@IanW
There is no guarantee satellite link will be active for 30 seconds even with multiple antennae.
Cost of Ka band equipment installation ranges from $100,000 - $200,000 per plane plus Inmarsat subscription fees are huge. Not many airlines can afford.
Read about DRS Technologies Automatic Deployable Flight Records ADFRs. Cheap ($20,000) and fail safe.
https://vimeo.com/109276008