Continued study of NAVSTAR/GPS for general aviation

11II11111111111111111111111111111111111111111111111111I111111111 3 1176 00138 6961 NASA-CR-159145 19800009756

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NASA CR-159145

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CONTINUED STUDY OF NAVSTAR/GPS FOR

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GENERAL AVIATION

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Final Report

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lIn', dt,l "" .... Y

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MAR 7 1980

Prepared for '

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National Aeronautics and Space Administration

langley Research Center

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Hampton, Virginia

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IRE SEA R C H T R I A N G L EPA R K, NOR THe A R 0 LIN A 27709

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CONTINUED STUDY OF NAVSTAR/GPS FOR GENERAL AVIATION

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Fi na 1 Report

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Prepared Under Contract NAS1-14719

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by

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R.D. Alberts and W.H. Ruedger

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Research Triangle Institute Research Triangle Park, North Carolina 27709

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Prepared for

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National Aeronautics and Space Administration Langley Research Center Hampton, Virginia

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December 1979

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I ACKNOWLEDGMENT

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This report was prepared by the Research Triangle Institute, Research Triangle Park, North Carolina, under Contract NASl-14719. The work has

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been administered by the Avionics Technology Research Branch of the Flight Instrumentation Division, Langley Research Center, National Aeronautics and

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Space Administration. Mr. H. J. E. Reid, Jr. served as Technical Represen- tative.

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Program studies began on 3 December 1976 and were completed on 2 March 1979. Mr. R. D. Alberts served as Laboratory Supervisor and

I Mr. W. H. Ruedger as Project Leader. Dr. J. G.Haidt provided a significant

contribution in conducting the ground augmentation analysis.

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ABSTRACT This report describes the activities conducted to provide systems

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engineering and planning support for examining the full potential of GPS for the general aviation community. The report presents a conceptual approach

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to this goal and discusses aspects of an experimental program to demonstrate these concepts. The report concludes with the observation that the true

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potential of GPS can only be exploited by utilization in concert with a data link. The capability afforded by the combination of position location and

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reporting stimulates the concept of GPS providing the aUXiliary functions of collision avoidance, and approach and landing guidance. A series of general

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recommendations for future NASA and civil community efforts in order to continue to support GPS for general aviation are included.

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TABLE OF CONTENTS

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Acknowledgment

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Abstract 1 1.0 Introduction

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2.0 Study Objectives •.

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5 3.0 Concepts •...

Introduction .• 3.1

6 . . . . . System Attributes 3.2

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. . . . . . 3.2.1 Loran [3-1J

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7 3.2.2 Omega [3-1J •.••• 8 3.2.3 Global Positioning System • 11 3.2.4 Continental Positioning System ••.•..•••

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Comparative Attributes .•••••• 3.3

16 . . . Data Link Advocacy .•••.•.•• 3.4

17 Clock Impact .••••••.••..•.. 3.5

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18 3.6 Comparative Concepts (Realization Potential) .•••• 3.7 28 Economic Considerations .••..•.•

33 Experimental Program Objectives ••..••. 4.0 I

4.1 Concept .

34 4.2 Evaluation Definition Guidelines 36 4.3 General Objectives .•••••••.•

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36 4.4 Specific Performance Evaluation Areas 39 4.5 Example Experiment Definition.

· . . • 43 4.6 GPS/Data Link Research Facility Considerations

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49 5.0 Recommendations ••••

51 . . . . . . . . . . . . Appendix A - Data Link Survey I

85 Appendix B - Ground Augmented GPS - Pseudolite Analysis.

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• • 141 Appendix C - RNAV System Survey . 187 Appendix 0 - Evaluation Facility Examples.

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References

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LIST OF FIGURES

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Figure Number Title Page

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3-1 Typi cal GPS Performance Over Inverted Range •

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3-2 Ground Augmentation Geometry. 23

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3-3 VDOP Contours in Vertical Runway

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Plane for Configuration IV: 3S (0, B , B ) and P (k=I.0) 24 L R

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3-4 VDOP Contours in Vertical Runway Plane for Configuration III: 3S (B , B , A) and P (k=I.0) 4

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L R

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3-5 Different i al GPS Geomet ry

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3-6 Low Cost GPS Receiver Block

Di agram . . . 31

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4-1 Experimental Program Philosophy 35

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4-2 On-Board Test Bed Configuration

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4-3 Evaluation Facility Functional Organization. 44

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4-4 Detailed Functional Flow Diagram. 45

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4-5 Analysis Module Description 48

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A-I JTIDS TDMA System Concept 56

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A-2 JTIDS Functional Requirements

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A-3 JTIDS Signal Timing and Waveform

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Structure 62

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A-4 JTIDS Terminal Configuration. 69

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A-5 DABS Transponder • 73

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A-6 DABS Interrogation and Reply Formats 74

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A-7 DABS Data Block Formats 77

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A-8 SM Interface Timing Di agram for DABS Transactions 80

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LIST OF FIGURES (continued)

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Figure Number Titl e

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Illustration of Landing Geometry Used B-1

103 . . . . . to Evaluate Pseudolite Augmentation

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B-2 Four-Satellite Cluster Serving as

105 . . . . Baseline Configuration ••••

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B-3 Variation of HOOP, VOOP, and TOOP with Cone Angle for Baseline Configuration.

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B-4 Variation of HOOP, VOOP, and TOOP with Elevation of Satellite #1 for Baseline Configuration •••••••••••••

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B-5 Variation of VOOP Along Glide Path for Three Alternative Beacon Configurations

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B-6 Variation of TOOP Along Glide Path for 112 Three Alternative Beacon Configurations

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Variation of HOOP with Cone Angle for B-7 Three Alternative Beacon Configurations at Touchdown Point ••••••••••

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B-8 Variation of VOOP with Cone Angle for Three Alternative Beacon Configurations

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at Touchdown Point •.••••..•. 115 B-9 Variation of TOOP with Cone Angle for

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Three Alternative Beacon Configurations at Touchdown Point •••••••••• B-10 Variation of HOOP with Cone Angle for

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Three Alternative Beacon Configurations 117 at Approach Point •••••••••••

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Variation of VOOP with Cone Angle for B-l1 Three Alternative Beacon Configurations 118 at Approach Point •••••••••.•

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Variation of TOOP with Cone Angle for B-12 Three Alternative Beacon Configurations 119 at Approach Point •••••••••••

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B-13 Variation of VOOP Improvement Factor with Cone Angl e for Two Pseudol ite Confi gurat ions

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at Touchdown Point ••••••••••••• Variation of VOOP Improvement Factor with B-14

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Cone Angle for Two Pseudolite Configurations 121 at Approach Point ••••••••••••••

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LIST OF FIGURES (continued)

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Titl e Figure Number Page

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Overall GPS System Concept ••• C-l

DNSS • • • • • • • • • • C-2 I

Composite Block Diagram, Time-Shared C-3 155 DNSS/621B Receiver ••

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RF Processor/L.O Synthesizer •• C-4

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161 Block Diagram, PN Demodulator C-5 Carrier Loop. C-6

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. . . . . Code Clock Derivation

164 C-7 166 Code Pre-positioning. C-8

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Search Process • • • • C-9

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LORAN-C Transmission Pulse 173 C-I0 Operation of LORAN-C Receiver C-ll

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Proposed LORAN-C Coverage of U.S. Coast C-12 177 Guard Implementation Program ••.•••

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Proposed LORAN-C System for Complete C-13 Coverage of Conterminious United States a nd Ala ska • • • • • • • • • • • • • • •

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183 Omega Navigation Signal Format. C-14

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Differential Omega Position Fix C-15

186 . . . . . . . Accuracy • • • • • • • • • • •

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Systems Engi neeri ng Laboratory 0-1 190 Organization ••••••••

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Interconnection of the Simulation 0-2 192 Facility Equipment •••••••

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197 Proposed AFAL Laboratory Configuration •• 0-3 Simplified Block Diagram of DAIS ITB 0-4 Faci 1 ity • • • • • • • . • • •

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LIST OF TABLES

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Table Number

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Page 3.4.1 GPS User Equipment Measurement Accuracies.

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3-1 621B Enroute Navigation System Accuracy Summary •

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3-2 621B ILS Navigation Accuracy Summary

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3-3 Navaid Matrix

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3-4 Candidate Ground-Based Navigation Systems

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Candidate Satellite-Based Navigation Systems . .

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3-5 Radar Altimeter Survey.

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3-6 Ground Transceiver Accuracy - Rationale 22

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3-7 GPS Navigation Error Summary. 26 •

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3-8 User System Cost Comparison ($1,000) 29

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3-9 Estimated Cost of GA Avionics

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4-1 Specific Performance Evaluation Areas 36

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4-2 Five Channel Receiver - Redundancy/

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Accuracy • . 40

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A-I JTIDS User Classes 57

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A-2 JTIDS System Characteristics 57

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A-3 Waveform Characteristics 61

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A-4 JTIDS (Class 3) Bands of Performance

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Characteristics 65 •

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A-5 Comparison of DABS vs. ATCRBS Characteristics 72

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C-1 GPS Error Budget •

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C-2 Required User Equipment Received Signal Levels 151

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LIST OF TABLES (continued)

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Table Number Title Page C-3 RF Link Calculation of User Received

Power . . . . . . 151

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C-4 Receiver Sensitivity (Theoretical) . . 160

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C-5 Sweep Parameters 168

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C-6 LORAN-C Costs (CCF) 179

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C-7 Summary LORAN-C Costs 180

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0-1 Laboratory Capabilities/Resources

Matrix . . . . . . . . 195

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1.0 INTRODUCTION

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The Research Triangle Institute, previously, under contract NASl-14302

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ll entitled IIpreliminary Study of NAVSTAR/GPS for General Aviation conducted a planning effort to focus attention on the applicability of the Global

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Positioning System for General Aviation. During that study, Department of Defense literature was reviewed and a description of the GPS concept, sys-

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tem definition, and implementation program was formulated. Comparative costs and achievable performances were examined for GPS and for conven-

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GPS to the general aviation community at a reasonable cost in a 1985 time frame.

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As a part of the previous study, recommendations were made for subse- quent activities which would continue to define the nature of NASA partici-

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pation in developing GPS technology for general aviation.

A major theme implicit in these recommendations is the concept of an

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integrated Communications Navigation and Identification capability for general aviation to be implemented over the next two decades. A low-cost

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navigation system as derived from GPS in concert with a low-cost data link also derived from existing military programs provides the cornerstone from which such a capability can be achieved.

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The current study continues to support this theme. Its general objective is to provide systems engineering and planning support for a

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low-cost, satellite-based communication/navigation capability to general aviation and to other NASA programs. Major tasks performed as a part of

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this study include the analysis of GPS navigation capability with ground augmentation, a cursory survey of data link systems for application to

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general aviation, and a comparison of various generic candidate navigation system concepts. In the performance of these tasks, the interplay between

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promising approaches to ground augmented navigation include systems requiring data link to the user aircraft.

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A major conclusion of this study is that the high accuracy available with GPS is not a requirement for enroute navigation; thus, GPS provides

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very little capability over conventional systems in this flight segment.

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It is in the terminal area environment, where advantage of GPS precision . can be taken, that a distinction occurs. It is the availability of the

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precision position that provides the advocacy of incorporating a data link with GPS. This results in the ability of the integrated system to provide

auxiliary functions which normally would not be available to General I

Aviation at a nominal cost. These functions include collision avoidance and guidance for approach and landing. Significant in this light is that

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to whatever degree is practical, these functions, can be self-contained.

The following sections of this report will develop the idea that

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operation of GPS in a differential mode presents a strategy for expanding the role of GPS for general aviation to that of providing auxiliary func-

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tions such as approach and landing guidance' and collision avoidance. The feasibility of this strategy is to be the subject of a future study.

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2.0 STUDY OBJECTIVES The basic objectives of this study were two: first, to determine the

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ultimate potential of GPS in serving the general aviation community, and second, to identify and define the research and development activities

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required to realize this potential. It will be discussed in subsequent sections that the potential for GPS in general aviation lies in the

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auxiliary application directed system functions which develop through integration with a data link. It will be recommended that this potential

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be demonstrated through the conduct of an experimental program including flight test.

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3.0 CONCEPTS

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3.1 Introduction

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The definition of concepts is based on drawing from conventional systems and then configuring in such a way as to support basic communi- cations and navigation functions. The extrapolation to considering the

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capability available when both functions are integrated leads to several innovative ideas. This section presents the background for these consider-

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ations. Included are brief system descriptions of basic navaids and data links followed by the definition of candidate system configurations.

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Detailed description of these basic systems are included as appendices. It should be noted that the system descriptive information has been extracted

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from the 1 iterature as referred to at the end of thi s ·report.

Reference 3-1 presents a survey of present and forecasted navigation

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technology through 1980. This discussion has been condensed and included here in that it tends to focus on generic capabilities and limitations as they relate to candidate avionics suites. While the referenced study was

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conditioned on military mission scenarios, it is felt that the carry-over to the general aviation requirements of enroute, terminal area, and landing

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is easily accomplished. For example, the military aircraft essentially has point "A" to point "B" navigation requirements with intervening way points

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as does general aviation; further, while in the target area, the military aircraft has weapon and material delivery guidance requirements similar to

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terminal area navigation for general aviation; finally, the military aircraft has similar or possibly more stringent landing guidance require-

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are considered. The following discussion addresses the LORAN, OMEGA, and GPS navaids as well as addressing a hypothetical satellite-based system

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directed at civil application. This discussion serves as the technology overview required to integrate the navigation side of a combined

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communications/navigation system with the goal of increased functional application.

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3.2 System Attributes 3.2.1 Loran [3-1J Loran-C is a low-frequency, long-range, all-weather, pulsed-hyper-

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bolic radio navigation system which is capable of providing horizontal position fixes of very high repeatable accuracy, but only moderate absolute

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accuracy. Loran-C is presently operational in several parts of the world and an extensive expansion program is currently underway. The military has

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been using Loran-C for the past 10 to 15 years; thus, numerous Loran-C user equipments are available. Normal operation of Loran-C is as a hyperbolic

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navigation system, although Loran-C can also be used in a direct ranging mode at the expense of increased complexity of user equipment. This direct

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ranging mode is capable of providing increased accuracy through a reduction in user/Loran transmitter geometry dependency errors.

Loran-C service in a region is provided by a chain of transmitting

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stations consisting of a master and three or more secondaries. Each chain can provide service to an area within 2500 km from the chain baselines.

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The U.S. Coast Guard is currently responsible for the operation of seven Loran-C chains throughout the world. Fourteen additional stations are

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planned to complete the coverage of the U.S. coastal Confluence Zone and a large portion of the northern hemisphere.

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Loran-D is similar to and compatible with Loran-C but designed for military tactical use. There are presently three Loran-D chains in the

U.S. and one in Europe. I

Performance Capabilities The Loran-C navigation system is capable of providing absolute naviga-

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tion (horizontal position) accuracies of 0.5 km rms or better and repeatable navigation accuracies of 15 - 90 m rms. The absolute accuracy

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can be improved by a factor of 2 or 3 using sophisticated user equipment and prior calibration of propagation abnormalities.

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The Loran-C system does not inherently provide for velocity measure- ments. However, the Loran-C time'difference (TO) measurements can be dif-

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ferentiated to provide velocity estimates, although the resultant accuracy will likely be poor as well as noisy.

The Loran-C user equipment can potentially be integrated with an I

inertial navigation system (INS) to provide velocity estimates with an

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accuracy set by the INS (typically 0.5 to 1.0 mps rms). This is, however, beyond the cost capability of most general aviation users.

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Performance Limitations The Loran-C navigation system is currently usable only within the designated coverage areas, and does not provide CONUS coverage.

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3.2.2 Omega [3-1J

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OMEGA is a terrestrial-based radio navigation system with global all weather coverage that is capable of providing moderately accurate position

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fixes. The system operates with eight very low frequency (VLF) transmitter stations located around the world. The eighth permanent transmitter, to be

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located in Australia, is scheduled to become operative in late 1979.

However, the present operational system provides coverage for most of the

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globe.

Performance Capabilities The Air Force OMEGA navigation equipment specification requires accu-

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racies of 4km or better, 50th percentile and 8km or better, 95th percentile. Such performance was indeed demonstrated on Air Force

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conducted performance tests. While the OMEGA system does not inherently provide for velocity measurements, the (military) airborne equipment should

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internally implement dead reckoning for velocity aiding of signal tracking and for minimizing of lane-jump probabilities during temporary loss of the

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OMEGA signal. The dead reckoning capability is enhanced by automatic compass heading and true airspeed inputs. System velocity errors

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commensurate with dead reckoning navigation from these devices are to be expected if they are installed in the aircraft. Maneuvering susceptibility of current system,design is restricted to sustained high bank angle

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conditions and is not expected to be an operational problem.

To increase the airborne navigation accuracy available with Ot1EGA

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(especially in high performance aircraft) an inertial navigation system (INS) can be integrated with the OMEGA navigation equipment. According to

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reference 3-1, OMEGA navigation equipment integrated with a 0.5 mps (CEP rate) inertial navigation system, position estimates can be obtained with

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an accuracy of 2km CEP. Also, since an INS is employed, velocity estimates are available with an rms accuracy of 0.5 - 1.0 mps.

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Performance Limitations Although OMEGA is a long-range) moderate accuracy global navigation system) it cannot be considered for (Military) use as a primary) autonomous

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navigation system in either strategic or close tactical operations. Its absolute accuracy is limited by sudden phase distortions due to ionospheric

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disturbances and by polar cap absorption effects) both of which though infrequent) can lead to temporary errors as large as 15km without special

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mechanizations. The civil OMEGA system should be regarded as a system for enroute navigation to a way point with transition required to other navaids

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such as GPS) Loran) and/or other routine terminal aids (TACAN) VOR) or PAR) when in the vicinity of the terminal area.

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3.2.3 Global Positioning System (GPS) GPS is a satellite-based radio navigation system designed to provide

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highly accurate navigation fixes to properly equipped users. This system is presently in development stage) but limited operation is scheduled to

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commence in the mid 1980's with full operation in the late 1980's. Several types of GPS user equipment are being designed to satisfy various user

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requirements. Preliminary designs are to emphasize commonality of equipments as much as possible in order to reduce costs.

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Performance Capabilities The accuracy with which a GPS user can determine his position and

velocity depends upon the measurement accuracy of his user equipment as I

well as the error in the satellites ephemeris and clock data received from the satellites. The measurement accuracy of the military user equipment

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depends upon the equipment type. The Air Force has identified three types of user equipment for the various Air Force aircraft missions. The Type X

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user equipment is designed for the tactical high dynamics mission) Type Y for the strategic mission) and Type Z for the benign transport mission.

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The user equipment makes pseudorange and pseudorange-rate measurements to the GPS satellites. These are not true range and range-rate measurements

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since the time the signals were initiated by the satellite is not known accurately. However) with four sets of these measurements) the user is able to estimate his position and velocity as well as the GPS system time.

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The pseudorange and pseudorange-rate measurement accuracies of the three

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types of user equipment as specified by the Air Force [3-2J are summarized in Table 3.4-1.

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Tab 1 e 3.4-1 GPS User Equ i pment j'1easurelnent Accuraci es

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Pseudo- Pseudo- Smoothing Range Range

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Remarks Times (sec) Type Rate (mps) (meters) P-Si gna 1 Cj A-Si gna 1 Set

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High Performance >0.1 0.006 15.0 1.5 x

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Medium Performance >0.1 0.006 15.0 1.5 Y

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LOVI Cost >0.1 0.006 15.0 z

The other major influence on the accuracy of position and velocity

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estimates are the errors in the satellite ephemeris and clock data. This data is required by the user in the solution of the GPS navigation equa-

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tions. These (pseudorange, ephemeris, and clock) errors tend to be the dominate source of errors and thus determine the overall system accuracy

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1 imits.

These pseudorange errors translate into equivalent position errors

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through standard geometrical translation. The Aerospace Corporation has indicated [3-3J that 99.9% of the time the error magnification effects of

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geometry are less than a factor of about four and that 99% of the time the magnification is less than a factor of three. The pseudorange accuracies shown in Table 3.4-1 then translate to position accuracies of about 5

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meters for the X and Y sets and 50 meters for the Z set.

The accuracies as discussed above represent user equipment specifi-

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cation numbers and the extent ion of these based on geometrical analyses.

It is of interest to compare these with actual results being obtained using

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the Yuma Proving Ground inverted range in conjunction with one or both of the first two satellites in orbit. Figure 3-1 shows representative data

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[3-4J indicating actual performance being attained with GPS. The results

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GPS POSITION ERROR (GPS - RTE) • 1ST RESULT

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C·141. 20 SEP 77 • REAL TIME COMPARISON • ONE SPACE VEHICLE CLOCK +25

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-25 X ERROR (m) "x = 3.36 (m) +25

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-25 Y ERROR (rn) "y = 4.18(m) +25

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Z ERROR (m) "z = 3.60(m) -25 '-----'-_-.L_--l..-_.L-_-.-L .. _.L .. _ . L._ .... l .... L_ .. _ 1 I .L_ .. .L.. _.l...----lIL o 5 10 15 20 TIME (MINUTES}

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Figure 3-1. Typical GPS Performance Over Inverted Range [3-4J

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are from a C-141 flying at an altitude of 6km. Presented are the observed

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differences between the GPS position estimates and the Real-Time Estimate (RTE) of trajectory. The author identifies the several spikes as resulting

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from tracking instrumentation performance degradation during turns. The author states that these transients disappeared on subsequent flights utilizing the Best Estimate Trajectory (BET).

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Performance Limitations GPS is, by comparat ive Ifleasure, relTlarkab ly free of obvi ous performance

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1 imitations. The major 1 imitation envisioned for the mil itary is stated to be interference by jamming. For the civil user this translates to inter-

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ference by such sources as rnult i path and/or 1oca lly i nterferri ng systems such as VORTAC, TACAN, DABS, and ATCRBS. The GPS signal structure provides

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some degree of immunity, but susceptibility nonetheless rernains. The evaluation of these effects is an aspect of the experimental proyrani

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discussed in subsequent sections. The military has posed a solution of utilizing highly directional steerable receiving antennas to avoid this occurrence. This is not viable for the civil community in that antenna

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placement, even omrnidirectional coverage, is problematical and increased complexity simply complicates the problem at increased cost.

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3.2.4 Continental Positioning Systerfl

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This system is a hypothetical system ~,hich is configured, here rather arbitrarily, to address the general aviation navigation requirements from a

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satellite-based navigation system perspective. The system is predicated both on providing coverage for the CONUS and Coastal Confluence Region (CCR) only and on relaxing the anti-jam requirement ~/hile retaining the

I

pseudorandom code ranging signal structure. The implication of CONUS and CCR is that coverage may be obtained \'iith a single sate11 ite constellation.

I

If this yJere to be at geostationary altitude, the number of satell ites to provide the necessary coverage is minimized (i.e., a total of four). This

I

implies loss of satellite redundancy as \-,e11 as the negation of improved accuracy through the smoothing available \Jith an overspecified system

I

empl oyi ng more than four satell ites such as cPS.

I

I

I

I

I

Performance Capabilities Since the Continental Positioning System is a hypothetical system, no true performance estimates are available. However, since the CONUS cover-

I

age assumption alluded to a single satellite constellation, it is of interest to compare this system concept to the NAVSTAR/GPS predecessor,

I

that is, the Air Force sponsored 621B system. This system is described in further detail in the Appendix C. 6218 was designed for simultaneous high

I

precision and anti-jam capability \vhich is reflected in the complex signal structure and as in GPS is overly sophisticated for general aviation. The

I

point made here is that 6218 demonstrates that a small number of satellites at geosynchronous altitude does provide the geometrical aspects of a viable

I

system. The ex~ension of this approach is therefore appl icable to general aviation.

The 621B system has also been tested at the Yuma inverted range [3-5J.

I

The basic objective of these tests was to validate the theoretical predic- tions of receiver bias and noise errors. These tests consisted of flying

I

two receivers (Hazeltine and Magnavox) and performing non-real-time naviga- t i on by recordi n9 range and range-rate to the ground transmitters and then

I

reconstructing a navigation solution post-fl ight. No attempt was made to compare the two receivers. Sample results of these tests are shown in

I

Tables 3-1 and 3-2. These data indicate that GPS equivalent performance can be achieved from the CONUS/geostationary approach to satellite naviga-

I

t i on system.

Performance Limitations This approach to satellite-based navigation has several disadvantages

I

above and beyond those addressed for GPS. Essentially, all of the limita- tions are geometry related. For example, there is no inherent constella-

I

tion redundancy, capability for constellation optimization for GOOP, or potential for improved accuracy through the use of more than four satel-

I

lites for a smoothed solution. Further, the increased range to geosynchro- nous orbit places heavier requirements on transmitter power and/or impacts

I

the received signal margin. The other limitations discussed for GPS such as multipath, interference, etc. also affect the Continental Positioning

System to some extent dependent on system parameters such as carri er I

frequency, signal structure, etc.

I

I

I

Tab1 e 3-1 621B Enroute Navi gat i on System Accuracy Summary [3-5J

I

I

POSITION MEAN RATE MEAN 1<1 1<1 m mps

I

X X HC -0.30 1.77 HC 0.03 0.18 -0.70 1. 86 MRL 0.03 0.15 MRL :

I

y

Iy

HC 0.12 3.14 HC -0.03 0.18 fvlRL 0.40 2.50 MRL 0.06 0.21

I

Z Z HC -1.25 3.66 HC 0.09 0.18

I

0.49 2.59 fvlRL 0.27 MRL -0.15

I

I

Table 3-2. 6218 ILS Navigation Accuracy Sumrnary [3-5J

I

POSITION ~lEAN 1 0" RATE MEAN 10"

I

m nus X X

I

HC 0.09 1.01 HC 0.06 0.34 fvlRL -0.34 2.23 r~RL 0.06 0.43 I

I

y y HC -0.82 3.47 HC -0.15 0.52 MRL 0.67 3.75 i~RL -0.06 0.67

I

Z z

I HC -0.40 4.05 HC -0.06 0.70

I

MRL -0.40 2.53 r~RL -0.12 0.61

I

HC - Ilazeltine Receiver MRL - Magnavox Receiver

I

I

I

I

I

3.3 Comparative Attributes In attempting to compare relative navigation systems, it is convenient to simply examine the matrix shown in Table 3-3 in rather general terms.

I

Table 3-4 presents a system comparison in considerable detail. Both LORAN-C and GPS possess

I

Navaid Matrix Table 3-3.

I

satell ite-based land-based systems systems

I

LORAN-C GPS precision systems

I

OMEGA Continental medium-accuracy Posit i oni ng systems System*

I

*postulated medium accuracy

I

sufficient accuracy to provide preclslon navigation for general aviation while the hypothetical Continental Positioning System could also be up-

I

graded to this category. All systems could conceivably be operated in a differential mode to further improve accuracy. It is not likely that any

I

system, with the exception of GPS, and then in a differential mode, could address the requirements of navigation during approach and landing due to

I

lack of vertical position as well as accuracy considerations.

All systems, with the exception of LORAN-C provide at least CONUS

I

coverage. LORAN-C presently does not provide interior CONUS coverage although the addition of five stations to accomplish this has been pro- posed. GPS and OMEGA provide global coverage and as such are supportive of

I

ICAO requirements.

Another interesting comparative feature among systems over and above

I

that of accuracy is the degree of inherent redundancy contained in the GPS system concept.

I

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Candidate Ground Based Navigation Systems [3-6J Table 3-4

I

I

LORAN-C OI'IEGA DIFF. DVIEGA FREQUENCY 90 to 110 ,liz 10.2, 11.3, 13.6 KHz 10.2,11.3,13.6 KHz FOR ST,\TfONS, MDNI TOR IIOT DETERfll NED COVERAGE 1500-2250 KI1 GROUND 15000 Kl1 PER STATION 20-200 1.1': FROM

I

5500 ,11 SKY\~AVE 1',011 ITOR STATION I<DRLOWIOE BY ~ STATIONS ACCURACY 20-90 M 5-IO Kl'. DAY I~2 K~l AT 4 Kfl N IGIIT 20-LOO KN READOUT 15 SEC FOR MANUAL POSITION UPDATE CUNTINUOUS

I

CONT. FOR AUTOMATIC 2 flIN. TO CONTINUOUS WE IGHT (KG) (100 (IOU (100 SIZE (C~3) (1,000,000 (1,000,000 <100,000 (10,000 (10,000 (10,000 COST ($)

I

EASE OF OPERATION SK ILLED/LIM ITED TRNG SrI LLEO/L WITEO TRNG SKILLED/LlfdTED TUNG SECURE USAGE NO REQUIREfIENT NO REQUIREMENT REQU IREI'IENT COMMUNICATION NOT REQUIRED NOT REqU WEO NOT RE~UIREO

I

SPEED (MPS) 25-850 25-e5 25-85 FUNCTION LONG-RANGE NAV LONG-RANGE NAV MEDIUM TU SHORT-RANGE OCEAN IC NAVIGATION IN AREAS CONUS ANO OCEAN IC '"HERE HIGIIER ACCURACY IS REQUIRED TNAN ONEGA

I

USERS CIVILIM = 4000 SIlIPS AND AIRCRAFT VESSELS IN COASTAL US AND FORE IGN CONFLUENCE REGION AIRCRAFT IN CONUS REMARKS GEDGRAPNICAL COVERAGE LANE AMB IGUITY EXPER IMUHJ1L LIMITED TO IIORTH

I

ATLANTIC AND COASTAL CONFLUENCE MEA Candidate Satellite Based Navigation Systems

I

cps GPS 621B FREQUENCY L-BArID L-BAND L-BAIlO

I I,ORLOWIDE CONUS

COVERAGE llORLDllIDE ACCURACY 10 11 (IIDUE) 1 10-25 :·1 100-250 i,1 REAOOUT 10 PER SEC (,~AX)2 (NOT AVAILAGLE) SECOIIDS 11 (fIRL//1P)3 LA CO;·iPAT I8lE WEIGHT (KG) (NCT AVAILAuLE)

I

SIZE (CM3) 200,000 (:.mL/MP)3 (NOT (IV,\ ILA~LE) GA CU'·IPMIeLE COST ($) 5000 (NCT AV,\llAGLE) -2500 EASE OF OPERATION SliILLED/LIMITED [RNG SKIllED/LII1ITLD TRtiG SKILLEIJ/LIr·IITlD mNG

I

SECURE USAGE REQU IRED REQU IRED NOT REQUIRED COMflUNICATION NOT REQU IIlED IIOT REQUIHED 1:aT ilEQOIlWl SPEED (MPS) TACTICAL REQ. TACT fl\l REQ. 0,\ CDi'IPAT 18ll

I

FUNCTION H1LITARY HIL ITARY CONUS NAVIGiHlOti USERS MILITARY, CIVIL !,:IUT/\RY l:IVIL/GENER:,L ,\VI/\TION MAR I NE UNDER CAilC[lLLO f1YPCTHli[CJ~l REI'IARliS DEVELUPNEIIT

I

1) HilJh OynJlllic USCI' L411ilJlolent 2) Depends un User [quiilf;terrt I 3) i·1aUlldvox [~eseJr'cl1 LdburdtOt·y :·:JrlfIJCr..

4) lIypothet icd I Pdrdilleters

I

I

I

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For LORAN and OMEGA a station failure may be overcome by selection of a new

I

station, the resulting degraded geometry provides poorer accuracy while a user equipment failure results in lack of solution. For the Continental

I

Positioning System, the loss of a satellite results in loss of navigation solution unless a dimension such as vertical may be derived from an

I

alternate system. This then results in degraded performance. Simultaneous tracking using one of a variety of multi-channel user equipments results in

I

gracefully degraded performance when other lower accuracy position information is available and only one channel fails. GPS contains redundancy both in the satellite constellation and in multi-channel user

I

equipment providing graceful degradation in performance for either mode of failure. The total system redundancy afforded by GPS elevates it above the

I

competing systems. It is this feature, in concert' with the accuracy and coverage afforded, which develops the advocacy for the GPS as a general

I

aviation navaid.

I

3.4 Data Link Advocacy The obvious conclusion of the preceding section is that GPS provides

superior coverage and accuracy for enroute navigation. However, GPS pro- I

vides very little additional useful enroute capability over that provided by OMEGA and LORAN-C. It is in the terminal area environment, where

I

advantage of GPS precision can be taken, that a distinction occurs. It is in the availability of the precision position that provides the advocacy of

I

incorporating a data link with GPS. This results in the ability of the integrated system to provide auxiliary functions which normally would not

I

be available to General Aviation at a nominal or perhaps any cost. These functions include collision avoidance and guidance for approach and land-

I

ing. Significant in this light is that to whatever degree is practical, these functions can be self-contained. There is no,theoretical requirement

for extensive ground instrumentation facilities (other than the monitor I

station in the case of differential GPS).

These functions are supported through the concept of position report-

I

ing for collision avoidance and through the operation in a differential mode for approach and landing. This latter point is discussed further in

I

subsequent sections. The explicit point to be made here is that it is

I

I

I

I

through the integration of a data link with GPS that the full potential of GPS for general aviation can be realized. GPS and a data link can provide

I

precision four-dimensional guidance and control globally as well as in the airspace environment which is notably characteristic of general aviation.

I

This environment includes those enroute and terminal areas which possess limited or in some instances no ground instrumentation for terminal area control and approach guidance.

I

3.5 Clock Impact

I

Recalling that, to achieve time synchronization, each user has a semi- accurate clock which is calibrated against ·the system master clock by 1)

I

inserting satellite clock offset (with respect to the master clock) into the telemetry data stream and 2) by using'a fourth satellite to solve a

I

four-dimensional geometry where the fourth dimension is user clock offset (with respect to the satellite clock), the user is afforded the avail- I ability of a very precise estimate of system time.

Addressing the accuracy to which this estimate can be provided, con- sider that the user bias is essentially computed to the range equivalent

I

accuracy of the pseudorange measurement magnified by the time-dilution-of- precision (TOOP). Magnovox [3-7J has indicated an error budget allocating

I

an RMS error of about 4 meters to the ranging measurement for the two fre- quency receiver. Further, Aerospace [3-3J has indicated a maximum percent

I

probable TOOP of about two for a 5° elevation mask for the Phase III fullup satellite configuration. This predicts that the range equivalent time

I

accuracy available to the user is on the order of the position error and for the specification goal of 10 meters this translates to about 30 nano- seconds.

I

The impact of clock availability is coupled with the data link advocacy of the preceding section. In order to support self-contained

I

functions such as collision avoidance and landing guidance, an interrogate and reply link concept is negated. An alternative which is applicable is

I

the use of a Time-Oivision-Multiple-Access link. The GPS can provide the synchronization to support such a link inherently.

I

I

I

I

3.6 Comparative Concepts (Realization Potential)

I

In order to assess the extent to which GPS may potentially provide capabilities above and beyond conventional area navigation systems, various

I

navigation concepts may be compared. The development of this comparison will be seen to point toward a single concept which should be explored in

I

depth.

Candidate approaches to area navigation are categorized into three

I

generic classes. These are: hyperbolic systems such as LORAN and Omega, satelfite systems such as GPS, and hybrid systems \'Jhich represent combina- tions or innovative systems. Representative of the latter are: hyperbolic

I

systems with GPS for altitude and time only, hyperbolic systems with a low cost altimeter for altitude, and ground augmented GPS for increased posi-

I

t i on accuracy.

Conventional Approaches -

I

Hyperbolic systems such as LORAN and Omega are quite limited in the potential they offer general aviation for capabilities beyond conventional

I

enroute navigation. They are essentially horizontal position finding sys- tems with no capability for altitude determination or time measurement.

I

They are medium to high accuracy systems (1.5 - 3 km for Omega and 150 - 300 m for LORAN). Omega is global but susceptible to ionospheric varia- tion, while LORAN operates primarily in the ground wave propagation mode

I

thus having limited coverage capability. The systems are ground station dependent with a very low level of redundancy for station outages. There

I

is essentially no inherent user redundancy with accompanying degraded operation except that potential exists for Omega to resort to single fre-

I

quency operation with the associated introduction of lane ambiguity which is potential degradation in quantum steps. Omega does offer the potential

I

of increased accuracy by taking advantage of the spatial correlation property of ionospheric variation and operating in a differential mode.

Satellite systems, on the other hand, offer expanded performance in I

the sense that altitude and possibly time are available as well as in- creased precision in the position estimate. This, however, is at the

I

expense of system cost - both to the user and facility (i.e., satellite constellation and ground monitor stations). Conceivably, user costs can be

I

reduced through technology development, probably at the expense of position I.

I

I

accuracy. The system is satellite dependent although a high level of re-

I

dundancy is offered by the GPS concept in that alternate satellites may be acquired, at the expense of GDOP degradation, in case of satellite outage.

I

The user equipment in satellite systems can provide inherent redun- dancy by receiver design at a cost. For example, the four or five channel

I high performance GPS receiver could conceivably be operated with one or

more channels failed if coarse position or time were available from alter- nate sources, or if the receiver were reconfigured to operate sequentially.

I

This advances the idea of an integrated system with graceful degradation as opposed to a dual redundant approach where redundancy is a cost-non-

I

effective feature until the occurrence of a failure. Further, if the four-channel graceful degradation approach is implemented, the" user can

I

acquire the additional features of the four channel receiver over a sequen- tial version essentially at no cost.

I

Hybrid system concepts may be divided into two categories. The first consists of conventional two-dimensional systems augmented to provide alti-

-I

tude information. The second consists of GPS with ground augmentation to improve accuracy. The motivation here is to examine system concepts which could extend the enroute capability of RNAV systems to the terminal area

I

environment by adding altitude measurement and improving accuracy.

Two concepts have been explored to augment conventional navaid systems

I

such as Omega and LORAN with altitude in order to upgrade their performance capabilities. These consisted, one, of adding a low-cost radar altimeter

I

and, two, adding a dual channel GPS receiver for altitude and time only.

Both concepts were discounted very quickly in that the horizontal accuracy

I

for both systems, and Omega in particular, is marginal with respect to other than enroute applications. LORAN has been observed to have repeat- able position accuracy of about 15 m but for self-contained navigation and

I

guidance, absolute accuracy is required. Further, a survey of current altimeters as shown in Table 3-5 indicates that while the required altitude

I

accuracy is achievable, cost (even anticipating a two-to-four-to-one reduc- tion with technology development) is prohibitive. Utilization of a dual

I

channel GPS receiver to provide altitude and time was also qUickly dismissed when it was observed that in effecting a four-dimensional solu-

I

tion to the GPS algorithm using position from a conventional navaid, the

I 19

I

Table 3-5. Radar Altimeter Survey MANUFACTURER 110DEL RANGE(m) ACCURACY FREQUENCY P~IR OOT WT(kg) V COST lin in BOtIZER 1'1i\R K -lOX 15- 30 +2m 4300 Mllz 16 W PK 2 12-30 voe 1.5A I'1J\X 2295 (PULSE) 30-150 +5% 150-750 +7% MINI-MARK 30-300 +7% 4300 NHz 3 ~, PK 14 or 28 0.6 A 1 9~5 (PULSE) VUC COLLINS ALT 50 o to +30 +2m 4300 I'1Hz 4' 28 VOC (24 W NOt·I) 5535 - (FM/CW) 30-150 +5% 150-600 +7% All 55 o to +30 +5% 4300 14Hz 4 28 VOC (30 ~I rlOM) 6635 - (FM/CW) 30-150 +5% 150-750 +7% HOFFMAN HRA-IOO 0-30 +110 4300 M~lz 5014W 3 14 or 28 0.8 A 3995 (PULSE 30-150 +2% VUC N DOPPLER) 150-750 +5% o KING KRA 10 < 30 +2m 2 28 VDC 0.2 A 2095 - - (- ) 30-150 +5% >150 +7% KRA 405 0-150 f!:.2m-or ~5% 4300 MHz 150 MW 5 28 VDC 0.85 A (FM/CW) 150-600 +7% - KOLLSW\N AHV-lO Typical 4300 11Hz 50 MW 28 VOC 1 ( lOW) 5500

-

(FM/CW) f!:..( IOcm+2%) l1aximulil - KS 250 0-30 +(20+5%) 4300 Mllz 50 MW 4 - - - (FM/CW) 0-1500 5%+0. 3m 0-7.5km 3%+lm 0-20km 1%+0.310 1/2%~O.3111 SPERRY M-100 15-150 3m or +5% 4300 HHz 14 VUC - 3 1 A C'J'JO (SHORT PULSE) 150-600 +7% (or) 28 VDC 0.5 1\ 600-750 +9% AI\-215 0-30 +lm 4300 HHz 70 ~I 28 VUC I.!.> 1\ 4 6350 (SHORT PULSE) 30-150 +3% 150-750 +4%

I

I

conventional navaid errors mapped directly into the altitude and time measurements. The net result is that augmented hyperbolic systems are not considered capable of supporting terminal area navigation or approach and

I

landing guidance.

Ground Augmented Approach -

I

Special attention has been given the development of GPS with ground ,augmentation to improve accuracy. One approach here was to look at GPS

I

with a'satellite transceiver located on the ground (i.e., a IIpseudolitell), probably near the end of the runway. This approach was felt to afford

I

increased accuracy on two fronts. First, the location of a transceiver on the ground was felt to significantly reduce the error in pseudorange for

I

that satellite. As an example, Table 3-6 shows the anticipated error reduction of a pseudolite (satellite-on-the-ground) over a satellite. The net improvement indicated is a ten-to-one improvement in RMS error in

I

pseudorange. Second, the location of the satellite beneath the aircraft was felt to significantly improve the geometry, especially in the

I

z-dimension. A detailed simulation was conducted to demonstrate the effects of a pseudolite augmented system and are included as Appendix B.

I

Various combinations of up to four satellites overhead and one on the ground as shown in Figure 3-2 were examined as a function of constellation

I

geometry (i.e., elevation angle) and for a simulated landing profile. The net results of the simulation were, however, not encouraging. For cases

I

where the aircraft was directly overhead, improvement of about two was evidenced from the increased accuracy of the pseudolite and a further improvement of again two was observed with the improved geometry. The

I

discouraging results were obtained when the aircraft was permitted to maneuver within the geometry. Figure 3-3 shows an example of the

I

dilution-of-precision in the vertical plane as an aircraft is allowed to fly an approach pattern to a runway threshold located at the center of the

I

y-axis. Notice that the geometry improves accuracy to the touchdown point, but thereafter quickly degrades as the aircraft is now lI ou tside" the

I

tetrahedron formed by the connecting lines between the three overhead satellites and the pseudolite. Another geometry where the aircraft is not

I allowed to exit the geometry is shown in Figure 3-4 and does not produce

this effect.

I

I

I

I

Table 3-6.

Ground Transceiver Accuracy - Rationale

I

I

ERROR SOURCE (10") .pseudolite Satellite EPHEMERIS ERROR 105m none

I

SATELLITE CLOCK ERROR O.9m O.45m TROPOSPHERIC DELAY ERROR 105m none

I

SPACE VEHICLE GROUP DELAY ERROR 3m 105m

I

MUL TIPATH ERROR L8m none IONOSPHERIC CORRECTION 15m none

I

INDUCED ERROR

I

I

TOTAL ERROR RMS 15.57 meter 1.57 meter

I

I

0" PSEUDOLITE '" '" 10 SATELLITE 0"

I

I

I

I

I

I

I

"

-------------------

SATELLITE CLUSTER Ranging Error: a N W B , B , A) CONFIGURATION: I 4S (0, R L (0, B , B , A) and P II 4s R L III: 3S (B , B , A) and P L R and P IV : 3S (0, B , B ) L R PSEUDOLITE Ranging Error: ka Figure 3-2. Ground Augmentation Geometry

I

I

z 1.0 I 1.8 ,",/2.1 2.0 1.0

I

I- E 1.8 ..:.::

=

I

1 . 0 c»

-=

:::t

I

-

.....

2.1 33.5

f- -

c

---

, r I I I I I y

I

8 6 4 2 -2 -4 -6 -8 Distance from Threshold in km

I

VDOP Contours in Vertical Runway Plane for Figure 3-3.

I

Configuration 1V:3S (O,BL,B ) and P (k=l.O) R

I

I

I

z 1.8 2.0

I

E ..:.::

=

I

c»

-=

:::t

I

-

- -

c: 2.4

I

-8 -6 in km Distance from Threshold

I

I

Figure 3-4. VDOP Contours in Vertical Runway Plane for Configuration 1II:35 (BL,BR,A) and P (k=l.O)

I

I

I

In addition, the negative system aspects of pseudolite augmentation

I

should be mentioned. The obvious dissident factor in pseudolite augmenta- tion is, of course, cost. It is not inconceivable that ground-based

I

satellite equivalent transmitters could be in the cost range of tens to hundreds of thousands of dollars--a probably prohibitive cost to be associ-

I

ated with the majority of small airports. Further, a significant technical problem exists with the deployment of a large number of additional GPS

I

transmitters. Each transmitter is' assigned a satellite identification code: this code is of sufficient length to accommodate only a limited number of transmitters (thirty-two). The system impact (on signal

I

structure) of a larger number of transmitters could be significant if each airport was to have a unique code. However, airports with sufficient

I

separation could conceivably use the same identification code.

Summarizing the pseudolite performance, then, the concept appears to

I

be susceptible to rapid geometry effects while providing only modest improvement in accuracy with attendant technology constraints.

I

Differential GPS - The second approach for ground augmentation considered was drawn from observing the improvement obtained by operating Omega in a differential

I

mode. The basic theme of differential GPS is to note, see Table 3-7, that the significant error sources in pseudorange measurement occur as bias

I

errors associated with the satellite and tropospheric propagation, and correlated errors associated with the ionospheric group delay error cor-

I

rection process. The total RSS error computed from Table 3-7 is 4.04 meters. If a differential mode is implement~ as shown in Figure 3-5 where

I

a monitor station is established to measure and correct a major portion of these errors, there is potential to reduce this to 1.75 meters. This

I

potential is dependent on the assumption that the error sources are cor- related over spatial dimensions comparable to the spacing between the monitor station and the user position. It also assumes temporal correla-

I

tion over time intervals on the order of the delay in computing the correction factor at the monitor and data linking this to the user air-

I

craft. The requirement for this data link is significant to the concept, for through it resides the capability of extending the auxiliary functional

I

capability to include guidance for approach and landing and for collision avoidance.

I

I

I

I

Table 3-7. GPS Navigation Error Summary [3-8]

I

Error Contributor Pseudorange Statistics Notes

I

Satellite Ephemeris 1.5 meter Bias Uncorre1ated be-

I

tween Satellites

I

Satellite Group and Clock 1.0 meter Bias Uncorre1ated be- tween Satellites

I

Pseudorange Noise 1.0 meter Markov Evaluated at

C/N = 30 db

I

O for P-Code

I

Range Quantization 0.226 meter White Noise

I

Range Mechanization Error 1.0 meter White Noi se

I

Evaluated at Ionospheric Dual Frequency 3.0 meter Markov C/N = 30 db O

I

for P-Code. No Averaging

I

at 5° Tropospheric Residual 1.0 meter Bias Evaluated elevation and

I

zero altitude

I

Multipath Error 1.0 meter White Noise

I

I

I

I

I

I

I

I

I satell ite(s)

I

I

I

I

user transversal

I

I

measured monitor position

+~

I

/ .

computed correction actual

I

factor monitor positi on measured

+ user

I

~sition • updated

I

user pos it ion

I

Figure 3-5. Differential GPS Geometry

I

I

I

I

I

GPS in a differential mode offers distinct perforrnance improvements in several different ~'1ays. It essentially removes the major portion of iono- spheric group delay correction and other bias type errors. It is very

I

likely much less expensive to implement a user receiver and data link than a ground-based satellite-equivalent transceiver (i.e., the pseudolite would

I

require the expensive clock carried on board the satellite). It would allow the user to be single frequency in that ionospheric correction would

I

not be requi red. It woul d overcome the rapi dly changi ng local geomet ry effects discussed previously.

I

In summary then, a single frequency GPS receiver operated in differen- tial mode can conceivably provide performance equal to or better than a

tHo-frequency receiver at the expense of a monitor station and a data link. I

The concept is further supportive of increased functional capability through designing the data link to also implement guidance for approach and

I

landing and collision avoidance. The recommendations in Section 5.0 ~·Jill indicate the desirability of developing further the feasibility of

I

differential GPS and experimentally demonstrating the increased functional capability resulting from this concept.

I

3.7 Economic Considerations

I

The economic considerations here emphasize the user segment of alter- nate navigation aids. Two general categories are discussed: one, the total system costs for current, satellite, and area navigation systems,

I

excluding facility acquisition costs; and two, the relative cost of single user equipment, again for the three categories. Facil ity acquisition costs

I

are not incl uded as the current system is just that, current. Land-based systems are in the impl ementat i on stages and GPS is soon to be an important

I

national resource.

I

Relative Total System Costs- Table 3-8 [3-6J indicates the user system cost comparison for the

I

three alternatives. The "current-base system" incl udes a majority of knovm or planned navigational components which are now in use or planned for use to satisfy global civilian and military navigational requirellients during

I

the 1975-1995 time frame. The "satell ite-based system" incorporates

I

I

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Table 3-8. User System Cost Comparison ($1,000) [3-6] Maintenance Costs Equi pment** (20 Yrs.) Replacement Total New User Costs Alternative Equipment New Current Cost*** Mai nt. of (20 Yrs) Costs Systems System* Replacement Equipment Current Based 3,753,165 2,075,000 1,457,000 2,801,000 1,040,000 11 ,126,165 System Satell ite Based 3,588,225 2,084,000 1,457,000 600,00 7,726,225

---

System Ground Based 3,665,444 2,133,000 1,457,000 720,000 7,975,444

---

~ystem *Cost of maintaining the current system during phase-in of alternative system.

**For the current system, existing equipment is assumed to be replaced at least once over the next 20 years.

***Includes satellite replacement costs (every 7 years).

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existing naviads such as DABS and MLS for- short-range requirements and in-

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cludes a satellite subcomponent for the long-range requirements. The "ground-based radio system" is simil ar to the sate1l ite-based system except that systems such as LORAN or OMEGA support the long-range requirement.

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The costs here include new user equipment acquisition costs, equipment repl acernent costs for the current base system, and ongoi ng maintenance for

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all three alternatives.

Notice that overall, the satell ite system cost is 40% less than the

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current system and on an equal with ground-based area navigation. In a cost-effective sense this weights the satellite system in a much more

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favorable position due to the increased performance at comparable cost.

Relative User Equipment Costs- I

The FAA [3-9J has indicated a receiver design cost allocation as shown in Figure 3-6. This results in an anticipated GPS receiver cost of about

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$2800. This is slightly high with respect to the predicted costs of on the order of $2000 contained in reference [C-IJ. Although the correct order of

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magnitude, technology developments in the area of microelectronics can certainly impact these costs for the 1985 time frame; although on a rela-

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tive basis, it can be argued that other navaid costs will similarly be affected. It should be pointed out, however, that to a certain extent

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there may be a lack of motivation to force a nev.J non-GPS microelectronic technology design when GPS is available and offers increased performance.

Thus, technology may actually drive GPS costs down, ~.Jhile not affecting

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other navaids.

In order to maintain perspective, relative costs for comparative

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navaids are shown in Table 3-9 [3-9J. The comparison is again sl ightly un- balanced in that the relative performance of the systems has not been

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weighted. Nonetheless, it does indicate that anticipated GPS user costs place it in a favorable candidacy for the general aviation user.

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Table 3-9. Estimated Cost of GA Avionics [3-9J

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GENERAL AVIATION LIST PRICE $ 900 VOR

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1,800 DME 1,000 RNAV Computer for VOR/DME 2,050 LORAN Receiver/Navigator

I 2,800 GPS Receiver/Navigator

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--------

------

- - - --

$:>0 ANTENNA r~----~/'-._---~\ $190 $25 PROCESSOR \r7 $185.

"UNIT KEYBOARD /'-.

\ r TRACKING $100 , ~II-

.. DISPLAY

CONVERTER/ ~ ~ COSTAS CDI CONTROL ...

..

IF ~ DRIVER Rf

r-+ LOOP

~ coRRELATOR , NAVIGATION ~ ~ $40.

DIGITAL DISPLAY w ......

+- ..

SYNTHESIZER ....

PRODUCTION COSTS =5700.

$25 I}WLIED LIST PRICE=S2800.

POWER SUPPLY Figure 3-6. Low Cost GPS Receiver Block Diagram [3-6]

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4.0 EXPERIMENTAL PROGRAM OBJECTIVES This section presents the objective~ of an overall program to establish

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an optimum user equipment configuration through experimental evaluation.

The task definition is directed at evaluation via flight test as had been

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the case for predecessor systems (i.e., 621B); however, during the develop- ment of these object ives, it became apparent that 1aboratory and/or

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"hot-bench" testing could provide va·1uab1e data vlithout the expense (and for Phase I GPS coverage, the inconvenience) of flight test. It is with regard

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to overall test and evaluation that these objectives have been defined.

The development of the objectives begins with evaluation concept

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definition, proceeds to establish experiment guidelines, provides a state- ment of general objectives supported by specific identified performance evaluation areas, and concludes with representative architectures for

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experiment definition.

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4.1 Concept The concepts addressed in the definition of an evaluation plan should

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focus on proving system worth and on demonstrating the unique capabilities provided to a given user group. Thus, the experiments should verify those

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quantitative measurables \'Ihich demonstrate navigation perforrllance and limi- tations specific to General Aviation requirements and utilization and

I further, demonstrate the increased scope of functional utilization available

to the General Aviation pilot through unique features of GPS. As an example of the first category, the evaluation program should demonstrate the achiev-

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able relative and absolute accuracies for a variety of flight conditions.

It should further explore those contributing factors such as multipath,

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shading, etc., which constrain or othen"ise infl uence the general aviation pil ot I s abil ity to use the system and shou1 d specifi cally address the advan-

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tages available over alternate navigation equipments. Examples of the second category include increased scope of utilization such as collision

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avoidance (when used with a data link), guidance for approach and landing, etc. Also considered in the second category is the redundancy availabiity I inherent in multichannel mechanized user equipment.

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4.2 Evaluation Definition Guidelines The following guidelines have been defined for the specification of evaluation procedures:

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1) The experiment objective will establish criteria whereby existing

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system requirements or procedures can be augmented or replaced. An example here is the use of GPS to provide position data in the terminal area, thus

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modifying (or replacing) the surveillance radar function.

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2) The experiment(s) will begin with the minimum configuration of a single channel receiver and reconfigure toward four channels to demonstrate

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the increased capability, performance, and redundancy. This is shown graphically in Figure 4-1. Incremental reconfiguration to upgrade system complexity is noted to increment user scope as well as cost simultaneously.

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3) The experiments will separately address the flight profile

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segments. These include enroute, terminal, approach, and landing as well as segment transitions. The role of GPS in general aviation operations is

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considered particularly directed at the terminal area and approach segments. Enroute does not exercise GPS capability over more conventional

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techniques, while landing probably requires augmentation to some degree.

Thus, the latter two segments are prioritized differently. The overall

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priority of flight segment in an evaluation program is then projected as: a) terminal area and approach

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b) landing (augmented) c) transitioning

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d) enroute

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4) The experiments will specifically address general aviation requirements, but will also emphasize the potential available to the

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commercial fleet and thus address total civil aviation.

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---_ .. --

- - - -- - -

- - - --

$ CAT III COMMERCIAL COMM& GA ENROUTE COMMERCIAL ANDGA TMA MINIMUM CONFIG- URATION w Ul Configuration Complexity* SELF CONTAINED , MLS!ILS TRANSITION GA ENROUTE - AUGMENTED RNAV LANDING (SINGLE CHANNEL SEQUENTIAL) * Redundancy/Accuracy Requirements and Management Figure 4-1. Experimental Program Philosophy

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4.3 General Objectives The general objectives of an evaluation program for satellite naviga-

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tion systems include the demonstration/evaluation of those features specific to navaids in the GA environment as well as the determination/demonstration

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of the redundancy management features. The demonstration/evaluation objec- tives may be conveniently partitioned into three categories representing enroute, terminal area, and approach and landing flight segments. Represen-

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tative investigation areas include:

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1) Enroute - The potential resident within a GPS configured system to replace and/or augment VOR/DME functions with RNAV capability,

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2) Terminal Area - Potential available for ~eplacing the ground sur-

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veillance function with position reporting, and

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3) Approach and Landing - Potential for and feasibility of providing precision approach guidance and self-contained landing to the general avia- t i on fl eeL

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4.4 Specific Performance Evaluation Areas

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Table 4-1 lists specific performance evaluation areas of significant impact to the general aviation user. The primary objective of these evalua-

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tion areas is to quantify performance and system requirements as directed to general aviation.

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Table 4-1. Specific Performance Evaluation Areas

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- POSITION AND VELOCITY ACCURACY (RELATIVE & ABSOLUTE) - ACQUISITION/REACQUISITION TIME

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- DROPOUTS - CONSTELLATION REVISION TIME

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- INPUT SIGNAL DYNAMIC RANGE - MULTIPATH/SHADING

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- L /L CONSIDERATION 1 2 - UPDATE RATE

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- CA/P CODE PERFORMANCE·

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Accuracy

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Of obvious importance in the achievement of a performance evaluation is the demonstration of system accuracy as a means of establishing a baseline

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from which comparisons may be made. These cornpari'sons span the ensemble of quantitative parameters of interest as well as being a means of evaluating the absolute accuracy of a given system.

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Acquisition/Reacguisition Time The time to acquire and more especially the time required to reacquire

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in the event of track loss is an important consideration over the general aviation flight profile. Dependent on the flight segment, lengthy reacqui-

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sition time could represent a serious problem. This is especially important since preceding sections have alluded to GPS in tile role of land-

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ing guidance and collision avoidance. It is also important to demonstrate that enroute navigation is not degraded by lengthy acquisition times.

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Dropouts Dropouts are perhaps not different in impact from reacquisition time except in a cause and effect respect. That is, signal dropouts cause the

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concern for reacquisition time. What is unique with respect to dropouts is

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the effect of the length of time signal is lost on the ability to reestab- lish track. For long signal outages, the initial conditions may degrade to the p9int of increasing the difficulty of their acquisition process.

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Constellation Revision Time Constellation revision results froln the need to overcome geometrical

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dilution of precision as the satellites progress in orbit and to'replace satellites which have dropped below 5° elevation. This requi~es that the interplay beh/een time to access the current geometry, time to predict the

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new geometry, and the time to acquire the new satellite or satellites, if required, be demonstrated. The performance of GPS is dependent on the data

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rate of this assessment as well as the total time to revise and the degree of degradation which can occur in these time frames.

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Input Signal Dynamic Range This area of evaluation simply demonstrates the levels at \thich dropout

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occurs and the effects of saturation in the event of strong signals.

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Multipath [4-1J Significant range errors can result when the line-of-sight signal and a delayed multipath signal are of comparable amplitude. The purpose of

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this specific evaluation area is to explore the multipath problem and its effects in ranging error. While emphasis is on effects at the user equip-

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ment, results are also applicable to the monitor station(s).

Operational environments typically encountered with GPS type equipment

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can cause satellite signals to be received over multiple paths and result in delayed signals having amplitudes comparable in amplitude to the line-of-

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sight signal. Delay of the indirect path may vary from a few nanoseconds to several microseconds. A PRN ranging system can track the direct and delayed

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signals independently, given sufficient delay of the multipath signal. How- ever, when the delay is less than about 1.5 code chip intervals, the delayed signal can cause errors in the signal tracking loop. Also, severe fading

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can occur when comparable amplitudes and very small delays are experienced.

For delays on the order of one-two chip intervals, error performance is a

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function of the signal-to-interference ratio, the multipath signal delay, and the relative phasing of the direct and delayed path carriers. The

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interference ratio is further a function of the receiving antenna pattern and its orientation, the transmission paths for the direct and reflected

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signals, and the characteristics of the signal reflecting surfaces.

Mu1tipath analyses have predicted GPS performance for selected situ-

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ation environments. This evaluation will provide supportive test data for these limited scope analyses and will extend performance evaluation to in- clude the effects of multipath from multi-specular and distributed diffuse

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scatterers.

Dua 1 Frequency Cons i derat ions [4-2., 4-3]

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The general aviation receiver will likely receive on only one frequency. As a result ionospheric refraction correction capability and

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choice of frequency are important issues. The correction capability con- sideration is obvious in that it ultimately impacts system accuracy (and is

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