Christopher Buck

The PARIS Ocean Altimeter In-Orbit Demonstrator

Mesoscale ocean altimetry remains a challenge in satellite remote sensing. Conventional nadir-looking radar altimeters can make observations only along the satellite ground track, and many of them are needed to sample the sea surface at the required spatial and temporal resolutions. The Passive Reflectometry and Interferometry System (PARIS) using Global Navigation Satellite Systems (GNSS) reflected signals was proposed as a means to perform ocean altimetry along several tracks simultaneously spread over a wide swath. The bandwidth limitation of the GNSS signals and the large ionospheric delay at L-band are however issues which deserve careful attention in the design and performance of a PARIS ocean altimeter. This paper describes such an instrument specially conceived to fully exploit the GNSS signals for best altimetric performance and to provide multifrequency observations to correct for the ionospheric delay. Furthermore, an in-orbit demonstration mission that would prove the expected altimetric accuracy suited for mesoscale ocean science is proposed.

Experimental Results of an X-Band PARIS Receiver Using Digital Satellite TV Opportunity Signals Scattered on the Sea Surface

A bistatic radar at X-band (11 GHz) using opportunity signals (Passive Reflectometry and Interferometry System (PARIS) concept implementing the interferometric technique) has been developed and tested in a coastal-based experimental setup. Digital satellite TV signals broadcast from geostationary orbit have been used as sources of opportunity, and correlation waveforms have been collected at different receiver bandwidths. Calibration algorithms to compensate instrumental errors have improved our observables and allowed us to obtain delay measurements with a precision of about 10 cm for wind speeds in the range of 3 m/s. A statistical analysis of the measurement noise is used to determine the quality of the obtained observables. This paper also discusses the opportunities of the PARIS concept applied at higher frequency band and with stronger signals.

An extension to the wide swath ocean altimeter concept

This paper presents a modification to the Wide Swath Ocean Altimeter allowing it not only to measure sea-surface currents directly, but also providing the possibility to correct for both baseline length and attitude errors.

Toward Consistent Satellite Calibration and Validation for GEOSS Interoperability

A significant challenge for the Global Earth Observation System of Systems (GEOSS) interoperability is the lack of consistency in the Earth observations from satellites developed, calibrated, and operated by different space agencies worldwide, and the potential for significant discrepancies among products exists. The Committee on Earth Observation Satellites (CEOS) and its Working Group on Calibration and Validation (WGCV) are taking specific steps to facilitate interoperability by developing data quality assurance strategies and conducting joint cross-calibration studies. In this study, the Antarctic Plateau Dome C site is used for cross-comparison of visible/near infrared, and microwave instruments. Observations from AVHRR, MODIS, Hyperion, AMSR-E, and other instruments were intercompared. The findings suggest that the site is stable with relatively low radiometric uncertainties, and is a good candidate for CEOS endorsed cal/val site for satellite cross-comparison to facilitate GEOSS interoperability.

ASAR instrument performance and product quality status

This paper presents the main characteristics of the advanced synthetic aperture radar (ASAR) instrument on board ENVISAT, ASAR products, the challenges in the ASAR calibration and product validation, the methodology used to perform the sensor performance monitoring and product calibration based on the special ASAR features and dedicated calibration sites and finally a summary on the product quality status will also be provided.

Calibration and early results of the ASAR on ENVISAT

This paper presents the approach for the in-flight calibration of the ENVISAT-1 ASAR and the verification of the ground processing facility PF-ASAR during the commissioning phase. The philosophy presented is a logical progression from the experience gained during calibration of the ERS SARs. The ASAR has a comprehensive internal calibration loop, which is described distinctly from the external calibration and characterisation. The antenna patterns of the various beams have been fully measured during on-ground flight-model testing and have been used for initial performance predictions. In-flight characterisation of the main beams is performed over the South American rainforest. As for ERS, absolute gain calibration is achieved using three fixed and one transportable precision calibration transponders situated in the Netherlands. These transponders are also capable of recording the azimuth beam patterns and supporting the external characterisation mode of ASAR.

Wind-Wave induced velocity in ATI SAR Ocean Surface Currents: First experimental evidence from an airborne campaign

Conventional and along-track interferometric (ATI) Synthetic Aperture Radar (SAR) sense the motion of the ocean surface by measuring the Doppler shift of reflected signals. Measurements are affected by a Wind-wave induced Artefact Surface Velocity (WASV) which was modelled theoretically in past studies and has been estimated empirically only once before with Envisat ASAR by Mouche et al., (2012). An airborne campaign in the tidally dominated Irish Sea served to evaluate this effect and the current retrieval capabilities of a dual-beam SAR interferometer known as Wavemill. A comprehensive collection of Wavemill airborne data acquired in a star pattern over a well-instrumented validation site made it possible for the first time to estimate the magnitude of the WASV, and its dependence on azimuth and incidence angle from data alone. In light wind (5.5 m/s) and moderate current (0.7 m/s) conditions, the wind-wave induced contribution to the measured ocean surface motion reaches up to 1.6 m/s upwind, with a well-defined 2nd order harmonic dependence on direction to the wind. The magnitude of the WASV is found to be larger at lower incidence angles. The airborne WASV results show excellent consistency with the empirical WASV estimated from Envisat ASAR. These results confirm that SAR and ATI surface velocity estimates are strongly affected by WASV and that the WASV can be well characterized with knowledge of the wind knowledge and of the geometry. These airborne results provide the first independent validation of Mouche et al., 2012, and confirm that the empirical model they propose provides the means to correct airborne and spaceborne SAR and ATI SAR data for WASV to obtain accurate ocean surface current measurements. After removing the WASV, the airborne Wavemill retrieved currents show very good agreement against ADCP measurements with a root mean square error (RMSE) typically around 0.1 m/s in velocity and 10° in direction. This article is protected by copyright. All rights reserved.

Paris Altimetry Precision Prediction with Galileo Signals-in-Space

This paper presents a complete analysis of the height precision of a GNSS-R space-based altimeter exploiting the forthcoming signals of the European Galileo satellite navigation system. The major differences between Galileo and GPS transmitted signals are shown and their impact on altimetric performance is carefully pinpointed, analysed and justified. Simulated cross-correlation altimetric waveforms are evaluated and the performance analysed by means of an analytical model for the evaluation of height measurement precision. The analysis shows that, thanks to the unique features in terms of bandwidth, TX power and modulation schemes, a GNSS-R altimeter based on Galileo signals-in-space could easily meet the requirements on precision for mesoscale ocean altimetry.

Status and trends for space-borne phased array radar

The first European SAR based on an active phased array was launched in March 2002 on board Envisat and is operational. Several SAR instruments with active array antennas are currently under development in Europe and new instrument concepts are under discussion for the next generation. How will new technologies, currently under development in the commercial and military sectors, influence radar design based on existing architectures and new concepts? This paper discusses the current technology status and identifies future trends.

P-band radar ice sounding in Antarctica

In February 2011, the Polarimetric Airborne Radar Ice Sounder (POLARIS) was flown in Antarctica in order to assess the feasibility of a potential space-based radar ice sounding mission. The campaign has demonstrated that the basal return is detectable in areas with up to 3 km thick cold ice, in areas with up to several hundred meters thick warm shelf ice, and in areas with up to 700 m thick crevassed glacier ice. However, major gaps in the basal return are observed, presumably due to excessive absorption, scattering from ice inclusions in the firn, low basal reflectivity, and the masking effect of the surface clutter. Internal layers are observed down to depths exceeding 2 km. The polarimetric data show that the internal layers are strongly anisotropic at a ridge, where the ice flow is supposed to be highly unidirectional. In case of space-based ice sounding, the antenna pattern cannot offer sufficient surface clutter suppression, but improved clutter suppression has been demonstrated with novel multi-phase-center techniques.