Gamma DIFF: Reference Manual

phase_sim_orb

NAME
phase_sim_orb - simulate unwrapped interferometric phase using DEM height and deformation rate using the orbit state vectors for baseline calculation.

SYNOPSIS
phase_sim_orb <SLC1_par> <SLC2R_par> <OFF_par> <hgt_map> <sim_phase> [SLC_ref_par] [def] [delta_t] [ph_mode]

<SLC1_par>	(input) SLC image parameter file of SLC-1 (reference SLC of interferometric pair)
<SLC2R_par>	(input) SLC image parameter file of SLC-2R (resampled SLC of the pair)
<OFF_par>	(input) ISP processing/offset parameter file (of interferometric pair)
<hgt_map>	(input) height map in the same geometry as the interferogram (enter - for none)
<sim_unw>	(output) simulated unwrapped interferometric phase
[SLC_ref_par]	(input) SLC parameter file of the image used for geometric co-registration (enter - for none)
[def]	(input) LOS deformation rate map (meters/yr, float, enter - for none)
[delta_t]	interferogram time interval (days, required for deformation modeling, enter - for none)
[int_mode]	(input) interferometric acquisition mode: 0: single-pass mode (Tandem-X) 1: repeat-pass mode (default)
[ph_mode]	phase offset mode: 0: absolute phase (default) 1: subtract phase offset that is a multiple of 2PI to improve phase precision

EXAMPLE
phase_sim_orb 19950421.slc.par 19960422.slc.par 19960421_19960421.off landers_dem.rdc 19960421_19960421.sim_unw - - 1

Simulates an unwrapped phase image for the specific geometry of an interferometric pair using a height map in the geometry of the interferogram. The interferometric phase is simulated for repeat-pass interferometry (int_mode = 1).

DESCRIPTION
phase_sim_orb simulates an interferogram based on a height map and a deformation rate map, both in SAR range-Doppler coordinates (RDC). The baseline for each point is calculated using the DEM height, slant range, doppler centroid, and state vectors. This approach is applicable for long image strips where the linear baseline model may be inadequate and residual quadratic phase trends may still be visible. 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) into 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.

Simulated interferometric phase is used primarily for generating 2-pass differential interferograms. 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.To calculate the deformation phase term the user must provide the LOS deformation rate and the number days between images used to calculate the specific interferogram pair.

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.

The process for calculation of a deformation rate map can be as follows. First obtain an initial differential interferogram using a DEM and state vectors. Unwrap this differential initial interferogram. In general this interferogram will have some residual phase trend. Next use base_ls (from the ISP) to estimate a residual baseline using the height map in SAR geometry and the unwrapped phase. The effective baseline for the residual phase should be relatively small and the linear model used by base_ls and phase_sim should be adequate. Add the two simulated interferograms together to get the net simulated interferogram using sub_phase. Subtract this net simulated phase from the original interferometric phase using sub_phase or generate the differential interferogram directly from the resampled SLC data using SLC_diff_intf.html. Filter and unwrap the differential interferogram. Then scale the unwrapped phase using dispmap to obtain the LOS deformation rate.

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 user has the option to specify the SLC parameter file of the SLC used as the geometric reference for a stack of images. If the geometric reference of the interferometric pair is other than SLC1, it is recommended to specify the parameter file for the geometric reference SLC. This reduces errors due to shifts in timing and sampling introduced by the resampling of SLC1 and SLC2 into the geometry of the reference SLC.

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.