ERS SAR Processing Levels

Introduction

The Synthetic Aperture Radar (SAR) data transmitted to Earth from an Earth observation satellite, is in a form unsuitable for immediate use by customers. ACRES can process this raw data in a number of ways, to produce products which comply with a range of standard processing levels suitable for use by clients. In the case of SAR data from the European Earth Resource Satellite (ERS), ACRES products are processed to conform to the European Space Agency's (ESA) specifications and formats.

The information shown here has been compiled to present an overview of the processing levels available. Further detailed information on processing levels may be obtained from ACRES, while information about SAR in general is available through NASA's Jet Propulsion Laboratory: What is Imaging Radar?

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Overview

There are three basic aspects associated with how SAR data is processed:

Data vs Image

• At the most basic level of processing, data is not interpretable as an image. An image is only formed after considerable processing of the data.

Orientation

• After this initial processing, the image can be oriented to the satellite path, or to a map projection.

Radiometric and Geometric Corrections

• These corrections include multi-looks (spatial averaging), incorporation of Ground Control Points and/or Digital Elevation Models. These are mentioned under the relevant categories below.
• There is a trade-off between radiometric accuracy and spatial resolution. For example, improved radiometric accuracy (resulting from an increased number of looks) will result in reduced speckle with lower spatial resolution.

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Processing Level Options

Raw Data

Data products at this level consist of minimally preprocessed raw SAR data suitable for input to another SAR processor. Such data is not able to be ingested by an image processing system, and is most likely to be used by research institutes interested in full SAR data processing, or as an interchange format for other SAR ground stations. The product contains all the required auxiliary data necessary for data processing.

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Single Look Complex (SLC)

This product can be considered the foundation for all other products below, but its use requires sophisticated SAR processing software. The data is in "slant range", which basically means that relationships between observed objects across track, are based on time differences between radar pulses. (This is in contrast to other processing levels in "ground range", meaning that observed objects are spatially related to the Earth as an approximate flat image).

Single Look Complex data maintains the phase information of the data. Its primary use is for interferometry, and for the development, calibration, testing and use of SAR algorithms used in post-processing.

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Path Image

This is a path oriented and system corrected product, being the basic product used for a variety of remote sensing applications. The European Space Agency uses the name "Precision Image" for this product, but ACRES has chosen not to adopt this name due to potential confusion with other ACRES products.

This product is noise reduced (3 looks) which results in less speckle on the image. The three looks also help to achieve the equal sized square pixels. The data can be calibrated by use of a calibration constant supplied in the header file of the product.

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Map Oriented Image

This is a map oriented and system corrected product. Instead of being in ground range projection, this product is aligned parallel to true North, and corrected using either an Equidistant Rectangular, UTM or AMG map projection, as selected by the user. This type of image is the most appropriate for integration into existing GIS databases.

This is a multilook product. The multilook process reduces the noise and results in less speckle on the image. There is however, an increase in coarseness of the spatial resolution.

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ERS SAR Price List

See ERS SAR Price List for various available products.