CE 778: Microwave Remote Sensing
Tutorial Exercise 3
Submission: 06/02/2025
Email: ce778.lab@gmail.com
SNAP - (Sentinel Application Platform) - is a tool ideal for Earth Observation Processing (EOP)
and analysis. It provides processing tools for all the three sentinel toolboxes (i.e sentinel 1,2,3).
The layer management system of the SNAP allows ESRI supported shapefiles to be added and
overlaid along with computing statistics and other important plots. Band arithmetic operations
could also be performed with easy bitmask definition and overlay. Operations like projection,
ortho-rectification and geo-coding are other important features of SNAP.
(1). To download SNAP go to >> http://http://step.esa.int/main/download/ then in All Toolboxes
menu select the correct OS framework (i.e windows 64 bit or windows 32 bit) and click
download.
Sentinel Image Details
Prof. J. INDU, Department of Civil Engineering, IIT Bombay
1. Mission Identifier: - S1A
2. Mode/Beam: - IW
3. Product Type: - SLC
4. 1: - Processing Level
5. S: - Product Class (Standard)
6. DV: - Dual VV
7. 20240125T010329: - Start Date/Time (Year, month, day, time in hour, minutes, seconds)
8. 20240125T010356: - Start Date/Time (Year, month, day, time in hour, minutes, seconds)
9. 052256: - Absolute Orbit Number
10. 06514B: - Mission Data take ID
11. 9B04: - Product unique ID
SAFE: - Product file extension.
In this Tutorial, we will refine the SAR image by applying a series of corrections to
enhance its accuracy and usability. First, we will apply the precise orbit file to update the
satellite's position and velocity information, improving the accuracy of the orbit state
vectors in the metadata. Next, we will perform radiometric calibration to ensure that pixel
values accurately represent the radar backscatter, making the image suitable for
quantitative analysis. Following this, we will apply radiometric terrain flattening to
eliminate radiometric biases caused by topographic variations, which can otherwise affect
classification accuracy. Finally, we will conduct geometric terrain correction using the
Range Doppler Terrain Correction method to correct distortions due to the sensor's tilt and
topographical variations, ensuring that the image aligns with real-world coordinates. The
final output will be projected using the UTM/WGS 84 coordinate system for consistency
in spatial representation.
Prof. J. INDU, Department of Civil Engineering, IIT Bombay
1. Go to File>>Open Product and Select image and click on Open.
2. Right click on product and click on RGB Image Window
3. Choose Red, Green and Blue Bands
Prof. J. INDU, Department of Civil Engineering, IIT Bombay
4. Go to Radar>>Apply Orbit File
The orbit state vectors provided in the metadata of a SAR product are generally not
accurate and can be refined with the precise orbit files which are available days-toweeks
after the generation of the product.
The orbit file provides accurate satellite position and velocity information. Based on this
information, the orbit state vectors in the abstract metadata of the product are updated.
• Click on run and by default it will be saved as image_orb filename
Prof. J. INDU, Department of Civil Engineering, IIT Bombay
5. Go to Radar>>Radiometric>>Calibrate And choose source product as your orbit
product.
The objective of SAR calibration is to provide imagery in which the pixel values can be
directly related to the radar backscatter of the scene. Though uncalibrated SAR imagery
is sufficient for qualitative use, calibrated SAR images are essential to quantitative use
of SAR data.
Typical SAR data processing, which produces level 1 images, does not include
radiometric corrections and significant radiometric bias remains. Therefore, it is
necessary to apply the radiometric correction to SAR images so that the pixel values of
the SAR images truly represent the radar backscatter of the reflecting surface. The
radiometric correction is also necessary for the comparison of SAR images acquired
with different sensors, or acquired from the same sensor but at different times, in
different modes, or processed by different processors.
• Click on run and by default it will be saved as image_orb_cal filename
In processing Parameters uncheck sigma0 and check beta0 as shown in figure below
Prof. J. INDU, Department of Civil Engineering, IIT Bombay
6. Go to Radar>>Radiometric>>Radiometric Terrain Flattening
And choose source product as your calibration product.
When land cover classification is applied to terrain that is not flat, inaccurate
classification result is produced. This is because that terrain variations affect not only
the position of a target on the Earth's surface, but also the brightness of the radar return.
Without treatment, the radiometric biases caused by terrain variations are introduced
into the coherency and covariance matrices. It is often seen that the classification result
mimic the radiometry rather than the actual land cover. This operator removes the
radiometric variability associated with topography using the Radiometric Terrain
Correction.
Prof. J. INDU, Department of Civil Engineering, IIT Bombay
Choose Digital elevation model and sampling method and click on run.
7. Go to Radar >> Geometric>>Terrain Correction >> Range Dopler Terrain Correction
And choose source product as your TF (Terrain Flattening) product.
Prof. J. INDU, Department of Civil Engineering, IIT Bombay
Due to topographical variations of a scene and the tilt of the satellite sensor, distances
can be distorted in the SAR images. Image data not directly at the sensor’s Nadir
location will have some distortion. Terrain corrections are intended to compensate for
these distortions so that the geometric representation of the image will be as close as
possible to the real world.
Go to processing parameters and in that select projection as UTM/WGS 84 as shown in
figure below.
Assignment
1. Download SLC image of your hometown of 2024. Perform all the corrections (Attach
screenshot of each Step).
Prof. J. INDU, Department of Civil Engineering, IIT Bombay
