Final approach guidance using an altimeter-aided Loran-C display system
Massachusetts Institute of Technology. Flight Transportation Laboratory
Joint University Program for Air Transportation Research.
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Introduction: The goal of this thesis is to test the flyability of a display system that uses a King Radio KEAO-346 altimeter and Micrologic ML-3000 LORAN-C receiver for final approach guidance and to model the dynamics of the altimeter and the Micrologic ML-3000 LORAN-C tracking loop. The altimeter and LORAN-C receiver provide position information that provides navigation in the vertical and horizontal planes, respectively, and that is displayed as glideslope deviation and XTK deviation to the pilot. The display system will be subject to flight tests that will have the twofold purpose of testing the flyability of the display system and of determining the dynamics of the navigation equipment. The flight tests will be a set of missed approaches to a runway with an ILS. The testing of the flyability of the display system will be a qualitative analysis of a pilot's reaction to the display form. The analysis will consist of comments from the pilot who flies the flight tests. The flight tests will be simply a set of missed approaches to a runway with an ILS. The system dynamics will be determined by comparing the recorded altimeter and LORAN-C navigation data with the simultaneously-recorded ILS navigation data. The glideslope angle from the ILS data will be compared to that of the arctangent of the altitude divided by the range. The localizer angle from the ILS data will be compared to that of the arctangent of the XTK error divided by the range. By also modeling the altimeter and Micrologic LORAN-C receiver dynamics, the data comparisons will provide information on not only system dynamics but also individual component dynamics. The flight tests will have the aim to excite the dynamics of the LORAN-C receiver by doing zig-zag patterns during the approach. The comparisons between the ILS and display system data will be done under the assumption that ILS dynamics are negligible with respect to the system dynamics. LORAN-C is a hyperbolic line-of-position (LOP) system by which a receiver can be located at the intersection of two hyperbolas. This is accomplished by measuring the difference in arrival times between two pairs of pulses emitted from three fixed transmitting sites as ground waves. The transmitting stations may be designated as Master M, Slave X, and Slave Y. One hyperbola is determined by the X minus M pair of stations, the other hyperbola by the Y minus M pair of stations. Through the use of cesium clocks, each station transmits precisely-timed, pulsed RF signals. A pulse transmitted by the Master is received by Slave X, which will synchronize itself to the Master and then transmit its own pulse a fixed time later. The Slave Y station, also synchronized to the Master, will transmit a fixed time after it receives the Slave X signal, in order to avoid ambiguities. LORAN-C pulses are transmitted on a 100 kHz carrier in groups of eight pulses and with a group repetition (Master-Slave X-Slave Y) rate ranging from 10 groups per second to 25 groups per second. The pulses in a group are spaced 1000pus apart. A LORAN-C chain, which is a group of stations with one master and at least two slave stations, is distinguished from others by its group repetition interval (GRI), which is the time (in tens of microseconds) that the chain cycles through its master-slave transmission sequence. Currently, there are sixteen LORAN-C chains throughout the world. For the New England area, the common LORAN-C chain is the 9960 chain or the chain that has the GRI of 99600pis. In practice, there are a number of ways that are used to locate oneself using LORAN-C. One method is to locate the actual time differences (TD's) given by a LORAN-C receiver on a special LORAN-C map. For modern receivers, the TD's can be displayed as latitude and longitude so that a special LORAN-C map is not required. Other methods that come as options on most modern receivers are to have the receiver display numerically the receiver's range and bearing to a recorded waypoint or to have the receiver display graphically cross-track error from a path determined by two waypoints (starting point and destination). Since LORAN-C can only provide navigation in the local horizontal plane because pulses are transmitted as ground waves, other means such as a barometric altimeter are necessary to provide vertical navigation data for final approach guidance. Over the past two decades, because of the increase in processing power and the corresponding decrease in cost, LORAN-C has become a viable option for aircraft navigation. The increase in processing power has increased the speed by which LORAN-C signals can be locked onto and has decreased the volume of the receiver so that it can be considered as an optional piece of equipment for the cockpit panel. Airborne units can be purchased for as little as $400 per unit, exclusive of antenna and installation costs. Errors in TD measurement are set by the signal-to-noise ratio (SNR) and by the dynamic response of the tracking loop of the user's receiver. Errors in position determination can result from warpage in the local line-of-position (LOP) or from coordinate conversions such as from TD's to lat-long. Reference 9 shows that for static tests, the repeatability accuracy in over 90% of the average area in the Northeast and Southeast United States is better than 80 meters, and that in 50% of the same coverage area, the accuracy is better than 40 meters. The dynamic response of LORAN-C is limited by the response of the receiver's tracking loops to noise and vehicle accelerations. Studies by the Department of Transportation and the State of Vermont showed that LORAN-C accuracy met FAA AC90-45A specifications (Reference 3: 'Approval of Area Navigation Systems for Use in the US National Airspace System ') for enroute, terminal area,and non-precision approach use. Non-precision approaches using LORAN-C have become more acceptable to FAA approval, as exemplified by their approvals in the recent past for LORANC non-precision approaches at Burlington, Vt. airport and at Hanscom Field in Bedford, Massachusetts. The thesis will follow the methodology of the following outline. Chapter 2 will introduce the display form and the manner in which it displays the navigation information. Chapter 3 will look in detail at the flight test data-taking equipment and methodology. Chapter 4 will explain how the altimeter and LORAN-C tracking loop were modeled. Chapter 5 will show the flight test results and the analysis that was done on the results using the modeling from Chapter 4. Chapter 6 will then provide a discussion of the display's flyability and the data analysis. Appendix A will explain an experiment that was used to test the static accuracy of the altimeter; Appendix B will explain in detail the construction and certification of the flight test pallet; and Appendix C will provide the computer documentation for the computer programs used in the display and for data analysis.
Cover titleMay 1986Also issued as an M.S. thesis, Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1986Includes bibliographical references (p. 116-117)
[Cambridge, Mass. : Massachusetts Institute of Technology], Flight Transportation Laboratory, 
FTL report (Massachusetts Institute of Technology. Flight Transportation Laboratory) ; R86-3
Airplanes, Navigation (Aeronautics), Landing