The short answer is to measure methane you rely on methane absorption features. To do that you need to be looking at a source through the methane plume, from a

The short answer is to measure methane you rely on methane absorption features. To do that you need to be looking at a source through the methane plume, from a satellite this source is the ground. From a ground instrument you can use the sun as a source, but it limits the times of day/direction/location you can measure from.