So I appreciate this is a bit old and from what I can see the post from shade has been criticized in other threads or forums, but I've recently picked up a P2 decided to give this a go anyway. No idea how many of these are even still out there...but I think it's still relevant.
TL;DR, it works insofar as getting you a few more satellites and is much quicker to lock on.
"A LOT" more satellites? No. Anything other than GPS satellites? Probably not. I have no way to tell. No idea how separate systems are translated in the context of what the Phantom's naza wants to see.
In my case, I was getting about 8-9 on the ground and 10 in the air. I locked pretty quickly with this with 11 initially and soon after 14 on the ground. I didn't see any increase at 100m. Now, this antenna that's used is huge. Without shaving anything more than the stock GPS puck mounts and the ribs around it, it smacks into the naza. The case will fit together, but puts some pressure on the naza and subsequently the board. How much pressure and whether it's detrimental to anything in the long run I can't say. A quick 10m flight was fine. Was it more stable? Seemed so, and it was a bit windier than it has been, but I'm sure YMMV. Would the smaller antenna be better (GP.1575.25.4.A.02)? Dunno yet. But I'm going to swap it over, also on the new unit and see how it compares.
So...there you go.
