# Detection of corner reflectors

Dear all,

We have set a corner reflector in a potential landslide area few days ago.
Because it need 12 images at last for PS analysis, I want to check the location and geometry of the corner reflector is correct or not first.
From a previous study (Froese et al., 2008), the paper shows a clearly location of the corner reflectors (white spots) in relation to the surroundings as following:

I have tried to used the intensity band of sentinel-1 SLC image to detect the location of the corner reflector. By comparing the intensity images before and after the setting of corner reflector, I couldnât get any information.

Could anyone give me some hints for how to check the set, geometry and location of the corner reflector is correct or not?

Thank you very much!

Hi Sharon,

You will find some information here - Sentinel incidence angle and pass - or if you provide the lat/long of your CR, its type (e.g. trihedral) and size and date of your image, I can provide the expected orientation of the CR and how bright it should be in your image.

Hi Peter,

Iâm trying to understand the discussions you provided.

And the information of the CR we set is as followed:
lat/lon: WGS84(24.98239315, 121.69172164)
type: trihedral
Size: 1m1m1m
date of image: 2020/02/23

Thank you very much!

Hi Peter,

I have checked the information in Heaven Above.

The altitude is only 12 degrees, which is different from the incident angle that I get based on a sentinel image product by SNAP. I used the product on 30-10-2017 with Range Doppler Terrain Correction on SNAP, and found the incidence angle for every point of the image in âProduct Explorerâ, âTie-Point Gridsâ, âincident_angleâ. The âincidenceAngleFromEllipsoidâ is about 43 degree and the âlocalIncidenceAngleâ is about 38 degree.

Please tell me where is the mistake? Thank you very much!

Hi Sharon,

I see from the S1-B planning (https://sentinel.esa.int/web/sentinel/missions/sentinel-1/observation-scenario/acquisition-segments) there was an acquisition over your site on 23rd Feb at 10:00:52 UT as shown below. Hopefully your location is that shown in the Google Earth screenshot and that this is the correct time for your data.

The data from Heavens-Above for this acquisition and your location near Taipei is:

This shows that the azimuth angle of your CR should be 260 deg (i.e. almost pointing west). The elevation angle for the boresight of your CR should be 47 deg. Normally the elevation of a CR is set using the base plate of the CR as illustrated below. The black lines are the base and back of the CR (as viewed from the side). The green line is the horizontal (or ground) while the red line is the CR boresight. Angle A is the angle from Heavens-Above (47 deg), B is the angle from boresight to the base plate of the CR (35.3 deg - not 45 deg) and C is the angle you need to set the CR elevation (47 - 35 = 12 deg). So you need to set the elevation of the base plate of your CR to 12 deg.

Please let me know if the above is understandable!

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Hi Peter,

Thank you for the detailed explanation!
The CR elevation I set was 2 deg (because I subtract 45 deg, and I know why is 35 deg now).
So, I think the acquisition on 23rd Feb couldnât use for detecting the CR location. We will reset the CR elevation and then check the intensity_VV band of GRDH product derived on 29rd Feb. Should I do any processing before check the location of the CR from the original GRDH product? Such as Thermal Noise Removal and Calibration step?

About the search results on Heavens-Above, I have a doubt. As I have set the observing location, why there are over one pass for the same date? What is the different? I thought it would have an interval of 12 days between each pass?

Thank you very much!

Hi Sharon,

We will reset the CR elevation and then check the intensity_VV band of GRDH product derived on 29rd Feb. Should I do any processing before check the location of the CR from the original GRDH product? Such as Thermal Noise Removal and Calibration step?

I see that there is a S1-A IW acquisition on the 29th over your site. Just to check if the CR is visible there is no need to do any processing. Due to the small size of your CR you might also want to check the SLC product (by looking at the intensity image) and again without any processing.

About the search results on Heavens-Above, I have a doubt.

HA is used to show when a satellite is above the horizon from the observer location. So for your location S1-B (or S1-A) is above the horizon 3 or 4 times per day. However, as the IW swath is fixed, the swath will not image your location 3 or 4 times per day - it will image further towards the east or towards the west. So you need to check the ESA planning to find the date & time (in UT) of the IW acquisiton over your site and then find that date & time (coverting UT to local time) in HA. In HA if you look at passes exactly 12 days apart, you will find the same azimuth and altitude angles.
Peter

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Hi Peter,

I will check both the GRDH and SLC products on the 29th, and then tell you the result.

Very thanks for your explanation and help!

Hi Peter,

I have tried to locate the CR from the GRDH products by images derived on 2020/02/29 (after reset the elevation of about 12 degrees of CR), 2020/02/23 (with the wrong elevation angle of about 2 degrees) and 2020/02/17 (before the set of CR).

I used the GCP tool to mark the closest pixel of my CR location (WGS84(24.98239315, 121.69172164)), I donât think the CR is visible.

And then I checked the SLC product.
Because I couldnât locate the CR position from the original Intensity_IW3_VV bands. The lon/lat of my CR is out of range from the coordinates show on the bottom-right of the window (I donât know why).

So, I applied deburst and Range-Doppler terrain correction steps. And then located the CR position as followed, including
2020/02/29 (after reset the elevation of about 12 degrees of CR)
2020/02/23 (with the wrong elevation angle of about 2 degrees)
2020/02/17, 2020/02/11, 2020/02/05 (before the set of CR).

According to this result, the CR is visible in both elevation angles of about 12 degrees and 2 degrees?
If the CR is visible from the image on 2020/02/29, the straight brighter pixels instead of one brighter pixel is owing to the Range-Doppler terrain correction step?

Thank you so much for helping me!

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Hi @Sharon,

Iâm trying to looking for in SLC product 3 corner reflectors. So, in your case, do you have applied to your images only deburst and range doppler terrain correction steps?

Thanks in advance.

Yes, I only applied the steps of deburst and range doppler terrain correction.

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Hi, I would like to place a corner reflector for monitoring a dam. I have followed all the explanations you have indicated above and have come to this conclusion.
I would like to know if what I did is right. Thanks in advance.
CR ASC.pdf (15.0 KB)
ASC.pdf (2.9 MB)
DESC.pdf (2.9 MB)
CR DESC.pdf (15.1 KB)

TRAJECTORY SATELLITES USED FIRST DSC AND THEN ASC

@peter.meadows

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Hi, The first thing to check is the passes for S1A & B over your site, for which there will be a maximum of 4 at your latitude for each of S1A & B during each repeat-period (12 days). Below shows the 4 passes for S1-A:

For the period from 4th Oct details of the four S1-A passes are:

• 4th Oct, 05:28:14 UT, descending, alt = 59 deg, azi = 101 deg
• 5th Oct, 17:14:45 UT, ascending, alt = 45 deg, azi = 261 deg
• 11th Oct, 05:20:03 UT, descending, alt = 49 deg, azi = 99 deg
• 12th Oct 17:06:25 UT, ascending, alt = 55 deg, azi = 260 deg.

S1-B will be the same except the date will be 6 days earlier or later. The alt and azi angles are from Heavens-Above for the location of the Dam (note HA uses your local time and not UT so the time in HA will be out by 1 or 2 hours).

The method in your PDF files is correct but the alt, azi angles are incorrect. So yes you orientate the CR so that the angle of the CR base is at an angle of the altitude angle - 35 deg and it is pointing towards the azimuth angle. So for the 4th Oct pass, the CR base is at an angle of 59-35 = 24 deg and at an azimuth angle of 101 deg (just S of E).

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I have a doubt, as for the site heavens above, it gives the azimuth with respect to the north or with respect to the inclination of the satellite?

thanks

HA gives the azimuth angle with respect to true/geographic north. True north can be found using a magnetic compass provided it has been corrected for magnetic declination for your location - see online declination calculator at NCEI Geomagnetic Calculators (noaa.gov).

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