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DESIGN of an IMAGE RADIATION MONITOR for ILS GLIDE SLOPE in the PRESENCE of SNOW a Dissertation Presented to the Faculty of The

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DESIGN of an IMAGE RADIATION MONITOR for ILS GLIDE SLOPE in the PRESENCE of SNOW a Dissertation Presented to the Faculty of The DESIGN OF AN IMAGE RADIATION MONITOR FOR ILS GLIDE SLOPE IN THE PRESENCE OF SNOW A Dissertation Presented to The Faculty of the Fritz J. and Dolores H. Russ College of Engineering and Technology Ohio University In Partial Fulfillment of the Requirement for the Degree Doctor of Philosophy by Frank Marcum August, 1995 ACKNOWLEDGMENTS The author wishes to thank Dr. Roger Radcliff for his support over the years and his help in organizing this material. Mr. Joe Shovlin is also recognized for his help in reviewing the technical papers associated with this material. TABLE OF CONTENTS Page LIST OF FIGURES GLOSSARY vii I. INTRODUCTION 11. PREVIOUS RESEARCH 111. DESCRIPTION OF GLIDE SLOPE AND PARAMETERS IV. CALCULATION OF FIELD FROM ANTENNA OVER GROUND A. The Optical Approximation B. Validity of the Optical Approximation V. GLIDE SLOPE PERFORMANCE VS. REFLECTION COEFFICIENT A. Null Reference B. Sideband Reference C. Capture Effect VI. EFFECT OF SNOW OVER GROUND VII. ANALYSIS OF SNOW EFFECTS A. Effects on Null Reference Glide Slope B. Effects on Sideband Reference Glide Slope C. Effects on Capture Effect Glide Slope D. Probability of Snow Type E. Effects of Rough Snow Surfaces and Terrain VIII. MONITOR DESIGN CONCEPT A. Monitor Error Budgets and Calibration B. Monitor Siting Criteria IX. CONCLUSIONS BIBLIOGRAPHY A. Ohio University Documents B. FAA Literature APPENDIX A. Tolerance Limits ABSTRACT LIST OF FIGURES Page 1. Vertical Position of Aircraft Relative to Course Deviation Indicator. 18 2. Two-Dimensional Geometry of Problem. 21 3. Geometry of Optical Problem. 4. Minimum Distances for Surface Wave and Space Wave Equality. 26 5. Characteristic Glide Slope Radiation Pattern. 27 6. Antenna Configurations for Glide Slopes. (Ref: Wilcox Glide Slope Manual.) 29 7. Multiple Reflections from a Multi-layered Image Plane. 3 5 8. Example of Reflection Coefficient for Snow of Increasing Depth over Ground. 39 9. Null Reference Cat I Tolerance Limits. 42 10. 3:l Sideband Reference Cat I Tolerance Limits. 43 11. 2.5:l Sideband Reference Cat I Tolerance Limits. 44 12. 4:l Sideband Reference Cat I Tolerance Limits. 44 13. Capture Effect Cat I Tolerance Limits. 45 14. Capture Effect Cat I11 Tolerance Limits. 46 15. Critical Snow Parameters that Cause Out of Tolerance Performance on Null Reference. Dielectric Constant = 1.4. 48 16. Critical Snow Parameters that Cause Out of Tolerance Performance on Null Reference. Dielectric Constant = 2.0. 48 17. Critical Snow Parameters that Cause Out of Tolerance Performance on 3:l Sideband Reference. Dielectric Constant = 1.4. 49 18. Critical Snow Parameters that Cause Out of Tolerance Performance on 3:l Sideband Reference. Dielectric Constant = 2.0. 49 19. Critical Snow Parameters that Cause Out of Tolerance Performance on Capture Effect, Cat I. Dielectric Constant = 1.4. 50 20. Critical Snow Parameters that Cause Out of Tolerance Performance on Capture Effect, Cat 111. Dielectric Constant = 1.4. 50 21. Conductivity vs. Relative Dielectric Constant for Dry Snow. 52 22. Roughness Reduction Factor. 54 23. Monitor Block Diagram. 56 vii GLOSSARY Alignment Error - difference between actual mean path angle and commissioned glide path angle. ~ypicallycaused by transmitter misalignment, antenna height error, or failure to account for sloping ground plane. AM - Amplitude Modulation. The amplitude of a radio frequency carrier is varied, or modulated in such a way that a simple detector circuit can receive the information encoded. Below Path Clearance - guarantees that the CDI needle will always be at full scale deflection when the pilot is well below the glide path and above any obstacles. Bend - long-duration deviation of the course from the nominal path angle. Typically caused by scatterer very close to the glide slope, or slow change in ground plane with distance. CDI - Course Deviation Indicator. A cockpit device that displays ddm scaled by a microamp value. An ammeter is used to locate the aircraft position relative to the on- course location, where a zero ddm value is measured. For example, if the pilot is below the glide path, the needle is above its centered reading. CSB - Carrier Plus Sideband. That portion of the ILS signal that contains 90 Hz and 150 Hz AM sidebands that are modulated in phase and broadcast vith the RF carrier. ddm - Difference in Depth of Modulation. For ILS, two audio viii tones (90 Hz and 150 Hz) are space modulated onto the radio frequency carrier. At different points in space, varying levels of each tone can be received. An ILS receiver measures the amplitude of each tone versus the carrier level; the depth of modulation. The difference in these modulation levels is converted to a bipolar voltage that drives a cockpit display for aircraft guidance. In the case of glide slope, a greater amount of 90 Hz (fly down) tone indicates that the pilot is too high; a greater amount of 150 Hz (fly up) tone indicates he is too low. FAA - Federal Aviation Administration. Glide Path Angle - the mean angular path along which a glide path receiver measures equal amounts of 90 Hz and 150 Hz tones or 0 ddm. ILS - Instrument an ding System, the current radio navigation landing aid. ILS consists of: 1. localizer, for horizontal alignment with a runway intercept point or runway centerline. It operates in the frequency range between 108 and 112 MHz; 2. glide slope, for vertical guidance for rate of descent to a point of decision or touchdown point on the runway. It operates in the frequency range between 329 and 336 MHz. The two combined steer the aircraft to a decision point at which time the pilot should be able to see the runway and complete his approach. Roughness - very short-duration, seemingly random deviations ix of the course from the nominal path angle. ~ypically caused by scatterers distant to the glide slope. SBO - SideBand Only. That portion of the ILS signal that contains only 90 Hz and 150 Hz AM sidebands. The 90 Hz tone is out of phase with that on the CSB. Space Modulation - a phased array of antennas can produce desired maxima and minima in space. The CSB and SBO signals on the glide slope are space modulated so that their sum produces a predominance of 150 Hz tone below the glide path and a predominance of 90 Hz tone above the path. Scalloping - medium-duration, cyclical deviations of the course from the nominal path angle. Typically caused by scatterer in the vicinity of the glide slope. Symmetry - a quality factor expressing the amount of equality of angular excursion above and below the path at the width points. Equality is defined at 50%. Width - the angular excursion between specific ddm levels equal to k0.08875 ddm for glide slope (equivalent to +75 uA) . 10 I. INTRODUCTION The problem of monitoring the performance of the Instru- ment Landing System (ILS) image-type glide slopes has been investigated for a number of years. Both experimental and theoretical studies have yielded information about system performance, but the problem was by no means completely solved. Major error sources contributing to glide slope performance were identified as electronic component drifts and/or failures, scattering from nearby reflective surfaces, and changes to the ground plane in the vicinity of the glide slope. Transmitter signal errors can affect the radiated antenna signals that form the glide path and course width. Radiated signal integrity is verified by integral monitoring [I]. An integral monitor samples the antenna currents, verifying that the transmitted signals are capable of generating the commis- sioned path angle and course width. Integral monitors are calibrated by flight measurements to determine what changes in transmitter signal cause the path and width to go out of tolerance. The integral monitor accurately senses transmitter and antenna changes that affect the far-field, but it cannot detect changes in the environment that affect the ground- reflected signal [2]. Reflective objects near the glide slope produce multipath errors, which cause roughness, course bends, and scalloping in the approach region. ILS critical areas [3] were estab- 11 lished to reduce multipath interference from objects such as structures, vehicles, and aircraft stopped on the ground. The ILS critical area is a region in front of each radio naviga- tion antenna system where these objects are restricted. This procedure reduces certain types of errors to what might be expected from terrain irregularities. There are two sources of radiation necessary to form the glide slope signal. These are the signal radiated directly from the antenna and its ground-reflected image. Addition of standing water or snow cover to the path-forming region of the ground plane in front of a commissioned facility can change the image radiation characteristics. Changes to the ground plane are currently addressed by the ILS Maintenance Manual [4]. This procedure calls for visual inspection of the critical area or use of a snow depth monitor [5]. The snow depth monitor is a sonar-like device that sends an alarm to the system when the ground plane rises due to snow. When the snow reaches a certain depth, a Notice to Airmen is published, advising pilots not to use the glide slope (forcing higher landing minima) until the snow can be removed from the path- forming region. The FAA does not measure the image radiation nor the electrical parameters of snow. One drawback to the snow depth monitor is that the measurement is highly localized. Only the snow depth near the monitor is measured. If snow drifts are present throughout the path-forming region, the effective depth in front of the glide 12 slope may be different than what is measured by the monitor. The now-defunct near-field monitor attemptedto determine the variance of path and width by signal measurements in the near-field of the antennas.
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