If someone has a WiFi only iPad, they will need an external GPS receiver. While it is true that the 3G iPads do have internal GPS, I have found much better results using the external. Here's why:
In order for the internal GPS to be "on", airplane mode must be off. Not really a problem, unless you're flying above or beyond cell service coverage areas. See, the iPad (or iPhone) will be searching for a cellular data signal, which is a high battery consumption function. If it doesn't get a good cell signal, it will keep searching until it does, which uses a lot of juice. When I fly, after I download the current weather, I put my iPad (and iPhone) in airplane mode, which also turns off WiFi and Bluetooth. I then go into my settings and turn the Bluetooth back on so it can find my external GPS. I then get much better battery life out of the iPad, about 8 hours during the day or 12 hours at night (because I dim the display).
You can certainly use the internal GPS, and it works well even at high altitudes, but I'd rather save my battery and use the external GPS.