*DEVELOPMENT OF A NEAR-ZONE COMPUTER MODEL FOR INVESTIGATION OF FEASIBILITY OF GROUND CHECKING THE CAPTURE-EFFECT GLIDE SLOPE , A Thesis Presented to the Faculty of the Graduate College of Ohio University In Partial Fulfillment of the Requirements for the Degree Master of Science Thierry LANGLOIS dlESTAINTOT *- JUt3 e +fsiGZi 1984 OHIO UTiIVERSITY LIBRARY TABLE OF CONTENTS Page I . INTRODUCTION AND BACKGROUND ............................. 1 A . Introduction ........................................ 1 B . Basic Theory of Image-Type Glide Slope Systems ....... 4 1 . Image theory ...................................... 5 2 . Near.field. far-field discrimination .............. 8 I1 . CEGS DESCRIPTION ........................................ 12 A . History .............................................. B . Capture Effect Glide Slope Operation ................. 111 . DEVELOPMENT OF A NEAR-ZONE CEGS MODEL ( TGCGS ) ......... A . Existing Models ...................................... B . Near-Zone Effects Considered by TGCGS ................ 1 . Sloping reflecting plane .......................... 2 . Finite conductivity ............................... 3 . Spherical wave propagation effect ................. 4 . Array factor ...................................... 29 5 . Reflector factor .................................. 33 6 . Ground plane surface roughness .................... 33 C . TGCGS Model Summary .................................. 37 D . Model Application .................................... 41 1 . Basic analytical approach ......................... 41 2 . Fault conditions .................................. 48 3 . Other perturbations ............................... 50 4 . Ground check location selection ................... 56 5 . Clearance signal checking ......................... 59 IV . DATA (MEASUREDAND ~DELED)............................. 61 A . Tamiami Airport ...................................... 61 B . 360-Degree Phase Proximity Point ..................... 64 V . CONCLUSIONS ............................................ 66 VI. RECOMMENDATIONS ......................................... 68 VII. ACKNOWLEDGEMENTS ........................................ 69 VIII. REFERENCES ........................................... 70 IX . APPENDICES ........................................... 73 A . Image-type Glide Slope Systems ....................... 73 B . Parameters Modeled by TGCGS .......................... 74 C . General Data for Tamiami Airport CEGS ................ 76 D . TGCGS Listing ........................................ 77 LIST OF FIGURES figure Page 1 . Geometry Illustrating a Possible Deficiency in Ground Measurements Caused by Terrain Irregularities Beyond the Measuring Point .......................................... 2 2 . Glide Slope Signals in Space: Above and Below Path CDI ...... 6 3 . Summary of Orientation and Excitation of Image Elements for Image Theory ............................................ 7 4 . Example of Image Theory Application: Current Element Parallel to the Ground Plane ................................. 9 5 . Composite CEGS Radiation Pattern ............................. 15 6a . Carrier Sideband. Ecs. Radiation Pattern ..................... 16 6b . Separate Sideband. EsS. Radiation Pattern .................... 16 7 . Clearance Signal Radiation Patterns .......................... 18 8 . DDM Structure With and Without Clearance Signal.............. 19 9 . Reference Coordinate System for TGCGS Model .................. 22 10 . Image Theory for Reflected Field ............................. 24 11 . Magnitude of the Horizontally-Polarized Reflection Coef- f icient vs . Incidence Angle for Various Ground Plane Electrical Constants (modeled) ............................... 26 12 . Geometry Illustrating How to Reduce Spherical Wave In- cidence into Infinite Series of Plane Wave Reflections ....... 27 13 . Spherical Wave Propagation Effect on Reflection Coefficient .4 (angle of incidence .80'. 0 .1 X 10 dm. E~ .3) .."" ... 30 14 . Antenna Products Company FA8976 .............................. 31 15 . Three-Element Colinear Antenna Array ......................... 32 16 . Modeled Array Factor vs . Azimuth Angle for Normal and Faulty System ...................................................... 34 17 . Image Theory Applied to Corner Reflector ..................... 35 18. Modeled Reflector Factor Magnitude vs. Elevation Angle ....... 36 19. Modeled Magnitude of the Roughness Coefficient vs. Angle for Various Roughness Parameters (roughness parameter = h) ....... 38 20. TGCGS Block Diagram.......................................... 39 21. Modeled CDI vs. Receiving Antenna Height at the 122244 Ground Check Point for a Normal System.............................. 40 22. Theoritical Airport Geometry with Grid Used for Primary TGCGS Tests........................................................ 43 23. CDI vs. Receiving Antenna Height for Grid Line X = 30,5m..... 44 24. CDI vs. Receiving Antenna Height for Grid Line X = 61m....... 45 25. CDI vs. Receiving Antenna Height for Grid Line X = 91.5m..... 46 26. CDI vs. Receiving Antenna Height for Grid Line X = 122m (in Front of the Antenna Mast) ................................... 47 27. Effect of Faults in CEGS System on Glide Path Performances... 49 28. Modeled CDI vs. Receiving Antenna Height for Normal System and for Broad and Narrow Alarms..............,............... 51 29. Modeled CDI vs. Receiving Antenna Height with -+ 15O Dephasing of the Lower Antenna......................................... 52 30. Modeled CDI vs. Receiving Antenna Height with -+ 15O Dephasing of the Middle Antenna......,.......................... 53 31. Modeled CDI vs. Receiving Antenna Height with -+ 15' Dephasing of the Upper Antenna.....,................................... 54 32. Modeled CDI vs. Receiving Antenna Height. Effect of Atten- uation in Antenna Currents..,................................ 55 33. Modeled CDI vs. Receiving Antenna Height for Various Reflect- ing Ground Electric Constants................,............. 57 34. Modeled CDI vs. Receiving Antenna Height for Various Snow Depths.....................................,................. 58 35. Clearance Signal Relative Strength for Normal and Fault Con- ditions.................................,................... 60 36. Control Points for ILS Site at New Tamiami Airport, Florida.. 62 37. CEGS Transmitter Mast Equipped with Three FA8976 Antennas.... 63 38. Measured and Modeled 01 vs. Elevation Angle for the 360' Phase Proximity Point Located Directly in Front of the Anten- na Mast.............................,...................... 65 I. INTRODUCTION AND BACKGROUND A. Introduction. Aircraft Instrument Landing Systems are designed to provide operational capability to permit the pilot of any appropri- ately equipped aircraft to approach and land at an airport during con- ditions of marginal visibility. Safety is clearly essential for air- craft operation. The performance record is impressive; these systems have been working for four decades and no accident has been attributed to ILS failure. Thus, the impetus for the work presented here is not safety, but cost. Currently, the integrity of ILS operation is maintained by costly periodic airborne masurements in addition to some form of near-f ield monitoring. A lower cost alternative, which is the topic of this paper, substitutes near-field ground measurements for the airborne measurements. A clearly apparent deficiency in ground measurements, as opposed to airborne measurements, is that path variations caused by terrain irregularities beyond the measuring point cannot be sensed. An illus- tration is shown in figure 1. To account for the effects of terrain irregularities using ground measurements, a program is outlined here which employs a terrain-sensitive glide slope model [l] in conjunction with near-field ground measurements to verify that a glide slope system is performing within safety tolerances. Terrain irregu larities beyond the measurement point resulting in possible path variation G.S. mast ground measurement ground I antenna I Figure 1 . Geometry Illustrating a Possible Deficiency in Ground Measurements Caused by Terrain Irregularities Beyond the Measuring Point. An additional consideration of the ground-based measurements is the sensitivity of the current near-zone monitor to environmental variations. Specifically, conditions can exist, such as snow on the ground plane [2], that cause the near-field monitor to sense an out- of-tolerance situation, although the far-field glide path is within tolerance. Circumstances such as this are clearly not in the best interests of safety, as they deprive the pilot of electronic guidance, often when it is most needed. To reduce the occurrence of unnecessary system outages resulting from the over-sensitivity of the near-field monitor, the measurement procedure presented in this paper can be per- formed as the monitor approaches on out-of-tolerance condition to determine whether the monitor indication is representative of the far- field signal or is merely influenced by some extraneous factor. The objective of this work is to investigate the feasibility of ground checking a glide slope facility both as a substitute for flight checking and for routine system maintenance. The means of this inves- tigation will be the use of computer models to study near-zone glide slope signal behavior and to correlate near-field measurements to far- field glide path characteristics. Basically, these measurements will be made by raising a horizontal dipole