Gamma DIFF&GEO: Reference Manual

S1_coreg_TOPS

NAME
S1_coreg_TOPS - Script to co-register a Sentinel-1 TOPS mode burst SLC to a reference burst SLC by iteratively applying intensity matching and "spectral diversity" methods.

SYNOPSIS
S1_coreg_TOPS <SLC1_tab> <SLC1_ID> <SLC2_tab> <SLC2_ID> <RSLC2_tab> [hgt] [RLK] [AZLK] [poly1] [poly2] [cc_thresh] [fraction_thresh] [ph_stdev_thresh] [cleaning] flag1 [RSLC3_tab]

<SLC1_tab>	(input) SLC_tab of S1 TOPS burst SLC reference (e.g. 20141015.SLC_tab)
<SLC1_ID>	(input) ID for reference files (e.g. 20141015)
<SLC2_tab>	(input) SLC_tab of S1 TOPS burst SLC slave (e.g. 20141027.SLC_tab)
<SLC2_ID>	(input) ID for slave files (e.g. 20141027)
<RSLC2_tab>	(input) SLC_tab of co-registered S1 TOPS burst SLC slave (e.g. 20141027.RSLC_tab)
[hgt]	(input) height map in RDC of MLI-1 mosaic (float, or constant height value; default=0.1)
RLK	number of range looks in the output MLI image (default=10)
AZLK	number of azimuth looks in the output MLI image (default=2)
poly1	polygon file indicating area used for matching (relative to MLI reference to reduce area used for matching)
[cc_thresh]	coherence threshold used (default = 0.8)
[fraction_thresh]	minimum valid fraction of unwrapped phase values used (default = 0.01)
[ph_stdev_thresh]	phase standard deviation threshold (default = 0.8 radian)
[cleaning]	flag to indicate if intermediate files are deleted (default=1 --> deleted, 0: not deleted)
flag1	flag to indicate if existing intermediate files are used (default = 0 --> not used, 1: used)
[RSLC3_tab]	(input) 3 column list of already available co-registered TOPS slave image to use for overlap interferograms

EXAMPLE
S1_coreg_TOPS 20141003.SLC_tab 20141003 20141015.SLC_tab 20141015.SLC_tab 20141015.RSLC_tab 20141003.hgt 10 2 20141003.mask.poly1 20141003.mask.poly2 0.7 0.02 0.8 1

Conducts a co-registration procedure for the S1 burst SLC slave (20141015.SLC_tab) to the S1 burst SLC reference geometry (20141003.SLC_tab). The output co-registered burst SLC slave is available as burst SLC (20141015.RSLC_tab) and as SLC mosaic (20141015.rslc, 20141015.rslc.par). Furthermore, quality information on the iterative co-registration steps (in 20141003_20141015.coreg_quality) and the differential interferogram (20141003_20141015.diff) are generated - the later also as quality check (with possible phase jumps at burst and sub-swath interfaces in the case of inaccurate co-registration or as a result of along-track motion or ionospheric effects).

S1_coreg_TOPS 20141003.SLC_tab 20141003 20141015.SLC_tab 20141015.SLC_tab 20141015.RSLC_tab 20141003.hgt 10 2 - - 0.7 0.02 0.8 1

Command example without providing polygon files to limit the area where the intensity matching (poly1 area) and spectral diversity (poly2 area) methods are applied.

DESCRIPTION
S1_coreg_TOPS supports running an S1 IWS co-registration and differential interferogram generation procedure that includes an iterative co-registration refinement estimated using intensity matching followed by an iterative co-registration refinement estimated using the spectral diversity method for the burst overlap areas as supported by S1_poly_overlap.

The main steps of the of S1 TOPS SLC co-registration procedure correspond to the procedure described in Section 6.1 of the Sentinel-1 Users Guide, including the calculation of a co-registration lookup table based on the orbit geometry and terrain height and refinements of this lookup table using the intensity matching and the spectral diversity methods. The script S1_coreg_TOPS automates the entire procedure up the the differential interferogram. In this it iterates the matching refinement until the azimuth correction determined is below a defined threshold (0.01 SLC pixel). After reaching this quality it iterates the spectral diversity method until the azimuth correction determined is below another defined threshold (0.005 SLC pixel).

As one important output an ASCII file containing quality information is generated. The name of this quality file is automatically generated from the reference and slave scene IDs using the style "ID1_ID2.coreg_quality". This file provides detailed information on the (iterative) co-registration refinements done, first using intensity matching and then using the spectral diversity method. For the intensity matching statistics of the matching are indicated. For the spectral diversity method information on the coherence and phase statistics are provided for each bursdt overlap area considered. Furthermore, the weighting factors used in the calculation of the global phase offset are indicated.

In such a co-registration process the slave IWS SLC is resampled several times to the master geometry. This resampling can be significantly accelerated if it is done only for a small section of the entire scene. To limit the area (resampled and) used for the intensity matching refinement iterations a polygon area (20150308.mask.poly1) can be indicated. In the case the LAT programs are available only the data of this area is then resampled for the intensity matching refinement iterations. Similarly, the program S1_poly_overlap is used (within S1_coreg_TOPS) to only resample the burst overlap areas when doing the co-registration refinement iterations using the spectral diversity method.

Furthermore, a second polygon area (20150308.mask.poly2) can be indicated to restrict the use of the spectral diversity method to suited areas. Here the focus is not on the accelaration of the resampling, but on avoiding low coherence and instable (moving) areas. The second polygon area can be used to exclude incoherence areas (e.g. ocean) as well as areas with significant movement in azimuth direction (e.g. glaciers) from the spectral diversity method. This is recommended as for areas with significant movement in azimuth direction phase offsets not related to the co-registration are expected.

Polygon areas can be defined using the LAT program polyras. Without LAT programs S1_coreg_TOPS ignores the indicated polygons and does the resampling for the entire area. While this ensures that the program still runs users without access to the LAT module cannot benefit from the related significant computational efficiency gain possible.

Once the final co-registration function (consisting of the lookup table that was updated with the intensity matching refinements, and the azimuth offset in the ISP offset parameter file "ID1_ID2.off" determined by iteratively using the spectral diversity method) the slave burst SLC is resampled to the reference burst SLC geometry. Furthermore, the mosaic SLC is calculated as well as the differential interferogram. To calculate the differential interferogram phase_sim_orb is used to calculate the topopgraphic phase and SLC_diff_intf to calculate the multi-look differntial interferogram.

The main reason that the differential interferogram ("ID1_ID2.diff") is calculated is that it should always be carefully checked by an operator for phase jumps at the burst interfaces. Such phase jumps are a clear indication for either a remaining co-registration error in the azimuth direction, or the presence of along-track motion that causes the phase jump, or the presence of ionospheric effects (along-track gradients in the ionospheric free electron concentration) that can also cause positional offsets in the azimuth direction which may then cause phase jumps.

As input a pair (master and slave) of two Sentinel-1 (S1) Interferometric Wide Swath (IWS) burst SLCs in their original geometry are provided. These burst SLCs consist of the sub-swath burst SLCs for one or several sub-swaths as defined in the SLC_tabs provided for the reference (e.g. 20141003.SLC_tab) and the slave (20141015.SLC_tab). For each sub-swath the SLC_tab contains one line with the burst SLC name (e.g 20141015.IW1.slc), the name of the SLC parameter file of the burst SLC (e.g 20141015.IW1.slc.par), and the related TOPS parameter file (e.g. 20141015.IW1.slc.TOPS_par). In this IW1 is used to indicate that it is for the first sub-swath (IW1).

The indicated ID parameters (e.g. 20141003 and 20141015) are used in the naming of the intermediate files automatically generated by the program.

Then an SLC_tab for the output slave scene that was resampled to the geometry of the reference burst SLC needs to be indicated. For an input slave SLC_tab 20141015.SLC_tab a possible output SLC_tab could be 20141015.RSLC_tab. For each sub-swath this output SLC_tab contains one line with the burst SLC name (e.g 20141015.IW1.rslc), the name of the SLC parameter file of the burst SLC (e.g 20141015.IW1.rslc.par), and the related TOPS parameter file (e.g. 20141015.IW1.rslc.TOPS_par). These are all new filename of not yet existing files to be used as names for the generated resampled (co-registered) data sets. The output SLC_tab can be created using manual editing or using commands as
echo "20141015.IW1.rslc 20141015.IW1.rslc.par 20141015.IW1.rslc.TOPS_par" > 20141015.RSLC_tab
echo "20141015.IW2.rslc 20141015.IW2.rslc.par 20141015.IW2.rslc.TOPS_par" >> 20141015.RSLC_tab
echo "20141015.IW3.rslc 20141015.IW3.rslc.par 20141015.IW3.rslc.TOPS_par" >> 20141015.RSLC_tab
.

The height map indicated has to be in the same slant range geometry as the MLI mosaic image of the reference scene (generated using the same number of range and azimuth looks as indicated on the command line. Instead of indicating an existing height file it is possible to indicate a constant height value (e.g. 0.1 meter) to be used as a constant height reference. As a consequence the initial transormation lookup table calculated based on the orbit geometry and the DEM will be of slightly lower quality. The level of degradation of the quality also depends significantly on the perpendicular baseline between the master and slave scenes. For longer baselines (> 100m) and in high mountainous areas using DEM heigth is strongly recommended.

To apply the refined azimuth offset refinement you need to rerun the slave burst SLC resampling, but using the output offset parameter file instead of the input offset parameter file (when running the the program SLC_interp_S1_TOPS).

In the second part of the iterative co-registration refinement

S1_coreg_TOPS runs the program S1_coreg_overlap to apply a spectral diversity method to all the available burst overlap regions of all the available sub-swaths (expect those areas masked by the indicated polygon poly2). On the command line of S1_coreg_TOPS it is possible to indicate relevant parameters used by S1_coreg_overlap such as the coherence threshold used, the minimum valid fraction of unwrapped phase values used, and the phase standard deviation threshold used.

The determined azimuth offset correction from S1_coreg_overlap is used to update the constant value of the azimuth offset polynomial in the offset parameter file.

S1_coreg_TOPS generates a large amount of intermediate files, including interferograms, coherence maps, coherence masks, unwrapped phases, etc. for every available burst overlap regions. Normally, these many intermediate files are directly deleted at the end of the program without looking at it in detail. In cases where the co-registration refinement result does not converge or where the quality number give indications of problems it is recommended not to deleted the intermediate files and check them carefully to be able to optimize the tresholds used. Deleting or not deleting the intermediate files is done by setting the cleaning flag to 1 (which is the default and which means the files are deleted) or to 0 to keep the intermediate files.

OPTIONS

Using S1_coreg_TOPS to do the co-registration for a slave acquired with a long temporal separation from the master scene may result in generally reduced coherence. As a way to optimize the coherence it is possible to indicate in addition to the master burst SLC a second already co-registered SLC (possibly one close in time to the slave considered). The program uses then this second scene to calculate the double-difference interferograms, while the master geometry is the one of the true master.

There is the possibility to set the flag1 parameter to 1 to use already existing files (e.g. the MLI mosaic of the reference scene or the looklup table calculated using rdc_trans. This option may be suited to gain time when testing..