I see that the mini-helicopter on Mars uses a blend of gyro inertial reference, corrected by terrain scanning to allow for gyro drift rates...
Unfortunately there was a software glitch which caused the terrain scans to be processed out of synch with the inertial system, causing violent oscillation until the terrain scan was disabled. The helicopter landed OK, but had used a lot of uneccessary power due to the nav errors.
TERCOM has moved on a long way since it first came into existence, as have optical sensors and system memory. With occasional rad alt updates, TERCOM aided INS looks like a good potential on board system?