Metadata property 'system:time_start' not found after scaling MODIS NPP data?

Question

I'm trying to re-scale an image collection using the solution provided here and then to reduce it using the timestamp as per this example. The script fails with the error:

NPP trend: Layer error: ImageCollection.reduce: Error in map(ID=2004_01_01): Image.metadata: Metadata property 'system:time_start' not found.

Can anyone tell me where I'm going wrong?

My script:

  var addTime = function(image) {
  return image.addBands(image.metadata('system:time_start')
    .divide(1000 * 60 * 60 * 24 * 365));
};

var scaleNPP = function(image) {
  return image.expression('float(b("Npp")/10000)')
};

var collection = ee.ImageCollection("MODIS/006/MOD17A3H")
  .filterDate('2004-01-01', '2010-07-01')
  .filterBounds(NNam)
  .map(scaleNPP);

var trend = collection.select(['system:time_start', 'Npp'])
  .reduce(ee.Reducer.linearFit());

Answer 1

When using the expression function you get a new image that has no properties. To copy the properties from the same image you are computing, you have to use copyProperties function.

var addTime = function(image) {
  return image.addBands(image.metadata('system:time_start')
    .divide(1000 * 60 * 60 * 24 * 365));
};

var scaleNPP = function(image) {
  return image.expression('float(b("Npp")/10000)')
              .copyProperties({source: image})
};

var collection = ee.ImageCollection("MODIS/006/MOD17A3H")
  .filterDate('2004-01-01', '2010-07-01')
  .filterBounds(NNam)
  .map(scaleNPP);

var trend = collection.select(['system:time_start', 'Npp'])
  .reduce(ee.Reducer.linearFit());

Answer 2

var addTime = function(image) {
  return image.addBands(image.metadata('system:time_start')
  .divide(1000 * 60 * 60 * 24 * 365));
};

var scaleNPP = function(image) {
  var nppScaled = image.expression('float(b("Npp")/10000)').rename('Npp_scaled');
  return image.addBands(nppScaled);
};

var collection = ee.ImageCollection("MODIS/006/MOD17A3H")
  .filterDate('2004-01-01', '2010-07-01')
  .filterBounds(geo)
  .map(addTime)//.aside(print) //uncomment this aside to print intermediate result
  .map(scaleNPP);
print(collection);

var trend = collection.select(['system:time_start', 'Npp_scaled'])
  .reduce(ee.Reducer.linearFit());

// Display the trend with increasing slopes in green, decreasing in red.
Map.addLayer(
    trend, //.clip(geometry),
    {min: 0, max: [-0.1, 0.1, 10], bands: ['scale', 'scale', 'offset']},'NPP trend');