By contrast with optical ones, radar images require before becoming accessible a period of time for processing. This may be longer or shorter according to the dimensions of the scene observed, sensor characteristics and processor performance. For a basis of comparison, in the mid-eighties a 100 x 100 km scene took up to nine hours' processing on a powerful computer. In the early nineties, for the same scene, a couple of hours were enough, while today it takes a quarter-hour. These improvements are due partly to more refined, optimized algorithms, but mainly to increased computer performance. Despite this astounding rise in computer capacity, the time to process an image is 60 times what the radar needs to record the scene. This means that by the time the image is available the satellite is at least 6000 km away from the area viewed. This limitation rules out use of SAR images for a whole range of applications that need real time or near real time.

The need to bridge the gap between the moment a scene is recorded and the moment its image is available has stimulated the search for innovative solutions, both algorithmically and in circuits. This is the context for the development of the single-bit processor. Creating it has meant both adopting non-standard processing and designing ad hoc hardware.

With the advantages described, the drawbacks should also be listed.

At the present time the components are cumbersome and need relatively high power; however, technological evolution promises to eliminate these drawbacks before too long. Additionally, producing them entails development costs that would not be incurred if commercial processors were used.

The single-bit processor supplies in real time the images taken by the SAR sensor, as well as, again in real time, the images in amplitude and phase format that are the starting point for generating the topographical products like the ground height maps. This latter processing is, however, done on conventional computers off line, i.e. not in real time. Data on the alignment of the antennas will also be supplied. For each piece of processing, data on image quality and processing parameters used will be supplied. As regards the interferometric products, image-coherence maps, interference fringes and unwrapped fringes will be supplied (DEM not georeferenced).

The interferometric products today take some 2 hours' processing for 50 x 50 km regions with spacing between one height point and the next at 25m. As an example of satellite survey capacity, two workers on the ground would take at least 5 years to do the same work (the basis for this estimate is 4 million measuring points, 1 minute's time for each individual measurement and a 24-hour working day, i.e. three shifts).

The precision data processing system and image archiving and distribution system (I-PAF)

Precision data processing

Radar images, by contrast with optical ones, are taken by recording a large number (some 2000 per second) of echoes reflected from the Earth's surface. The signals picked up are very similar to holographic data and accordingly require highly sophisticated, laborious processing. Once two radar images of the same area have been obtained, interferometric processing can be done. This consists of determining the time differences in echo return time for each pixel taken (radar signal phase differences). This operation is particularly delicate, since small errors introduced by the system can lead to unacceptable errors in the final product. With an eye to limiting possible errors, the mission incorporates various error correction strategies:

• The navigation system allows high-precision recording of position and velocity data for the platform and at the same time keeps a check on the dimensions of the interferometer, consisting of a 60-meter long arm with two antennas at its end.

• The data are recorded in "strips" some 50 km wide, always including two edges of sea. This expedient allows the data processed to be "anchored" to a very exact reference, the sea.

• Since the recordings are made on ascending and descending orbits, the data strips overlap in so-called crossover zones. In these zones further checks and corrections can be done.

Today's data processing technology allows the system to be run on conventional systems. Processing is divided into:

• Transcription on to "computer-compatible" magnetic support of data coming from the mission, recorded on high-density tapes;

• Processing of the calibration signals for the onboard instruments and of the platform position and velocity information;

• Synthesis of the interferograms and subsequent geometrical processing to derive the height datum;

• In the final stage in the processing chain, the height data obtained from the interferograms are mosaicized to form the products for distribution.

The result of processing the data acquired can be differentiated into two categories: image products and interferometric products.

The image products cover all the products obtained from acquiring and processing the signals coming from one individual antenna. These images will have a format and quality comparable with the products derived from the two previous missions. It will accordingly be possible to analyze the data from the three missions in a broader context, and in some cases to secure further results from the scientific experiments conducted during the first two X-SAR missions.