I would think that MRT had the presence of mind to use some at least lightly encrypted data format and ambiguous modulation rather than sending NMEA using a
widely used modulation method.
I would simply buy a Pocketlink and use the time you will waste decoding the data from their reasonably priced system to go hawking!
Even if you do manage to decode it, an SDR is not exactly a conveniently portable way to track your bird in the field.