I think that in order to calculate your latitude and longitude position using what amount to high-tech sextant readings you have to know your local down vector to a high degree of accuracy. That is, the system judges the position of the Blackbird by comparing its local down vector with the angles to the star sightings. If the down vector measurement is off by an arcsecond, the measured latitude and longitude position on the earth will also be off by an arcsecond (at sea level, about 600 feet).

It seems to me that there's an interesting problem that the Blackbird goes so fast that its down vector changes dramatically over the course of a typical flight, so I'm wondering how it accurately updates its down vector reference. Jets can go through arbitrary and wobbly 3-d trajectories, accelerating in any direction, so it can't be a simple measurement of gravity. It would have to be a accelerometer combined with inertial and gyro and elevation readings all summed up.