We use the iSailor App with the Bad Elf GPS on Bluetooth. Works great! I especially like the Anchor alarm on iSailor. That alone would be a good reason to own it. By using the Bad Elf you are not tied into a cellular system and it works anywhere. We used it when transiting across the Gulf. Good luck!
Any 3G capable (cellular
capable) iPad has a built in GPS.
YOU DO NOT NEED A CELL PLAN FOR THE INTERNAL GPS TO WORK.
The built in GPS chip will work
anywhere with no external GPS, wi-fi or cell plan.
I have never had a cell plan fon any of our iPads. If I need internet on my iPad, outside the range of wi-fi, I tether the iPad to my iPhone for internet access and use the phones 3G or 4G data. Our 3G capable iPads still operate the GPS
hundreds of miles from any cell or wi-fi service just as accurately as out fixed mount plotters.
If you
do not have a 3G enabled iPad, and you have the basic wi-fi only model, then you
will need an external GPS but if you have a 3G iPad there is no need for any external GPS. To me the additional expense, for a boat owner, to upgrade to a 3G capable iPad is well worth the additional up-front $130.00 expense. By doing so it eliminates an additional piece of gear to rely on, that has potential to fail or drop a BT signal from or have a battery die etc. and it usually costs less than purchasing a BT external GPS..
Last spring during one of the iPad software upgrades, IOS 8.3 to be specific, many of my customers using an external GPS puck (most use Bad Elf or Dual) lost that capability. Even in the next revision, to IOS 8.4, some still had issues if they had older BT GPS models. During this time the 3G capable iPads suffered no loss of GPS use.