Smoothen the Sentinel-1 Time Series Graph Using Google Earth Engine

Question

In order to study the growth pattern of crops, I tried to smoothen the Sentinel-1 time series graph using the following code but got the error.

Error:

ImageCollection (Error)
ImageCollection.reduce: Error in map(ID=null):
Image.select: Can't add a band named 'VV' to image because a band with this name already exists. Existing bands: [VV].

Code:

Map.addLayer(Bhagwanpur, {}, 'Bhagwanpur', false)
Map.centerObject(Bhagwanpur, 12)
            
Map.addLayer(GT, {}, 'March GT', false)

var CreateBuffer = function(feature, buffer_size) {
  return feature.buffer(buffer_size)
  }

var buffer_size= 20

var GT_Buffer = GT.map(function(ft){return CreateBuffer(ft, buffer_size)});
Map.addLayer(GT_Buffer, {}, 'GT_Buffer')
Map.centerObject(GT_Buffer, 15)
print('GT_Buffer',GT_Buffer)


/*----------------------------------------------------------------------------------------------------------*/

// Importing the Sentinel-1 Images:
var Sen= ee.ImageCollection("COPERNICUS/S1_GRD")
          .filterDate('2021-05-01', '2022-07-15')
          .filterBounds(Bhagwanpur)
          .select (['VV','VH'])
          .filter(ee.Filter.eq('instrumentMode', 'IW'))

print('Sen',Sen)


var addVVbyVH = function(image) {
  var vv = image.select('VV');
  var vh = image.select('VH');
  var VVbyVH = (vv).divide(vh).rename('VVbyVH');
  return image.addBands(VVbyVH);
};

var Sen = Sen.map(addVVbyVH);
print('Sen',Sen)


/*-------------------------------------------------------------------------------------------------------------*/
//Visualization of Images: 
Map.addLayer(Sen.first().clip(Bhagwanpur), {'min': [-25, -20, 0], 'max': [5, -10, 5]}, 'S1 RGB', false)

/*------------------------------------------Smoothen the Sentinel-1 graph------------------------------------------------------------------*/

function smoother(t){
  // helper function to apply linear regression equation
  function applyFit(img){
      return img.select('VV').multiply(fit.select('scale')).add(fit.select('offset'))
              .set('system:time_start',img.get('system:time_start')).rename('VV');
  }
  t = ee.Date(t);
  
  var window = Sen.filterDate(t.advance(-windowSize,'month'),t.advance(windowSize,'month'));
    
  var fit = window.select(['VV','VV'])
    .reduce(ee.Reducer.linearFit());
    
  return window.map(applyFit).toList(5);
}

// function to reduce time stacked linear regression results
// requires that a variable 'fitIC' exists from the smooter function

function reduceFits(t){
  t = ee.Date(t);
  return fitIC.filterDate(t.advance(-windowSize,'day'),t.advance(windowSize,'day'))
              .mean().set('system:time_start',t.millis()).rename('VV');
}

var dates = Sen.aggregate_array('system:time_start');

var windowSize = 1; //days on either sides

var fitIC = ee.ImageCollection(dates.map(smoother).flatten());

var smoothed = ee.ImageCollection(dates.map(reduceFits));
// print('smoothed',smoothed)

// var smooth_mos = smoothed.mosaic()

// merge original and smoothed data into one image collection for plotting
// var joined = ee.ImageCollection(smoothed.select(['vv'],['smoothed'])
//                 .merge(Sen.select(['vv'],['original'])));


/*------------------------------------// Plot the Graph----------------------------------------*/

print(smoothed);
var scale = smoothed.first().projection().nominalScale();

print("scale", scale);


var chartvv = ui.Chart.image.seriesByRegion(
     Sen,
     GT_Buffer.filter(ee.Filter.eq('Field_ID', 5)),
    ee.Reducer.mean(),
    'VV',
    10)
    
var chartv2 = chartvv.setSeriesNames(['VV'])
        .setChartType('LineChart') 
        .setOptions({
          title: 'Sentinel-1 time series',
          hAxis: {title: 'Date', titleTextStyle: {italic: false, bold: true}},
          vAxis: {
            title: 'dB values',
            titleTextStyle: {italic: false, bold: true}
          },
          lineWidth: 5,
          colors: ['#fc0303', '#17fc03','#0324fc'],
          curveType: 'function'
        });
        
print(chartv2)

Code link: https://code.earthengine.google.co.in/6f7665b85f5fe82319ebdb9e04141c74

Answer 1

Line 57:

var fit = window.select(['VV','VV'])

You can't select the same band twice. Perhaps you meant to use 'VH' for the second band.