-128 to +128 Values for VV Readings

Apps / Services Python SDK
1

Hi,

I have used the following script to download SAR data. I have realized now that the values of VV range from -128 to +128.

VH start from 0 as expected.

Is there any explanation for those values of VV?

def read_sentinel1_image_field_date(_bbox, _geometry, _time_interval, _data_folder):
    utm_bbox = to_utm_bbox(_bbox)  
    size = bbox_to_dimensions(utm_bbox, 5)  
    geometry = Geometry(_geometry, crs=CRS.WGS84)

    evalscript = """
    //VERSION=3
    function setup() {
      return {
        input: ["VV", "VH", "shadowMask", "dataMask" ],
        output: { id:"default", bands: 4}
      }
    }

    function evaluatePixel(samples) {
      return [samples.VV, samples.VH, samples.shadowMask, samples.dataMask]      
    }    
    """


    request = SentinelHubRequest(
        data_folder= _data_folder,
        evalscript=evalscript,
        input_data=[
            SentinelHubRequest.input_data(
                data_collection=DataCollection.SENTINEL1_IW,
                time_interval=_time_interval,
                other_args={"processing": {"orthorectify": "True",
                                           "speckleFilter": {"type": "LEE", "windowSizeX": 3, "windowSizeY": 3},
                                           "backCoeff": "GAMMA0_TERRAIN"}}
            ),
        ],
        responses=[
            SentinelHubRequest.output_response('default', MimeType.TIFF),
        ],
        bbox=_bbox,
        geometry=geometry,
        size=size,
        config=config
    )

    try:
        response = request.get_data(save_data=True)
        filename_d = request.get_filename_list()
        return filename_d
    except Exception as e:
        print('Error getting sentinel imagery {0}'.format(e))        
        pass
        return 0
2

Hi Felipe,

Without knowing the AOI or the Time range used in the request it is difficult to say why the values were unexpected. We may be able to help more with that info :slightly_smiling_face: As I’m sure you know, there is plenty of info about the data in our documentation.

3

Right, so if those values are unexpected, I suppose I can set everything that is negative to 0 without losing available information for that given area of interest? @william.ray

4

If this happens very rarely you could use this as a solution. I would suggest analysing if there are any spatial patterns with with the observed unexpected values before doing this though. If they are common then it may be a processing error.

In addition, which backscatter coefficient are you outputting the Sentinel-1 GRD VV & VH?

5

Understood. Yes, this happens rarely.
I am getting the raw values from that script, Sentinel-1 GRD VV, VH, shadow and mask data.
In this case, -128 would be a raw data return for VV without any processing.

6

OK, thanks for the info. I would certainly recommend reading my colleague’s post about the different SampleTypes you can encounter when accessing satellite imagery in our APIs. Each sensor will have a different native Sample Type so it is an important concept to get your head around. You can read the post here.

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