Gamma MSP: Reference Manual

ASAR_AP_proc

NAME
ASAR_AP_proc - Generates MSP sensor parameter and processing parameter files from Level 0 ASAR Alternating Polarization (AP) Mode data. This program also reformats the raw SAR signal data for the two channels to be compatible with the MSP. The entire level 0 AP data set is converted to 8-bit I/Q unsigned binary complex samples in two files corresponding to the two polarization channels.

SYNOPSIS
ASAR_AP_proc <L0> <INS> <SAR_par1> <SAR_par2> <PROC_par1> <PROC_par2> <raw1> <raw2> <ant_gain1> <ant_gain2> [loff] [nl] [roff] [nr]

<L0>	(input) Level 0 Image Mode ASAR data file (ASA_IM_0P)
<INS>	(input) ASAR instrument characterization file (ASA_INS_AX)
<SAR_par1>	(output) MSP SAR sensor parameter file 1
<SAR_par2>	(output) MSP SAR sensor parameter file 2
<PROC_par1>	(output) MSP processing parameter file 1 (example p<20071121>.slc.par)
<PROC_par2>	(output) MSP processing parameter file 1 (example p<20071121>.slc.par)
<raw1>	(output) byte aligned 8-bit I/Q raw data 1
<raw2>	(output) byte aligned 8-bit I/Q raw data 1
<ant_gain1>	(input) antenna pattern file 1 (derived using program ASAR_XCA)
<ant_gain2>	(input) antenna pattern file 2 (derived using program ASAR_XCA)
[loff]	offset to first line to process (default = 0, enter - for default)
[nl]	number of lines to process(default = to end of input, enter - for default)
[roff]	offset to first range sample to process (default = 0, enter - for default)
[nr]	number of range samples to process (default = to end of line, enter - for default)

EXAMPLE

ASAR_AP_proc ASA_APH_0CNPDE20050131_211902_000000362034_00200_15284_1759.N1 ASA_INS_AXVIEC20041215_180208_20030211_000000_20051231_000000 ASAR_20050131_HH.par ASAR_20050131_HV.parp20050131_HH.slc.par p20050131_HV.slc.par20050131_HH.raw 20050131_HV.raw ASAR_IS4_HH_antenna.gain ASAR_IS4_HV_antenna.gain

DESCRIPTION
ASAR_AP_proc reads the AP mode ASAR Level 0 data files and generates the MSP processing parameter (p*.slc.par) and sensor parameter files along with reformatting the raw data. Level 0 AP files have names that begin with (ASA_APC_0..., ASA_APH_0..., ASA_APV_0...). These files contain both raw data and meta data describing when the data were acquired and the imaging mode (IS1 - IS7). Alternating Polarization data is a dual polarization acquisition mode where alternate echo bursts comprising of approximately 200 echoes are recorded. After recording in one polarization mode for approximately 200 echoes (e.g.HH), the polarization mode is changed to HV for the next burst.. Detailed information on the AP Level 0 format is contaned in the ASAR Product Handbook available on-line at http://envisat.esa.int. in Section 2.5.

The ASAR AP mode data are stored as records in the raw data files with each record corresponding to an echo. Each echo consists of a header consisting of timing and error information and radar parameters followed by a set of 64-byte packets containing the echo samples. These echo samples are stored using block floating point quantization (FBAQ) that effectively increases the dynamic range of the data to 8 bits from the 4 bits used to encode the data in the Level 0 data. Each packet consists of 63 complex samples (4-bit I/4-bit Q) and a single byte that is an index into a set of lookup tables. These lookup tables map the 4-bit values to the corresponding 8-bit values.

The output of ASAR_AP_proc is two sets of raw data, one for each of the two polarization modes. Blank lines are inserted in one raw data file, when the other polarization mode is active. This way the timing is preserved. The data set comprised of SAR_par1, PROC_par1, raw1 and ant_gain1 correspond to the first of the two polarizations in the polarization mode specification ( i.e. HH in APH H/HVmode, VV in APV V/VH mode, and HH for APC HH/VV mode). Processing proceeds for the two raw data sets as for a normal SAR images with the exception that autofocus may not work with this type of data and may not be required..

The set of 256 look up tables needed to generate decompressed samples and other ASAR calibration and system parameters are stored in the Instrument Characterization File (ASA_INS_AXVIEC...). These files must be downloaded from ESA and the correct file selected for the date that the data were aquired. The ESA website to download both the INS files and the external characterization file (ASA_XCA...) is http://envisat.esa.int/services/auxiliary_data/asar .

The external characterization file is required to extract the antenna pattern as measured on the ground and is used to compensate for the antenna gain across the swath in the SAR processing. The program in the ISP called ASAR_XCA and is used to extract the appropriate antenna pattern (the ant_gain1 and ant_gain2 files on the command line).

The ASAR instrument shifts the echo digitization window approximately every 15 seconds along-track. ASAR_AP_proc initially scans the entire data set to determine the maximum shift from start to finish. and shifts the data in the output such that the starting time for all echoes is the same for the entire raw data set.. No samples are lost in this process because the data are extended to accomodate the range of the shift. Every echo is decompressed and written to the appropriate raw file specified .

The remain optional parameters on the command line specify what portion of the raw data file that will be processed by the MSP. If these are omitted, then the entire raw data set is processed. Default values for the number of lines nl corresponds to all remaining echoes in the file beginning at loff lines. Similarly, the default number of samples nr, is all samples to the end of the line beginning at roff samples.