Remove single (un-mosaicked) tiles after mosaicking image collection by day

Question

I want to create a time-series of Sentinel-2 images that cover my entire study area (instead of individual tiles, since the study area spans 2 tiles). I mosaicked the image collection by day using the following function (where S2 is the Sentinel-2 image collection):

    function mosaicByDate(collection){
  var imagelist = S2.toList(S2.size())
  var unique_dates = imagelist.map(function(image){
    return ee.Image(image).date().format("YYYY-MM-dd")
  }).distinct()
  var imagelist_mosaic = unique_dates.map(function(d){
    d = ee.Date(d)
    var image = collection
      .filterDate(d, d.advance(1, "day"))
      .mosaic()
    return image.set(
        "system:time_start", d.millis(), 
        "system:id", d.format("YYYY-MM-dd")).copyProperties(image, image.propertyNames())
  })
  return ee.ImageCollection(imagelist_mosaic)
}

var S2_mosaic = mosaicByDate(S2)

The function works perfectly, but since there are some days where only one of the tiles is acquired (so there is no mosaicking), the time-series of images shows only half the study area for these dates. I created a filmstrip to show what this looks like.

I now would like to remove the image dates for which there is only one tile... anyone have a suggestion how do this? Unfortunately it is not consistantly every other image, there are subsequent one-tile dates as well. The image below corresponds to the imagedates of the filmstrip shown above.

Answer 1

When you set the time and id, also set a property with the number of scenes in the collection (get that before the mosaic), then filter out the images without enough scenes.

var imagelist_mosaic = unique_dates.map(function(d){
    d = ee.Date(d)
    var result = collection
      .filterDate(d, d.advance(1, "day"))

    var size = result.size();
      
    return image.mosaic().set(
        "system:time_start", d.millis(), 
        "system:id", d.format("YYYY-MM-dd"),
        "count", result.size())
      .copyProperties(image, image.propertyNames())
  })
  return ee.ImageCollection(imagelist_mosaic).filter("size > 1")
}