I've never heard of anything lighter than 4-5g; would definitely be interesting to hear of something smaller.
Flightsketch has one in beta testing.
Fits in a BT-20 tube.
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I can tell you definitively that GPS performance with a smaller board is not as good as with a larger one. Ideally, the ground plane for a patch antenna should be at least twice the size of the patch antenna in both dimensions, with no active components, just a ground plane. Pretty much nobody does that... board space is a precious commodity. Sometimes you have to accept reduced performance if "other" things (like size) take precedence.
Northern Ohio Tripoli (NOTRA) member Andrew Kleinhenz uses the M-P, he’s a member of TRF and is on the NOTRA FB page as well. Hopefully he doesn’t mind me throwing his name out there...I just got the Marco Polo Advanced and it works great it tests. Now to fly it in a rocket. Anyone using the Marco Polo? What's the best way to mount it?
I played with some small GPS chipsets like as could be had with a cellphone and they really need to be out in the open to function well.
If used in a rocket, might have an issue upon touchdown if the GPS patch is facing the dirt or off to the side. Not as much of a problem with the larger GPS patch.