GPS works in the dark. and in the rain.
However, GPS technology that is readilly available in WalMart cannot be relied upon to create an intricate circle.
A handheld GPSs accuracy varies tremendously. 3 meters +/- would be about what I would expect on an average day in an area with clear skies and good
sat coverage overhead.
However, I've seen those same systems give me a +/- accuracy of up to 25 meters.
Nope, if you are going to do it with GPS you need surveying grade equipment. Millimeter accuracy... and that stuff doesn't come cheap. 20k will get
you a good base model.
(and I know that the crops aren't laid down to millimeter accuracy, but they are certainly under the 3 meter accuracy and those handhelds simply
aren't that good)...
Often times debunkers of this phenomenon simply don't know what they are saying when they try to debunk it.
What I also find intriguing is how they almost always overlook critical evidence because it causes them to question their PRECONCIEVED ideals that if
it is laid down here on Earth then men MUST be responsible for the mystery... afterall, people claim to do this all the time.
Yet they can never, or never will, duplicate their results given similar circumstances.
I'm still waiting for that video of one of those circle maker organizations actually creating one of these glyphs, like the human-butterfly glyph,
under the cover of darkness without sticking out like a sore-thumb on the hillside.
Because I feel very strongly that whomever is making those glyphs, if they are indeed humans, would need quite a few people and would require a lot of
lights in order to observe what they were doing...
Unless of course these pranksters have 3rd gen. nightvision and 20-60k dollar GPS equipment.
). 
