Ken McNamara sent a message inquiring about what GPS to use with your new Pixhawk 2.1: I believe this is a great subject for a new discussion. Right now, I believe there is only the "HERE" GPS that is available as a "plug-and-play" solution for the PH2.1 because of the DF-17 plugs; but I think any U-Blox module will work if you can plug it in or solder your own pigtails. From experience, the Ublox Neo M8N works really good in 'less than perfect' conditions: It got a decent lock INDOORS and acquiring a 'warm fix' took only a few seconds. Here is the module I am talking about:
The 'HERE' modules are awesome but quite big; also they include LED indicators and an arm button.