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WHAT IS A RADAR ALTIMETER INDIVIDUAL ECHO?
The EnviSat Radar Altimeter, in its nominal operation, provides averaged waveforms at
the rate of 18 Hz (one averaged waveform over 100 individual echoes, or every 55.7
milliseconds). It has also the capability to provide limited bursts of individual,
unaveraged echo samples data in phase (I) and quadrature (Q), at the full rate (1800 Hz).
These individual echoes are not processed on-board in the same way the nominal 18 Hz RA-2
waveforms are. The needed processing is performed on-ground by means of a processor
implemented ad-hoc to provide the users with reconstructed individual echoes, in the
same format as the original averaged ones, currently present in the RA2 Level 1b products
and in a data set of the Level 2 SGDR products.
The on-ground processor constructs the individual echoes from the raw data emulating the on-board processing performed on the averaged echoes before transmitting them on ground. Furthermore, a Level 1b processing is performed on the individual waveforms (in order to obtain the attenuation or AGC, the sigma0 scaling factor and other parameters), with some variations with respect to the algorithms actually used for the averaged echoes. Recently, some major modifications have been implemented in this software in order to collocate IEs L1b records with SGDRs, for precise geolocation parameters. An extra calibration of successive IE waveforms in terms of a-b tracker time branch has been also added in order to equalise window delay of successive IEs in a burst and be able to easily compare successive IEs waveforms and use the available phase information. Once done so, the Individual Echoes are fully processed and instrument calibrated for validation and science exploitation purposes. It has been demonstrated that through the full rate data it is possible to discover some features than cannot be seen with the averaged data. Among other things it is foreseen the use of these data with transponders for the sigma_0 calibration. Moreover, it is the first time in altimetry that we have echoes that contain the information of the phase with an increased resolution of 3.5m alone the track. This is a great potential for new science studies. INDIVIDUAL ECHOES COVERAGE
The coverage of the Individual Echoes is rather homogeneous around the world. Next figure illustrates
the IEs acquired between 25 November 2004 and 5 February 2005. Normally one second of Individual Echoes
is collected and transmitted every 180 seconds, the time it take to download a full IE memory buffer into
the regular RA-2 source packet data. Upon request, over specific targets, 55.7 milliseconds of IEs can be
instead stored on board and transmitted every 8-9 seconds. This is planned in advance by means of specific
macro-commands.  PROJECT PRESENTATION
At the end of 2004, the European Space Agency in ESRIN launched a pioneer project
aimed at processing, on ground, the RA2 raw Individual echoes, in order to emulate
the on-board processing performed by the instrument on the averaged echoes and to seed
scientific investigation on this novel data set.
A parallel contract was signed with two independent consortiums, in order to implement two Individual Echoes processors and analyse their output results. The first consortium was led by CLS (Collecte Localisation Satellites at Toulouse F) and composed of PILDo Labs (at Barcelona E) in charge of the processor implementation (along with CLS) and geophysical applications, UCL (University College London in UK), LEGOS in Toulouse F and IFREMER in ? F for the geophysical applications. The second consortium was led by NOCS (National Oceanographic Centre at Southampton, UK), and composed of SciSys (Scientific Systems at Chippenham UK) in charge of the processor implementation and DMU (De Montfort University at Leicester UK), in charge, along with NOCS, of the geophysical applications. The processor produced by SciSys has been implemented on a GRID architecture at ESRIN, such that the processor, input and output data do not have to be physically at each users’ place, avoiding problems with large amount of data downloading and local processor’s launch procedures. The links to the IE output data present inside this web site, in fact, are coming from the SciSys processor runs. OBJECTIVE
The main objective of the ESA Individual Echoes project is to provide the scientific
community with reconstructed Ku band individual echoes (128 samples per waveform)
such that they can be easily used for further scientific applications.
The first ambition is to obtain around 3 years of reprocessed IE data with world-wide coverage such that all kinds of scientific applications (over all types of surfaces) can be performed with extensive sets of data. |
