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The Federal Aviation Administration (FAA) air traffic control system relies directly on aircraft locations provided by the long range en route surveillance radars. The accuracy of the radars is an important factor in determining the overall performance of the system. To support the planned modernization of the air traffic control system a study was conducted to measure the accuracy of the radar tracking function of the current system. The aircraft radar tracks were compared to the positions produced by the Global Positioning Satellite System (GPS), which was considered the true aircraft position. The GPS data was available from the FAA's Reduced Vertical Separation Minimum Certification Program. Utilizing the host air traffic management data distribution system at each air route traffic control center that captures the radar tracking data, 265 flight's of radar tracking data were compared to their GPS positions. Three distance metrics were used. The time coincident straight line distance, referred to as the horizontal track error, and its two orthogonal components: cross track error (side to side error) and along track error (longitudinal error) were calculated. A total of 54,170 measurements were taken. This resulted in an average horizontal error of 0.69 nautical miles, an average (unsigned) cross track error of 0.12 nautical miles, and an average (unsigned) along track error of 0.67 nautical miles.