Roll Calibration for CryoSat-2: a comprehensive approach

International audienceCryoSat-2 is the first satellite mission carrying a high pulse repetition frequency radar altimeter with interferometric capability on board. Across track interferometry allows the angle to the point of closest approach to be determined by combining echoes received by two antennas and knowledge of their orientation. Accurate information of the platform mispointing angles, in particular of the roll, is crucial to determine the angle of arrival in the across-track direction with sufficient accuracy. As a consequence, different methods were designed in the CryoSat-2 calibration plan in order to estimate interferometer performance along with the mission and to assess the roll’s contribution to the accuracy of the angle of arrival. In this paper, we present the comprehensive approach used in the CryoSat-2 Mission to calibrate the roll mispointing angle, combining analysis from external calibration of both man-made targets, i.e., transponder and natural targets. The roll calibration approach for CryoSat-2 is proven to guarantee that the interferometric measurements are exceeding the expected performance

Improved Arctic Sea Ice Freeboard Retrieval From Satellite Altimetry Using Optimized Sea Surface Decorrelation Scales

A growing number of studies are concluding that the resilience of the Arctic sea ice cover in a warming climate is essentially controlled by its thickness. Satellite radar and laser altimeters have allowed us to routinely monitor sea ice thickness across most of the Arctic Ocean for several decades. However, a key uncertainty remaining in the sea ice thickness retrieval is the error on the sea surface height (SSH) which is conventionally interpolated at ice floes from a limited number of lead observations along the altimeter's orbital track. Here, we use an objective mapping approach to determine sea surface height from all proximal lead samples located on the orbital track and from adjacent tracks within a neighborhood of 30–220 (mean 105) km. The patterns of the SSH signal's zonal, meridional, and temporal decorrelation length scales are obtained by analyzing the covariance of historic CryoSat-2 Arctic lead observations, which match the scales obtained from an equivalent analysis of high-resolution sea ice-ocean model fields. We use these length scales to determine an optimal SSH and error estimate for each sea ice floe location. By exploiting leads from adjacent tracks, we can increase the sea ice radar freeboard precision estimated at orbital crossovers by up to 20%. In regions of high SSH uncertainty, biases in CryoSat-2 radar freeboard can be reduced by 25% with respect to coincident airborne validation data. The new method is not restricted to a particular sensor or mode, so it can be generalized to all present and historic polar altimetry missions

On the influence of coastal mesoscale dynamics on the jellyfish trajectories and distributions

IMAGE DU MOIS AVISO: http://www.aviso.oceanobs.com/en/news/idm/2012/may-2012-jellyfish-on-the-move/print.html ** Résumé de la conférence: http://www.coastalt.eu/files/sandiegoworkshop11/5CA-WS_summary.pdfInternational audienceOceanic mesoscale plays a key role in modulating large-scale circulation, heat fluxes transfer and primary production enhancement. Such hydrodynamic processes are also crucial for jellyfish transport and distribution along the Mediterranean coastal areas. Investigating the relationships between jellyfish distribution and mesoscale hydrodynamic processes therefore provides a rational to understand the influence of such physical structures on the dynamics of regional ecosystems, at the interface between the open ocean and the continental shelf. Nevertheless, the high spatial and temporal variability associated with coastal mesoscale motions makes them difficult to study with sparse in-situ observations. Alternative options rely on developing methodologies based on the combination of multi-sensor platforms in conjunction with numerical simulations. In this respect, we use an advanced Lagrangian particle tracking code developed at LOPB (Marseille, France) to simulate jellyfish trajectories from both a 3D circulation regional model and currents derived from satellite observations. These are obtained by a high resolution altimetric current mapping tools developed at IMEDEA (Majorques, Spain). The large scale signals (~100 km) are removed by subtracting the gridded Sea Level Anomaly maps (AVISO) to improve along track data. In a second step, the residuals are submitted to an objective analysis scheme with correlation scales adjusted to smaller mesoscale and coastal dynamics. Our approach allows us to characterize the main mesoscale features and exchange between the Ligurian Sea and the Gulf of Lion and to infer possible main pathways of jellyfish trajectories

Possible Triplet Electron Pairing and an Anisotropic Spin Susceptibility in Organic Superconductors (TMTSF)_2 X

We argue that (TMTSF)_2 PF_6 compound under pressure is likely a triplet superconductor with a vector order parameter d(k) \equiv (d_a(k) \neq 0, d_c(k) = ?, d_{b'}(k) = 0); |d_a(k)| > |d_c(k)|. It corresponds to an anisotropic spin susceptibility at T=0: \chi_{b'} = \chi_0, \chi_a \ll \chi_0, where \chi_0 is its value in a metallic phase. [The spin quantization axis, z, is parallel to a so-called b'-axis]. We show that the suggested order parameter explains why the upper critical field along the b'-axis exceeds all paramagnetic limiting fields, including that for a nonuniform superconducting state, whereas the upper critical field along the a-axis (a \perp b') is limited by the Pauli paramagnetic effects [I. J. Lee, M. J. Naughton, G. M. Danner and P. M. Chaikin, Phys. Rev. Lett. 78, 3555 (1997)]. The triplet order parameter is in agreement with the recent Knight shift measurements by I. J. Lee et al. as well as with the early results on a destruction of superconductivity by nonmagnetic impurities and on the absence of the Hebel-Slichter peak in the NMR relaxation rate.Comment: 4 pages, 1 eps figur

CryoSat ocean product quality status and future evolution

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

CryoSat Ice Baseline-D validation and evolutions

The ESA Earth Explorer CryoSat-2 was launched on 8 April 2010 to monitor the precise changes in the thickness of terrestrial ice sheets and marine floating ice. To do that, CryoSat orbits the planet at an altitude of around 720 km with a retrograde orbit inclination of 92∘ and a quasi repeat cycle of 369 d (30 d subcycle). To reach the mission goals, the CryoSat products have to meet the highest quality standards to date, achieved through continual improvements of the operational processing chains. The new CryoSat Ice Baseline-D, in operation since 27 May 2019, represents a major processor upgrade with respect to the previous Ice Baseline-C. Over land ice the new Baseline-D provides better results with respect to the previous baseline when comparing the data to a reference elevation model over the Austfonna ice cap region, improving the ascending and descending crossover statistics from 1.9 to 0.1 m. The improved processing of the star tracker measurements implemented in Baseline-D has led to a reduction in the standard deviation of the point-to-point comparison with the previous star tracker processing method implemented in Baseline-C from 3.8 to 3.7 m. Over sea ice, Baseline-D improves the quality of the retrieved heights inside and at the boundaries of the synthetic aperture radar interferometric (SARIn or SIN) acquisition mask, removing the negative freeboard pattern which is beneficial not only for freeboard retrieval but also for any application that exploits the phase information from SARIn Level 1B (L1B) products. In addition, scatter comparisons with the Beaufort Gyre Exploration Project (BGEP; https://www.whoi.edu/beaufortgyre, last access: October 2019) and Operation IceBridge (OIB; Kurtz et al., 2013) in situ measurements confirm the improvements in the Baseline-D freeboard product quality. Relative to OIB, the Baseline-D freeboard mean bias is reduced by about 8 cm, which roughly corresponds to a 60 % decrease with respect to Baseline-C. The BGEP data indicate a similar tendency with a mean draft bias lowered from 0.85 to −0.14 m. For the two in situ datasets, the root mean square deviation (RMSD) is also well reduced from 14 to 11 cm for OIB and by a factor of 2 for the BGEP. Observations over inland waters show a slight increase in the percentage of good observations in Baseline-D, generally around 5 %–10 % for most lakes. This paper provides an overview of the new Level 1 and Level 2 (L2) CryoSat Ice Baseline-D evolutions and related data quality assessment, based on results obtained from analyzing the 6-month Baseline-D test dataset released to CryoSat expert users prior to the final transfer to operations