CryoSat-2 is a satellite of the European Space Agency that was launched in April 2010. It is intended to monitor changes in the thickness of the marine ice, floating in the polar oceans, and to measure variations in the thickness of the vast ice sheets that overlie Greenland and Antarctica.
It carries an innovative radar altimeter called SIRAL (Synthetic Aperture Interferometric Altimeter). It has two radar antennas, which met the measurement requirements for ice-sheet elevation and sea-ice freeboard with unprecedented accuracy. The satellite orbits the planet at an altitude of around 720 km with a retrograde drifting orbit inclination of 92° and a “quasi” repeat cycle of 369 days (30 days sub-cycle). CryoSat-2 is therefore able to reach latitudes up to 88° covering more than 4.6 million km2 of unexplored areas over the poles when compared with previous polar missions carrying an altimetry.
In its first nine years of operations, CryoSat has achieved its mission objectives and has provided high-quality of data for a number of Earth science applications and opened up new research streams and triggered new scientific questions which have emerged from the previous phases.
The purpose of this paper is to provide a general overview of the mission status and provide programmatic highlights in is news extended phase until 2021.

CryoSat-2 is ESA's first satellite mission dedicated to measuring changes in the polar land ice and sea ice cover. Following its launch in April 2010, we have examined the performance of the instrument over the continental ice sheets of Antarctica and Greenland, the Artic Ocean, and, for the purposes of calibration, over the oceans. We have confirmed the engineering performance at system level of the interferometer, and that the mission has met its original scientific requirements in relation to land and sea ice. This presentation summarises the key outcomes of the mission performance, and presents a series of example case studies where CryoSat-2 data have been applied to study changes in Earth's land and sea ice cover. We show that (CryoSat has been able to detect changes in the mass of the Antarctic and Greenland ice sheets with an accuracy that greatly exceeds that of the preceding 20 years of conventional satellite altimetry, and that important changes have occurred in these regions. We show that CryoSat has been able to quantify changes in the volume of sea ice across the entire northern hemisphere, and that unexpected changes have occurred. And we show that by exploiting the missions full interferometric capability, CryoSat has been able to detect changes in the volume of rugged, glaciated terrain, that were beyond the capability of past altimeters. These measurements underpin key fundamental climate data records and also support a wide range of operational services.

Launched in 2010, the European Space Agency’s (ESA) polar-orbiting CryoSat satellite was the first SAR/SARin altimeter concept specifically designed to measure changes in the thickness of the sea ice and the elevation of the ice sheets and mountain glaciers. Going beyond its ice-monitoring objective, CryoSat has also demonstrated to be a valuable source of observations for geodesic, hydrologic and oceanographic applications by measuring multi-thematic and high-resolution geophysical parameters. To enable their full scientific and operational exploitation, the CryoSat products continuously evolve and need to be routinely quality-controlled and thoroughly validated. This paper provides an overview of the CryoSat product evolutions and data quality assessment, covering all Cal/Val activities performed by ESA and its partners over the ice and ocean surfaces. Like in any operational environment, the implementation of new CryoSat processing baselines is however by itself often complex and time-consuming, limiting high reactivity toward most recent outcomes from R&D activities. ESA therefore intends to develop a new paradigm allowing producing and distributing simplified, thematic, regional and rapidly evolving CryoSat products in parallel to the consolidated PDGS ones. This concept of "CryoSat ThEMatic PrOducts” (aka Cryo-TEMPO) aims at maximizing the use of the mission for multiple thematic applications. The implementation of Cryo-TEMPO will bring together multi-disciplinary EO scientists and altimetry experts in order to define new standards for CryoSat products to be used as inputs for multi-mission high-level products and data assimilation systems. Cryo-TEMPO shall be QA4EO-compliant by including documented and fully traceable quality indicators following the general guidelines and principles established by the CEOS/GEO. Cryo-TEMPO will also follow the recommendations provided by CryoSat Scientific Expert Groups regarding the format and content of these products over the sea-ice, the land ice, the polar ocean and hydrology domains.

The ESA Earth Explorer CryoSat-2 was launched on 8 April 2010 and from an altitude of just over 700 km and reaching latitudes of 88 degrees, monitors precise changes in the thickness of terrestrial ice sheets and marine ice. The aim of the CryoSat-2 mission is to determine variations in the thickness of the Earth's marine ice cover and understand the extent to which the Antarctic and Greenland ice sheets are contributing global sea level rise. CryoSat Baseline-D ice data will be released in spring next year and will include several evolutions as well as a new data format (NetCDF) compliant with other space-based observations. This paper provides one of the first detailed quantitative and qualitative analysis of the CryoSat ice Baseline-D data over the land ice and sea ice domains, covering all relevant mode-dependent Level-2 geophysical parameters (e.g. freeboard, ice sheet elevations, Polar sea level anomaly) and their comparisons with previous Baseline products and other source of ground and/or space-based observations. The Baseline-D upgrades will bring significant improvements to the quality of Level-1B and Level-2 products relative to the previous baseline products, which in turn are expected to have a strong positive impact on the scientific exploitation of the CryoSat mission over the cryosphere.

The main payload of CryoSat is a Ku-band pulsewidth limited radar altimeter, called SIRAL (Synthetic interferometric radar altimeter). that is equipped with two antennas for single-pass interferometric capability.
Due to the fact that SIRAL is a phase coherent pulse-width limited radar altimeter, a proper calibration approach has been developed. In fact, not only corrections for transfer function, gain and instrument path delay have to be computed (as in previous altimeters), but also corrections for phase (SAR/SARIn) and phase difference between the two receiving chains (SARIN only). To summarize, SIRAL performs regularly four types of internal calibrations:
- CAL1 in order to calibrate the internal path delay and long-term power drift.
- CAL2 in order to compensate for the instrument IF transfer function.
- CAL4 to calibrate the interferometer.
- AutoCal, a specific sequence used to calibrate the gain and phase difference for each AGC setting.

After about 8 years of operational activity of the CryoSat satellite, the performance of the SIRAL instrument are revealed to be in line or better than the expected one.
In fact the calibration products, that have been designed to model a wide range of imperfections of the instrument, can be analyzed to highlight whether and how the instrument is changing over the time also as function of its thermal status. It is worth underlining here that each variation of the instrument measured by the calibration data is compensated in the Level1 processing. Inspecting the temporal evolution of the calibration data, SIRAL has been verified to be stable during its life. The performance of the SIRAL will be presented together with the outcomes of the stability analysis on the calibration data, in order to verify that the instrument has reached the requirements and that it is maintaining the performance over its life.

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 have been periodically performed about once a year.
The end-to-end calibration strategy for the CryoSat interferometer 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. 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).
One of the contributions to the inaccuracy of the angle of earliest arrival measured by the CryoSat interferometer is expected to arise from uncorrected thermal deformation of the star tracker-bench-antenna assembly. A first assessment of this contribution will be given starting from the analysis of the roll mispointing angles returned by the Star Trackers mounted at each side of the antenna bench.

Launched on the 8th of April 2010 from the Baikonur Cosmodrome in Kazakhstan, ESA Earth Explorer mission CryoSat 2 has successfully achieved its mission objectives, producing an exceptional wealth of data in a field of growing scientific interest. The CryoSat-2 mission has been extended until end 2019 thanks to several factors: the outstanding science results, the excellent platform performances and the extraordinary status of the fuel budget. Now, with NASA’s recently launched ICESat 2 mission in September 2018, a unique opportunity has been identified of high scientific interest.

The main CryoSat 2 instrument is the SAR/Interferometric Radar Altimeter (SIRAL), whereas ICESat-2 is equipped with the Advanced Topographic Laser Altimeter System (ATLAS). The combined operations of these two payloads allows a significant improvement in the retrieval of sea ice thickness in the Arctic and Antarctic latitudes, as well as a better understanding of the ongoing changes in the cryosphere due to the effects of global warming. In order to benefit from the combined use of these two instruments, independent observations must be taken over the same geographical areas. In addition to that, the time difference between the observations is constrained as well, and depends on the particular scientific application. This paper summarises the results of an analysis conducted by the CryoSat 2 Mission Control Team, in collaboration with the scientific community and the ICESat-2 team. The purpose of the analysis is to explore the different possibilities available in order to exploit the synergies of these two missions, including the operational approach to achieve it.

The CryoSat 2 satellite is operated following a near polar reference orbit, with a 92 degrees mean inclination of the orbital plane. The spacecraft follows a ground track repeat cycle of 5344 orbits in 369 nodal days, leading to a mean altitude of 722 km. On the other hand, ICESat-2 will be operated at a much lower altitude (484 km), following a ground track repeat cycle of 1387 orbits in 91 nodal days, with a very similar inclination of the orbital plane. The geometry of the orbital planes leads to numerous ground track crossovers over the polar areas where the footprint of both instruments overlap. The crossovers occurrence has been analysed in terms of their spatial distribution and timing characteristics, which provides a measure of the scientific gain that can be obtained keeping CryoSat 2 in its current orbit.

In order to increase the scientific return from these operations further options have been investigated, which imply an orbital change of the CryoSat-2 satellite. The main challenge in the synchronization of both ground tracks is given by the large difference in orbital period of the two satellites. However, a resonance with the ICESat-2 orbital period can be achieved close to the current CryoSat-2 altitude. This orbital change leads to footprint overlaps of both instruments along ample geographical areas, taking place at regular intervals. The operational implementation and feasibility of the required orbital change has been studied, as part of this analysis, and it discussed in this paper as well.