script.js

//VERSION=3 (auto-converted from 1)
/*

Surface Soil Moisture (SSM) retrieval using change detection approach.

Author Details: 
Narayana Rao B.
206-MRSLab, CSRE,
IIT Bombay, India.

A detailed explanation of the implemented algorithm can be found in the following articles.

Wagner, W., Lemoine, G., Borgeaud, M. and Rott, H., 1999. 
A study of vegetation cover effects on ERS scatterometer data. 
IEEE Transactions on Geoscience and Remote Sensing, 37(2), pp.938-948.

B. Bauer-Marschallinger et al., 
"Toward Global Soil Moisture Monitoring With Sentinel-1: Harnessing Assets and Overcoming Obstacles," 
in IEEE Transactions on Geoscience and Remote Sensing, vol. 57, no. 1, pp. 520-539, Jan. 2019.

*/

function setup() {
  return {
    input: [{
      bands: [
          "VV",
          "VH"
      ]
    }],
    output: { bands: 3 },
    mosaicking: "ORBIT"
  }
}

function filterScenes (scenes, inputMetadata) 
  {
    return scenes.filter(function (scene) {
    // Considering 36 months data to avoid seasonal variations in masking
    return scene.date.getTime()>=(inputMetadata.to.getTime()-36*30*24*3600*1000) ;  // Data from 36 months to current date
    });
  }

function evaluatePixel(samples, scenes) 
  {  
    var count = 0;
    var Mv = 0;
    var max = 0;
    var min =2.0;
    var sum_VV = 0;

    for (var i=0;i<samples.length-1;i++) 
      {
          max = samples[i].VV > max ? samples[i].VV:max; // Calculating all time maximum--Wet index 
          min = samples[i].VV < min ? samples[i].VV:min; // Calculating all time minimum--Dry index 
          sum_VV += samples[i].VV; 
          count++;
      }
    // Overall range of intensity values Anologous to 0-100% soil moisture 
    var sensitivity  = max-min; 
    // If overall averge is more than 6dB i.e., High intensity always usually urban areas.
    // Generating urban area mask using -6dB threshold
    urban_mask = 10*Math.log10(sum_VV/count) > -6 ?  0 : 1; 
    // If overall averge is less than 17dB i.e., low intensity always usually water bodies.
    // Generating permanent water body mask using -17dB threshold
    water_mask = 10*Math.log10(sum_VV/count) < -17 ?  0 : 1; 
    // Assuming change in bckscatter intensity only because of change in soil moisture.
    Mv = ((samples[0].VV) - min)/(sensitivity);
    Mv = Mv*water_mask*urban_mask;// Applying urban and permanent water body mask

    /*

    Assigning colormap for enhanced visualisation

    */
    var v = Mv;
    var vmin = 0;
    var vmax = 0.6;
    var diffv = vmax - vmin;
    
    var r = 0.0;
    var g = 0.0;
    var b = 0.0;

    if (v < vmin){
      v = vmin;
    }
    if (v > vmax){
      v = vmax;
    }
    //Threshold values for colorMap
    var T1 = 0.1;
    var T2 = 0.3;
    var T3 = 0.4;
    var T4 = 0.5;
    
    var Thresh_1 = (vmin + T1 * diffv);
    var Thresh_2 = (vmin + T2 * diffv);
    var Thresh_3 = (vmin + T3 * diffv);
    var Thresh_4 = (vmin + T4 * diffv);

    if (v <= 0) 
    { 
      r=1;
      g=1;
      b=1;
    }
    else if (v < Thresh_1)   {  r = 0.5 +  (v - vmin) / (Thresh_1 - vmin) / 2; } 
    else if (v < Thresh_2)
    {
       r = 1;
       g = (v - Thresh_1) / (Thresh_2 - Thresh_1);
       b = 0;
    } 
    else if (v < Thresh_3) 
    {
       r = 1 + (Thresh_2 - v) / (Thresh_3 - Thresh_2);
       g = 1;
       b = (v - Thresh_2) / (Thresh_3 - Thresh_2);
    } 
    else if (v < Thresh_4)
     {
       r = 0;
       g = 1 + (Thresh_3 - v) / (Thresh_4 - Thresh_3);
       b = 1;
    } 
    else {  b = 1.0 + (Thresh_4 - v) / (vmax - Thresh_4) / 2;  }
     return [r, g, b];

  }