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This letter describes a novel approach to retrieve the pitch of the CryoSat satellite by the analysis of Synthetic Interferometric Radar ALtimeter (SIRAL) acquisitions in synthetic aperture radar (SAR) or SAR-interferometric mode over ocean. The power distribution of the single-look echoes, which are gathered for each sample location on the Earth s...
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... spherical surface, e.g. the ocean, the power of the single look echoes in a surface sample stack is modulated by the along-track antenna pattern. Moreover, according to [11], the look angle for a given surface sample is here defined as the angle at which the surface sample itself is seen with respect to the nadir direction, as it is sketched in Fig. ...
Context 2
... Fig. 1, the satellite has been represented as flying pitched, so that the along-track antenna pattern has it maximum at angle η, i.e. the pitch, with respect to the nadir direction, recalling that CryoSat antennas ideally point towards the nadir. The same sign convention used in [10] is here adopted, so that the satellite flying nose down ...
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Citations
... However, different studies revealed that the mispointing angles in the early versions of the CryoSat-2 Level1b were affected by static offsets [4][5][6], which are now compensated [7]. Both the roll and pitch biases are related to inaccuracies in the rotations applied to the Star Tracker quaternions to translate them from their internal reference frame to the interferometer baseline one. ...
CryoSat-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.
... When represented as a function of the look angle, the center of the RIP is the result of the sum of the pitch mispointing and surface slope, as depicted in Fig. 6. Hence, over open ocean, where the ocean surface slope is of the order of millidegrees, the center of the RIP from zero gives a reliable estimation of the platform pitch mispointing (Scagliola et al., 2015). ...
... Prior to Baseline-D, the Ice Baseline-C processors were installed on the operational and reprocessing platforms and Baseline-C L1B products were produced and distributed to users from 1 April 2015 (Scagliola and Fornari, 2015). During this period some issues were identified, and the scientific community suggested a series of evolutions that have been taken into consideration when updating the L1B processors for Baseline-D. ...
... In Baseline-D, two additional stack characterization parameters (also known as beam behavior parameters) have been added to the SAR and SARIn L1B products: the stack peakiness and the position of the center of the Gaussian function that fits the range-integrated power of the single-look echoes within a stack, as a function of the look angle. The stack peakiness (Passaro et al., 2018) can be useful to improving the sea ice discrimination and the position of the center of the Gaussian function that fits the range-integrated power of the single-look echoes within a stack as a function of the look angle (Scagliola et al., 2015). In radar altimetry, the window delay refers to the two-way time between the pulse emission and the reference point at the center of the range window. ...
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.
... When represented as a function of the look angle, the center of the RIP is the result of the sum of the pitch mispointing and surface slope, as depicted in Fig. 6. Hence, over open ocean, where the ocean surface slope is of the order of millidegrees, the center of the RIP from zero gives a reliable estimation of the platform pitch mispointing (Scagliola et al., 2015). ...
Just as CryoSat-2, Sentinel-3 embarks on board a radar altimeter (SRAL) with the novel Synthetic Aperture Radar (SAR) mode that enables higher resolution and more accurate altimeter-derived parameters in the coastal zone, thanks to the reduced along-track footprint. Exploiting the SAR data in the recent years, many researchers have already proven that the performance of SAR altimetry with specific coastal retrackers is superior to collocated Pseudo-Low Resolution Mode (PLRM) coastal altimetry algorithms but they also pointed out that residual errors due to land contamination are still present in the very proximity of the land (0-3 km).
The objective of this work is to further improve these results by exploiting extra information provided by SAR altimeters, namely the so-called Range Integrated Power (RIP), the new waveform built by a simple integration of the Doppler beams in the range direction. The RIP characterizes the backscattering state of the ground cell, towards which all the Doppler beams have been steered. These developments lead to a new retracker, here coined SAMOSA++, in which the RIP, as computed from the L1B-S data, is converted into a surface backscattering profile and directly integrated in the SAMOSA retracker as part of the model formulation itself. In this way, the modified SAMOSA model is automatically and autonomously able to cope with the different return waveform shapes from different surface types: either diffusive or specular. The mean square slope computed from the RIP is also estimated, representing a new output of the retracker.
The performance of this new retracker is here cross-compared against its previous version, SAMOSA+, and against the standard Sentinel-3 marine PDGS (Payload Data Ground Segment) SAR retracker (SAMOSA2) in both coastal zone and open ocean in order to ensure a seamless transition between these zones.
The new retracker SAMOSA++ is validated in the North East Atlantic region, where appropriate in situ validation data are available. The retrievals from the new retracker are cross-compared against the network of tide gauges and buoys in the German Bight and versus the output of the GCOAST Helmholtz-Zentrum Geesthacht (HZG) regional circulation and wave model. In addition, sea level estimates derived with different ocean tide and wet path delay geophysical correction models are compared. Results indicate that in this region the best geophysical correction models are the FES2014b tide model and the GPD+ wet tropospheric correction that incorporates data from the Sentinel-3 on-board radiometer.
Analyses show that both SAMOSA+ and SAMOSA++ ensure the continuity of the PDGS SAR Marine retracker in the open ocean, leading to clear improvements in the coastal zone, larger for SAMOSA++ than for SAMOSA+. In summary, the new SAMOSA++ retracker retrieves more accurate altimetric parameters in the coastal zone, with a better consistency with respect to regional ocean models and in situ data.
... The analysis of the CryoSat SARin acquisitions from interferometer calibration campaigns revealed that the roll mispointing angle measured by the Star Trackers is affected by a bias with respect to the angle of arrival measured by SIRAL . Other investigations were performed to quantify also the bias between the pitch that is reported by the Star Trackers and the actual satellite pitch (Galin et al., 2014), (Scagliola et al., 2015). The results of these analysis allowed to increase the accuracy of the attitude information for CryoSat, which is a crucial information also for oceanographic applications (Rodríguez et al., 1989) (Ray et al., 2015) Recently, in (Gray et al., 2017) it has been proposed a different approach for the calibration of the CryoSat interferometer based on swath processing of SARin acquisitions over glaciers. ...
Exploiting the interferometric capability of CryoSat, 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. This paper presents an assessment of the achievable end-to-end performance for the across-track angle that is retrieved by the CryoSat SARin acquisitions. In orbit calibration campaigns are periodically performed to monitor the performance and to calibrate the interferometer. We started from the analysis of the data acquired during the interferometer calibrations campaigns to verify if the current performance of CryoSat is in line with the system requirements. The analysis revealed that the interferometric measurements of CryoSat are exceeding the expected performance and that, by proper processing, it is possible to achieve further improvements for the accuracy on the retrieved across-track angle.
... Recalling that in a surface sample stack each single look echo has been acquired from a different position of the instrument along the orbit, it results to be scaled in power by the antenna as function of the look angle ξ, that is here defined as the angle at which the surface sample itself is seen with respect to the nadir direction. According to [4], on a uniformly rough spherical surface, e.g. the ocean, the power of the single look echoes in the stack X(τ, ξ n ) with n = 1, · · · , N being N the number of single look echoes in the stack, is modulated by the two-way along-track antenna pattern G az : ...
... According to [4], the two-way along-track antenna pattern for CryoSat can be modeled as ...
... The estimated pitchη in (4) was computed by compensating the pitch, as measured from the Star Tracker, with a pitch bias. The pitch bias was estimated similarly to [4] building from each SAR stack data a sub-stack of 64 looks and multilooking 100 consecutive sub-stacks together in order to knock-down the speckle noise. The outcome was that the pitch bias resulted in the combination of a constant term equal to 0.055 • and a sinusoidal function of the time, due to the seasonal solar illumination on the Star Trackers. ...
One of themain benefits of the along-track processing in Synthetic Aperture Radar altimetry [1] is the speckle reduction that is achieved by averaging all the observations accumulated for a same scattering area. Being the different observations obtained by looking at the same scattering area from different positions of the instrument along the orbit, they result to be modulated by the along-track antenna pattern. By compensating the along-track antenna pattern before averaging, a higher speckle reduction can be achieved. This paper is aimed at presenting this processing method and at evaluating the possible improvement in the sense of the effective number of looks and of the precision of the physical parameters retrieved from the power waveforms.
Abstract. 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. For 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 days (30 days sub-cycle). 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 27th 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 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 m to 0.1 m. The improved processing of the star tracker measurements implemented in Baseline-D has led to a reduction of the standard deviation of the point-to-point comparison with the previous star tracker processing method implemented in Baseline-C from 3.8 m to 3.7 m. Over sea ice, the Baseline-D improves the quality of the retrieved heights in areas up to ~ 12 km inside the Synthetic Aperture Radar Interferometric (SARIn or SIN) acquisition mask, which is beneficial not only for freeboard retrieval, but for any application that exploits the phase information from SARIn Level-1 (L1) products. In addition, scatter comparisons with the Beaufort Gyre Exploration Project (BGEP, https://www.whoi.edu/beaufortgyre ) 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 m to −0.14 m. For the two in-situ datasets, the Root Mean Square Deviation (RMSD) is also well reduced from 14 cm to 11 cm for OIB and with a factor 2 for 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 analysing the 6-month Baseline-D test dataset released to CryoSat expert users prior the final transfer to operations.
Synthetic Aperture Radar ALtimeter (SARAL) is a new generation radar altimeter and has the best height precision now. As using synthetic aperture technique, the height precision of SARAL is improved by one fold. Based on studying the height precision of Conventional Radar Altimeter (CRA) and SARAL, a novel comparison method is developed to process the airborne flight experiment data. And the precision comparison result shows that the height precision of SARAL is increased by one fold.
Synthetic aperture radar (SAR) altimeters reduce the along-track footprint size exploiting the coherence of the transmitted pulses and achieve at the same time a noise reduction. Consequently, a large effort has been aimed at the formulation of theoretical models that apply to SAR altimeters, in order to fully exploit the improvement in spatial resolution obtained from the along-track aperture synthesis. This letter presents a novel semianalytical waveform model for SAR interferometric altimeters that preserves high accuracy even in the presence of mispointing. Starting from the waveform model proposed by Wingham et al. that provides a unified formulation for pulse-limited and SAR interferometric altimeters which can only be computed numerically, here, we describe a semianalytical approximation for small variations of the mispointing angles around an arbitrary combination of pitch and roll angles (μ̂, θ̂). The proposed semianalytical waveform model allows to reduce the high dimensionality of the model proposed by Wingham et al. and it has been proven to be accurate for variations of mispointing angles up to 0.4 deg around the (μ̂, θ̂). The performance of the proposed formulation has been evaluated on simulated data from Sentinel-6 configuration and on real data from CryoSat-2 SARin acquisitions over ocean.
