ESA ERS-1 SAR

This section is still under construction.

ERS-1 and JERS-1

ERS-1 and JERS-1 are remote sensing satellites launched by the European Space Ag ency and NASDA of Japan, respectively. Both satellites carry Synthetic Aperture Radar instruments which are used to image the Earth's surface. The Alaska SAR Facility (ASF) has an agreement with both Space Agencies to receive data from th ese two sensors and process it into images. ERS-1 data can only be received wit hin the viewing range of the ASF downlink antenna, which covers the state of Ala ska, the Bering Strait, North-West Canada, the Gulf of Alaska, and the ice sheet up to the North Pole. JERS-1 data can be received by ASF for any part of the w orld. Data for each sensor can also be ordered direct from the European (ESRIN) or Japanese (NASDA EOC) data distribution facilities. Some restrictions and cost s may apply.

The ERS-1 SAR is a C-band (6 cm wavelength) instrument, operating with VV polari zation. It's orbit height is about 800 km and the range of incidence angles wit hin a typical image is fixed at between 20 degrees for near range and 26 degrees for range range. A typical image area is 80 x 80 km.

The JERS-1 SAR is an L-band (24 cm wavelength) instrument, operating with HH pol arization. It's orbit height is about 570 km and the range of incidence angles within a typical image is fixed at between 32 degrees at near range and 38 degre es at far range. A typical image area is 80 x 75 km.

The ERS-1 SAR was designed primarily to observe features on the ocean, such as w aves or ice sheets. The wavelength and incidence angle selected for ERS-1 are a lso good for observing some, but not all, features on land, such as mountainous terrain or agricultural fields. The JERS-1 SAR was designed primarily to collec t data over land which would be useful to geologists. The wavelength, incidence angle and polarization selected for JERS-1 are not ideal for observing ocean fe atures.

ASF distributes hi-res (high resolution) and lo-res (low-resolution) data produc ts. The hi-res data product has 8192 by 8192 12.5 x 12.5 m pixels, with 25-30m r esolution and takes up 64 Megabytes of disk space. The lo-res ASF data product has 1024 by 1024 100 x 100 meter pixels, with 200 meter resolutionand takes up 1 Megabyte of disk space. Many samples (about 256) have been averaged to produc e each pixel in the lo-res data products. Each pixel represents normalized rada r cross section (s0) for either L-band HH or C-band VV at the appropriate incide nce angle. The lo-res ASF products can be displayed by the MacSigma0 program.

The main difference between the lo-res ERS-1 and JERS-1 images versus those from an airborne system like the AIRSAR is the size of the imaged area, which is muc h larger for the satellite images. In addition, the resolution of the lo-res da ta is much larger than for the AIRSAR data. Also, the range of incidence angles for the satellite data is only about 6 degrees across the imaged swath, while f or the AIRSAR the range of incidence angles is about 40 degrees for a typical im age. This means that the backscatter variation with incidence angle across the swath is more pronounced for the AIRSAR data. The geometry for collecting satel lite SAR data is similar to the geometry illustrated in the last section for the AIRSAR. The height of the platform, however, is significantly different. Sinc e the AIRSAR platform (a DC-8 aircraft) flies at around 10 km altitude, to image a swath 10 km wide, an incidence angle variation of about 40 degrees is typical . For the satellite platforms, with altitudes greater than 500 km, an 80 km swa th width can be observed with only a 6 degree variation in incidence angle acros s the swath.

For more information on the Alaska SAR Facility processor, see:

"User's Guide To Products", Jet Propulsion Laboratory Document D-9362, Version 1 .0, Jet Propulsion Laboratory, Pasadena, California, January 1992.

ERS-1, JERS-1 Data Formats (ASF)

An ERS-1 or JERS-1 data set processed at the Alaska SAR Facility contains three files: the .i.ERS-1, JERS-1:data file; itself, a .i.trailer file;.i.ERS-1, JERS -1:trailer file;, and a .i.leader file;.i.ERS-1, JERS-1:leader file;. The data file contains a small header and the data itself. In the data, there is general ly some number of bytes at the beginning of each line which are ignored. This n umber is typically 12. The following is a typical data file:

The leader and trailer files are both of the same format. This format is called the .i.CEOS format;.i.ERS-1, JERS-1:CEOS format;. In this format, files contai n a sequence of "records" which are saved end-to-end. Each "record" is a partic ular type. Encoded in the beginning of each record are the record type and reco rd length. The parameters contained within each "record" and their specific loc ations depend on the record type. Unfortunately, the encoded record type is not consistent. The best method of locating records is by using the .i.CEOS format :File Descriptor Record; which is always the first record in either file. This record contains information about the other records contained within the file. Also, the sequence of "records" is sometimes not actually stored end-to-end but may be separated by an extra byte. The MacSigma0 program was written to check a nd compensate for this extra byte.

The leader and trailer files are identified as ".ldr" (or ".LDR") and ".trl" (o r ".TRL", ".tlr", or ".TLR") files. In other words, if the data file is called "Beaufort", then the trailer file should be either "Beaufort.trl" or "Beaufort.T RL". If the data file is called "Bonanza Creek.dat", then the trailer file is e xpected to be either "Bonanza Creek.trl", "Bonanza Creek.TRL", "Bonanza Creek.tl r", or "Bonanza Creek.TLR".

The following are references for the CEOS header format:

"Alaska SAR Facility (ASF): System Interface Specification", Jet Propulsion Labo ratory Document D-5267 Revision A, Jet Propulsion Laboratory, Pasadena, Californ ia, May 1992.

SAR Data Products Standard Committee on Earth Observing Satellites Working Group on Data Revision 2.0, 10 March 1989 (available from the user services group at the Alaska SAR Facility at 907-474-74 87)

Updated 8/25/94
bruce.chapman@jpl.nasa.gov