Passive position fix system

A passive position fix system on a vehicle, the vehicle having a geographic position over the earth, comprising:

• (a) gyro means for providing a first gyro position of the vehicle;
• (b) map storage means for receiving the first gyro position and in response providing map-sets associated with geographic positions in the vicinity of the first gyro position provided by the gyro means, each map-set comprising a stored set of gravity gradients and a gravity anomaly at an associated geographic position on the earth;
• (c) gravity sensor means for providing a GSS-set, the GSS-set comprising a sensed set of gravity gradients values and a gravity anomaly value at the geographic position of the vehicle;
• (d) a preprocessor for storing the map sets and for serially reading out the map-sets and for storing the GSS-set and for reading out the GSS-set along with a map-set;
• (e) comparator means for receiving the GSS-set and for serially receiving the map-sets and for determining when there is a match between the GSS-set and a map-set and for providing a displacement vector based on which map-set the GSS-set;
• and
• (f) processor means for placing a second gyro position in the gyro means based on the first gyro position and based on the displacement vector.