Envisat ASAR

Envisat drifting phase

New

At the end of October 2010, the orbit of the Envisat satellite was lowered by 17 km, putting it into a new "drifting phase" or "Extended Mission" orbit with a 30-day repeat cycle. In this new orbit, they did not do any orbit inclination adjustments so the satellite plane started to drift. At the ESA Living Planet Symposium of June-July 2010, they announced that they had modified the plan for the "drifting phase" orbit of Envisat to have the "fixed nodes" of the orbit at 38 degrees N on the descending tracks and 38 degrees S on the ascending tracks instead of the previous plan to have them both at the equator. This means that we generally get good baselines within about 4-6 degrees of latitude from those two fixed nodes. At other locations, the baselines increase rapidly with time but 30-day or 60-day interferograms will probably be OK.

Envisat functioned completely in the new orbit from November 2010 to April 2012. Of course, all of the new "drifting phase" tracks are completely different and can't be used with any old data from the original phase. It was like starting a new mission. Unfortunately, ESA lost contact with Envisat on 8 April 2012.

There is no change to ROI_pac required to process the new Envisat data. Two Envisat "Extended Mission" or "2010+" orbit interferograms have been processed with ROI_pac, see http://www-radar.jpl.nasa.gov/CrustalDef/san_fran .

For more information about the Envisat Orbital Change (effective 22 October 2010): ESA Technical Document

Envisat processing

ROI_pac can process image mode (IM) (also called stripmap) raw (or Level 0) data from the ASAR instrument on Envisat. The distributed ROI_pac is not presently capable of processing the Alternating Polarization (AP) or Wide Swath (WS) modes, although there is an effort to develop a ScanSAR interferometry processor for the Envisat WS data and an alternative technique is described below. ROI_pac is not designed to process the SLC (Level 1) data products from Envisat, but a very preliminary package for importing SLC products into ROI_pac is available on the ContribSoftware page.

Envisat ASAR Level 0 (raw) data is a single file (the standard ESA name for the stripmap data is in the form ASA_IM__0*.N1 or if you get data from WInSAR it may have the form ENV1*.baq) so you can just copy this to your computer.

The make_raw_envi.pl script is now included in the ROI_pac version 3.0.x releases, along with a program asa_im_decode written by SeanBuckley that actually reads the Envisat IM data. The script can use either the orbits from ESA (called the Doris orbits) or the Delft (ODR) orbits for Envisat. Note that the script extracts the orbit information from the ODR, POR or VOR orbits and stores them in a hdr_$date.rsc file with the raw data. For the rest of ROI_pac processing, you use $orbit_type=HDR. This script can concatenate or merge multiple scenes.

To use the ESA Doris orbits (contact the ESA Help Desk EOHelp@esa.int for information on where to get these) you need to set the environment variables POR_DIR and VOR_DIR to point to directories of the POR (preliminary orbits) and VOR (precise orbits), and give the DOR parameter to the make_raw_envi.pl call. The ESA Doris POR orbits are usually available within about three days after acquisition and the VOR orbits are available about six weeks after acquisition. The ROI_pac software will first look for a VOR orbit for the desired date, and if there is no VOR orbit then it looks for a POR orbit.

Note that the ESA Doris orbit files (DOR_VOR and DOR_POR) cover a little more than one day. The first date in the DOR filename is the date that the orbit was processed, not the date of the orbit data. The time covered by the orbit file is specified by the second two dates in the filename. For example, this DOR file:

DOR_VOR_AXVF-P20040226_162200_20031228_215528_20031230_002328

Was processed on 2004/02/26, but the orbit data covers the time between 2003/12/28T21:55:28Z and 2003/12/30T00:23:28Z or basically the date 2003/12/29 plus a little bit of the day before and after. The Envisat DOR orbit files always cover one full day plus a small part of the day before and after, so the date that the file covers is not actually one of the dates listed in the filename. You have to either subtract one day from the third date or add one to the second date in the filename to see the day covered by the orbit file.

In addition to the data file and the orbit files, you also need to have the set of Envisat instrument calibration (INS) files on your computer. Create a INS directory and download all of the ASA_INS* files from http://earth.eo.esa.int/services/auxiliary_data/asar/current/ into the directory. Note that the INS files for older data (pre 2007) may be compressed, so you will have to gunzip or unzip them if they are. Then set the environment variable INS_DIR to point to that directory. The make_raw_envi.pl script will determine which of the INS files to use. Note that you need to update the INS files every year or when there is a change in the calibration of the Envisat ASAR instrument.

Another script make_raw_envi_subset.pl and associated programs are not part of the 3.0 release but here under ContribSoftware. This script is for extracting a subset of a long "floating pass" product of ASAR Level 0 data.

Envisat ScanSAR

A new set of code has been contributed by CunrenLiang at Peking University to process Envisat WS data in ROI_pac using the zero-pulse padding technique, see the ContribSoftware page to download it, and the EnvisatScanSar page for more details.

older information

There are also some older versions of the Perl scripts and a program to do the "make_raw" import into ROI_pac available at:

http://winsar.stanford.edu/software/envisat/

http://www.geo.cornell.edu/eas/PeoplePlaces/Faculty/matt/ROI_PAC4ENVISAT.tar (a slightly newer version)

Envisat (last edited 2012-12-12 17:36:26 by user-vcauugs)