Get Sentinel 1 Images with sigma nought backscatter values in dB

[jpsuarezq54 (Juan Suarez)]

Actually, I am working with GLF-CR neuronal network with my own SAR images. I tried all ways to download data in that format but it was no posible for me.

The neuronal networks data works in this way:
-Load de SAR image with values in dB btw -34 or more to 0 in float32 format. Sentinel-1 Global Backscatter Model
-Them clipped btw [-35,0] and [-25,0] the VV and VH bands respectily.
-In the last step, normalize the image btw [0,1]

I have this code:

evalscript = """
//VERSION=3
function setup() {
  return {
    input: ["VV","VH"],
    output: { id: "default", bands: 2, sampleType: "FLOAT32" },
  }
}

function evaluatePixel(samples) {
  var vvdB = toDb(samples.VV)
  var vhdB = toDb(samples.VH)
  return [vvdB, vhdB]
}

// displays VV in decibels from -20 to 0
function toDb(linear) {
  // the following commented out lines are simplified below
  // var log = 10 * Math.log(linear) / Math.LN10
  // var val = Math.max(0, (log + 20) / 20)
  return Math.max(0, Math.log(linear) * 0.21714724095 + 1)
}
"""

bbox = BBox(bbox=[xx,xy,yy,yx], crs=CRS.WGS84)
def get_alldata(time_interval):
    return SentinelHubRequest(
        evalscript=evalscript,
        input_data=[
            SentinelHubRequest.input_data(
                data_collection=DataCollection.SENTINEL1_IW_DES.define_from("sd", service_url=config.sh_base_url),
                time_interval=time_interval,
                other_args={"dataFilter": {"resolution": "HIGH"},"processing": {"backCoeff":"SIGMA0_ELLIPSOID","orthorectify": True,"demInstance": "COPERNICUS"}}
            ),
        ],
        responses=[SentinelHubRequest.output_response("default", MimeType.TIFF),],
        bbox=bbox,
        size=aoi_size,
        config=config,
    )

I get the images btw 0 and 2 but no in dB.

How can i download the data in that format?

[william.ray]

Hi Juan,

I’m not exactly sure what you are trying to do but if you are trying to obtain all the Sentinel-1 imagery for a certain time period then the following evalscript should help you out:

//VERSION=3

function setup() {
  return {
    input: ["VV", "VH", "dataMask"],
    output: [{
      id: "VV",
      bands: 1,
      sampleType: "FLOAT32"
    },
    {id: "VH",
      bands: 1,
      sampleType: "FLOAT32",
  },
    {id: "Ratio",
      bands: 1,
      sampleType: "FLOAT32"
  }],
    mosaicking: "ORBIT"
  };
}

var timestamps = []

function updateOutputMetadata(scenes, inputMetadata, outputMetadata){
    outputMetadata.userData = {
          "Dates":  [...new Set(timestamps)]
    }
}

function preProcessScenes (collections) {
  // Sort scenes by dateFrom in ascending order
  collections.scenes.orbits.sort(function (s1, s2) {
    var date1 = new Date(s1.dateFrom)
    var date2 = new Date(s2.dateFrom)
    return date1 - date2
  })
  return collections
}

function updateOutput (outputs, collections) {
  outputs.VV.bands = collections.scenes.length
  outputs.VH.bands = collections.scenes.length
  outputs.Ratio.bands = collections.scenes.length
}

function toDb(linear) {
        return 10 * Math.log(linear) / Math.LN10
    }   

function evaluatePixel(samples, scenes) {

  var VV_bands = []
  var VH_bands = []
  var Ratio_bands = []

  for (let i = 0; i < scenes.length; i++){
    if (samples[i].dataMask === 1){
      VV_bands.push(toDb(samples[i].VV))
      VH_bands.push(toDb(samples[i].VH))
      Ratio_bands.push((toDb(samples[i].VH))/(toDb(samples[i].VV)))

    } else {
      // No data = - 9999
      VV_bands.push(-9999)
      VH_bands.push(-9999)
      Ratio_bands.push(-9999)
    }
    //throw new Error (JSON.stringify(scenes[i]))
    timestamps.push(JSON.stringify(scenes[i].date))
  }
  return {
    VV: VV_bands,
    VH: VH_bands,
    Ratio: (Ratio_bands),
  }
}

Please do ask more questions if you are still not getting your desired result.

[jpsuarezq54 (Juan Suarez)]

No, I just wanna get the Sentinel 1 data values like Sentinel-1 Global Backscatter Model btw -18dB to -4.5dB or like the values of SEN12MS-CR dataset where described: Sentinel-1 SAR: 2 channels corresponding to sigma nought backscatter values in dB scale for VV and VH polarization.

The posted code is an example of parameters that how I’m download the data.