TYPES OF SAR DATA PRODUCTS
Product Types
SAR data can be made into a multitude of different product types. Some are better suited for human exploitation whereas others are better exploited by algorithms and signal-processing tools.
On this page we describe the different SAR data products that ICEYE provides. Technical details about the format of these products, with some tips on how the use them are provided in the section on 'Technical Information'.
Info
The section "What is SAR?" provides a review of the technologies mentioned here.
Complex Images
Unlike optical imagery, each pixel in a SAR image is represented as a complex number. The complex number represents two critical components of SAR imaging, amplitude and phase. The amplitude of a pixel is a measure of the amount of RADAR energy reflected back to the satellite and displayed as a grey-scale value when we look at a SAR image. The phase component is a measure of the position of the RADAR wave after it has interacted with all the scatterers within the pixel. After image formation, the phase information is usually discarded which reduces the size of the SAR image. Some advanced SAR capabilities such as interferometric SAR (INSAR) or coherent change detection (CCD) makes use of the phase information and therefore needs the complex image samples.
All ICEYE images are available as Complex Datasets (with the exception of SCAN mode collections). More details about complex image file formats can be found in the 'Technical Information' section.
Warning
Complex images, by their nature, are always single-look. This means that they contain the native resolution of the collection. ICEYE satellites always collect much finer resolution in the azimuth direction than the range direction. (eg a SPOT image has an azimuth resolution of 25cm whereas the slant range resolution is 50cm.) We also save our complex images in the slant-plane. This means that if you view the image in a picture viewing package (eg GIMP), the image will appear distorted as the square pixels on your screen are actually representing rectangular pixels on the ground. ICEYE CPX images also have a full geotif header and contain RPCs so any GIS viewer (eg QGIS) will render the image correctly on your screen. This is the best way to view the finest resolution in ICEYE SAR images.
Complex Image Parameters
| PARAMETER | STRIP | SPOT | SPOT FINE | DWELL | COMMENTS |
|---|---|---|---|---|---|
| Focusing plane | Slant Plane | Slant Plane | Slant Plane | Slant Plane | |
| Slant range resolution [m] | 0.5 to 2.5 | 0.5 | 0.25 | 0.5 | Note 1 |
| Slant azimuth resolution [m] | 3 | 0.25 | 0.1 | 0.05 | |
| Impulse response weighing function (peak side level) | Uniform (-13.3dB) | Uniform (-13.3dB) | Uniform (-13.3dB) | Uniform (-13.3dB) | |
| Slant Range Sample Spacing [m] | 0.4 to 2.4 | < 0.4 | < 0.2 | < 0.4 | Note 1 |
| Slant Azimuth Sample Spacing [m] | 1.6 | < 0.2 | < 0.09 | < 0.05 | |
| Slant range product format | HDF5 + XML | HDF5 + XML | HDF5 + XML | HDF5 + XML | |
| SLC Product Size [GB] | 3.4 to 2.9 | 0.6 to 7.2 | < 15 | < 15 | |
| Dynamic Range (bits per pixel) | 16(uint) 32(Float) | 16(uint) 32(Float) | 16(uint) 32(Float) | 16(uint) 32(Float) | Note 3 |
Info
Complex Images are used for 'interferometric SAR' such as the formation of digital elevation models (DEM), land subsidence monitoring or coherent change analysis.
Amplitude Images
These are the familiar SAR gray-scale images with amplitude-only pixels. They are “multi-looked” to reduce the grainy effect of speckle, at the cost of slightly lower resolution. Amplitude images are projected to the ground surface and can be oriented with respect to the sensor or produced on an ellipsoid-based map projection. ICEYE produces amplitude images in the natural range-azimuth sensor orientation because they offer the most flexibility in exploitation. To be consistent with conventional terminology, these sensor-oriented images are called Ground Range Detected (GRD). This term may change in the future to be something more meaningful.
Info
Amplitude images are most useful for rapid observation of a location regardless of lighting or weather conditions.
Amplitude Image Parameters
| PARAMETER | STRIP | SPOT/ SPOT EXTENDED AREA / DWELL | SPOT FINE | SCAN | COMMENTS |
|---|---|---|---|---|---|
| Nominal Ground Resolution [m] | 3 | 1 | 0.5 | 15 | Note 2 |
| Ground Range Resolution [m] | < 3 | 1.5 to 0.9 | 0.73 to 0.43 | < 15 | Note 1 |
| Ground Azimuth Resolution [m] | < 3 | < 1 | < 0.5 | < 15 | |
| Impulse response weighing function (peak side level) | Taylor Weighting (-20dB) | Taylor Weighting (-20dB) | Taylor Weighting (-20dB) | Taylor Weighting (-20dB) | |
| Ground Range Sample Spacing [m] | 2.5 | 0.5 | 0.25 | 6 | |
| Ground Azimuth Sample Spacing [m] | 2.5 | 0.5 | 0.25 | 6 | |
| Range Looks | 1 | 1 | 1 | 1 | |
| Azimuth Looks | 1 to 2 | 1 to 20 | 5 | 1 | Note 3 |
| Product format | Geotiff + XML | Geotiff + XML | Geotiff + XML | Geotiff + XML | |
| GRD Product Size [MB] | 700 | 250 to 1500 | < 2500 | 800 | |
| Dynamic Range (bits per pixel) | 16(uint) 32(Float) | 16(uint) 32(Float) | 16(uint) 32(Float) | 16(uint) 32(Float) | Note 4 |
Notes and Explanations
- Slant Range Resolution: For Strip mode the transmitted bandwidth is varied to make sure that the resolution on the ground remains the same. For Spot mode the maximum bandwidth is transmitted at all times. This means that the slant resolution for Spot images is constant and the ground resolution changes with incidence angle.
- Nominal Ground Resolution: Nominal ground resolution for all Spotlight modes (Spot, Spot Entended Area, Dwell and Spot fine) is the ground resolution at a 30° incidence angle.
- Azimuth Looks: Spot amplitude images are produced using 4 looks, Spot Extended Area 1 to 2 looks, and Dwell 20 looks.
- Complex Dynamic Range: A complex number with 16bit I and 16bit Q values. 32 bit float values can be provided by request.
References
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European Space Agency. The sentinel application platform - snap. Accessed 2020 Dec 17. URL: http://step.esa.int/main/toolboxes/snap/. ↩