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  1. C GPS for IFR Operations Version 3.01 November 5, 2002 Thomas B. Bahder, Ph. D., CFII Copyright 2002 by Thomas B. Bahder All Rights Reserved. No part of this document can be modified. Permission is hereby granted to disseminate this document in its entirety, free of charge, for educational purposes only. No fees can be charged for this document, except nominal reproduction fees. For profit, no part of this document may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the author.

  2. Notice of Disclaimer The author has made every attempt to present correct and accurate information in this tutorial. However, the author assumes no liability for any events that result from the use or misuse of the information presented here. Ultimately, it is every pilots responsibility to use the information provided by official FAA publications, such as the FAA Approved Flight Manual, Flight Manual Supplement, the AFD (Airport Facilities Directory), NOTAM’s, Aeronautical Charts, Advisory Circulars, the Aeronautical Information Manual and the Manual provided by the manufacturer of the GPS receiver. Software Version Used This tutorial is based on two GPS receivers: The II Morrow, Inc. GX60 receiver and GX series simulator, version 3.0, with the following option selected, GX60, IFR, and APPR, and The Garmin GNS 430 receiver and Garmin 400 Series Windows Trainer simulator with software version 3.00. At the time of this writing, GPS WAAS and LAAS were not yet authorized for IFR flight.

  3. Introduction This tutorial presents the basic information that pilots need to use GPS to fly IFR (Instrument Flight Rules) in the U.S. National Airspace System. The tutorial assumes a basic understanding of navigation and instrument approach procedures using the traditional ground-based navaids, such as VOR, NDB, and ILS. Two receivers are used as examples of GPS receivers: the II Morrow Apollo GX60 receiver and the Garmin GNS430 receiver. However, this tutorial does not go into the level of detail required to operate each specific model of GPS receiver, or any other specific model of GPS receiver. This information must be obtained from the receiver manufacturer’s manual for that particular receiver as well as the FAA Approved Flight Manual and/or Flight Manual Supplement for the particular aircraft.

  4. CONTENTS Part I -- GPS System Overview …………………………………………………….……………..5 Part II -- Flying IFR ……………………………………………………………….……………..30 Part III -- GPS Instrument Approaches ……………………………………………………....…40 Part IV -- Flying Nonprecision GPS Approaches …………………………………………….…69 IFR GPS Simulator and Aircraft Checkout: GX60 ……………………………………….……92 Part V -- Flying Approaches with Garmin 430 ………………..…………………………………93 IFR GPS Simulator and Aircraft Checkout: GNS430………………………………………….100 World Wide Web Sites……………………………………………………………………….……101 References ………………………………..……………………………………….………………. 102 page

  5. Part I -- GPS System Overview

  6. Evolution of Satellite Navigation • The science and technology of navigation has occupied man for generations. Initially, man navigated on the flat surface of the Earth using existing landmarks. Next, man learned that the surface of the Earth is really the surface of a sphere, and that the Earth orbits our sun. This refinement in our understanding gave rise to celestial navigation, which is based on carrying a good clock and making angular observations of the natural stars. A drawback of celestial navigation is that the sky must be clear enough to see the stars. With the development of radios, a new class of navigation aids was borne. At first, these aids consisted of Earth-based radio beacons, such as VORs, LORAN, and Omega. Finally, man created Earth satellites that send out radio beacons that are not obscured by clouds. These artificial satellites allow highly precise and reliable navigation in the vicinity of the Earth. • Historically, there were several satellite systems that were primarily designed for navigation: • Transit -- ~1960’s operational system, U. S. Navy/Johns Hopkins Applied Physics Laboratory (APL) • Timation -- ~1960’s experimental system, U. S. Naval Research Laboratory, U. S. Navy/APL • 621B -- 1960’s study program , U. S. Air Force, Aerospace Corp. • GPS (Global Positioning System -- NAVSTAR) -- US Air Force • 1973 GPS program was established by JPO (Joint Program Office) • 1978 Air Force Began Launches of Block I (experimental) satellites • 1989 First Launch of Block II (operational) satellites • 1995 GPS FOC (Full Operational Capability) announced • GLONASS (Global Navigation Satellite System) Russian version of GPS, developed essentially in parallel with GPS

  7. Ground Tracking Stations Ground Tracking Stations The GPS System Segments Space Segment User Segment GPS Receiver Master Control Station Control Segment

  8. The GPS Satellite Constellation(Space Segment) • 24 Satellites in nearly circular orbits around Earth • 6 orbital planes • 4 satellites in each orbital plane • 20,200 km altitudes (about 4 Earth radii) • Orbital periods are 11 hours 58 minutes • Each satellite carries 4 atomic clocks, one operational and three spares • Constellation is designed so that at least 4 or 5 satellites are in view from anywhere on Earth Graphic used by permission from Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of Texas at Austin.

  9. GPS Control Segment Stations track GPS satellite signals and relay them to the Master Control Station in Colorado Springs Master Control Station Determines GPS satellite positions (ephemeris) and uploads this information into each satellite Satellites broadcast their positions to GPS users

  10. GPS Block IIR Satellite -- Artist Conception

  11. GPS Block II Satellite

  12. Launch of GPS Satellite Maiden Launch of Delta II Booster with SVN 14 as Payload SVN 14 inside Delta II Booster

  13. GPS Receiver (User Segment) • GPS receiver identifies a given satellite by matching the (unique) code from that satellite • Data superimposed on the code gives satellite position (ephemeris) and GPS system time at which the signal was emitted • Receiver has a clock and it calculates how much time it took the signal to travel from the satellite to receiver • The speed of light is 186,000 miles/second, and the distance from satellite to receiver is computed • Corrections are made due to slowing down of signal through atmosphere • Using the computed distances to 4 satellites, the receiver computes its 3-dimensional position by mathematical triangulation

  14. GPS Altitude Displayed by Receiver: CAUTION • WGS-84 (World Geodetic System 1984) is the system of coordinates used by GPS • In the WGS-84 system of coordinates, an ellipsoid is defined to approximate the average mean sea level. • Computed GPS altitude is the height above the WGS-84 ellipsoid, not True MSL altitude. • CAUTION: The altitude above the WGS-84 ellipsoid can be different from the MSL altitude on the aircraft altimeter by hundreds of feet. This is a serious consideration during instrument approaches. Use your altimeter! • Typically, GPS vertical position is less accurate than GPS horizontal position. Mean Sea Level is NOT the same as the WGS-84 ellipsoid

  15. II Morrow GX60 GPS Receiver TSO-C129 A1 certified for Enroute, Terminal and Nonprecision Approach Operations

  16. The GPS System Segments (continued) • The GPS consists of three segments: SPACE, CONTROL and USER • The SPACE segment • Consists of 24 operational satellites in six orbital planes, (four satellites in each plane). The satellites operate in approximately circular 20,200 km (10,900 nm) orbits at an inclination angle of 55 degrees and with a 12-hour period. From a given point on Earth, each satellite appears at the same place every 23 hours 56 minutes. • The CONTROL segment • Consists of five Monitor Stations (Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs), three Ground Antennas, (Ascension Island, Diego Garcia, Kwajalein), and a Master Control Station (MCS) located at Falcon AFB in Colorado. The monitor stations passively track all satellites in view, accumulating ranging data. This information is processed at the MCS to determine satellite orbits and to update each satellite's navigation message. Updated information is transmitted to each satellite via the Ground Antennas. • The USER segment • Consists of GPS users on the ground and in the air with receivers capable of locking on to the satellite signals. • Some receivers have RAIM (see next page)

  17. RAIM -- Receiver Autonomous Integrity Monitoring • GPS receivers for critical applications, such as IFR flight, have RAIM algorithms that check integrity of GPS data • Integrity -- The ability of a system to provide timely warnings to users when the system should not be used for navigation • To perform the RAIM function, at least one extra satellite, in addition to the ones used for navigation, must be tracked by the receiver • RAIM algorithms need a minimum of 5 satellites, 4 for position determination plus one extra • Alternative RAIM scheme: 4 satellites plus baro-aiding provided by barometer -- the altimeter setting must be entered manually • Some receivers are capable of isolating which satellite signal is corrupt and can remove it from the position solution. These receivers need 6 satellites, or 5 satellites plus baro-aiding. • Without RAIM capability, the pilot has no assurance of accuracy of GPS position

  18. Causes of RAIM Outages • Insufficient number of satellites • Poor satellite geometry leads to large position errors • Change in aircraft dynamics, such as banking, obscures the GPS satellite signal (which must be received line-of-sight)

  19. SPS (Standard Positioning Service) Available to all users, such as civilians, FAA, and other civilian organizations Positioning Accuracy: 100 meters horizontal position, 95 % of the time 300 meters horizontal position, 99.99 % of the time C/A code (Course Acquisition Code) Broadcast on Single Frequency L1: 1575.42 MHz Accuracy of signal is intentionally degraded by U. S. DoD to the 100 meter level, called SA (selective availability) PPS (Precise Positioning Service) Only available to U. S. Department of Defense and other Authorized Users Positioning Accuracy: ~16 meters horizontal position, 98 % of the time P-code, (Precise Code, or Y-code (encrypted)) Broadcast on Two Frequencies L1: 1575.42 MHz L2: 1227.6 MHz The two frequencies, L1 and L2, allow better compensation for atmospheric effects (allows measurement of ionospheric delay) GPS Positioning Services At least 4 satellites must be simultaneously tracked to compute position in 3-dimensions

  20. Determining User Position • 1. Consider two satellite clocks, labeled A and B, that are located at known positions and are synchronized. • 2. Assume that you (the user) are located somewhere between clocks A and B, and that you have a clock that is synchronized with clocks A and B • 3. Assume that you simultaneously receive radio signals from clocks A and B, and these signals tell you the time at which the signals left clocks A and B, respectively. • 4. Using the known time of arrival of the signals at your position, and the time the signals left each clock, you can figure out the distance that each signal traveled (shown by the arrows), since each signal travels at the speed of light, 186,000 miles/second. You can then draw a sphere centered at each clock with a radius equal to the distance traveled from that clock. • 5. The intersection of the two spheres is a circle. Your position is somewhere on this circle. • 6. We need a third satellite, to resolve the ambiguity in position along the circle. B A

  21. C Determining User Position (continued) P2 B A P1 • 7. Adding the simultaneous reception of signals from a third satellite C, adds another sphere centered at C. • 8. The intersection of the circle (shown in dark) with the sphere centered at C gives our probable position as two points, shown as P1 and P2. • 9. Finally, the simultaneous reception of a fourth satellite (not shown) gives another sphere, which resolves the ambiguity in position between points P1 and P2. • 10. When radio waves pass through the atmosphere, the waves are delayed. These delays are taken into account to produce an accurate user position.

  22. GPS Signal Details • The coarse/acquisition (C/A) code has a 1.023 MHz chip rate, a period of one millisecond (ms) and is used by civilians for navigation. This code is also used by DoD, primarily to acquire the P-code. • The precision P-code has a 10.23 MHz rate, a period of seven days and is the principal navigation ranging code for DoD. • The Y-code (encrypted form of the P-code) is used in place of the P-code whenever the anti-spoofing (A-S) mode of operation is activated. • The C/A code is available on the L1 frequency and the P-code is available on both L1 and L2. The different satellites all transmit on the same frequencies, L1 andL2, but each satellite has a unique code. • A GPS receiver locks onto and tracks a given satellite by correlating an internally replicated code (inside the receiver) for that given satellite, with that coming from space. If the correlation is successful, the receiver is able to track the satellite. • Due to the spread spectrum characteristic of the signals, the system provides a large margin of resistance to interference. • Each satellite transmits a navigation message containing its orbital elements, clock behavior, system time and status messages. In addition, an almanac is also provided which gives the approximate data for each active satellite. This allows the user set to find all satellites once the first has been acquired.

  23. Differential GPS This basic idea of differential GPS is used in WAAS and LAAS, and all other augmentation schemes. The aircraft receives the GPS signals as well as the broadcast corrections from the GPS receiver station, improving availability and integrity of the stand-alone GPS system position.

  24. WAAS (Wide Area Augmentation System) Will provide enroute navigation plus Category I-type approaches for most airports WAAS Capable Receiver Needed WAAS is a GPS-based navigation and landing system that will provide precision guidance to aircraft at thousands of airports and airstrips where there is currently no precision landing capability. Systems such as WAAS are known as satellite-based augmentation systems (SBAS). WAAS is designed to improve the accuracy and ensure the integrity of information coming from GPS satellites. The FAA is moving directly to a Lateral Navigation/Vertical Navigation (LNAV/VNAV) capability using WAAS with expected capability in 2003. Concurrently, the FAA will evaluate the approach to achieve Global Navigation Satellite System (GNSS) Landing System (GLS) capability in later years. LAAS (Local Area Augmentation System) Will provide navigation capability in areas where WAAS is not available plusCategory II/III approaches and landings at selected locations LAAS Capable Receiver Needed Similar to WAAS, LAAS will broadcast its correction to the standard GPS signals, but will use a VHF radio datalink from a ground-based transmitter located at each airport. It is expected that the end-state configuration will pinpoint the aircraft's position to within one meter or less. Beyond Category III, the LAAS will provide the user with a navigation signal that can be used as an all weather (airport) surface navigation capability for taxing. The LAAS is intended to complement the WAAS and function together to supply users of the U.S. NAS with seamless satellite based navigation at locations where the WAAS is unable to meet existing navigation and landing requirements (such as availability). In addition, the LAAS will meet the more stringent Category II/III requirements that exist at selected airports. FAA GPS Augmentations

  25. WAAS –How it Works EXCERPT TAKEN FROM WEB SITE: http://gps.faa.gov/Programs/index.htm Unlike traditional ground-based navigation aids, WAAS will cover a more extensive service area. Wide area ground reference stations (WRS) have been linked to form a U.S. WAAS network. Precisely surveyed ground reference stations receive signals from GPS satellites and any errors in the signals are identified. Each station in the network relays the data to one of two wide area master stations (WMS) where correctional information for specific geographical areas is computed. A correction message is prepared and uplinked to a geostationary communications satellite (GEO) via a ground uplink station (GUS). This message is broadcast on the same frequency as GPS (L1, 1575.42 MHz) to future GPS/WAAS receivers on board aircraft flying within the broadcast coverage area of WAAS. The WAAS will improve basic GPS accuracy, to approximately 7meters vertically and horizontally; system availability, through the use of geostationary communication satellites (GEOs) carrying navigation payloads; and to provide important integrity information about the entire GPS constellation.

  26. LAAS – How it Works EXCERPT TAKEN FROM WEB SITE: http://gps.faa.gov/Programs/index.htm Local Area Augmentation System (LAAS) ground facility (LGF) includes 4 Reference Receivers (RR), RR antenna pairs, redundunt Very High Frequency Data Broadcast (VDB) equpment feeding a single VDB antenna, and equipment racks. These sets of equipment are installed on the airport property where LAAS is intended to provide service. The LGF receives, decodes, and monitors GPS satellites information and produces correction messages. To compute corrections, the ground facility calculates position based on GPS, and then compares this position to their known location. Once the corrections are computed, a check is performed on the generated correction messages to help ensure that the messages will not produce misleading information for the users. This correction message, along with suitable integrity parameters and approach path information, is then sent to the airborne LAAS user(s) using the VDB from the ground-based transmitter. Airborne LAAS users receive this data broadcast from the LGF and use the information to assess the accuracy and integrity of the messages, and then compute accurate Position, Velocity, and Time (PVT) information using the same data. This PVT is utilized for the area navigation (RNAV) guidance and for generating ILS-look-alike guidance to aid the aircraft on an approach.

  27. International Initiatives Europe’s Galileo(EXCERPT TAKEN FROM http://www.esa.int/export/esaSA/GGGMX650NDC_navigation_2.html ) Galileo will be Europe’s own global navigation satellite system, providing a highly accurate, guaranteed global positioning service under civilian control. It will be inter-operable with GPS and GLONASS, the two other global satellite navigation systems. A user will be able to take a position with the same receiver from any of the satellites in any combination. By offering dual frequencies as standard, however, Galileo will deliver real-time positioning accuracy down to the metre range, which is unprecedented for a publicly available system. It will guarantee availability of the service under all but the most extreme circumstances and will inform users within seconds of a failure of any satellite. This will make it suitable for applications where safety is crucial, such as running trains, guiding cars and landing aircraft. The first experimental satellite, part of the so-called Galileo System Test Bed (GSTB) will be launched in late 2004. • Canadian WAAS • Japanese MTSAT = MULTI-FUNCTIONAL TRANSPORT SATELLITE, for DGPS for Pacific region (see http://www.mlit.go.jp/koku/ats/e/mtsat/role/01.html) • European Geostationary Navigation Overlay System (EGNOS), (Excerpt taken from: see http://www.esa.int/export/esaSA/GGG8YN4UGEC_index_0.html) EGNOS receives signals from the GPS and GLONASS satellites and, using specialised hardware, adds a correction factor which makes them accurate to five metres or better. The signals are then beamed back into space, and broadcast by three civilian geostationary satellites. EGNOS-receiving equipment, fitted into vehicles, picks up this precise tracking information.

  28. Some History -- GPS Date Rollover Issues • The GPS Joint Program Office has determined that all generations of GPS satellites are unaffected by the Year 2000 (Y2K) and GPS End of Week (EOW) Rollover Issues. However, the Civil GPS users may need to verify that their receivers and applications will work properly through these events. • Y2K Issue: • The Year 2000 rollover issue, commonly referred to as the Y2K problem, stems from the fact that many computer programs are using a two digit date field and assume the year is 19xx. When the year 2000 arrives, a two digit date becomes '00' and could be interpreted as an invalid date. • End Of Week (EOW) Rollover Issue: • The EOW rollover problem is really a problem that occurs every 20 years. GPS system time, which counts weeks, started counting on midnight 5/6 January 1980, in modulo 1024 (0-1023). On midnight 21/22 August 1999, the GPS week will rollover from week 1023 to week 0000. This could be interpreted as an invalid date.

  29. GPS as Sole Means of Navigation Recent study requested by FAA and carried out by Johns Hopkins Applied Physics Laboratory concluded that GPS is adequate for “Sole Means of Navigation” Concerns: Integrity, satellite availability (failure scenarios), and jamming (intentional and non-intentional) ===> Implication for future of navigation---The future is GPS!

  30. Part II -- Flying IFR

  31. General Requirements for GPS Operations under IFR • GPS navigation equipment must be approved in accordance with TSO C-129 • Hand-held GPS are NOT approved for IFR • GPS operation must be conducted in accordance with FAA approved Flight Manual or Flight Manual Supplement • Aircraft using GPS must be equipped with approved, operational alternate means of navigation appropriate to the flight • Active monitoring of alternate nav equipment is not required if GPS receiver uses RAIM • Active monitoring of alternate nav equipment is required when RAIM capability is lost • Aircraft navigating by IFR approved GPS are considered RNAV aircraft, i.e., use /G suffix in ATC flight plan (AIM Table 5-1-2) • Prior to a GPS IFR flight, pilot must review GPS NOTAM’s (DUATS has GPS NOTAM’s)

  32. GPS NOTAM’s • GPS satellite outages are issued as GPS NOTAM’s (Notices to Airmen) for known or scheduled outages • The effect of satellite outage on intended flight operation cannot be determined unless the receiver has a RAIM algorithm that allows excluding a given satellite that is predicted to be out of service, e.g., II Morrow GX60 has this capability. • DUATS, FAA briefers, and Automated Flight Service Stations will provide GPS RAIM availability during briefings. Get RAIM prediction if flying a GPS departure procedure. • Satellites are referenced by their PRN (Pseudo-Random Noise) code number. • EXAMPLE: To obtain GPS NOTAMs from GTE DUATS: http://duats.com Choose “Abbreviated WX Briefing”, and fill in all required questions, select location as GPS, uncheck all items, but check the box for “Notices to Airmen (NOTAMs)” You will see GPS NOTAM’s such as: ******** NOTAMs ******** !GPS 05/026 GPS PRN 4 OTS WEF 9905250600-9905251800 !GPS 05/027 GPS PRN 27 OTS WEF 9905270430-9905271100

  33. General Requirements for GPS Operations under IFR(continued) • GPS for oceanic operations can be used as soon as avionics are installed, provided general requirements are met (see AIM Table 1-1-7 on GPS IFR Equipment Classes/Categories). • GPS domestic en route and terminal IFR operations can be conducted as soon as avionics are installed and general requirements are met • II Morrow GX60 receiver is GPS IFR Equipment Category/Class TSO-C129 A1 (oceanic, en route, terminal, and nonprecision approach capable) • GPS Approach Overlay Program authorizes pilots to use GPS avionics under IFR for flying existing nonprecision instrument approaches, except localizer (LOC), localizer directional aid (LDA), and simplified directional facility (SDF) procedures. • Authorization to fly GPS approaches is limited to U.S. airspace. • FAA administrator must authorize GPS use in other airspace • GPS instrument approaches outside U.S. airspace must be authorized by the appropriate sovereign authority

  34. Alternate Airport Requirements • If your destination has no instrument approach, then you must file an alternate airport. • If the weather at your destination requires that you file an alternate destination, then that alternate airport must have an instrument approach that is NOT a GPS approach. • However, when arriving at the alternate destination, you may elect to use any approach procedure, including a GPS instrument approach.

  35. Two Categories of GPS Approaches There are two categories GPS approaches for general purpose civilian use: • GPS Overlay Approaches -- associated with GPS Approach Overlay Program • Stand Alone Approaches -- Designed from the start with GPS in mind

  36. GPS Approach Overlay Program • Purpose of the Approach Overlay Program is to Transition from ground-based to satellite-based navigation facilities for approaches • Overlay Program allows pilots to fly existing VOR, VOR/DME, NDB, NDB/DME, TACAN, and RNAV nonprecision instrument approach procedures • Overlay Program is limited to U.S. Airspace • GPS equipment may be used to fly all nonprecision approaches, except LOC, LDA, and SDF procedures. • Approach Procedure Must be retrieved from the receiver’s database. If not in the database, the procedure cannot be flown. • Required alternate airport must have an approach procedure other than GPS (or LORAN-C). • Approach Overlay Program has three Phases

  37. Phases of Approach Overlay Program • Phase I (Ended in February 1994, when FAA declared GPS operational for civil operations) • Phase II (“GPS” is not included in procedure name) • Began on February 17, 1994 • Certified GPS equipment can be used as the primary IFR flight guidance to fly an overlay to an existing nonprecision aproach, such as VOR, VOR/DME, NDB, NDB/DME, TACAN, and RNAV • Underlying ground-based navaid(s) for the approach must be operational • Avionics in aircraft for the ground-based navaid(s) must be installed and operational • Monitoring of the ground-based navaid(s) for the underlying approach is not necessary • During the approach, the avionics for the the ground-based navaid(s) need not be operating if RAIM is providing integrity. • Approach should be requested by existing published approach procedure name, such as VOR RWY 24. • Phase III ( procedure name includes “or GPS” ) • Began April 24, 1994 when first IAP were published to include “or GPS” in name • Neither the traditional aircraft avionics nor the underlying navaid(s) need be installed, operational or monitored to fly the approaches • For GPS systems that do not use RAIM, the traditional avionics and ground-based navaid(s) that provide the equivalent integrity must must installed and operational during the approach • For any required alternate airport, the traditional avionics and ground-based naviaid(s) that define the instrument approach and route to the alternate must be installed and operational • Approach should be requested by GPS procedure name, such as GPS RWY 24.

  38. GPS NOTAMUse of GPS as Substitute for NDB and DME • Operators in the U. S. NAS are authorized to use GPS equipment certified for IFR operations in place of ADF and DME equipment for the following operations: • Determining the aircraft position over a DME fix. • Flying a DME arc. • Navigating to/from an NDB. • Determining the aircraft position over an NDB. • Determining the aircraft position over a fix made up of a crossing NDB bearing. • Holding over an NDB. • Approved IFR GPS instrument approach operations include: • Locating DME fixes and Locator Outer Markers (LOMs) • Flying DME arcs • Determining NDB cross-bearing fixes • Navigating to/from and holding over NDBs • See complete text of this NOTAM in Published NOTAMs or Special Notices section of Airport Facilities Directory.

  39. GPS Position Accuracy • Selective Availability (SA) is a method by which the U. S. Department of Defense (DoD) purposely degrades the accuracy of the civilian GPS signal (C/A code). This is typically the largest source of error in GPS. • DoD has announced that SA will be discontinued within a decade, as of March 29, 1996. • When SA is active (most of the time) the DoD guarantees that the horizontal accuracy will not be degraded beyond 100 meters (328 feet) 95% of the time, and 300 meters (984 feet) 99% of the time. • The accuracy of GPS is also affected by satellite geometry, i.e., triangulation to obtain user position is more accurate for widely-spaced satellites than for satellites clustered tightly together • Other smaller sources of error are: • satellite ephemeris, satellite clock, ionosphere delay, troposphere delay, multipath, receiver electronics errors due to thermal noise and receiver design • Other errors of C/A code receivers are due to solar sun spot activity, potentially significant in extreme northern or southern regions, e.g. Alaska.

  40. Part III -- GPS Instrument Approaches

  41. Area Navigation (RNAV) Instrument Approaches • RNAVApproaches are based on Waypoints • Waypoint -- A predetermined geographical position (used for route definition and/or progress reporting purposes) that is defined by lattitude/longitude. • RNAV equipment can be based on existing VOR’s , LORAN-C, or GPS, or some other navigation system. The basic idea is that a mathematical point is constructed with known coordinates, called a waypoint. • The approach course is defined by waypoints instead of by traditional ground-based navaids

  42. Codable vs. Non-Codable Approach Procedures • An aircraft is not authorized to fly any IFR approach unless that Instrument Approach Procedure is retrievable from the navigation data base. • Certain FAR Part 97 nonprecision instrument approaches may present an unresolvable coding situation, i.e., an approach may be determined to be not codable by the database coding agency or the manufacturer of the navigation equipment. This means that, for a given GPS receiver, certain approaches may not be in the GPS navigation data base, due to inconsistency with the design of the receiver, e.g., an approach may be inconsistent with algorithms used in receiver.

  43. Two Types of Waypoints • Fly-by Waypoint • Used when aircraft should begin a turn to the next course prior to reaching the next waypoint separating the two route segments. • Approach waypoints (except for the Missed Approach Waypoint (MAWP) and Missed Approach Hold Waypoint (MAHWP) are normally Fly-by waypoints. • This turn anticipation is compensated by airspace and terrain clearance. • Fly-over Waypoint • New approach charts depict Fly-over waypoints as circled waypoints • Used when aircraft must fly over the waypoint prior to starting the turn

  44. Waypoint Abbreviations in Approaches • IAF -- Initial Approach Fix • IF -- Intermediate Fix • FAF -- Final Approach Fix • IFAF -- Combination Initial Approach Fix/Final Approach Fix • MAP or MAWP -- Missed Approach Point • MAHP or MAHWP -- Missed Approach Hold Waypoint • CF069 -- Course Fix, e. g., on radial 069 • RW36 -- Runway 36 threshold waypoint, often the MAP

  45. Conventional vs. GPS Navigation Data • Slight differences may exist between the course portrayed on navigational (approach or enroute) charts and the GPS navigational display on the receiver when flying an overlay approach or along an airway. • Magnetic tracks defined by VOR radials are determined by magnetic variation at the VOR; however, a GPS receiver may use the magnetic variation at the current position. This may lead to small differences in the magnetic courses. • Variation in distances will occur, since GPS distance-to-waypoint values are along track (straight line) distances (ATD), while DME distances are slant range distances. DME Distance GPS Distance

  46. RNAV Terminology and Nav Display Angle Off Course displayed on VOR RNAV equipment, including GX60, displays Cross Track Deviation, NOT Angle Off Course

  47. Flight Plan Elements • A Flight Plan is a Sequence of waypoints • The Flight Plan has a direction-- there is a starting waypoint and an ending waypoint • GX60 receiver stores up to 30 flight plans, each can have up to 20 legs • Name of Flight Plan on its Title Page • Each flight plan has a Title Page (first page) containing the Name of the Flight Plan • Active Flight Plan • At any time, there is one flight plan that is the Active Flight Plan • The Active Flight Plan may be ACTIVE or INACTIVE • Activate a Flight Plan by going to Title page of flight plan, and press SELECT, and press ENTER Flight Plan

  48. GX60 Flight Plan Elements • Active Flight Plan Location • In the GX60, there is a special location for the Active Flight Plan. When a stored flight plan is Activated, the stored flight plan is copied onto the Active Flight Plan Location • Activating Flight Plan • Activate a Flight Plan by going to Title page of the flight plan, and press SELECT, and use large and small knobs to choose ACTIVATE, then press ENTER. • If your aircraft position is as shown, and you activate the Flight Plan (or reactivate it), automatic sequencing of waypoints occurs. The Active Leg becomes W29 - ESN. The TO-waypoint becomes ESN. • Active Leg • At any time (when Flight Plan is Active), one leg is the Active leg, here it is W29--ESN. • TO-waypoint • At any time (when Flight Plan is Active), one waypoint is the TO-waypoint, here ESN.

  49. Waypoint Sequencing in Flight Plan • As aircraft passes each dashed line (which bisects the angle between flight plan legs) a new leg becomes the active leg, and a new waypoint becomes the current To-waypoint. • Here, Active Leg is W29-ESN, current To-Waypoint is ESN • When an instrument approach is Loaded, and Enabled, the waypoints that define the approach are automatically put on the Flight Plan waypoint sequence. • The waypoints that define the instrument approach become part of the regular flight plan, and waypoints sequence as described above.

  50. GX60 Waypoint Sequencing With Duplicate Waypoints • Consider a Flight Plan with duplicate waypoints: ANP - ESN - ESN - OXB • If a Flight Plan leg contains two waypoints that are located at the same physical place (here these two points are ESN), then on passing ESN the new To-waypoint will be OXB. This situation can occur by mistake, and causes no problem. • However, it commonly occurs on GPS approaches when the Initial Approach Fix (IAF) and Final Approach Fix (FAF) coincide, see for example W29 GPS RWY 11. The coincident IAF/FAF occurs when a procedure turn must be done. • In order to prevent problems with automatic waypoint sequencing during procedure turns, an OBS/HOLD button is provided to permit manual pilot intervention of waypoint sequencing. The pilot can turn waypoint sequencing ON and OFF (toggle switch) by pressing the OBS/HOLD button.