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The data flow through the system and the operations and quality control is performed by software tools for 
data and facility management which are implemented on workstations that have access to all subsystems via 
the LAN. These functions might again be supported by an expert system. 
A variety of SAR Products will be derived with the SAR Data Processing Chain. Starting with Raw Spacecrafi 
Telemetry Data (level 0), recorded on HDDT, plus orbit and attitude data, the ISAR transcription task gen- 
erates Annotated Raw Data Sets (level 1), which can be extracted on request. The subsequent SAR processing 
task generates a variety of Bulk Products while the Geocoding task finally generates Geocoded Products from 
Bulk Products. 
Bulk Products (level 1.5) are digital images which are earth located. The image data sets consist of pixel arrays 
ordered in range and azimuth, representing a segment of the ground swath. The radiometric information is 
as accurate as possible. The earth location is derived from a precise orbit and the best attainable attitude 
information. Geometric manipulations are restricted to an optional slant range to ground range conversion. 
Geocoded Products comprise Corrected Products, Precision Products and Map Products. Corrected Products 
(level 2A) are derived from Bulk Products in ground range. Such products are of use for scenes taken over 
the open ocean, coastal areas and flat terrain. Depending on the area, none to many GCP's are used for the 
rectification resulting in quite different absolute and relative geometric accuracies. Several map projections 
such as UTM, stereographic and others can be selected. Precision Products (level 2B) and Map Products (level 
2C) are both derived from Bulk Products in slant range. Such products will be generated for areas with 
moderate to strong variations of terrain elevation. The rectification makes use of Digital Elevation Models to 
avoid severe geometric distortions and shall achieve absolute and relative geometric accuracies in the order 
of the pixel sizes (30 m for ERS-1). However, the accuracy finally achieved depends very much on the 
accuracy of the DEM used. While a precision product would be derived from a single scene, a map product 
could originate from several scenes applying mosaicking techniques. The Map Product in a proposed scale 
of 1 : 200,000 can directly be compared with existing topographic or thematic maps. 
3. The Preprocessing System 
According to the system description given in chapter 2 Preprocessing is the first step in the production pro- 
cedure of SAR imagery. The term Preprocessing designates the task that converts raw data sets plus auxiliary 
information to annotated raw data sets and to digital image data sets. The ERS-1 satellite will carry the first 
SAR sensor which is designed to work operationally for several years. This is the most significant difference 
to all spaceborne SAR systems flown up to now. From that it is clear that existing Preprocessing systems like 
GSAR [Bennett, 1981] with an average throughput figure of one product per day will not be adequate to 
process a reasonable amount of data. Therefore a high throughput capability represents a hard requirement for 
the processor. Because the output product of the Preprocessor can be used as an input to the Geocoding system 
this latter task can best be performed if the Preprocessor products are of best possible quality. Therefore both 
requirements - high precision and high throughput - are very important to the Preprocessor. 
The term high precision will be applied in a many layered way. First of all it shall be a requirement which 
is closely related to product quality. In this context pixel location accuracy in cross-track and along-track 
direction shall be a key issue. These parameters are effected essentially by the accuracy of the earth model, 
the state vector of the satellite and the attitude measurements. 
It is required to achieve a location accuracy of 20 m in cross-track and 150 m in along-track for all level 1.5 
products (acquired over slow varying terrain slope). In the case of ERS-1 the GEM-6 earth model 
[Klinkrad, 1985] will be the standard model which is characterized by the earth oblateness coefficient and the 
semi major axes. The state vector gives all necessary information about the position of the satellite in all three 
axes as well as the velocity vector. The Mission Management and Control Center of ESA delivers the predicted 
orbit with an accuracy of 36 m, 51 m and 1300 m (radial, cross-track, along-track). The restituted orbit values 
are 25 m, 25 m and 100 m. However one product offered by the German PAF will be a refined state vector 
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