Extracting Sentinel-1 values from polygons

Question

I'm trying to extract Sentinel 1 VH values for each day of a selected period, and for each polygon of a feature collection:

var sentinel1 = ee.ImageCollection('COPERNICUS/S1_GRD')
  //filtering by polarisation
                 .filter(ee.Filter.listContains('transmitterReceiverPolarisation', 'VH'))
                 //filter by image collected in interferometric wide swath mode
                 .filter(ee.Filter.eq('instrumentMode', 'IW'))
                 //filter by the period
                 .filterDate("2020-12-24","2020-12-28");

var S1_ic = sentinel1.select('VH');

var features = [
  ee.Feature(ee.Geometry.Polygon([
    [11.45721391, 44.04473365],
    [11.45782721, 44.04426541],
    [11.45719584, 44.04382759],
    [11.45690026, 44.04404892],
    [11.45655093, 44.04430347]
    ]), {name: '1'}),
  ee.Feature(ee.Geometry.Polygon([
    [11.45568411, 44.04495992],
    [11.45628790, 44.04450235],
    [11.45565213, 44.04406050],
    [11.45505275, 44.04452208]
    ]), {name: '2'})
  ];
  
var polygons = ee.FeatureCollection(features);
print (polygons)
Map.addLayer(polygons)
Map.centerObject(polygons)
  

var tVH = ee.FeatureCollection(S1_ic.map(function (img) {
  return img.reduceRegions({
    collection: polygons, 
    reducer: ee.Reducer.mean(),
    scale: 10}
    );
})).flatten();

Export.table.toDrive({collection: tVH, folder: 'Export GEE'});

here is the code link: https://code.earthengine.google.com/6b31a91109a92777329249370c739c61.

I'm quite new using GEE and I don't get how to download the data with one single record for each day, instead of having several records for the same day.

Answer 1

One nice trick that allows obtaining the desired output is converting the ImageCollection to a multiband single image and then apply the reduce regions to this single image. An extra tip: you should apply also a spatial filter to the collection in order to include only images that have information of the region of interest. Here's the code:

var sentinel1 = ee.ImageCollection('COPERNICUS/S1_GRD')
                 //filtering by polarisation
                 .filter(ee.Filter.listContains('transmitterReceiverPolarisation', 'VH'))
                 //filter by image collected in interferometric wide swath mode
                 .filter(ee.Filter.eq('instrumentMode', 'IW'))
                 //filter by the period
                 .filterDate("2020-12-24","2020-12-28")
                 // Filter by polygons
                 .filterBounds(polygons);

var S1_ic = sentinel1.select('VH');
// Convert image collection to a multiband single image
var sentinel1TS = S1_ic.toBands();

var tVH = ee.FeatureCollection(sentinel1TS.reduceRegions({
    collection: polygons, 
    reducer: ee.Reducer.mean(),
    scale: 10})
);


Export.table.toDrive({
  collection: tVH, 
  folder: 'Export GEE',
  description: 'tsVH',
  fileFormat: 'CSV'
});