ASAR GROUND PROCESSOR VERIFICATION AND WAVE VALIDATION

The European Space Agency has selected an Advanced Synthetic Aperture Radar operating at C-band (5.331 GHz) as a payload for the forthcoming ENVISAT mission. The ASAR instrument modes of operation ensure continuity of ERS SAR and feature enhanced capability in terms of coverage, range of incidence angles and polarisations. This paper presents the key features of the ASAR system. The ASAR Ground Processor is presented highlighting the concepts for its development and the performance achieved versus the ESA specifications. The selection of processing algorithm for each product is discussed based on the image quality requirements and the throughput. The processor validation using ERS data is introduced, together with examples of simulated ASAR products and their quality measurements. Finally the validation plan of the newly developed ASAR products is detailed and pre-launch validation results presented. ASAR SYSTEM DESIGN OVERVIEW The ASAR system design has been inherited from ERS, this is why the ENVISAT platform is yaw steered and, thanks to the provision of star sensor, the attitude control of the platform is expected to be better than ERS. ASAR will benefit from the availability of an on-board Solid State Recorder allowing up to 10 minutes recording of High Rate modes (100Mbit/s) at any point around the orbit. The DORIS orbit determination system will allow for accurate geolocation of all ASAR products produced in near real time and off-line. The ASAR instrument design uses an active array antenna with 320 Transmit Receive modules (T/R) to produce a versatile position of the imaged swath by beam steering in elevation. Additionally, a large swath coverage (400km) is achieved using the ScanSAR technique at medium resolution (150m) or low resolution (1 km). All operating modes feature the possibility to select H or V polarisation on transmit and receive (HH or VV) and the alternating polarisation sub-modes allow to use a combination of both (HH and VV, or HH and HV, or VV and VH). Moreover, an improved wave measurement mode is offered to the users with 100 km spacing between vignettes and the capability to select the position alternating between 2 positions in any swaths (see Fig. 1). Additionally, ASAR is equipped with a programmable digital waveform generator, which allows to optimize the radiometric quality of the products versus signal to noise ratio. As a further improvement compared to ERS, an 8 bit ADC associated with a Flexible Block Adaptive Quantization scheme to be used in 8:4, 8:3 and 8:2 compression ratios will allow for a larger dynamic range of the input signals in order to limit saturation problems. ASAR features a temperature compensation scheme to compensate for drifts observed at T/R level around the orbit. Furthermore the instrument frequency stability and datation have been improved to support interferometric applications. A detailed description of the ASAR instrument is provided in [1] [7]. The ASAR has five modes of mutually exclusive operation, which can be classified in two categories: Fig. 1. ASAR Modes of operation • Low data rate modes for Global Monitoring and Wave modes with an operational capability of up to 100% of the orbit. Both modes are systematically recorded on-board and the on-board tape recorder is dumped every orbit when visible by an ESA station (KIRUNA station in Sweden) or via the Artemis Data Relay Satellite to the ESA ESRIN station in Italy. • High data rate (100Mb/s) for Image, Alternating Polarization and Wide Swath modes with operation time up to 30 minutes per orbit (including 10’ in eclipse). The High Rate mode data are recovered according to one of the following schemes: real time transmission via an X-band link to ESA or alternative station, real time transmission via the Ka-band link using Artemis Data Relay Satellite to the ESA ESRIN station in Italy, or alternatively recorded on board on the Solid State Recorder and dumped via Xor Ka-band link when in visibility of an ESA station. ASAR DATA PROCESSING The development of the ASAR processor and products is based on three concepts mainly derived from the ERS experience: • The need for users to have identical products independent of which production facility generate them. • The preservation of the specifications of the existing ERS SAR high-resolution products (SLC, PRI, GEC) and the introduction of new products (medium resolution and browses). • The capability to generate a large amount of products in near real time. Following the above concepts, ESA has developed with Alcatel Space Industries and MDA an ASAR Generic Processor (PF-ASAR) able to process data from all the ASAR modes. PF-ASAR will be installed in the ESA Payload Data Handling Stations (Kiruna and ESRIN-Frascati), in the ENVISAT Processing and Archiving Centers (PACs) and in the national stations offering ESA ASAR services. The use of PF-ASAR will insure product compatibility between the different processing centers (same format and processing algorithm) and will simplify product validation. One of the key features of PF-ASAR is the capability to process acquired data to generate medium resolution (150 m resolution, example in Fig. 2) or low resolution products (1 km resolution) and their corresponding browse images in stripline without geometric or radiometric discontinuity. The stripline image products contain data from an entire acquisition segment up to 10 minutes for Image, Alternating Polarisation or Wide Swath modes and up to a complete orbit for the Global Monitoring mode. PF-ASAR will be used to insure the systematic processing in near real time of all received high rate data generating medium resolution and browse products. All Wave mode or Global Monitoring mode data will be also systematically processed in near real time. Furthermore PF-ASAR will allow to process High Resolution Products from Image or Alternating Polarisation acquisitions (Precision Images, Single Look Complex or Ellipsoid Geocoded products) in near real time or off line depending on user requests. Fig. 2 shows an example of Precision Image (IMP) and Ellipsoid Geocoded Image (IMG). PF-ASAR is targeted to run on an IBM Power 3 multi-processor computer system and is designed to insure a throughput linearly proportional to the number of processing nodes available in the processing station. Fig. 2. Example of IMM Product (left) located on the Walgreen Coast in Antarctica, IMP Product of Bathurst Island in Canada (top right) and IMG Product of the Bay of Naples in Italy (bottom right). Processing Algorithms The Range Doppler algorithm is used for the image mode precision (PRI), geocoded (GEC) and complex (SLC) products. Range Doppler is used in order to meet the stringent image quality requirements while still maintaining good throughput. A modified version of the Range Doppler algorithm [2] was developed for the Alternating Polarisarion (AP) SLC product. The algorithm was chosen such that the output product would be useful for SAR Interferometry. The algorithm is a modified phase preserving Range Doppler approach. Although the aperture covers a little more than 5 bursts, (i.e. between 2 and 3 bursts are available per target for each polarisation) the processed Doppler bandwidth is made equal to four burst bandwidths (multi-burst processing) so as to keep as much of the signal information as possible while maintaining radiometric quality. The resulting product has a modulated impulse response function (IRF) in azimuth due to a segmented azimuth spectra for each target but contains all the necessary information required for SAR Interferometry (InSAR). This modulation can be removed during InSAR post-processing using the short FFT approach [3] to select the common spectra between the two image pairs. The Range Doppler is also used to produce the SLC imagettes for wave mode. These imagettes are used as input to the wave mode cross spectra algorithm. The cross spectra algorithm was developed by NORUT Information Technology [4]. This algorithm is an improvement over the one used for ERS since it allows the wave direction ambiguity to be resolved and compensates for the signal to disturbance ratio which is in the order of 20 dB higher. The latter also makes the product well suited for obtaining a unique solution for the underlying swell spectrum. In most of the cases, wave systems are detected and a unique solution of the wave inversion problem is achievable without using external information. PF-ASAR is currently being updated to include this new inversion algorithm developed by NORUTIFREMER [5] and generate systematically the ocean wave spectrum Level 2 product. The SPECAN algorithm is well suited to burst-mode data and as a result is used for most AP mode products and for WS and GM mode products. The algorithm is also used to process medium resolution products, which are produced systematically and therefore benefit from the efficiency of the SPECAN algorithm. PF-ASAR includes a Doppler Centroid Estimator with specified accuracy of 50Hz for image and wave modes (like ERS) and 25Hz in ScanSAR modes in order to limit radiometric errors in azimuth [6]. Internal Calibration Processing on the ASAR Ground Processor During preprocessing PF-ASAR performs, for all imaging modes, FBAQ signal decoding (from 2, 3 or 4 bits back to 8 bits) and corrects for possible downlink bit errors on the source packet annotations. Once decompressed, I/Q science data are subject to an I/Q correction (bias, differential gains, non-orthogonality). As with ERS, any necessary corrections for nonlinearity may be applied in the ground processor using pre-launch instrument ADC characterization. Prior to processing, PFASAR also performs noise power estimation from the instrument noise measurements. The ASAR instrument’s internal path transfer function is derived by the ASAR internal calibration scheme [7]. This is achieved by dedicated calibration signal paths to each T/R module and special calibration pulses