Loss of system:time_start Property in Sentinel-1 ImageCollection After Applying Refined Lee Filter in Google Earth Engine

Question

I'm working with Sentinel-1 imagery in Google Earth Engine and I've encountered an issue. My goal is to filter the Sentinel-1 collection, apply the Refined Lee filter, and calculate VV/VH ratios. However, I've noticed that the system:time_start property is lost after applying the Refined Lee filter to the ImageCollection. I would like to get it back after applying the Refined Lee filter

Here is the script I'm using:

// Define the study region
var geometry = ee.Geometry.Polygon(
  [[[176.4802920098029, -38.489215944147574],
    [176.51016108939274, -38.489215944147574],
    [176.51016108939274, -38.47530795883661],
    [176.4802920098029, -38.47530795883661],
    [176.4802920098029, -38.489215944147574]]]
);
var studyRegion = geometry;

// Function to get Sentinel-1 collection
function get_s1_col(studyRegion, start_date, end_date) {
    var s1_col = ee.ImageCollection('COPERNICUS/S1_GRD')
        .filter(ee.Filter.eq('instrumentMode', 'IW'))
        .filter(ee.Filter.eq('orbitProperties_pass', 'DESCENDING'))
        .filterDate(start_date, end_date)
        .filterBounds(studyRegion);
    return s1_col;
}

// Function for Refined Lee filter
var refinedLee = function(image) {
  // (Refined Lee filter implementation)
};

// Function to calculate VV/VH ratio
var calculateVVVHRatio = function(image) {
    var vv = image.select('VV').toFloat();
    var vh = image.select('VH').toFloat();
    var ratio = vv.subtract(vh).divide(vv.add(vh)).rename('VV_VH_ratio');
    var combined_image = ee.Image.cat([vv, vh, ratio]);
    combined_image = combined_image.rename(['VV', 'VH', 'ratioVV_VH']);
    return combined_image;
};

// Function to clip each image in the collection
var clipCollection = function(image) {
  var clippedImage = image.clip(studyRegion);
  return clippedImage;
};

// Process Sentinel-1 collection
var process_s1_collection = function(studyRegion, start_date, end_date) {
    var s1_collection = get_s1_col(studyRegion, start_date, end_date);
    print('s1_collection: ', s1_collection);
    var s1_lee_sigma = s1_collection.map(refinedLee);
    print('s1_lee_sigma: ', s1_lee_sigma);
    var s1_vvRatioVH = s1_lee_sigma.map(calculateVVVHRatio);
    var clip = s1_vvRatioVH.map(clipCollection);
    return clip;
};

// Create 2019-2024 collection
var s1_2019_2024 = process_s1_collection(studyRegion, '2019-01-01', '2024-01-31');
print('s1_2019_2024:', s1_2019_2024);

The full link of my script here: https://code.earthengine.google.com/d1b517214adc45d7aa4ac09e719cad32

Answer 1

There are operations you can make with an image that causes you to lose its properties (like a simple multiplication). If you want them back, you can simply copy them back from the original image. In your case, you made your life a bit harder by redefining the image variable inside your refinedLee() function. It forces you to extract the properties before you redefine the variable.

var refinedLee = function(image) {
  var props = image.toDictionary(image.propertyNames())
  ...
  return powerToDb(ee.Image(result)).set(props)
}

https://code.earthengine.google.com/d7b2f2ee66f9f3a80bf8b4563fe95052