Sentinel-3 single image per day

Question

I have created a list of dates from which I would like to get Sentinel3 images, only to find that I have multiple images per day.

How can I reduce it to 1 image per day?

I intend to do use median for the merging of the photos.

var region = ee.FeatureCollection([ee.Feature(Map.getBounds(true))])

//Dates of Interest
var start = ee.Date("2018-04-01");
var finish = ee.Date("2018-05-01");

//------------------- Data bases ---------------------//
var sen3 = ee.ImageCollection('COPERNICUS/S3/OLCI')
  .filterDate(start, finish)
  .filterBounds(region)
  .select(['Oa17_radiance', 'Oa08_radiance'])
  .map(function(image){
     return image
     .clip(region)
   })

//reference  dates
var sen2_for_dates = ee.ImageCollection('COPERNICUS/S2_SR')
  .filterDate(start, finish)
  .filterBounds(region)
  .map(function(image){
    return image
    .clipToCollection(region)
    .updateMask(image.select('QA60').not())
    .addBands(image.normalizedDifference(['B8','B4']).rename('ndvi'))
  })
  .select('ndvi')

//------------------ Unique dates ----------------------//
var sen2_dates = sen2_for_dates.map(function(image){
  return image.set('simpleDateMillis', ee.Date(ee.Date(image.date().format('YYYY-MM-dd'))))
});
var listMillisSimple = ee.List(sen2_dates.aggregate_array('simpleDateMillis'));
var uniqueDatesSimple = listMillisSimple.distinct();
var SEN2_day0 = uniqueDatesSimple

//THIS IS THE lIST OF DATES I WOULD LIKE TO GET IT's IMAGES
var SEN3_day0 = uniqueDatesSimple.map(function(date){
  return ee.Date(date).advance(0, 'day');
});

var sen3_dates = sen3.map(function(image){
  return image.set('simpleDateMillis', ee.Date(ee.Date(image.date().format('YYYY-MM-dd'))))
});

//The ImageCollection that should be containing the images from the specific dates
var allImagesInRangeS30 = sen3_dates.filter(ee.Filter.inList("simpleDateMillis", SEN3_day0))

I get these structures: SEN3_day0 - List of 6 elements, dates (as needed). sen3_dates - ImageCollection of 409 images (to be filtered by the list of dates in SEN3_day0). allImagesInRangeS30 - ImageCollection of 75 images (needs to be 6, as the list).

Answer 1

My usual approach for this is to map over a list of day offsets, from start to finish. For each, filter the collection down to that date and make a composite. I add an empty property to be able to filter out days without imagery. It relies on median() returning a 0 band image when the collection is empty.

var region = ee.FeatureCollection([ee.Feature(Map.getBounds(true))])

var start = ee.Date('2018-04-01')
var finish = ee.Date('2018-05-01')

var days = finish.difference(start, 'days')
var dayOffsets = ee.List.sequence(0, days.subtract(1))
var allImagesInRangeS30 = ee.ImageCollection(dayOffsets
  .map(function (dayOffset) {
    var dayStart = start.advance(dayOffset, 'days')
    var dayFinish = dayStart.advance(1, 'days')
    var composite = ee.ImageCollection('COPERNICUS/S3/OLCI')
      .filterDate(dayStart, dayFinish)
      .filterBounds(region)
      .select(['Oa17_radiance', 'Oa08_radiance'])
      .median()
    return composite
      .set('empty', composite.bandNames().size().eq(0))
      .set('system:time_start', dayStart.millis())
  }))
  .filterMetadata('empty', 'equals', 0)

print(allImagesInRangeS30)

https://code.earthengine.google.com/e72a75cd645a8780b6b8deae6e06af80

Answer 2

How would I use this code to make a time series of the averaged value for each day? Right now I have:

print(ui.Chart.image.series(allImagesInRangeS30, geom, ee.Reducer.mean(), 9e9));

but I get error "Error generating chart: No properties to chart."