Improved navigation by combining VOR/DME information with air or inertial data

The primary navigation aid for civil aircraft flying in the U.S. airspace, as well as the airspaces of most of the developed countries of the world, is the VOR/DME system. Using VOR and DME measurements, bearing and range relative to a fixed ground station can be determined onboard the aircraft. Even though this is a good navigation system, reductions in air traffic congestion and air controller workloads could be realized if still more accurate onboard navigation were available. Current practice is to use information from a single VOR/DME station. This work is concerned with determining the improvement in navigational accuracy obtainable by combining VOR/DME information (from one or two stations) with air data (airspeed and heading) or with data from an inertial navigation system (INS) by means of a maximum-likelihood filter. It was found that the addition of air data to the information from one VOR/DME station reduces the RMS position error by a factor of about 2, whereas the addition of inertial data from a low-quality INS reduces the RMS position error by a factor of about 3. The use of information from two VOR/DME stations with air or inertial data yields large factors of improvement in RMS position accuracy over the use of a single VOR/DME station, roughly 15 to 20 for the air-data case and 25 to 35 for the inertial-data case. As far as position accuracy is concerned, at most one VOR station need be used. When continuously updating an INS with VOR/ DME information, the use of a high-quality INS (0.01 deg/hr gyro drift) instead of a low-quality INS (1.0 deg/hr gyro drift) does not substantially improve position accuracy. Accurate in-flight alignment of an INS platform can be accomplished in about 30 minutes by using VOR/DME information. The accuracy of inflight alignment when using two DME's is about the same as for ground alignment, whereas when using one VOR/DME, alignment is less accurate by a factor of 2 or 3. Although the need for initial in-flight alignment of INS's onboard commercial aircraft is questionable, realignment of the system before a transoceanic flight could result in significant improvements in navigational accuracy. This might permit a reduction in separation requirements over the North Atlantic routes. Also, iii Preceding Page blank realignment after a transoceanic flight would result in more accurate position, velocity, and attitude information in the terminal area. Periodic realignment (every one or two hours) of a high-quality INS during a transcontinental flight results in significant reductions in position and velocity errors over unaided-inertial operation or the use of position display resets. If a realignment is performed just prior to entering the terminal area, accurate position, velocity, and attitude information would be available for approach and landing without reliance upon VOR/DME information. The performance of the air-data filter was found to be rather insensitive to wide variations in error model statistics. Also, in general, when combining VOR/DME information with air or inertial data, the suboptimal filter resulting when the DME bias errors are neglected performs nearly optimally.