MrDK
I agree with you, in fact I've got the necessary data from that message down to 35 bytes (the commas are not transmitted if the fields are fixed.) It can be compacted a bit further by using binary notation instead of ASCII characters.
9V-SRN,370,0,475,161,-2.6921,105.9659,2233
The receiving end can have a lookup table to fill in whatever flight number 9V-SRN is on, what kind of plane it is and who the Crew are.
This won't take much bamdwidth. That's why Inmarsat will do it for free.