Gamma DIFF: Reference Manual

phase_sim

ANSI-C program: phase_sim.c

NAME
phase_sim - simulate unwrapped interferometric phase using DEM height, linear baseline model, and linear deformation rate for single or repeat-pass interferograms

SYNOPSIS
phase_sim <SLC1_par> <OFF_par> <baseline> <hgt_map> <sim_phase> [ph_flag] [bflag] [def] [delta_t] [int_mode] [SLC2R_par] [ph_mode]


EXAMPLE
phase_sim 19960421.par 19960421_19960422.off 19960421_19960422.base landers.hgt_map 19960421_19960422.sim_unw 0 1 0

Simulates an unwrapped phase image for the specific geometry of an interferometric pair using a height map in the geometry of the interferogram. The simulated interferogram is unflattened and uses the precision baseline.

DESCRIPTION
phase_sim simulates an interferogram based on a height map in SAR geometry, linear baseline model, and possibly a deformation rate map, both in SAR range-Doppler coordinates (RDC). To calculate the deformation phase the user must also provide the number days between images used to calculate the specific interferogram pair. The imaging geometry of a specific interferometric pair is used for the phase calculation. This geometry is defined by the SLC parameter file of the reference SLC image (containing information on the SAR sensor, the processing applied, and the orbit geometry) the baseline file (containing the information on the interferometric baseline to be used for the simulation), and the ISP processing/offset parameter file (containing information on the selected SLC image section and the multi-looking applied in interferometric processing). In the case of interferograms all generated using the same reference image, use the parameter file of the reference image as the SLC1_par argument.

The height map in SAR geometry is obtained by resampling a digital elevation model (DEM) to SAR coordinates. The geometric transformation is determined by simulation of a SAR intensity image (in SAR geometry) from the DEM. The simulated SAR intensity image is then registered to a real SAR intensity image using a cross correlation registration approach for real valued data. This step is required to remove errors resulting from small errors in the orbit geometry. If a - is entered for the height file, then all heights are set to 0.0. The number of range and azimuth looks of the height map should be the same for the output interferogram.

Simulated phase images are mainly used for 2-pass differential interferometry. In 2-pass differential interferometry the phase corresponding to the terrain topography is simulated and subtracted from an interferometric pair which contains both differential and topographic phase components. Another potential application of a simulated phase image isto help generate a higher quality DEM with interferometry. The simulated phase is subtracted from the interferogram and the residual phase is unwrapped and processed to generate a higher resolution product. Using the differential interferogram facilitates filtering and phase unwrapping to achieve higher accuracy in the final DEM product.

Notice, that the baseline geometry described in the baseline file is may not be perfect and have residual quadratic phase trends. One approach to remove these trends is to use quad_fit and quad_sub.

The process for calculation of a deformation rate map using phase_sim can be as follows. First obtain an initial simulated interferogram using a DEM, without the deformation model in the call to phase_sim. Next subtract this simulated interferogram from the original interferogram using sub_phase or generate the differential interferogram directly from the resampled SLC data using SLC_diff_intf.

Note that the the residual phase is the combination of residual baseline error, deformation, and tropospheric phase. Therefore it is important to select a time interval for the interferogram that is sufficiently long that the deformation phase dominates over baseline error and tropospheric phase.

The interferometric mode (int_mode) option permits selection of either single or repeat pass mode. The single pass interferometric phase is half as large as the repeat pass simulated phase because the path difference is half that compared to repeat pass interferometry.  Tandem-X data are acquired in single-pass mode where one satellite transmits and the other receives. In this case int_mode must be set to 0 to correctly simulate the interferometric phase.

phase_sim can be used for cross-platform interferometry between ERS and ASAR where the sensors have different carrier frequencies. To do this this the SLC parameter file of SLC2R must be provided on the command line.

OPTIONS
Setting the phase flattening flag (ph_flag) to 0 results in an unwrapped phase in the geometry of the original interferogram (without flat Earth phase trend removed).  Setting the flattening flag to 1 simulates an unwrapped phase image with the ellipsoidal Earth phase trend removed.

There is also the ph_mode command line option to subtract a multiple of 2PI from the simulated interferogram to improve the accuracy of the single-precision phase data stored in the output file. When there is a large difference in slant range, the magnitude of the interferometric phase can become large enough (~200000) such that there is a a measurable loss of accuracy in the phase data due to the single-precision representation.  The value of the phase that is subtracted from the entire simulated interferogram is calculated from the simulated phase at the center of the interferogram with a height of 0.0. This phase value is then rounded to the nearest multiple of 2PI. All internal calculations related to the phase that are internal to the program are performed using double-precision.

SEE ALSO

quad_fit, quad_sub. dispmap, sub_phase, SLC_diff_intf, sub_phase, typedef_ISP.h typedef_DIFF.h