Retired datacomms guy here.

I'd expect they're making better use of bandwidth than you'd think. I'd look to something like H.264 (thinking video compression not single image as there'd be multiple - most probably overlapping - views of whatever's of interest.

Having had a (very small!) bit of interaction with folks at the NSA (AKA 'No Such Agency') I can tell you there are some very smart people there - and signal compression is very related to encryption / decryption. I have no idea what they've tuned it to, but H.264 out of the box gets a 500x-ish ratio.

Just my $0.02

And thanks for putting all this in one place!