Since 8 April 2010 ESA's Earth Explorer mission CryoSat-2 has been measuring marine and land ice thickness variations with a dedicated altimeter. With the effects of a fast-changing climate becoming apparent, it is increasingly important to understand exactly how Earth's ice fields are responding and regulate climate and sea level. It is therefore of utmost importance that the quality of the CryoSat-2 altimeter data meets the highest accuracy and precision level, not only over the ice caps and sea-ice surface but over the oceans as well. The SIRAL altimeter of CryoSat-2 is capable of measuring high-resolution geophysical parameters from the open ocean to the coast. In this paper, we validate the CryoSat-2 geophysical ocean product, Baseline-B (GOP). The analyses focus on long-term monitoring, validation, and cross-calibration of these ocean altimeter data with all the concurrent altimeter data in the Radar Altimeter Database System RADS and with independent in-situ tide gauge measurements from the Permanent Service for Mean Sea Level PSMSL. The objective is to evaluate the stability of the CryoSat-2 measurement system and the identification and explanation of biases and bias drifts. A persistent monitoring of these biases in time is important to establish the data's usefulness for investigation of long-term sea level and ice topography/volume changes. The results constitute the final status of the GOP baseline-B product for use as essential climate variables. GOP ocean data has a -6.3±0.2 cm range bias w.r.t. to all calibrated satellites in RADS and a drift of 0.14 mm yr−1 w.r.t. Jason-2. GOP data has significant wave height and sea state biases close to that of the Jasons’, and seems to be better handling the LRM-SAR transitions than the RADS CryoSat-2 product. GOP data w.r.t. PSMSL tide gauge data has an average correlation of R = 0.83, a mean st. dev. of σ = 6.30 cm, and a drift of 0.11 mm yr−1, equivalent to 40 Gt yr−1 ice sheet mass loss. It is concluded that GOP CryoSat-2 performs better than RADS CryoSat-2, which seems to suffer from a 480-day platform temperature cycle. GOP is also on a par with the altimeter data from the Jason reference missions, and well suited for ocean and climate studies.@en

The main objectives of this paper are to present the status of the CryoSat ocean products and to give an overview of all associated quality control and validation activities. Launched in 2010, the polar-orbiting European Space Agency's (ESA) CryoSat mission was primarily developed to measure changes in the thickness of polar sea ice and elevation of the ice sheets. Going beyond its ice-monitoring objective, CryoSat is also a valuable source of data for the oceanographic community. The satellite's radar altimeter can measure high-resolution geophysical parameters from the open ocean to the coast. To enable their full scientific and operational exploitation, the ocean products continuously evolve and need to be quality-controlled and thoroughly validated via science-oriented diagnostics based on multi-platform in situ data, models and other satellite missions. In support to ESA, the CryoSat ocean validation teams conduct this quality assessment for both the near real time and offline ocean products, both over short time scales (daily and monthly monitoring) and long-term stability (annual trends). Based on the outcomes from these quality analyses and feedback from scientific oceanographic community, ESA intends to upgrade the CryoSat Ocean processing chain for Autumn 2017.@en

Swarm is the European Space Agency (ESA)'s first Earth observation constellation mission, which was launched in 2013 to study the geomagnetic field and its temporal evolution. Two Langmuir probes aboard each of the three Swarm satellites provide in situ measurements of plasma parameters, which contribute to the study of the ionospheric plasma dynamics. To maintain a high data quality for scientific and technical applications, the Swarm products are continuously monitored and validated via science-oriented diagnostics. This paper presents an overview of the data quality of the Swarm Langmuir probes' measurements. The data quality is assessed by analysing short and long data segments, where the latter are selected to be sufficiently long enough to consider the impact of the solar activity. Langmuir probe data have been validated through comparison with numerical models, other satellite missions, and ground observations. Based on the outcomes from quality control and validation activities conducted by ESA, as well as scientific analysis and feedback provided by the user community, the Swarm products are regularly upgraded. In this paper, we discuss the data quality improvements introduced with the latest baseline, and how the data quality is influenced by the solar cycle. In particular, plasma measurements are more accurate in day-side regions during high solar activity, while electron temperature measurements are more reliable during night side at middle and low latitudes during low solar activity. The main anomalies affecting the Langmuir probe measurements are described, as well as possible improvements in the derived plasma parameters to be implemented in future baselines.

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ESA’s Earth Explorer CryoSat-2 precisely measures the changes in the thickness of marine ice floating on the polar oceans and variations in the thickness of the vast ice sheets that overlie Greenland and Antarctica. The data delivered by the CryoSat-2 mission completes the picture to determine and understand the ice role in the Earth system in general and climate change in particular. For this, the quality of the satellite orbit, the measurements of the altimeter, and all required corrections have to meet the highest performance; not only over the ice caps and sea-ice surface but also over the oceans. As Cryosat-2 ocean products continuously evolve they need to be quality controlled and thoroughly validated via science-oriented diagnostics based on multi-platform in situ data, models and other (altimeter) satellite missions. The rationale for this is based on the new CryoSat-2 scientific roadmap, which specifically addresses the key technical and scientific challenges related to the long-term monitoring of sea-level and ocean circulation changes in the context of Global Warming. This also involves opportunities for synergy with missions like ICESAT-2 and the upcoming Copernicus CRISTAL mission. In this context, the objective of our research is the long-term monitoring of the level-2 CryoSat-2 Geophysical Ocean Product (GOP), by evaluating the stability of the measurement system and identifying potential biases, trends and drifts over the ocean, through calibration and comparisons with concurrent ocean altimeter data, supported by the Radar Altimeter Database System (RADS). Independently, we also address this by comparing the GOP geophysical parameters with external models and in situ measurements such as the ones from selected sets of tide gauges. The very precise determination of the orbital height is part of the research activity but dealt with in a separate paper. For our activity we persistently monitor, analyze and identify systematic errors in the observations, estimated (trends in) biases in range, significant wave height, backscatter, wind speed and sea state bias, and timing biases. An important finding is that GOP CryoSat-2 Baseline C data seem to have a range bias of -2.82 cm and no apparent drift w.r.t. altimeter (Jason) reference missions (< 0.1 mm/yr). The comparison with tide gauges is based on monthly averaged sea level from the PSMSL archive, for which we conclude that GOP data has a correlation of better than 0.84 with a selected set of 185 PSMSL tide gauges, a mean standard deviation better than 5.8 cm, and an average drift of -0.19 mm/yr, which translates to an overall drift of +0.11 mm/yr when taking a global GIA correction of +0.3 mm/yr into account. We conclude that Cryosat-2 GOP represents a (long-term) stable measurement.@en