Sentinel-1A GRD product

Microwave Toolbox
#23

Hi,

I can confirm that the conversion from imaging slant range coordinates to ground range coordinates, a slant to ground projection is performed onto an ellipsoid (the WGS84 ellipsoid) corrected using terrain height, which varies in azimuth and is constant in range.

You mention that you want to reproject - to what projection do you want to use and why?

Peter

#24

Hi,

thank you for your response. I work in the Arctic area, especially in
the arctic ocean. So I have to use for example a polar map projection.
Normally I reproject to EPSG 3413 (stereographic). I canā€™t use the
conventional lat/long or UTM visualization because in this very high
latitudes the distortion is very big.
But I donā€™t understand what the ellipsoid correction of the S1TBX is
doing with the GRD data. After the reprojection I have changed lat/long
coordinates than before the correction. So I guess the ellipsoid
correction tool is shifting the image, although there are no terrain or
DEM values in the ocean.

Best regards

Felix

1 Like
#25

It uses the average scene height and also applies the Earth
Gravitational Model. With this and the orbits, it then applies the same
Range Doppler algorithm as in the terrain correction.

1 Like
#26

Hi,
I wonder how to reproject from GRN to UTM too.
I used ā€œSAR-Simulation_Terrain_Correctionā€, the ocean will be mask out,
Without the SAR-Simulation_Terrain_Correction I canot perform when I perform the ā€œReprojectionā€.
How I reprojection without maskout the ocean

#27

Hi
I think you have to disable the ā€œMask out areas without elevationā€ button in the Range Doppler Terrain Correction. Itā€™s set by default.

#28

Thanks Felix_L, that is work.
How we can run this in command line?

#29

From the command line you need to set the graph parameter nodataValueAtSea to false

#30

Can you show example the command line look like? Thanks

#31

Given that SLC to GRD is projected onto an ellipsoid, how come GRD is still in a satellite type geometry? That is on the ascending passes, the image appears flipped north - south compared to a map, and on the descending passes the image appears flipped east-west.

If I apply an ellipsoid correction to GRD, I then get a familiar looking map geometry.

Sarah

#32

The projection of slant range (i.e. in the direction between the satellite and a point on the ground for SLC products) to ground range (i.e. on the surface of the earth for GRD products) means that the product is still in satellite coordinates (i.e. range and azimuth). If the SLC product were geocoded, then the orientation would be north, east etc. rather than range and azimuth.

The convention used for S1 products is that the first pixel in the product is at the earliest azimuth and earliest range times. Most images are displayed with the first pixel at the upper left of the image. So for ascending S1 products, the first pixel will be the southern most pixel and so the image will be displayed flipped north-south (but east-west is OK). For descending products the opposite is true.

If I apply an ellipsoid correction to GRD, I then get a familiar looking map geometry

This looks as if this is geocoding the product.

4 Likes
#33

Thanks Peter. Thatā€™s very helpful.

#34

Hello! Iā€™m working with IW GRDH images.

do you guys know if I need to calibrate (in sigma0 values) before or after applying the speckle filter?

Iā€™ve been told that I cannot apply the filter in a image with sigma0 values. Is that true?

Anyway, do I need to calibrate it at all?

And, at last, when I calibrate, the sigma0 band comes in linear scale, right? I wanted them in dB scale, but it is not, since there is no negative value when I choose to see the pixel info.

Thanks!

#35

Calibrate first.

Linear to/from dB conversion can be done afterwards.

#37

laismoreira1m
Ok, so the right order would be:

Calibrate with Sigma 0 values
Speckle filter
Terrain Correction
Convert from linear to dB.
At the step 3, should I mark the box of apply radiometric normalization? Considering that my data is already in sigma 0ā€¦

thanks

#38

Ohā€¦ and do I need to apply orbit file before calibrating?

thanks!

#39

when you have calibrated the data and you already have Sigma0 you donā€™t need to select the ā€œradiometric normalizationā€ in the Terrain Correction module.

Application of the orbit files is not mandatory but often improves the geometric accuracy of your product. It should be done at the very beginning.

  1. Orbit File
  2. Calibrate to Sigma0
  3. Speckle Filter
  4. Terrain Correction
  5. Convert to dB
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Oil spill application on SNAP
#40

Hello

where will you put thermal noise correction in above sequence?

Thanks

#41

I am talking about ā€œIW GRDā€ data.

#42

as early as possible Iā€™d say.

2 Likes
#43

Thanks

I have to classify different types of crops. Should i do terrain filtering and terrain correction .

For what kind of specific purpose we use these two operation?

regards