Method of correcting measurement errors caused by clock deviations in a

Abstract

Conventional secondary radar systems use mechanically rotating antennas to radiate a concentrated beam that rotates in a horizontal plane. Data exchange between ground stations and airborne transponders can only take place if the aircraft is struck by the antenna lobe. The distance from the aircraft to the ground station is determined from the signal transit time. Directional information is derived from the antenna position. The secondary radar system according to the invention uses an omnidirectional antenna, so that data can be exchanged between ground station and aircraft at any time. To determine the position of the aircraft, interrogation signals are transmitted by a single active ground station such that the reply signal from a transponder falls within the common system time frame of the ground station. The times of arrival of the reply signals at different ground stations are then directly proportional to the distances from the aircraft to these ground stations. Measurement errors caused by clock deviations are corrected by determining any deviation of the clocks of two ground stations from each other. To correct clock deviations, two interrogation/reply cycles are used in direct succession in opposite directions over a common transmission path. The two transmission periods will differ by twice the deviation of the clocks in the two ground stations. The measured deviation is used to correct the measured transit time values and/or to synchronize the clocks. Synchronization of several ground stations with a master station can be achieved by using successive interrogation/reply cycles between the master station and all other stations.