CryoSat Interferometer: End-to-End Calibration and Achievable Performance
1Aresys, Italy; 2ESA ESTEC; 3ESA ESRIN
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 calibration campaigns following the approach described in  have been periodically performed about once a year.
The end-to-end calibration strategy for the CryoSat interferometer, described in , uses the ocean surface as the known external target. In fact, the interferometer can be used to determine the across-track slope of the overflown surface and the slope of the ocean surface can be considered as known starting from the geoid. Denoting by β the across-track slope of the ocean and assuming that the knowledge error of the geoid slope is negligibly small, β can be compared with the across-track slope derived from CryoSat SARin Level1b products which results in β’=η(θ-χ) where η is a geometric factor, θ is the angle of earliest arrival measured by the CryoSat interferometer and χ is the baseline roll angle. By comparison of the expected across-track slope β and the measured across-track slope β’, the accuracy and the precision of the angle of arrival θ measured by the CryoSat interferometer can be assessed. It is worth noticing here that the accuracy of the measured across-track slope β’ is also dependent on the knowledge of the baseline orientation. As discussed in , starting from β and β’ a calibration function F(θ) for the angle of arrival can be computed.
In our analysis, the accuracy (i.e. the closeness of the measurement to the true value) and of the precision (i.e. the closeness of agreement among a set of measurements) of the CryoSat interferometer have been assessed starting from the residual errors on the measured angle of arrival, that have been obtained after compensation of the calibration function F(θ).
In addition, 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.
 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
CryoSat SAR/SARin Level1b Products: Assessment of BaselineC and Improvements towards BaselineD
1Aresys, Italy; 2ESA ESTEC; 3ESA ESRIN
CryoSat was launched on the 8th April 2010 and is the first European ice mission dedicated to the monitoring of precise changes in the thickness of polar ice sheets and floating sea ice. Cryosat carries an innovative radar altimeter called the Synthetic Aperture Interferometric Altimeter (SIRAL), that transmits pulses at a high pulse repetition frequency thus making the received echoes phase coherent and suitable for azimuth processing. This allows to reach a significantly improved along track resolution with respect to traditional pulse-width limited altimeters.
CryoSat is the first altimetry mission operating in SAR mode and continuous improvements in the Level1 Instrument Processing Facility (IPF1) are being identified, tested and validated in order to improve the quality of the Level1b products. The current IPF, Baseline C, was released in operation in April 2015 and the second CryoSat reprocessing campaign was jointly initiated, taking benefit of the upgrade implemented in the IPF1 processing chain but also of some specific configurations for the calibration corrections.
In particular, the CryoSat Level1b BaselineC products generated in the framework of the second reprocessing campaign include refined information for what concerns the mispointing angles and the calibration corrections.
This poster will thus detail thus the evolutions that are currently planned for the CryoSat BaselineD SAR/SARin Level1b products and the corresponding quality improvements that are expected.
CONFORM: CryOsat Netcdf FORmat Migration for Ice and Ocean Products
1ACS, Italy; 2RHEA c/o ESA; 3ARESYS, Italy; 4CLS, France; 5MSSL, UK; 6ESA, Italy; 7IDEAS+/Telespazio, UK
One of the major evolutions planned in 2017 for both CryoSat Ice and Ocean products is the switch to NetCDF, a much more user-friendly format than the current Earth Explorer format (EE). The EE format is no more suited for today’s evolution and maintenance needs whereas the netCDF format is the standard “de-facto” for most advanced and modern Earth Observation missions. The ongoing format migration activity for Cryosat products is called CONFORM (CryOsat Netcdf FORmat Migration). CONFORM has been designed considering the Climate and Forecast conventions as well as the guidelines used in other altimetric missions. One of the objectives is to ensure good homogeneity and harmonization between CryoSat and previous (reprocessed ERS1/2, Envisat-Phase F), new (Sentinel-3) and future (Sentinel 6) ESA altimetric missions. However, at the same time, the proposed design could also contribute to the definition of innovative conventions and standard for some particular fields specific to CryoSat, the first ESA ice-oriented mission. Demonstration of new ocean and ice CryoSat NetCDF products will be organized aside the presentation of this poster whereas CONFORM data sample will be made available –on demand - to all participants of the North-American CryoSat Science Meeting
Transponder Calibration in SAR/SARIN Altimetry: from CryoSat-2 to Sentinel-3
1isardSAT Ltd., United Kingdom; 2RHEA / ESA, The Nederthlands; 3RHEA / ESA, Italy; 4ESRIN / ESA, Italy
Transponders are commonly used to calibrate absolute range from conventional altimeter waveforms because of it characteristic point target radar reflection. The waveforms corresponding to the transponder distinguish themselves from the other waveforms resulting from natural targets, in power and shape.
ESA has deployed a transponder available for the CryoSat project (a refurbished ESA transponder developed for the ERS-1 altimeter calibration). It is deployed at the KSAT Svalbard station: SvalSAT. Another transponder has been deployed by Technical University of Crete for the Sentinel 3 calibration in the island of Crete.
For CryoSat-2 , we are using the transponder to calibrate SIRAL’s range, datation, and interferometric baseline (or angle of arrival) to meet the missions requirements .
For Sentinel-3 we are using the transponder to calibrate SRAL’s range, datation to meet the missions requirements .
In these calibrations, we are using 3 different type of data: the raw L1A data, the stack beams before they are multi-looked (L1BS), and the multi-looked waveform products (L1B) .
Ideally the comparison between (a) the theoretical value provided by the well-known target, and (b) the measurement by the instrument to be calibrated; provides us with the error the instrument is introducing when performing its measurement . When this error can be assumed to be constant regardless the conditions, it will provide the bias of the instrument. And if the measurements can be repeated after a certain period of time, it can also provide an indication of the instrument drift.
The performances of the CryoSat-2 altimeter, the SIRAL (Synthetic aperture interferometer radar altimeter), have been monitored since 2010 with the Transponder measurements. The range and datation biases from the processor have been corrected in the initial Baselines and now with the Baseline C the long-term trends can be performed in order to evaluate the aging of the instrument.
For Sentinel-3 altimeter, the first data with the Crete TRP have been analysed and the first performances assessment will be made before the end of the Commissioning Phase.
On this presentation the main results with the CryoSat-2 and Sentinel-3 altimeters will be shown.
 C.R. Francis, “CryoSat Mission and Data Description”, CS-RP-ESA-SY-0059.
 CryoSat Science and Mission Requirements Document, CS-RS-UCL-SY-001.
 D.J.Wingham, et al.: “CryoSat: A mission to determine the fluctuations in Earth’s land and marine ice fields”, Advances in Space Research 37 (2006) 841–871.
 Sentinel-3 Mission Requirements Document, EOP-SMO/1151/MD-md.
 SIRAL2 Calibration using TRP: Detail Processing Model – DPM; ISARD_ESA_CR2_TRP_CAL_DPM_030.
Squeezing SARIn Capabilities for Complex Scenarios: L1 & L2 Processing Improvements
1isardSAT Ltd., United Kingdom; 2isardSAT S.L., Spain
The CryoSat mission is designed to determine fluctuations in the mass of the Earth’s land and the marine ice fields. Its primary payload is a radar altimeter that operates in different modes optimised depending on the kind of surface: Low resolution mode (LRM), SAR mode (SAR) and SAR interferometric mode (SARin). This radar is named SIRAL: Synthetic aperture interferometer radar altimeter . The SARIn mode uses two antennas allowing to compute cross-track angles at which signals arrives.
For scenarios with large topographical variations over short distances, a received SARIn waveform can sometimes have more than one peaks, so different elevations can be retrieved from a single waveform. Over certain regions with certain x-track slope conditions, the elevation profile x-track can be retrieved using the phase difference information.
Firstly with data from ASIRAS  and secondly with CryoSat-2  data, the computation of surface elevation profiles over ice caps and glaciers using the so called "swath processing" proved the powerful capabilities of the SARIn mode, improving the spatial resolution of the elevation measurements.
Improvements to the L2-swath processing approach have been investigated and implemented within the ESA founded projects CryoSat+ for Topography and CryoSat+ for Mountain Glaciers. The processing improvements are not only focused in the retracking algorithms but also on the algorithms that are applied in the early stages of the processing chain in order to clean the stack data, multi-looked waveforms and phase difference information as much as possible.
These new algorithm and improvements, initially designed for glaciers, have now been evaluated over other areas such as Coastal and Inland Waters.
 C.R. Francis, “CryoSat Mission and Data Description”, CS-RP-ESA-SY-0059.
 Hawley R.L., et.al.: Ice-sheet elevations from across-track processing of airborne interferometric radar altimetry, Geophys. Res. Lett., 36, L25501, doi:10.1029/2009GL040416, 2009.
 Laurence Gray et.al., Interferometric swath processing of Cryosat data for glacial ice topography , The Cryosphere, 7, 1857-1867, doi:10.5194/tc-7-1857-2013
Broadview Radar Altimetry Toolbox
1isardSAT Ltd., United Kingdom; 2DEIMOS Engenharia, Portugal; 3TU Delft, Faculty of Aerospace Engineering, The Netherlands; 4Deimos/ ESRIN , Italy; 5Serco / ESRIN , Italy; 6ESA / ESRIN, Italy
The universal altimetry toolbox, BRAT (Broadview Radar Altimetry Toolbox) which can read all previous and current altimetry missions’ data, incorporates now the capability to read the upcoming Sentinel-3 L1 and L2 products.
ESA endeavoured to develop and supply this capability to support the users of the future Sentinel-3 SAR Altimetry Mission. BRAT is a collection of tools and tutorial documents designed to facilitate the processing of radar altimetry data.
This project started in 2005 from the joint efforts of ESA (European Space Agency) and CNES (Centre National d’Etudes Spatiales), and it is freely available at http://earth.esa.int/brat. The tools enable users to interact with the most common altimetry data formats. The BratGUI is the front-end for the powerful command line tools that are part of the BRAT suite. BRAT can also be used in conjunction with MATLAB/IDL (via reading routines) or in C/C++/Fortran via a programming API, allowing the user to obtain desired data, bypassing the data-formatting hassle. BRAT can be used simply to visualise data quickly, or to translate the data into other formats such as NetCDF, ASCII text files, KML (Google Earth) and raster images (JPEG, PNG, etc.). Several kinds of computations can be done within BRAT involving combinations of data fields that the user can save for posterior reuse or using the already embedded formulas that include the standard oceanographic altimetry formulas.
The Radar Altimeter Tutorial, that contains a strong introduction to altimetry, shows its applications in different fields such as Oceanography, Cryosphere, Geodesy, Hydrology among others. Included are also “use cases”, with step-by-step examples, on how to use the toolbox in the different contexts. The Sentinel-3 SAR Altimetry Toolbox shall benefit from the current BRAT version. While developing the toolbox we will revamp of the Graphical User Interface and provide, among other enhancements, support for reading the upcoming S3 datasets and specific “use-cases” for SAR altimetry in order to train the users and make them aware of the great potential of SAR altimetry for coastal and inland applications. As for any open source framework, contributions from users having developed their own functions are welcome.
The Broadview Radar Altimetry Toolbox is a continuation of the Basic Radar Altimetry Toolbox. While developing the new toolbox we will revamp of the Graphical User Interface and provide, among other enhancements, support for reading the upcoming S3 datasets and specific “use-cases” for SAR altimetry in order to train the users and make them aware of the great potential of SAR altimetry for coastal and inland applications. As for any open source framework, contributions from users having developed their own functions are welcome.
The first Release of the new Radar Altimetry Toolbox was published in September 2015. It incorporates the capability to read S3 products as well as the new CryoSat-2 Baseline C.
The second Release of the Toolbox, in October 2016, will have a new graphical user interface and other visualisation improvements.
Sea Ice App
CPOM, United Kingdom