A-terrain contour matching navigation system for an aircraft, the system comprising: an inertial navigation system 110 configured to output a navigation solution including a horizontal position of the aircraft and a vertical inertial altitude of the aircraft; a clearance altimeter 120 configured to output a clearance altitude of the aircraft above a terrain; a memory 130 programmed with a contour map 132; and a processor 140 coupled to the inertial navigation system, the clearance altimeter 120 and the memory 130; wherein the processor 140 is programmed with a reference basket selector 141 to select a reference basket area from the memory 130; wherein the processor 140 is programmed to calculate a first single sample point comprising the horizontal position of the aircraft and an altitude sample, the altitude sample based on a difference between the vertical inertial altitude and the clearance altitude; wherein the processor 140 is programmed with a correlation function 143 that correlates the first single sample point over the reference basket area to determine a position in the reference basket area corresponding to a predicted position of the aircraft; characterized in that the processor 140 is configured to determine a correlation quality; wherein when the correlation quality does not achieve a pre-determined quality threshold, the processor 140 is programmed to calculate an additional sample point and the correlation function 143 correlates the additional sample point over the reference basket area to determine the position in the reference basket area corresponding to the predicted position of the aircraft and further determine the correlation quality; and wherein when the correlation quality does achieve a pre-determined quality threshold, the processor 140 is programmed with a position error output function 146 that calculates a horizontal position error and vertical position error based on the position in the reference basket area corresponding to the predicted position of the aircraft; wherein the reference basket area includes an area of the terrain under the aircraft, as mapped by the contour map, which includes a longitude and latitude point provided by the inertial navigation system, and also includes an area around the longitude and latitude point based on uncertainty in the longitude and latitude information. an inertial navigation system 110 configured to output a navigation solution including a horizontal position of the aircraft and a vertical inertial altitude of the aircraft; a clearance altimeter 120 configured to output a clearance altitude of the aircraft above a terrain; a memory 130 programmed with a contour map 132; and a processor 140 coupled to the inertial navigation system, the clearance altimeter 120 and the memory 130; wherein the processor 140 is programmed with a reference basket selector 141 to select a reference basket area from the memory 130; wherein the processor 140 is programmed to calculate a first single sample point comprising the horizontal position of the aircraft and an altitude sample, the altitude sample based on a difference between the vertical inertial altitude and the clearance altitude; wherein the processor 140 is programmed with a correlation function 143 that correlates the first single sample point over the reference basket area to determine a position in the reference basket area corresponding to a predicted position of the aircraft; characterized in that the processor 140 is configured to determine a correlation quality; wherein when the correlation quality does not achieve a pre-determined quality threshold, the processor 140 is programmed to calculate an additional sample point and the correlation function 143 correlates the additional sample point over the reference basket area to determine the position in the reference basket area corresponding to the predicted position of the aircraft and further determine the correlation quality; and wherein when the correlation quality does achieve a pre-determined quality threshold, the processor 140 is programmed with a position error output function 146 that calculates a horizontal position error and vertical position error based on the position in the reference basket area corresponding to the predicted position of the aircraft; wherein the reference basket area includes an area of the terrain under the aircraft, as mapped by the contour map, which includes a longitude and latitude point provided by the inertial navigation system, and also includes an area around the longitude and latitude point based on uncertainty in the longitude and latitude information.