Interferometry Module for Digital Elevation Model Generation

Transcription

1 Interferometry Module for Digital Elevation Model Generation In order to fully exploit processes of the Interferometry Module for Digital Elevation Model generation, the European Space Agency (ESA) has distributed these sample data sets; any other use of this material is prohibited. Data Set Description the data set is separated into two parts: 1. Input Data (InSAR_Input.zip) is used to run this exercise, it includes the following binary data and corresponding header files: a. 7_11_02_slc Master SLC data b. 16_01_03_slc Slave SLC data c. start_dem Reference Digital Elevation Model 2. Output Data (InSAR_Output.zip) includes intermediate products, and also contains a possible selection of Ground Control Points (e.g.orb_ref file) to be used in the Orbital Refinement step. Input and Output data can be downloaded from the sarmap server by using the following ftp account information. Host Name: ftp.sarmap.ch User Name: sarscape_datasets Password: zxcvbnm More information regarding the data is available in the Data Format document, reading this document prior to starting the exercise is suggested. The data set consists of: An ENVISAT ASAR Single Look Complex data pair acquired in IS4 mode (SAR incidence angle around 34 ) and VV polarization over the Las Vegas, NV area. The satellite acquisition dates are November 7, 2002 and January 16, The images provided for this exercise have already imported and resized and the area coverage is approximately 14x14km. A GTOPO30 extract, over sampled to a 15 meter grid size, which is used as reference low resolution Digital Elevation Model, this was generated using the Digital Elevation Model Extraction tool. pg. 1 of 5

2 The goals of this exercise are: - To estimate the orbital related parameters. - To generate an interferogram. - To flatten the interferogram, separating the differential phase, by using the reference Digital Elevation Mode. - To filter the flattened interferogram and to generate the coherence images. - To unwrap the differential phase. - To refine the orbital parameters and correct the phase offset. - To re-flatten the flattened and filtered interferogram using corrected orbital parameters (optional). - To generate the InSAR Digital Elevation Model. The exercise can be completed in two ways: 1. Using the input master/slave data already imported with the verified SORIS satellite orbits. 2. Using the input master/slave data already imported with the standard satellite orbits, which are stored in the ENVISAT ASAR original product. Refer to the Basic Module>Data Import>Standard Formats for more detailed information about standard products and DORIS orbits. The following exercises will make it easier to understand the processing functionalities as well as the difference introduced by using precise orbits in the interferometric processing chain. 1. Without DORIS orbits entering the relevant files stored in the No_DORIS folder. 2. With DORIS orbits entering the relevant files stored in the DORIS folder. Processing Steps: Baseline Estimation Allows the user to obtain information about the baseline values and other orbital parameters of the input SAR pair, this is not a mandatory function. Interferogram Generation Master intensity and Salve intensity images are generated from the two Single Look Complex input data, co-registered, and multi-looked interferogram. For this data set, multi-looking factors of 1 (range) and 4 (azimuth) will created a quasi-square pixel (around 15m). The products will be automatically named by adding the relevant suffix to the output file names. pg. 2 of 5

3 If the Digital Elevation Mode is use as the input (optional), the flattened interferogram (_dint), the slant range DEM (_srdem), and the synthetic interferogram (_sint) will be generated among the output products. In this case, the Interferogram Flatting step has not occurred and the Adaptive Filter and Coherence Generation step will follow. Digital Elevation Model/Elliposoid Interferogram Flattening The previously generated interferogram is split into the synthetic phase and the residual (differential) phase. The input Digital Elevation Model is re-projected onto the master SAR geometry and it is generated as a slant range output product. Comparing the results from the first processing run (without DORIS orbits) and the results from the second processing run (with DORIS orbits); will clearly indicate the improvements in terms of orbital accuracy. The regularly distributed low frequency interferometric fringes, which are visible from the lower left corner to the upper right corner of the differential phase file, will disappear in the second processing run. The fringes are due to small orbital inaccuracies, which do not exist in DORIS data. The output products are automatically named by adding the relevant suffix to the root name. Adaptive Filter and Coherence Generation The flattened interferogram (differential phase) is filtered, using the boxcar method, and the scene coherence is generated. Coherence data values can vary between 0 and 1 in proportion to the temporal correlation between master and slave SAR acquisitions. Output products are automatically named by adding the relevant suffix to the root name. Phase Unwrapping The differential phase is unwrapped using a region growing algorithm. A coherence threshold is used to avoid (or minimize) unwrapping errors. Low threshold values (between 0.15 and 0.2) are suggested in order to limit interruptions during the growing process; in this case 0.18 was used. The unwrapping phase is automatically named by adding the suffix _upha to the input file name. Orbital Refinement Corrections are made to orbital parameters, note that this process is required when precise orbits data (DORIS data) are available to correct the phase offset. New files are not generated, but the orbit correction parameters are written in the header file of the input unwrapped phase (_upha.sml>interferometric_processing). Making a copy of the header file before executing this step will enable the old and new header files to be compared upon process completion. pg. 3 of 5

4 A Ground Control Points file must be previously generated; also the Ground Control Points need to be well distributed throughout the entire scene in order to avoid areas of residual topographic fringes. The figure below is an example of Ground Control Points (white dots) distribution and location. Interferogram Re-Flattening This step can be executed in order to visually assess the quality of the Orbital Refinement and is not required for Digital Elevation Mode generation. Output products are automatically named by adding the relevant suffix to the root name. Phase to Height The unwrapped phase is re-combined with the synthetic phase and it is converted to height and geocoded into the specified map projection. If the Interferogram Re- Flattening step has been performed, the input to the Phase to Height must be the original edited unwrapped phase (not re-flattened). With respect to the cartographic reference system the following parameters should be set: pg. 4 of 5

5 Country: UTM-GLOBAL Hemisphere: NORTH Projection: UTM Zone: 11 Referenced height: 0.0 Output grid size: 15m Output products are named by adding the relevant suffix to the root name. Notes: Look Up Table related problems can be experienced when displaying the data in the ENVI view. The problem is solved by properly stretching the image histogram using the relevant function from the Enhance pull down menu of the ENVI image window. Tiff files are intended only for visualization, the original histogram is stretched to 8 bit by means of a pre-defined scaling formula (see Tools>GenerateTiff ). The interferograms are shown in grey scale; in order to visualize the interferometric fringes in RGB the corresponding tiff file must be created. The.sml and.hdr are the product header files, the.sml contains all the processing related information and the.hdr contains the information for product compatibility with ENVI. It does not matter which of the input data are selected as master and which is slave, it is mandatory that the initial order is maintained throughout the entire processing chain. pg. 5 of 5