A very cool project!
I found a reddit AMA from one year ago at https://www.reddit.com/r/privacy/comments/gfw3ab/were_the_de...
They want to do it with MSS on the Ka-Band.
With the low number of satellites and the low orbit (400-700km) you'll need an unobstructed horizon. It could be great at sea!
Edit: I originally had a question here wondering how they expect to get the radio frequency licenses...