Seamless Synthetic Aperture Radar Archive API
Search and Download Data from Multiple Archives
The Seamless Synthetic Aperture Radar (SAR) Archive (SSARA) Application Programming Interface (API) enables users to search and download:
- Master and paired (slave) granules from the SAR archives at ASF and University NAVSTAR Consortium (UNAVCO)/Western North America InSAR (WInSAR);
- Corresponding Digital Elevation Models (DEMs) from Open Topography and tropospheric data from Jet Propulsion Lab (JPL); and
- Sample standardized InSAR data products from archives at ASF and WInSAR/UNAVCO.
SSARA API Services
SSARA API Keywords
Vertex Search and Download
ASF provides users the ability to search, evaluate, and download InSAR pairs via the Baseline Tool in the Vertex interface.
Users are able to:
- Identify stacks of SAR granules suitable for interferometric processing;
- Assess the perpendicular and temporal baseline distribution of a stack by interacting with the online Baseline Tool and
- Select pairs for download
Selecting this option will show those granules that belong to an InSAR stack over the geographic region of interest.
Using the Path & Frame option when constructing a search allows a user to see results that closely match their requirements. The Path & Frame option requires that you already know which path corresponds to your area of interest.
Searching by Path allows the user to dramatically narrow their search, and speed up search results. Useful when you know the path(s) associated with your AOI.
Helpful info regarding repeat passes:
Platform | Number of orbits before the satellite revisits the same area | Number of days before the satellite revisits the same area
- ALOS: 671 (46 days)
- R1: 343 (24 days)
- E1: 501 (35 days)
- E2: 501 (35 days)
- Sentinel-1A: 175 (12 days) (6 days as S-1 constellation)
- Sentinel-1B: 175 (12 days) (6 days as S-1 constellation)
An InSAR pair consists of two granules, a Master image, and a Paired image, that can form an interferogram.
An InSAR stack is composed of all granules that cover the same geographic region, are from the same platform (see exceptions in the next question), and were acquired with the same beam mode. Theoretically, any two granules in an InSAR stack may be used to create an interferogram, as long as the baseline is not beyond a certain critical length.
In principle, data from all satellites in the ASF archive should be suitable for InSAR, as long as the image pair adheres to some very basic rules: the data needs to be acquired by the same satellite, in the same beam mode, and with the same look direction.
There are a few exceptions to this rule. Sentinel-1A and Sentinel-1B can be used interchangeably. The tandem mode of the ERS-1 and ERS-2 satellites provide data very suitable for InSAR applications because both satellites meet the criteria above and have a favorable temporal baseline (data acquired one day apart). All the data in the archive, except RAMP data, are right looking. This means that RAMP data cannot be combined with any other Radarsat-1 data, even if those had been acquired with the same beam mode.
ScanSAR data, available for R1 or ALOS PALSAR, are another special case, as they are formed by combining several beam modes in one data set. This requires special processing techniques and is at this stage considered research-level processing.
- Fine Beam (FN1-5)
- Standard Beam (ST1-7)
- Wide Beam
- Extended High Beam
- Extended Low Beam
- ALOS PALSAR
- Fine Beam Single Polarimetric (FBS)
- Fine Beam Dual Polarimetric (FBD)
- Fine Beam Polarimetric
- Sentinel-1A/Sentinel-1B (IW)
Baseline length is the temporal and spatial distance between two satellite observations:
- Perpendicular Baseline (B┴) is the spatial distance between the first and second observations measured perpendicular to the look direction. It gives an indication of the sensitivity to topographic height, the amount of decorrelation due to phase gradients, and the effectiveness of the phase unwrapping. The longer B┴, the weaker the coherency and the lower the sensitivity to height changes (Hanssen, R. Radar Interferometry: Data Interpretation and Error Analysis. Kluwer Academic Publishers; 2001. 308 p.)
- Temporal Baseline is the time difference between the first and second acquisition. To minimize decorrelation, the temporal baseline should be as short as possible. However, this may not be the case for studies of continuous deformation such as earthquake or volcano monitoring.
- Critical baseline is the maximum horizontal separation between two satellite orbits at which the Interferometric correlation becomes zero. It is a function of wavelength (λ), incidence angle (θinc), topographic slope (ζ), range bandwidth (BR), and range (R):
Standard interferometric processing techniques can only be applied when the baseline of the interferometric pair is well below the critical baseline. Advanced interferometric processing techniques, such as persistent scatterer analysis, are able to overcome this limitation.
The Baseline Tool is an interactive tool that allows users to visualize the perpendicular and temporal baseline distribution of all granules within a given InSAR stack. To access this tool select “Baseline” button in the search results page of Vertex.
- Critical Baseline: Shown as a gray box on the plot. Represents the maximum baseline viable for interferometry.
- Master Granule: The default Master granule is the interferable granule corresponding to the granule you have selected to analyze. It is highlighted by a black dot on the baseline plot and by the blue radio button in the baseline table (RAW, GRD, L1, L1.5 products may display in the Analyze box, but will not display in the table or plot).
- Paired Granules: Granules represented by gray dots.
- Selected Granules: Granules that have been selected for download appear as blue dots in the plot, and have a check mark in the download column of the table.
- Baseline Filters
- Acquisition Date: Click on the blue box and use sliders to filter by date.
- Perpendicular Baseline: Click on the blue box and use sliders to filter by baseline.
- Temporal Baseline: Click on the blue box and use sliders to filter by time.
- Granule Information
- Queue: Click to add to queue for download by Download script.
- Set As Master: Click gray dot on plot, then click Set As Master to change master granule.
- Download: Click to download granule in the Granule Information box.
- Baseline Table
- Download Select: Check the box to queue product for download via the download script.
- Master Select: Click the radio button to set the master.
- Export CSV: Exports the baseline table to CSV.
- Download Script: Download a script which will download products checked in the download column.
- One-Click Download: Click the blue down-arrow to immediately download the product.
- Column Sorting: Click at top of column to sort.