L. Phalippou

SIRAL, a high spatial resolution radar altimeter for the Cryosat mission

SIRAL (SAR Interferometer Radar Altimeter) is the new spaceborne altimeter designed for CryoSat mission. This ESA mission, planned for 2004, will be used to estimate-on a global scale- the fluctuations in mass of sea-ice and land-ice. The novelty of SIRAL concept with respect to conventional pulse-limited altimeter, is the implementation of Doppler processing for along-track resolution enhancement and also of interferometry, used to locate the echo in the across-track direction. The innovative technical features of SIRAL are presented hereafter with regards to the function requirements and also the expected performance.

Re-tracking of SAR altimeter ocean power-waveforms and related accuracies of the retrieved sea surface height, significant wave height and wind speed

This paper describes the simulation of the power- waveforms acquired in by a radar altimeter operating in SAR mode over ocean surfaces, including the effects of the radar transfer function and of the geophysical ocean parameters, namely the sea surface height( SSH), the sea surface wave height (SWH) and the surface wind speed (WS). The performances of the SAR mode with respect to the retrieval of the ocean geophysical variables (SSH, SWH, WS) are then computed using the Cramer-Rao estimation bounds. It is shown that the radar to ocean range estimation provided by the SAR mode is improved by more than a factor of two compared with conventional Ku band altimeters. Improvements on SWH and wind speed are also discussed.

Overview of the performances and tracking design of the SIRAL altimeter for the CryoSat mission

This paper presents the principle of the measurements, the main system features and the basic ground processing of SIRAL. Simulations of the main performances in the different modes are also discussed including the bidimensional impulse response (range and azimuth). A major scientific requirement is to ensure the continuity of the measurements. This is particularly challenging for the on-board tracker over the steepest parts of the Antarctica. As a consequence a new tracking algorithm has been designed for SIRAL with the expertise of CNES. The algorithm has been derived from the analysis of simulated echoes using an Antarctica DEM and a radar echo simulator. The tracking design and performance are briefly described.

SIRAL: the radar altimeter for CryoSat mission, under development

SIRAL (SAR Interferometer Radar Altimeter) is the new spaceborne altimeter designed for CryoSat mission. The instrument is currently in the development phase, in ALCATEL SPACE, which encompasses an Engineering Model and a Flight Model. This ESA mission, planned for 2004, will be used to estimate - on a global scale - the fluctuations in mass of sea-ice and land-ice. This paper discusses the flexibility of the instrument and in particular its capability to operate in various modes (conventional altimeter, SAR and interferometer modes), and gives main results from the pre-developments and breadboard activities.

End to end performances of a short baseline interferometric radar altimeter for ocean mesoscale topography

The swath radar altimeter concept has been revisited - in the framework of an ESA study - with the objective of defining an optimal functioning point with respect to mission performance while keeping the instrument complexity as low as possible. We show that a swath altimeter concept, with a shorter interferometric baseline than WSOA and moderate power can meet the requirements of mesoscale topography mapping. The paper presents a complete sea surface height (SSH) error budget for the mission

High spatial resolution radar altimeter for ocean and ice-sheet monitoring

The monitoring of the ice-sheet height over Antarctica and Greenland is of primary importance for climate studies. The required height accuracy is of the order of a few centimetres. Current spaceborne altimeters are pulse-limited and therefore fail to deliver high accuracy surface height measurements over land and ice because of pulse spreading and slope induced errors. Moreover, sudden changes in the topography often result in a loss of track. Several techniques have been investigated during the past ten years in order to improve the spatial resolution of radar altimeters and therefore the accuracy of elevation estimates over terrains. Most of these techniques result in a significant increase in cost and complexity compared to a conventional altimeter. This paper presents the results of a recent feasibility study for a spaceborne high-spatial resolution radar altimeter (HSRRA) performed by Alcatel and the University College of London for the European Spatial Agency (ESA). This paper gives an overview of the concept and of the processing scheme. The theoretical altimeter waveforms derived for a flat tilted surface are also presented and discussed.

Spaceborne P-band radar for ice-sheet sounding: design and performances

The Antarctic ice sheet is one of the most prominent physical features on our planet. It represents an area of about 14 millions km/sup 2/. The average ice thickness being around 2200 m (and up to 4500 m), the Antarctic ice sheet contains a 30 million km/sub 3/ ice volume corresponding to about 70 m of sea level. However, this huge reservoir of water is still partially unknown and due to its large extent, only observations from space can give some global insight into the structure of the ice sheet. The Antarctic ice sheet is an exceptional archive of the past climates and a major actor of the Earth water cycle. Each ice layer corresponding to one period of time keeps its own chemical characteristics linked to past atmospheric events. The typical temporal scale involved allow for a description of past climate over a few climatic cycles, or a few 100000-year events. A dedicated mission has been proposed to European Space Agency as a candidate for an Earth Explorer Opportunity mission. This mission is based on a P-band radar instrument with a nadir-looking geometry, working at 435 MHz with a bandwidth of 6 MHz (imposed by ITU regulation), allowing to sound the vertical structure of the ice from the surface down to the bedrock. A feasibility study of the mission was done in Alcatel Space under ESA contract with an emphasis on the radar instrument. For coherent reflection and deep sounding (corresponding to the so-called Fresnel zone), the useful signal coming from any ice layer or from the bedrock overlap with lateral incoherent surface scattering. The surface clutter is reduced by the antenna pattern in the across-track direction (typical aperture is lower than 5/spl deg/) and by Doppler filtering in the along-track direction. These beam requirements impose to have a very long antenna in the across-track direction. The typical dimension of the antenna is 10 m /spl times/ 1 m. This work summarizes the principle of the instrument, and the associated antenna solution. Main performances are also presented.

Siral the radar altimeter for the cryosat mission, pre-launch performances

The development phase of the SAR Interferometer Radar Altimeter (SIRAL) is in the final stage in Alcatel Space and during year 2004 the Engineering and the Flight Models is tested before delivery to the Mission Prime ASTRIUM GmbH. This ESA mission will be used to estimate - on a global scale - the fluctuations in mass of sea-ice and land-ice. This paper gives a detailed presentation of the microwave and processing units developed, followed by a survey of the pre-launch performances.

Future radar altimeter concepts for ocean applications

Alcatel Space is currently leading a stuffy for ESA for reviewing the user needs in ocean and ice altimetry and for proposing and studying new altimeter concepts. The paper presents the results of this study focusing on ocean mesoscale sea surface topography (sea state applications not addressed) applications and swath interferometric radar altimeter concepts.

Preliminary design of the SWIMSAT radar for the measurement of ocean wave spectra

The main objective of SWIMSAT is to measure the directional spectra of waves thanks to a space-borne radar. The instrumental concept involves a real-aperture radar using a multi-beam conical scanning, in order to provide measurements of the spectral properties of the wave field, and to estimate the profile of radar cross-section depending on incidence (0/spl deg/ to 10/spl deg/) and azimuth (0/spl deg/ to 360/spl deg/). The paper provides an overview of the mission and of the measurement principle, and the status on instrument design.