I don't get it. With a budget of millions for these UAVs was it so hard to fit the thing with an IRS unit, or even an older INS unit to augment the GPS? Then

I don't get it. With a budget of millions for these UAVs was it so hard to fit the thing with an IRS unit, or even an older INS unit to augment the GPS? Then one could add a single if statement checking if the GPS location has diverged greatly from where it was in the last NMEA statement? What am I missing here???