Google Earth Engine time series analysis sediment error

Question

I am a beginner in learning about GEE. I am now calculating sediment using Sentinel 2a satellite images in GEE, but I encountered an issue as follows:

NDSSI Mosaic: Layer error: ImageCollection.mosaic: Error in map(ID=null): 
Date: Parameter 'value' is required.

Below is the code I have:

    // Display catchment boundary
    Map.addLayer(boundary, {}, 'Boundary');
    
    // Import the global surface water layer
    var gsw = ee.Image("JRC/GSW1_3/GlobalSurfaceWater");
    
    // Select the 'max_extent' band
    var water = gsw.select('max_extent').clip(boundary); 
            
    // Keep the water areas only (1)
    var masked = water.updateMask(water);
    
    // finally masking out 0 values and retain only water areas (1)
    var masked = water.selfMask();
    Map.addLayer(masked, {}, 'Water');
    
    //////
    var masks2a = function (image) {
      var qa = image.select('QA60');
      var cloudBitMask = 1 << 10;
      var cirrusBitMask = 1 << 11;
      var mask = qa.bitwiseAnd(cloudBitMask).eq(0)
        .and(qa.bitwiseAnd(cirrusBitMask).eq(0));
      return image.updateMask(mask).divide(10000);
      };
    
      // Function to calculate NDSSI and add time as a band.
    var addVariables = function(image) {
      // Get the timestamp and convert it to a date.
      var timestamp = ee.Number(image.get('system:time_start'));
      var date = ee.Date(timestamp);
      var yearStart = ee.Date.fromYMD(date.get('year'), 1, 1);
      var fractionalYear = date.difference(yearStart, 'day').divide(365);
      
      // Calculate NDSSI.
      var ndssi = image.normalizedDifference(['B2', 'B8']).rename('NDSSI');
      
      // Add NDSSI and fractional year as bands.
      return image.addBands(ndssi)
                  .addBands(ee.Image(fractionalYear).rename('fractional_year').toFloat());
    };
    
    var filtermasks2a = ee.ImageCollection('COPERNICUS/S2')
      .filterBounds(boundary)
      .filterDate('2020-01-01', '2023-10-30')
      // Pre-filter to get less cloudy granules.
      .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 20))
      .map(masks2a)
      .map(addVariables)
                      
    // Display RGB parameters
    var rgb = filtermasks2a.select('B4', 'B3','B2');
    
    
    // Plot a time series of NDSSI at a single location.
    Map.centerObject(boundary, 15);
    
    // Load and define a continuous palette
    var palettes = require('users/gena/packages:palettes');
    
    // Choose and define NDSSI display palette
    var palette = palettes.colorbrewer.RdBu[10];
    var viz = {min: -1, max: 1, palette: palette};
    
    // Display the NDSSI map
    Map.addLayer(filtermasks2a.select('NDSSI'), viz, 'NDSSI Mosaic');

Is there a solution?

Answer 1

This fails because addVariables() expects the image to have a system:time_start property. You lost all properties in masks2a(). Just tweak that function, to add the properties back:

var masks2a = function(image) {
  var qa = image.select('QA60')
  var cloudBitMask = 1 << 10
  var cirrusBitMask = 1 << 11
  var mask = qa.bitwiseAnd(cloudBitMask).eq(0)
    .and(qa.bitwiseAnd(cirrusBitMask).eq(0))
  return image.updateMask(mask).divide(10000)
    .copyProperties(image, image.propertyNames())
}

https://code.earthengine.google.com/a17aae96293dd56503726e2aeff69535