ERS SAR Processing Options

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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. Geoscience Australia 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 ERS (European Earth Resource Satellite), Geoscience Australia products are processed to conform to the European Space Agency (ESA) specifications and formats.

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

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 able to be interpreted as an image. An image is formed only 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

Processing level options

Raw data

Data products at this level consist of minimally pre-processed 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.

Raw Data Characteristics
Scene area 100km (range) × 110km (azimuth)
Pixel / line spacing 7.9m × 3.9m
Pixel depth 16 bits (8bits I; 8bits Q)
Total product volume ~300MBytes
Projection Slant range
No. of looks 1

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.

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

Raw Data Characteristics
Scene area 100km × 100km (approx.)
Pixel / line spacing 7.9m × 3.9m
Pixel depth 16 bits I; 16 bits Q
Total product volume ~300Mbytes
Projection Slant range
No. of looks 1

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 (ESA) uses the name 'Precision Image' for this product, but we have chosen not to adopt this name because of the potential for confusion with other 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.

Raw Data Characteristics
Scene area 100km (range) × at least 102.5km (azimuth)
Spatial resolution 25m range; 15m azimuth
Pixel size 12.5m × 12.5m
Total product volume 131.3Mbytes
Projection Ground range
Effective no. of looks 3

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, depending on which is 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. However, there is an increase in coarseness of the spatial resolution.

Raw Data Characteristics
Scene area (nominal) 100km (range) × at least 102.5km (azimuth)
Spatial resolution 25m range; 15m azimuth
Pixel size 12.5m × 12.5m
Total product volume 144Mbytes
Projections UTM, AMG, Equidistant Rectangular
Effective No. of looks 3

ERS SAR Price List

A list of products is available from ERS Prices.

Topic contact: earth.observation@ga.gov.au Last updated: February 10, 2012