how do I get my iPad2 to use an external GPS receiver

I have a Marine Navionics Chart app on my iPad. I have bought an external Blue Tooth GPS receiver. The iPad has recognised the device. I wish to use this to improve the accuracy of my location when at sea. When I have "Location Services" off my iPad does not seem to want to use the external receiver (a GNS 1000 device).

How can I get it to use this device?