There are many external GPS options available for the iPad, iPad 2, iPad 3, iPhone and similar devices. We have done the leg work for you and narrowed it down to the three best for pilots:
The WiFi model of the iPad does not have a built-in GPS antenna – an external GPS is a MUST if you require location services. The 3G model does include an internal GPS receiver, but it designed for casual ground use. It is not highly reliable in the air – accuracy that is less than precise and a tendency to drop the GPS signal. Accuracy and reliability are ESSENTIAL to help you get the most from your aviation apps.
These external GPS products are extremely accurate (WAAS-enabled) and highly reliable.