Two microwave transponders have been operating in west Crete and Gavdos to calibrate international satellite radar altimeters at the Ku-band. One has been continuously operating for about 8 years at the CDN1 Cal/Val site in the mountains of Crete, and the other at the GVD1 Cal/Val site on Gavdos since 11 October 2021. This ground infrastructure is also supported at present by four sea-surface Cal/Val sites operating, some of them for over 20 years, while two additional such Cal/Val sites are under construction. This ground infrastructure is part of the European Space Agency Permanent Facility for Altimetry Calibration (PFAC), and as of 2015, it has been producing continuously a time series of range biases for Sentinel-3A, Sentinel-3B, Sentinel-6 MF, Jason-2, Jason-3, and CryoSat-2. This work presents a thorough examination of the transponder Cal/Val responses to understand and determine absolute biases for all satellite altimeters overflying this ground infrastructure. The latest calibration results for the Jason-3, Copernicus Sentinel-3A and -3B, Sentinel-6 MF, and CryoSat-2 radar altimeters are described based on four sea-surface and two transponder Cal/Val sites of the PFAC in west Crete, Greece. Absolute biases for Jason-3, Sentinel-6 MF, Sentinel-3A, Sentinel-3B, and CryoSat-2 are close to a few mm, determined using various techniques, infrastructure, and settings.

A microwave range transponder has been operating at the CDN1 Cal/Val site on the mountains of Crete for about 6 years, to cal-ibrate international satellite radar altimeters in the Ku-band. This transponder is part of the European Space Agency Permanent Facility for Altimetry Calibration, and has been producing a continuous time series of range biases for Sentinel-3A, Sentinel-3B, Jason-2, Jason-3 and CryoSat-2 since 2015. As of 18-Dec-2020, the CDN1 transponder has allowed calibration of the new operational altimeter of Sentinel-6 MF satellite as it flies in tandem with Jason-3. This work investigates range biases derived from the long time series of Jason-3 (and subsequently that of Sentinel-6 MF since both follow the same orbit) and tries to isolate systematic and random con-stituents in the produced calibration results of the transponder. Systematic components in the dispersion of transponder biases are iden-tified as of internal origin, coming from irregularities in the transponder instrument itself and its setting, and of external cause arising from the altimeter, satellite orbit, Earth's position in space, geodynamic effects and others. Draconic harmonics, principally the 58-day period, play a significant role in the transponder results and create cyclic trends of maximum amplitude of 17 mm at times in the cal-ibration results. The attitude of the satellite body as it changes for solar panel orientation contributes an offset of about 7 mm when yaw rotation is off its central position, and the atmospheric, water mass and non-tidal ocean loadings are responsible for an annual sys-tematic signal of 10 mm. At the time of writing, all other constituents of uncertainty seem random in nature and not significantly influ-ential, although humidity requires further investigation in relation to the final transponder calibration results.(c) 2022 COSPAR. Published by Elsevier B.V. All rights reserved.

Poseidon-4 is a dual-frequency redundant radar altimeter, embarked onboard the Sentinel-6 Michael Freilich (MF) European Commission Copernicus Programme satellite. In this article, we assess Poseidon-4 main instrumental improvements and performances with the presentation of the more important outcomes from the in-flight internal calibration modes and an external calibration analysis over a transponder. The instrumental performances of the radar altimeter are excellent for both radar chain sides: Poseidon-4 delivers a range/azimuth instrument impulse response (IR) with the highest quality and fidelity in the era of space-borne radar altimetry, and its thermal noise response is almost just random noise. A power decay of the level of the transmitted power in the Ku-band has been detected both for the nominal and redundant sides, which is larger than expected, though it will not violate the requirement of the minimum signal-to-noise ratio over the ocean at the end of the satellite design lifetime. The innovative CAL1 ECHO CAL calibration mode allows for very precisely characterizing the sensitivity of the instrument IR to the in-orbit temperature variations and, thus, correcting for it in the science data as standard practice.

The main payload of the Sentinel-6 Michael Freilich (S6-MF) is the Poseidon-4 Ku- and C-band Radar Altimeter launched Nov 18th 2020 was developed to continue the ocean surface topography reference data record that commenced in 1992 (TOPEX/Poseidon), was followed by Jason-1 (2001), Jason-2 (2008) and Jason-3 (2016). The satellite has been developed to, as a minimum, meet in-orbit performances of the Jason-2 and adopts, for the first time, requirements concerning estimated Global Mean Sea Level (GMSL) drift and stability. The design of S6 has also taken into account the in-orbit performances of the CryoSat-2/Sentinel-3 missions and their altimeter designs. Integrated into the design is the capability to allow for further on-ground processing enhancements that now demonstrate focused processing capabilities that can be performed globally for evolving future applications. This paper provides an overview of the key performances of the mission at the end of its commissioning and tandem phase with Jason-3, that has also allowed for the first time the characterization of both nominal and redundant chains of the altimeter. It is now demonstrated that the mission meets all its commissioning objectives and all mission level requirements are met, though long-term estimates of GMSL stability requires a longer time series of around 3 years. The Poseidon-4 design and its performances has provided the basis for future missions, such as CRISTAL and potentially for the Sentinel-3 and 6 Next Generation systems.

This article describes the Copernicus Sentinel-6 satellite mission and early in-orbit results. The mission is designed to address the needs of the European Copernicus programme for precision sea level, near-real-time measurements of sea surface height, significant wave height, and other products tailored to operational services in the climate, ocean, meteorology and hydrology domains. It is designed to provide enhanced continuity to the very stable time series of mean sea level measurements and ocean sea state started in 1992 by the TOPEX/Poseidon (T/P) mission and follow-on Jason-1, Jason-2 and Jason-3 satellite missions. The mission is implemented through a unique international partnership with contributions from NASA, NOAA, ESA, EUMETSAT, and the European Union (EU). It includes two satellites that will fly sequentially (separated in time by 5 years). The first satellite, named Sentinel-6 Michael Freilich (S6-MF), launched from Vandenburg Air Force Base, USA on 21st November 2020. The main payload is the Poseidon-4 dual frequency (C/Ku-band) nadir-pointing radar altimeter providing synthetic aperture radar (SAR) processing in Ku-band to improve the signal through better along-track sampling and reduced measurement noise. The altimeter has an innovative interleaved mode enabling radar data processing on two parallel chains, one with the SAR enhancements and the other furnishing a “Low Resolution Mode” that is fully backward-compatible with the historical TOPEX/Poseidon and Jason measurements, so that complete inter-calibration between the state-of-the-art data and the historical record can be assured. Early in-orbit performance data are presented.

Given the considerable range of applications within the European Union Copernicus system, sustained satellite altimetry missions are required to address operational, science and societal needs. This article describes the Copernicus Sentinel-6 mission that is designed to provide precision sea level, sea surface height, significant wave height, inland water heights and other products tailored to operational services in the ocean, climate, atmospheric and land Copernicus Services. Sentinel-6 provides enhanced continuity to the very stable time series of mean sea level measurements and ocean sea state started in 1992 by the TOPEX/Poseidon mission and follow-on Jason-1, Jason-2 and Jason-3 satellite missions. The mission is implemented through a unique international partnership with contributions from NASA, NOAA, ESA, EUMETSAT, and the European Union (EU). It includes two satellites that will fly sequentially (separated in time by 5 years). The first satellite, named Sentinel-6 Michael Freilich, launched from Vandenburg Air Force Base, USA on 21st November 2020. The satellite and payload elements are explained including required performance and their operation. The main payload is the Poseidon-4 dual frequency (C/Ku-band) nadir-pointing radar altimeter that uses an innovative interleaved mode. This enables radar data processing on two parallel chains the first provides synthetic aperture radar (SAR) processing in Ku-band to improve the received altimeter echoes through better along-track sampling and reduced measurement noise; the second provides a Low Resolution Mode that is fully backward-compatible with the historical reference altimetry measurements, allowing a complete inter-calibration between the state-of-the-art data and the historical record. A three-channel Advanced Microwave Radiometer for Climate (AMR-C) provides measurements of atmospheric water vapour to mitigate degradation of the radar altimeter measurements. The main data products are explained and preliminary in-orbit Poseidon-4 altimeter data performance data are presented that demonstrate the altimeter to be performing within expectations.Y

The threat of sea level rise to coastal communities is an area of significant concern to the well-being and security of future generations. Environmental policy actions and decisions affecting coastal states are being made now. Given the considerable range of applications, sustained altimetry satellite missions are required to address operational, science and societal needs. This article describes the Copernicus Sentinel-6 mission that is designed to address the needs of the European Copernicus programme for precision sea level, near-real-time measurements of sea surface height, significant wave height, and other products tailored to operational services in the climate, ocean, meteorology and hydrology domains. It is designed to provide enhanced continuity to the very stable time series of mean sea level measurements and ocean sea state started in 1992 by the TOPEX/Poseidon (T/P) mission and follow-on Jason-1, Jason-2 and Jason-3 satellite missions. The mission is implemented through a unique international partnership with contributions from NASA, NOAA, ESA, EUMETSAT, and the European Union (EU). It includes two satellites that will fly sequentially (separated in time by 5 years). The first satellite, named Sentinel-6 Michael Freilich, launched from Vandenburg Air Force Base, USA on 21st November 2020. The main payload is the Poseidon-4 dual frequency (C/Ku-band) nadir-pointing radar altimeter providing synthetic aperture radar (SAR) processing in Ku-band to improve the signal through better along-track sampling and reduced measurement noise. The altimeter has an innovative interleaved mode enabling radar data processing on two parallel chains, one with the SAR enhancements and the other furnishing a "Low Resolution Mode" that is fully backward-compatible with the historical T/P and Jason measurements, so that complete inter-calibration between the state-of-the-art data and the historical record can be assured. A three-channel Advanced Microwave Radiometer for Climate (AMR-C) developed by NASA JPL provides measurements of atmospheric water vapour that would otherwise degrade the radar altimeter measurements. An experimental High Resolution Microwave Radiometer (HRMR) is also included in the AMR-C design to support improved performance in coastal areas. Additional sensors are included in the payload to provide Precise Orbit Determination, atmospheric sounding via GNSS-Radio Occultation and radiation monitoring around the spacecraft. Early in-orbit performance data are presented.

This work presents the latest calibration results for the Copernicus Sentinel-3A and -3B and the Jason-3 radar altimeters as determined by the Permanent Facility for Altimetry Calibration (PFAC) in west Crete, Greece. Radar altimeters are used to provide operational measurements for sea surface height, significant wave height and wind speed over oceans. To maintain Fiducial Reference Measurement (FRM) status, the stability and quality of altimetry products need to be continuously monitored throughout the operational phase of each altimeter. External and independent calibration and validation facilities provide an objective assessment of the altimeter’s performance by comparing satellite observations with ground-truth and in-situ measurements and infrastructures. Three independent methods are employed in the PFAC: Range calibration using a transponder, sea-surface calibration relying upon sea-surface Cal/Val sites, and crossover analysis. Procedures to determine FRM uncertainties for Cal/Val results have been demonstrated for each calibration. Biases for Sentinel-3A Passes No. 14, 278 and 335, Sentinel-3B Passes No. 14, 71 and 335, as well as for Jason-3 Passes No. 18 and No. 109 are given. Diverse calibration results by various techniques, infrastructure and settings are presented. Finally, upgrades to the PFAC in support of the Copernicus Sentinel-6 ‘Michael Freilich’, due to launch in November 2020, are summarized.

Abstract. One of the candidate missions in the evolution of the Copernicus Space Component (CSC) is the Copernicus polaR Ice and Snow Topography ALtimeter (CRISTAL). The aim of this mission is to obtain high-resolution sea-ice thickness and land ice elevation measurements and includes the capability to determine the properties of snow cover on ice to serve Copernicus’ operational products and services of direct relevance to the Polar Regions. The evolution of the CSC is foreseen in the mid-2020s to meet priority user needs not addressed by the existing infrastructure, and to reinforce the Copernicus services by expanding the monitoring capability in the thematic domains of anthropogenic emissions (CO2), polar and agriculture/forestry/emergency. This evolution will be synergetic with the enhanced continuity of services foreseen with the next generation of the existing Copernicus Sentinels. New high-priority candidate satellite missions have been identified by the European Commission (EC) for implementation in the coming years to address gaps in current capability and emerging user needs. This paper describes the CRISTAL mission objectives, main mission requirements driving its design, the payload complement currently under development and its expected contributions to the monitoring of important components of Earth’s cryosphere.

Abstract. The Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL) mission is one of six high-priority candidate missions (HPCMs) under consideration by the European Commission to enlarge the Copernicus Space Component. Together, the high-priority candidate missions fill gaps in the measurement capability of the existing Copernicus Space Component to address emerging and urgent user requirements in relation to monitoring anthropogenic CO2 emissions, polar environments, and land surfaces. The ambition is to enlarge the Copernicus Space Component with the high-priority candidate missions in the mid-2020s to provide enhanced continuity of services in synergy with the next generation of the existing Copernicus Sentinel missions. CRISTAL will carry a dual-frequency synthetic-aperture radar altimeter as its primary payload for measuring surface height and a passive microwave radiometer to support atmospheric corrections and surface-type classification. The altimeter will have interferometric capabilities at Ku-band for improved ground resolution and a second (non-interferometric) Ka-band frequency to provide information on snow layer properties. This paper outlines the user consultations that have supported expansion of the Copernicus Space Component to include the high-priority candidate missions, describes the primary and secondary objectives of the CRISTAL mission, identifies the key contributions the CRISTAL mission will make, and presents a concept – as far as it is already defined – for the mission payload.

Satellite altimeters have been producing, as of 1992, an amazing and historic record of sea level changes. As Europe moves into full operational altimetry, it has become imperative that the quality of these monitoring signals with their uncertainties should be controlled, fully and properly descripted, but also traced and connected to undisputable standards and units. Excellent quality is the foundation of these operational services of Europe in altimetry. In line with the above, the strategy of the Fiducial Reference Measurements for Altimetry (FRM4ALT) has been introduced to address and to achieve reliable, long-term, consistent, and undisputable satellite altimetry products for Earth observation and for sea-level change monitoring. FRM4ALT has been introduced and implemented by the European Space Agency in an effort to reach a uniform and absolute standardization for calibrating satellite altimeters. This paper examines the problem and the need behind the FRM4ALT principle to achieve an objective Earth observation. Secondly, it describes the expected FRM products and services which are to come into being out of this new observational strategy. Thirdly, it outlines the technology and the services required for reaching this goal. And finally, it elaborates upon the necessary resources, skills, partnerships, and facilities for establishing FRM standardization for altimetry.

This paper develops a model of the synthetic aperture, interferometric satellite radar altimeter echo power, and echo cross-product. The model uses the smallness of the satellite pitch and roll angles, and the limited range of satellite altitude to provide a semianalytical echo model, whose numerical dimensions are limited to two in the synthetic aperture case or three in the interferometric case at large roll angles, making its application to extensive data practical. By not implementing the synthetic beam formation, it is demonstrated that the model recovers the conventional case of a pulse-limited altimeter. A theoretical description of the fluctuations in the multilooked, interferometric cross-product, and the synthetic aperture and pulse-limited powers is given by extending the model to describe correlations between individual looks in each case. The model offers the potential to retrieve ocean-surface parameters from synthetic aperture and pulse-limited altimeter data simultaneously, with obvious application to the new generation of altimeters onboard CryoSat-2, Sentinel-3, and Sentinel-6 during the transition to operational synthetic aperture radar altimetry.

The launch of CryoSat-2 heralded a new era of interferometric Synthetic Aperture Radar altimetry over the Polar Ice Sheets. The mission's novel SAR interferometric (SARIn) mode of operation has enabled monitoring of rapidly changing coastal regions, which had been challenging for previous low resolution altimeters. Given the growing requirement to continue the 25-year altimeter record, there is now a need to assess the differences between existing SAR and SARIn altimeter datasets, with a view to understanding the impact on ice sheet retrievals of the different radar hardware and processing methodologies. Uniquely, CryoSat-2 data can be processed both with and without interferometric information, offering the opportunity to directly compare the SAR and SARIn products generated by the current ground segment. Here, we provide a first comparison of these Level-2 datasets, and evaluate their capacity to measure ice sheet elevation and elevation change. We find that the current interferometric product has substantially improved precision, accuracy and coverage compared to its non-interferometric counterpart, yielding a similar to 35% improvement in the root-mean-square-difference (RMSD) of elevations recorded at orbital cross-overs, and a similar to 30% lower RMSD of elevation rates relative to Operation IceBridge airborne altimeter measurements. This analysis demonstrates the value that the interferometer adds to the current CryoSat-2 configuration, and highlights the importance for non-interferometric SAR Level-2 processing of the auxiliary data used to identify the location of the echoing point. These results provide a benchmark of the relative performance of the Level-2 interferometric and non-interferometric products currently produced by the ground segment, which will help to inform the design and implementation of a future polar radar altimeter mission. (C) 2017 COSPAR. Published by Elsevier Ltd.

This letter describes the retrieval of the pitch of the CryoSat-2 satellite from the full bit rate (FBR) SAR mode data that are collected over the ocean by the SAR/Interferometric Radar Altimeter (SIRAL). Starting with the FBR SAR mode data, we form beams at the forward and backward look angles with respect to the nadir direction. If the satellite is flying pitched, the asymmetrical weighting of the echo power in the forward-looking and backward-looking beams that resulted from the along-track antenna gain pattern is used to measure the pitch of the satellite. The method depends on accurate knowledge of the orbit and the altimeter echoes only. In consequence, we are able to compare the results with the contemporaneous pitch value that is measured by the onboard star trackers, and particularly, we are able to determine a bias in the star tracker pitch measurements. We find that the star trackers' pitch measurements are biased, with a constant offset of 0.055 ° ± 0.0073 °. Because the star tracker attitude is used to inform the onboard control system, this bias implies that CryoSat-2 is flying nose up, with an average pitch of about 0.055 °.

We analyze data acquired by the CryoSat‐2 interferometric radar altimeter and demonstrate its novel capability to track topographic features on the Antarctic Ice Sheet. We map the perimeter and depth of a 260 km2 surface depression above an Antarctic subglacial lake (SGL) and, in combination with Ice, Cloud and land Elevation Satellite laser altimetry, chart decadal changes in SGL volume. During 2007–2008, between 4.9 and 6.4 km3 of water drained from the SGL, and peak discharge exceeded 160 m3 s−1. The flood was twice as large as any previously recorded and equivalent to ~ 10% of the meltwater generated annually beneath the ice sheet. The ice surface has since uplifted at a rate of 5.6 ± 2.8 m yr−1. Our study demonstrates the ability of CryoSat‐2 to provide detailed maps of ice sheet topography, its potential to accurately measure SGL drainage events, and the contribution it can make to understanding water flow beneath Antarctica.

We have derived digital elevation models (DEMs) over the western part of the Devon Ice Cap in Nunavut, Canada, using "swath processing" of interferometric data collected by Cryosat between February 2011 and January 2012. With the standard ESA (European Space Agency) SARIn (synthetic aperture radar interferometry) level 2 (L2) data product, the interferometric mode is used to map the cross-track position and elevation of the "point-of-closest-approach" (POCA) in sloping glacial terrain. However, in this work we explore the extent to which the phase of the returns in the intermediate L1b product can also be used to map the heights of time-delayed footprints beyond the POCA. We show that there is a range of average cross-track slopes (~ 0.5 to ~ 2°) for which the returns will be dominated by those beneath the satellite in the main beam of the antenna so that the resulting interferometric phase allows mapping of heights in the delayed range window beyond the POCA. In this way a swath of elevation data is mapped, allowing the creation of DEMs from a sequence of L1b SARIn Cryosat data takes. Comparison of the Devon results with airborne scanning laser data showed a mean difference of order 1 m with a standard deviation of about 1 m. The limitations of swath processing, which generates almost 2 orders of magnitude more data than traditional radar altimetry, are explored through simulation, and the strengths and weaknesses of the technique are discussed.

This paper describes the calibration of the CryoSat-2 interferometer, whose principal purpose is to accurately measure the height of the Antarctic and Greenland ice sheets. A sequence of CryoSat-2 data acquisitions over the tropical and midlatitude oceans were obtained between June and September 2010, from the SIRAL "A" and redundant SIRAL "B" radars operating in their "SARIN" mode, during a sequence of satellite rolls between -0.6° and 0.4°. Using the arrival angle of the echo relative to the interferometer baseline, the attitude of the satellite determined by the star trackers, and estimates of the ocean surface across-track slope from the EGM08 geoid, we determined the errors in the interferometer estimate of surface slope as functions of the roll angle and ocean surface waveheight. These were found to be in close agreement with the theoretical description. The scale factor of the interferometric measurement of angle was determined to be 0.973 ± 0.002. We estimate the accuracy of the across-track slope measurement of the interferometer by applying this scale factor to the measured phase. In applying this scale factor to the measurements, the across-track slope of the marine geoid was obtained with an accuracy of 26 μrad at 10 km and 10 μrad at 1000 km. We conclude that the instrument performance considerably exceeds that needed for the accurate determination of height over the sloping surfaces of the continental ice sheets. The results also demonstrate that CryoSat-2 provides the first observations of the instantaneous vector gradient of the ocean surface, and that the normal-incidence interferometric configuration has a greater potential for the measurement of the ocean across-track slope than has been previously recognized.

Satellite records show a decline in ice extent over more than three decades, with a record minimum in September 2012. Results from the Pan‐Arctic Ice‐Ocean Modelling and Assimilation system (PIOMAS) suggest that the decline in extent has been accompanied by a decline in volume, but this has not been confirmed by data. Using new data from the European Space Agency CryoSat‐2 (CS‐2) mission, validated with in situ data, we generate estimates of ice volume for the winters of 2010/11 and 2011/12. We compare these data with current estimates from PIOMAS and earlier (2003–8) estimates from the National Aeronautics and Space Administration ICESat mission. Between the ICESat and CryoSat‐2 periods, the autumn volume declined by 4291 km 3 and the winter volume by 1479 km 3 . This exceeds the decline in ice volume in the central Arctic from the PIOMAS model of 2644 km 3 in the autumn, but is less than the 2091 km 3 in winter, between the two time periods.