ANSI-C program: base_ls.c
NAME
base_ls - Least-squares
estimation of interferometric baseline based on the Ground
Control Point (GCP) data.
SYNOPSIS
base_ls <SLC_par> <OFF_par> <gcp_ph>
<baseline> [ph_flag] [bc_flag] [bn_flag] [bcdot_flag]
[bndot_flag] [bperp_min] [SLC2R_par]
| <SLC_par> | ISP parameter file of the reference SLC |
| <OFF_par> | ISP offset/interferogram parameter file |
| <gcp_ph> | ground control point data + extracted unwrapped phase values (text format) |
| <baseline> | baseline parameter file |
| [ph_flag] | restore range phase ramp (default=0: do not restore 1: restore) |
| [bc_flag] | cross-track baseline component estimate (0:orbit derived 1:estimate from data, default=1) |
| [bn_flag] | normal baseline component estimate (0:orbit derived 1:estimate from data, default=1) |
| [bcdot_flag] | cross-track baseline rate estimate (0:orbit derived 1:estimate from data, default=1) |
| [bndot_flag] | normal baseline component estimate (0:orbit derived 1:estimate from data, default=1) |
| [bperp_min] | minimum perpendicular baseline required for L.S. estimation (m, default= 10.) |
| [SLC2R_par] | (input) parameter file of resampled SLC, required if SLC-2 frequency differs from SLC-1 |
EXAMPLE
base_ls 1352.slc.par 1352_1610.off 1352_1610.gcp_data
1352_1610.base 1 1 1 1 0
DESCRIPTION
base_ls computes a precise estimate of the
interferometric baseline using ground control points of known x
and y image coordinates, the corresponding terrain height, and
the corresponding unwrapped interferometric phase. A non-linear
least-squares solution algorithm is used to determine the
baseline parameter values. The baseline model uses Track,
Cross-Track, and Normal (TCN) coordinates. At each point along
the SAR track of the reference image the TCN basis is
recalculated.
The computed baseline (in TCN coordinates) is written to the baseline file and is used to calculate heights and for differential interferometry. Statistics for quality control of the least-squares fit are computed and displayed. It is possible to estimate the baseline from either the original or flattened unwrapped interferometric phases. The command line parameter ph_flag specifies if the unwrapped phase reflects the original interferogram or has been flattened by removal of the phase trend using a smooth spherical earth model. If the unwrapped phase is from a differential interferogram created by subtraction of a phase model generated using the DIFF/GEO program phase_sim, then the simulated phase must be added back to the unwrapped phase to give the unflattened, uwrapped interferometric phase. Adding back the phase can be done using the DIFF/GEO program sub_phase.
For very short baselines the solution tends to get instable because of the negligible influence of the topographic height on the phase. To avoid problems a minimum perpendicular baseline (e.g. 20m) can be set. For shorter baselines the precision baseline is set to the initial baseline estimate. If the baseline is very small, the program base_init can be used on the complex valued interferogram to estimate the perpendicular baseline component given the orbital fringes. The parallel baseline component can be esitmated from the offset measurments.
The gcp_ph data file is created using the gcp_phase. It contains the height and unwrapped phase information. The heights can be extracted from a DEM in SAR coordinates matching the unwrapped phase interferogram using the DIFF/GEO program extract_gcp.
The user can specify the baseline components that base_ls will estimate by setting the appropriate flags on the command line. bc_flag and bn_flag refer to the cross-track and normal baseline components while bcdot_flag and bndot_flag are the respective rates of change in units of m/sec. If the data are very noisy, better results may be obtained by reducing the number of estimated parameters. Generally at least one bn_flag and bn_dot should be selected. Adding the other components should reduce the residual errors. If adding or removing a component causes only a very small change in the error, that baseline component is redundent.
If the two SLCs used to form the interferogram are from
sensors with different carrier frequencies (e.g. ERS and ENVISAT)
then the SLC parameter file of the second scene must be specified
on the command line.
SEE ALSO
typedef_ISP.h, gcp_phase, base_orbit, base_init