time series. A number of developments are underway for extending the capabilities of satellites and in situ observing systems. These include the generalization of directional measurements, an easier exchange of moored buoy data, the measurement of waves on drifting buoys, the evolution of satellite altimeter technology, and the measurement of directional wave spectra from satellite radar instruments. For each of these observing systems, the stability of the data is a very important issue. The combination of the different data sources, including numerical models, can help better fulfill the needs of users.

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Section: Surface Roughness and Wind Speed From Altimetersmentioning

confidence: 99%

Observing Sea States

et al. 2019

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“…Any altimeter errors, for example, in satellite ranges, sea state bias, wet tropospheric path delays, marine geoid, tides, and geographically correlated errors have to be determined and controlled by reference values provided by permanent calibration facilities (Cal/Val sites) on the earth surface. Absolute calibration is performed by permanent Cal/Val sites (Bonnefond et al 2010, Mertikas et al 2010, Watson et al 2009) exactly placed underneath, or adjacent to overflying satellites and close to the sea, but also calibration can be accomplished with microwave transponders on land (Pierdicca et al, 2013. In addition, relative calibration techniques using for example, global tide-gauge networks (Dong et al 2002, Mitchum 1998 or multi-mission cross-over calibrations (http://www.aviso.oceanobs.com, Bosch et al 2014) are employed.…”

Section: Introductionmentioning

confidence: 99%

First Calibration Results for the SARAL/AltiKa Altimetric Mission Using the Gavdos Permanent Facilities

et al. 2015

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Downloaded by [University of California, San Diego] at 02:50 13 June 2015 ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 2 This work presents the first calibration results for the SARAL/AltiKa altimetric mission using the Gavdos permanent calibration facilities. The results are covering one year of altimetric observations from April, 2013 till March 2014, and include 11 calibration values for the altimeter bias. The reference ascending orbit No. 571 of SARAL/AltiKa has been used for this altimeter assessment. This satellite pass is coming from south and nears Gavdos, where it finally passes through its west coastal tip, only 6 km off the main calibration location. The selected calibration regions in the south sea of Gavdos range from about 8 km to 20 km south off the point of closest approach. Several reference surfaces have been chosen for thisaltimeter evaluation based on gravimetric, but detailed regional geoid, as well as combination of it with other altimetric models. Based on these observations and the gravimetric geoid model, the altimeter bias for the SARAL/AltiKa is determined as mean value of -46mm ±10mm, and a median of -42mm ±10mm, using GDR-T data at 40 Hz rate. A preliminary cross-over analysis of the sea surface heights at a location south of Gavdos showed that SARAL/AltiKa measure less than Jason-2 by 4.6 cm. These bias values are consistent with those provided by Corsica, Harvest, and Karavatti Cal/Val sites. The wet troposphere and the ionosphere delay values of satellite altimetric measurements are also compared against in-situ observations (-5 mm difference in wet troposphere and almost the same for the ionosphere) determined by a local array of permanent GNSS receivers, and meteorological sensors.

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“…We note that one different type of active transponder has been built, which retransmits a replica of the received signal after a delay corresponding to a path length of several kilometres, to eliminate clutter. This is intended to calibrate the received echo power [6].…”

Section: Introductionmentioning

confidence: 99%

A Trihedral Corner Reflector for Radar Altimeter Calibration

Gibert

Gomez-Olive

Garcia-Mondéjar

et al. 2023

IEEE Trans. Geosci. Remote Sensing

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A trihedral corner reflector has been used to evaluate the capability of passive reflectors to calibrate radar altimeters such as the Poseidon-4 altimeter on board Sentinel-6A. The reflector location, placed on the top of a mountain ridge and about 4-km off-nadir of the Sentinel-6A subsatellite track, allows capturing echoed signals with a Signal-to-Clutter Ratio around 40 dB when processing the received data with a Fully-Focussed SAR algorithm. Results obtained show a range bias of 33.9 mm with 8.5 mm of standard deviation and datation bias of -2.3 μs with standard deviation of 1.8 μs for the measurement campaign between September 2021 and April 2022. Such values are comparable to what is currently achieved by means of active transponders and therefore it is demonstrated that passive reflectors may be of interest to support radar altimeter regular calibration.

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Transponder calibration of the Envisat RA-2 altimeter Ku band sigma naught

Cited by 8 publications

References 27 publications

Observing Sea States

Observing Sea States

First Calibration Results for the SARAL/AltiKa Altimetric Mission Using the Gavdos Permanent Facilities

A Trihedral Corner Reflector for Radar Altimeter Calibration

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