I used bad elf on a trip from Algarve to Azores and it was DGPS all the way with 3m accuracy. I have the plug in rather than the blue tooth. It's quite clever as you can put the IPad to sleep but the bad elf has an internal battery and remains acquired for quite some time so when you fire up the IPad the position is fixed. Even unplugging it for a few minutes and plugging back in it was locked almost immediately. The only downside is it sticks out the end of your iPad but my aqua-pack was big enough for it to fit inside plugged in.
I've got a GNS1000 - about £70 - a bluetooth type so stays down below (where the signal is still good) and connects wirelessly to the iPad. No problem and makes a handy independent electronic backup to the main chartplotter.
Disclaimer: I don't have an ipad so don't really know what I'm talking about. But...
I believe it might depend on the app you're using it with.
This article on the digital yacht site: http://digitalyacht.net/2013/01/22/getting-nmea-data-to-an-ipad-or-iphone/
Darned new forum posting rules are like hypertext never even happened :-(..
..suggests that iNavX (but not navionics) will read GPS info from an external network nmea feed. So if you're pumping out your other nav data over wifi and can include GPS sentences in the stream, you're sorted without additional hardware. Obviously there are rather cheaper and arguably better (though not simpler) ways to do it than the digital yacht solution
If you have an iphone(?), you can tether it to your ipad (via bluetooth) and use your phone's 3G to create a wifi hotspot. I've done it with my mac but not tried on ipad, but think it should work in the same way. You need to switch on hotpsot and bluetooth setting in your phone. Switch on bluetooth on your ipad. Recognise the device. Then when you switch on wifi on your ipad it should come up with your iphone in the available wifi networks. The password for it is on the phone. Obviously this will eat through your data allowance though so you need to make sure you have a good phone contract with lots of data included.
I had just the same issue. I bought the Dual XGPS150A - £78 from Amazon http://www.amazon.co.uk/gp/product/B006M49G80/ref=wms_ohs_product_img?ie=UTF8&psc=1
- and it's great. No wires - it connects via bluetooth to the iPad - and picks up satellites very efficiently. I keep mine next to the chart table on my Bavaria 30, and even though it's in the saloon with no sight of the sky the GPS works like a dream. I'm using Navionics, and am really impressed by the setup. I can even take the iPad into the cockpit, and the Dual still maintains connection.
The Dual has a built in battery, and is recharged via the usual USB socket system.