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Global Positioning System

Introduction:

  • Global Positioning System (GPS) GPS is a satellite-based navigation system composed of a network of satellites placed into orbit by the United States Department of Defense (DOD)
  • GPS was originally intended for military applications, but in the 1980s the government made the system available for civilian use. GPS works in all weather conditions, anywhere in the world, 24 hours a day. A GPS receiver must be locked onto the signal of at least three satellites to calculate a two-dimensional position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user’s three-dimensional position (latitude, longitude, and altitude). Other satellites must also be in view to offset signal loss and signal ambiguity. The use of the GPS is discussed in more detail in Chapter 17, Navigation. Additionally, GPS is discussed in the Aeronautical Information Manual (AIM)
  • The Global Positioning System (GPS) was developed in 1978 and monitored by the U.S. Department of Defense (DOD)
  • GPS provides a very precise, global navigation service, which is unaffected by weather, allowing for point-to-point navigation
  • Operates on the L-band: 1100 - 1600
  • Distance measured indirectly with time
  • Properly certified GPS equipment may be used as:
    • A supplemental means of IFR navigation for domestic en route, terminal operations, and certain instrument approach procedures (IAPs)
  • This approval permits the use of GPS in a manner that is consistent with current navigation requirements as well as approved air carrier operations specifications

System Overview:

  • The Global Positioning System is a satellite-based radio navigation system, which broadcasts a signal that is used by receivers to determine precise position anywhere in the world
  • The 24 satellite constellation is designed to ensure at least five satellites are always visible to a user worldwide
  • A minimum of four satellites is necessary for receivers to establish an accurate three-dimensional position
  • The receiver uses data from satellites above the mask angle (the lowest angle above the horizon at which a receiver can use a satellite)
  • The Department of Defense (DOD) is responsible for operating the GPS satellite constellation and monitors the GPS satellites to ensure proper operation
  • Every satellite’s orbital parameters (ephemeris data) are sent to each satellite for broadcast as part of the data message embedded in the GPS signal
  • The GPS coordinate system is the Cartesian earth-centered earth-fixed coordinates as specified in the World Geodetic System 1984 (WGS−84)

System Availability and Reliability

  • Global Positioning System:

    • The status of GPS satellites is broadcast as part of the data message transmitted by the GPS satellites
    • GPS status information is also available by means of the U.S. Coast Guard navigation information service: (703) 313−5907, Internet: http://www.navcen.uscg.gov/
    • Additionally, satellite status is available through the Notice to Airmen (NOTAM) system
  • Global Navigation Satellite System:

    • GNSS operational status depends on the type of equipment being used
    • For GPS-only equipment TSO-C129 or TSO­C196(), the operational status of non-precision approach capability for flight planning purposes is provided through a prediction program that is embedded in the receiver or provided separately

Receiver Autonomous Integrity Monitoring (RAIM):

  • RAIM is the capability of a GPS receiver to perform integrity monitoring on itself by ensuring available satellite signals meet the integrity requirements for a given phase of flight
    • RAIM provides immediate feedback to the pilot that without, provides no assurance of the GPS position integrity
  • This fault detection is critical for performance-based navigation (PBN) (see Performance-Based Navigation (PBN) and Area Navigation (RNAV), for an introduction to PBN), because delays of up to two hours can occur before an erroneous satellite transmission is detected and corrected by the satellite control segment
  • RAIM Requirements:

    • In order for RAIM to determine if a satellite is providing corrupted information, at least one satellite, in addition to those required for navigation, must be in view for the receiver to perform the RAIM function
      • RAIM requires a minimum of 5 satellites, or 4 satellites and barometric altimeter input (baro-aiding), to detect an integrity anomaly
      • Baro-aiding is a method of augmenting the GPS integrity solution by using a non-satellite input source in lieu of the fifth satellite
    • Some GPS receivers also have a RAIM capability, called fault detection and exclusion (FDE), that excludes a failed satellite from the position solution; GPS receivers capable of FDE require 6 satellites or 5 satellites with baro-aiding
      • This allows the GPS receiver to isolate the corrupt satellite signal, remove it from the position solution, and still provide an integrity-assured position
    • To ensure that baro-aiding is available, the current altimeter setting must be entered into the receiver as described in the operating manual
    • Do not use the GPS derived altitude due to the large GPS vertical errors that will make the integrity monitoring function invalid
  • RAIM messages vary somewhat between receivers; however, generally there are two types:
    1. The first type of message indicates that there are not enough satellites available to provide RAIM integrity monitoring
      • The GPS navigation solution may be acceptable, but the integrity of the solution cannot be determined
    2. Another type indicates that the RAIM integrity monitor has detected a potential error and that there is an inconsistency in the navigation solution for the given phase of flight
      • Without RAIM capability, the pilot has no assurance of the accuracy of the GPS position

Selective Availability:

  • Selective Availability (SA) was a method by which the accuracy of GPS is intentionally degraded
    • This feature was designed to deny hostile use of precise GPS positioning data
    • SA was discontinued on May 1, 2000, but many GPS receivers are designed to assume that SA is still active
    • New receivers may take advantage of the discontinuance of SA based on the performance values in ICAO Annex 10

Components:

  • Space segment – Satellites
  • Control segment – Ground based monitoring
  • User segment – Aircraft (antennas and receiver/processors)

Function:

  • Concept of ranging and triangulation
  • Uses satellites above the mask angle (lowest angle above the horizon at which it can use a satellite)
  • Each satellite transmits a course/acquisition (CA) code
  • Travels at 186,000 miles per second
  • The signal also contains the satellites position (ephemeris)
  • At least 4 satellites are required to provide a three-dimensional position (latitude, longitude, and altitude) and time solution
  • 24 Satellites in the constellation (1 for each time zone with about 6 back-ups) orbit the earth at 11,000 NM
  • A pseudo-random code timing signal and data message that the aircraft processes to obtain satellite position and status
  • By knowing the location of each satellite and matching timing with atomic clocks on the satellites the aircraft equipment can measure the time and thus position
  • 5 satellites ensured to be in view at all times
  • Monitored on the ground by 5 monitoring stations, 3 ground antennas and a master control station
  • Each GPS transmits a specific code called a Course/Acquisition (CA) code
  • The GPS receiver matches each satellite’s CA code with an identical copy of the code contained in the receivers database. By shifting its copy of the code in a matching process, and by comparing this shift with its internal clock, the receiver can calculate how long it took the signal to travel from the satellite to the receiver

Distance Measuring:

  • Receiver listens for the signal from the satellite
  • Each satellite transmits specific course acquisition code back to the receiver
  • Receiver calculates distance (pseudo-range) from satellite using time signal traveled and speed it traveled at (approx. speed of sound)

Trilateration:

  • Needs minimum of 4 satellites to give accurate 3-D position
  • Uses position information obtained from satellites and pseudo-range to determine position in space
  • Accurate timing is important

Correction for Errors:

  • System automatically selects best satellites
  • Most errors can be factored out using math and modeling
  • Ground antenna corrections (Master Control Station)

Using GPS IFR:

  • Strict regulations – database, antenna placement, no hand-helds
  • Using GPS IFR – must be equipped with an operational alternate means of navigation
  • Must have RAIM
  • Pilot must be familiar with GPS system on aircraft
  • Flight plans – if destination does not have an instrument approach, or only has a GPS approach, you must file an alternate

Navigation Modes:

  • En-route: ± 5nm (1 dot = 1 nm)
  • Approach ± 1 nm (1 dot = .2 nm)
  • Inside 2 NM from FAWP ± .3nm (1 dot = .06 nm) – smooth transition
  • Missed = ± 1 NM (if initiated prior to MAWP)
  • Turns should not be initiated until past the MAWP

Differential GPS:

  • 2 Control Centers
  • Over 60 remote broadcast sites
  • Reference receivers correct bias errors at one location with measuring bias errors at a known position
  • Error reduction: accuracy within 1-3 meters (SPS)
  • Gets less accurate as you travel farther away from reference receivers

Selective Availability (SA) and Reliability:

  • SA is a method by which the accuracy of GPS is intentionally degraded in order to deny hostile use of precise GPS positioning data
    • SA was discontinued on May 1, 2000, but many GPS receivers are designed to assume that SA is still active
    • New receivers may take advantage of the discontinuance of SA based on the performance values in ICAO Annex 10, and do not need to be designed to operate outside of that performance

VFR Operations:

  • GPS navigation has become a great asset to VFR pilots by providing increased navigational capabilities and enhanced situational awareness
    • Although GPS has provided many benefits to the VFR pilot, care most be exercised to ensure that system capabilities are not exceeded
    • VFR pilots should integrate GPS navigation with electronic navigation (when possible), as well as pilotage and dead reckoning
  • GPS receivers used for VFR navigation vary from fully integrated IFR/VFR installation used to support VFR operations to hand-held or leg strapped/dash mounted devices
    • The limitations of each type of receiver installation or use must be understood by the pilot to avoid misusing navigation information [Figure 2]
    • Most receivers are not intuitive
    • The pilot must learn the various keystrokes, knob functions, and displays that are used in the operation of the receiver
    • Some manufacturers provide computer-based tutorials or simulations of their receivers that pilots can use to become familiar with operating the equipment
  • Critical Areas of Concern:

    • RAIM Capability

      • VFR GPS panel mount receivers and hand-held units have no RAIM alerting capability which prevents pilots from being alerted to the loss of integrity
      • A systematic cross-check with other navigation techniques would identify this failure, and prevent a serious deviation
      • Be suspicious of the GPS position if a disagreement exists between the two positions
    • Database Currency:

      • Databases must be updated for IFR operations and should be updated for all other operations
      • Without a current database the moving map display may be outdated and offer erroneous information around critical airspace areas, such as a Restricted Area or a Class B airspace segment
      • Without the update, it is the pilot’s responsibility to verify the waypoint location referencing to an official current source, such as the Chart Supplement U.S., Sectional Chart, or En Route Chart
    • Antenna Location:

      • The antenna location for GPS receivers used for VFR and IFR operations may differ
        • In many VFR installations of GPS receivers, antenna location is more a matter of convenience than performance
        • In IFR installations, care is exercised to ensure that an adequate clear view is provided with the satellites
      • If an alternate location is used, some portion of the aircraft may block the view of satellites from the antenna, causing a greater opportunity to lose navigation signal
      • This is especially true in the case of hand-held receives, such as the Garmin GPSMAP 696 Color Portable Aviation GPS
        • Typically, suction cups are used to place the GPS antennas on the inside of cockpit windows
        • While this method has great utility, the antenna location is limited to the cockpit or cabin only and is rarely optimized to provide a clear view of available satellites
        • Consequently, signal losses may occur in certain situations of aircraft-satellite geometry, causing a loss of navigation signal
        • These losses, coupled with a lack of RAIM capability, could present erroneous position and navigation information with no warning to the pilot
      • While the use of a hand-held GPS for VFR operations is not limited by regulation, modification of the aircraft, such as installing a panel or yoke-mounted holder, is governed by 14 CFR Part 43
        • Consult with your mechanic to ensure compliance with the regulation, and a safe installation
  • Do not solely rely on GPS for VFR navigation but instead as a supplement
    • No design standard of accuracy or integrity is used for a VFR GPS receiver
    • VFR GPS receivers should be used in conjunction with other forms of navigation during VFR operations to ensure a correct route of flight is maintained
    • Minimize head-down time in the aircraft by being familiar with your GPS receiver’s operation and by keeping eyes outside scanning for traffic, terrain, and obstacles
  • GPS Waypoint Names
    GPS Waypoint Names
  • VFR Waypoints:

    • VFR waypoints provide VFR pilots with a supplementary tool to assist with position awareness while navigating visually in aircraft equipped with area navigation receivers
      • The uses of VFR waypoints include providing navigational aids for pilots unfamiliar with an area, waypoint definition of existing reporting points, enhanced navigation in and around Class B and Class C airspace, and enhanced navigation around Special Use Airspace
      • VFR pilots should rely on appropriate and current aeronautical charts published specifically for visual navigation
      • If operating in a terminal area, pilots should take advantage of the Terminal Area Chart available for that area, if published
      • The use of VFR waypoints does not relieve the pilot of any responsibility to comply with the operational requirements of 14 CFR Part 91
    • VFR waypoint names (for computer-entry and flight plans) consist of five letters beginning with the letters "VP" and are retrievable from navigation databases [Figure 1]
      • The VFR waypoint names are not intended to be pronounceable, and they are not for use in ATC communications
      • On VFR charts, stand-alone VFR waypoints will be portrayed using the same four-point star symbol used for IFR waypoints
      • VFR waypoints collocated with visual check points on the chart will be identified by small magenta flag symbols
      • VFR waypoints collocated with visual check points will be pronounceable based on the name of the visual check point and may be used for ATC communications
      • Each VFR waypoint name will appear in parentheses adjacent to the geographic location on the chart
      • Latitude/longitude data for all established VFR waypoints may be found in the appropriate regional Chart Supplement U.S.
    • VFR waypoints may not be used on IFR flight plans
      • VFR waypoints are not recognized by the IFR system and will be rejected for IFR routing purposes
    • Pilots may use the five-letter identifier as a waypoint in the route of flight section on a VFR flight plan
      • Pilots may use the VFR waypoints only when operating under VFR conditions
      • The point may represent an intended course change or describe the planned route of flight
      • This VFR filing would be similar to how a VOR would be used in a route of flight
    • VFR waypoints intended for use during flight should be loaded into the receiver while on the ground
      • Once airborne, pilots should avoid program ming routes or VFR waypoint chains into their receivers
    • Pilots should be especially vigilant for other traffic while operating near VFR waypoints
      • The same effort to see and avoid other aircraft near VFR waypoints will be necessary, as was the case with VORs and NDBs in the past
      • In fact, the increased accuracy of navigation through the use of GPS will demand even greater vigilance, as off-course deviations among different pilots and receivers will be less
      • Regardless of the class of airspace, monitor the available ATC frequency closely for information on other aircraft operating in the vicinity
RVSM Aircraft Suffixes
RVSM Aircraft Suffixes
No-RVSM Aircraft Suffixes
No-RVSM Aircraft Suffixes

IFR Use of GPS:

  • General Requirements:

    • GPS navigation equipment used for IFR operations must be approved in accordance with the requirements specified in Technical Standard Order (TSO) TSO−C129(), TSO−C196(), TSO−C145(), or TSO−C146(), and the installation must be done in accordance with Advisory Circular AC 20−138, Airworthiness Approval of Positioning and Navigation Systems
      • Equipment approved in accordance with TSO−C115a does not meet the requirements of TSO−C129. Visual flight rules (VFR) and hand-held GPS systems are not authorized for IFR navigation, instrument approaches, or as a principal instrument flight reference
    • Aircraft using un-augmented GPS (TSO-C129() or TSO-C196()) for navigation under IFR must be equipped with an alternate approved and operational means of navigation suitable for navigating the proposed route of flight
      • Examples of alternate navigation equipment include VOR or DME/DME/IRU capability
      • Active monitoring of alternative navigation equipment is not required when RAIM is available for integrity monitoring
      • Active monitoring of an alternate means of navigation is required when the GPS RAIM capability is lost
    • Procedures must be established for use in the event that the loss of RAIM capability is predicted to occur
      • In situations where RAIM is predicted to be unavailable, the flight must rely on other approved navigation equipment, re-route to where RAIM is available, delay departure, or cancel the flight
    • The GPS operation must be conducted in accordance with the FAA−approved aircraft flight manual (AFM) or flight manual supplement. Flight crew members must be thoroughly familiar with the particular GPS equipment installed in the aircraft, the receiver operation manual, and the AFM or flight manual supplement
      • Operation, receiver presentation and capabilities of GPS equipment vary. Due to these differences, operation of GPS receivers of different brands, or even models of the same brand, under IFR should not be attempted without thorough operational knowledge
      • Most receivers have a built-in simulator mode, which allows the pilot to become familiar with operation prior to attempting operation in the aircraft
    • Aircraft navigating by IFR−approved GPS are considered to be performance-based navigation (PBN) aircraft and have special equipment suffixes
      • File the appropriate equipment suffix [Figure 1/2]
      • If GPS avionics become inoperative, the pilot should advise ATC and amend the equipment suffix
    • Prior to any GPS IFR operation, the pilot must review appropriate NOTAMs and aeronautical information
  • Database Requirements:

    • The onboard navigation data must be current and appropriate for the region of intended operation and should include the navigation aids, waypoints, and relevant coded terminal airspace procedures for the departure, arrival, and alternate airfields
    • Further database guidance for terminal and en route requirements may be found in AC 90-100(), U.S. Terminal and En Route Area Navigation (RNAV) Operations
    • Further database guidance on Required Navigation Performance (RNP) instrument approach operations, RNP terminal, and RNP en route requirements may be found in AC 90-105(), Approval Guidance for RNP Operations and Barometric Vertical Navigation in the U.S. National Airspace System
    • All approach procedures to be flown must be retrievable from the current airborne navigation database supplied by the equipment manufacturer or other FAA−approved source
      • The system must be able to retrieve the procedure by name from the aircraft navigation database, not just as a manually entered series of waypoints
      • Manual entry of waypoints using latitude/longitude or place/bearing is not permitted for approach procedures
    • Prior to using a procedure or waypoint retrieved from the airborne navigation database, the pilot should verify the validity of the database
    • This verification should include the following preflight and inflight steps:
      • Preflight:

        • Determine the date of database issuance, and verify that the date/time of proposed use is before the expiration date/time
        • Verify that the database provider has not published a notice limiting the use of the specific waypoint or procedure
      • Inflight:

        • Determine that the waypoints and transition names coincide with names found on the procedure chart
          • Do not use waypoints which do not exactly match the spelling shown on published procedure charts
        • Determine that the waypoints are logical in location, in the correct order, and their orientation to each other is as found on the procedure chart, both laterally and vertically
          • There is no specific requirement to check each waypoint latitude and longitude, type of waypoint and/or altitude constraint, only the general relationship of waypoints in the procedure, or the logic of an individual waypoint's location
        • If the cursory check of procedure logic or individual waypoint location, specified above, indicates a potential error, do not use the retrieved procedure or waypoint until a verification of latitude and longitude, waypoint type, and altitude constraints indicate full conformity with the published data
    • Air carrier and commercial operators must meet the appropriate provisions of their approved operations specifications
      • During domestic operations for commerce or for hire, operators must have a second navigation system capable of reversion or contingency operations
      • Operators must have two independent navigation systems appropriate to the route to be flown, or one system that is suitable and a second, independent backup capability that allows the operator to proceed safely and land at a different airport, and the aircraft must have sufficient fuel (reference 14 CFR 121.349, 125.203, 129.17, and 135.165)
        • These rules ensure the safety of the operation by preventing a single point of failure
        • An aircraft approved for multi-sensor navigation and equipped with a single navigation system must maintain an ability to navigate or proceed safely in the event that any one component of the navigation system fails, including the flight management system (FMS)
        • Retaining a FMS-independent VOR capability would satisfy this requirement
      • The requirements for a second system apply to the entire set of equipment needed to achieve the navigation capability, not just the individual components of the system such as the radio navigation receiver
        • For example, to use two RNAV systems (e.g., GPS and DME/DME/IRU) to comply with the requirements, the aircraft must be equipped with two independent radio navigation receivers and two independent navigation computers (e.g., flight management systems (FMS))
        • Alternatively, to comply with the requirements using a single RNAV system with an installed and operable VOR capability, the VOR capability must be independent of the FMS
      • To satisfy the requirement for two independent navigation systems, if the primary navigation system is GPS−based, the second system must be independent of GPS (for example, VOR or DME/DME/IRU)
        • This allows continued navigation in case of failure of the GPS or WAAS services
        • Recognizing that GPS interference and test events resulting in the loss of GPS services have become more common, the FAA requires operators conducting IFR operations under 14 CFR 121.349, 125.203, 129.17 and 135.65 to retain a non-GPS navigation capability consisting of either DME/DME, IRU, or VOR for en route and terminal operations, and VOR and ILS for final approach
        • Since this system is to be used as a reversionary capability, single equipage is sufficient

Oceanic, Domestic, En Route, and Terminal Area Operations:

  • GPS IFR operations in oceanic areas is only permitted when approved avionics systems are installed
    • TSO-C196() users and TSO-C129() GPS users authorized for Class A1, A2, B1, B2, C1, or C2 operations may use GPS in place of another approved means of long-range navigation, such as dual INS [Figure 1/2]
      • Aircraft with a single installation GPS, meeting the above specifications, are authorized to operate on short oceanic routes requiring one means of long-range navigation in accordance with AC 20­138, Appendix 1
  • Conduct GPS domestic, en route, and terminal IFR operations only when approved avionics systems are installed
    • Pilots may use GPS via TSO−C129() authorized for Class A1, B1, B3, C1, or C3 operations GPS via TSO-C196(); or GPS/WAAS with either TSO-C145() or TSO-C146()
      • When using TSO-C129() or TSO-C196() receivers, the avionics necessary to receive all of the ground-based facilities appropriate for the route to the destination airport and any required alternate airport must be installed and operational
    • Ground-based facilities necessary for these routes must be operational
      • GPS en route IFR operations may be conducted in Alaska outside the operational service volume of ground-based navigation aids when a TSO-C145() or TSO-C146() GPS/wide area augmentation system (WAAS) system is installed and operating
      • WAAS is the U.S. version of a satellite-based augmentation system (SBAS)
        • In Alaska, aircraft may operate on GNSS Q­routes with GPS (TSO­C129 () or TSO­C196 ()) equipment while the aircraft remains in Air Traffic Control (ATC) radar surveillance or with GPS/WAAS (TSO­C145 () or TSO­C146 ()) which does not require ATC radar surveillance
        • In Alaska, aircraft may only operate on GNSS T­routes with GPS/WAAS (TSO­C145 () or TSO­C146 ()) equipment
      • Ground-based navigation equipment is not required to be installed and operating for en route IFR operations when using GPS/WAAS navigation systems
        • All operators should ensure that an alternate means of navigation is available in the unlikely event the GPS/WAAS navigation system becomes inoperative
      • Q-routes and T-routes outside Alaska
        • Q­routes require system performance currently met by GPS, GPS/WAAS, or DME/DME/IRU RNAV systems that satisfy the criteria discussed in AC 90-100(), U.S. Terminal and En Route Area Navigation (RNAV) Operations
        • T­routes require GPS or GPS/WAAS equipment
  • GPS IFR approach/departure operations can be conducted when approved avionics systems are installed and the following requirements are met:
    • The aircraft is TSO-C145() or TSO-C146() or TSO-C196() or TSO-C129() in Class A1, B1, B3, C1, or C3; and
    • The approach/departure must be retrievable from the current airborne navigation database in the navigation computer
      • The system must be able to retrieve the procedure by name from the aircraft navigation database
      • Manual entry of waypoints using latitude/longitude or place/bearing is not permitted for approach procedures
    • The authorization to fly instrument approaches/departures with GPS is limited to U.S. airspace
    • The use of GPS in any other airspace must be expressly authorized by the FAA Administrator
    • GPS instrument approach/departure operations outside the U.S. must be authorized by the appropriate sovereign authority
GPS IFR Equipment Classes/Categories
GPS IFR Equipment Classes/Categories
GPS Approval Required/Authorized Use
GPS Approval Required/Authorized Use
    • As the production of stand-alone GPS approaches has progressed, many of the original overlay approaches have been replaced with stand-alone procedures specifically designed for use by GPS systems. A GPS approach overlay allows pilots to use GPS avionics under IFR for flying designated nonprecision instrument approach procedures, except LOC, LDA, and simplified directional facility (SDF) procedures. These procedures are identified by the name of the procedure and "or GPS" (for example, VOR/DME or GPS RWY15). Other previous types of overlays have either been converted to this format or replaced with stand-alone procedures. Only approaches contained in the current on-board navigation database are authorized. The navigation database may contain information about non-overlay approach procedures that is intended to be used to enhance position orientation, generally by providing a map, while flying these approaches using conventional NAVAIDs. This approach information should not be confused with a GPS overlay approach. (See the receiver operating manual, AFM, or AFM Supplement for details on how to identify these approaches in the navigation database.)

GPS Approach Procedures:

  • As the production of stand-alone GPS approaches has progressed, many of the original overlay approaches have been replaced with stand-alone procedures specifically designed for use by GPS systems. The title of the remaining GPS overlay procedures has been revised on the approach chart to "or GPS" (e.g., VOR or GPS RWY 24). Therefore, all the approaches that can be used by GPS now contain "GPS" in the title (e.g., "VOR or GPS RWY 24," "GPS RWY 24," or "RNAV (GPS) RWY 24"). During these GPS approaches, underlying ground-based NAVAIDs are not required to be operational and associated aircraft avionics need not be installed, operational, turned on or monitored (monitoring of the underlying approach is suggested when equipment is available and functional). Existing overlay approaches may be requested using the GPS title, such as "GPS RWY 24" for the VOR or GPS RWY 24. NOTE− Any required alternate airport must have an approved instrument approach procedure other than GPS that is anticipated to be operational and available at the estimated time of arrival, and which the aircraft is equipped to fly

Waypoints:

  • GPS approaches make use of both fly-over and fly-by waypoints. Fly-by waypoints are used when an aircraft should begin a turn to the next course prior to reaching the waypoint separating the two route segments. This is known as turn anticipation and is compensated for in the airspace and terrain clearances. Approach waypoints, except for the MAWP and the missed approach holding waypoint (MAHWP), are normally fly-by waypoints. Fly-over waypoints are used when the aircraft must fly over the point prior to starting a turn. New approach charts depict fly-over waypoints as a circled waypoint symbol. Overlay approach charts and some early stand alone GPS approach charts may not reflect this convention
  • Since GPS receivers are basically "To−To" navigators, they must always be navigating to a defined point. On overlay approaches, if no pronounceable five-character name is published for an approach waypoint or fix, it was given a database identifier consisting of letters and numbers. These points will appear in the list of waypoints in the approach procedure database, but may not appear on the approach chart. A point used for the purpose of defining the navigation track for an airborne computer system (i.e., GPS or FMS) is called a Computer Navigation Fix (CNF). CNFs include unnamed DME fixes, beginning and ending points of DME arcs and sensor final approach fixes (FAFs) on some GPS overlay approaches. To aid in the approach chart/database correlation process, the FAA has begun a program to assign five-letter names to CNFs and to chart CNFs on various FAA Aeronautical Navigation Products (AeroNav Products). These CNFs are not to be used for any air traffic control (ATC) application, such as holding for which the fix has not already been assessed. CNFs will be charted to distinguish them from conventional reporting points, fixes, intersections, and waypoints. The CNF name will be enclosed in parenthesis, e.g., (CFBCD), and the name will be placed next to the CNF it defines. If the CNF is not at an existing point defined by means such as crossing radials or radial/DME, the point will be indicated by an "X." The CNF name will not be used in filing a flight plan or in aircraft/ATC communications. Use current phraseology, e.g., facility name, radial, distance, to describe these fixes
  • Unnamed waypoints in the database will be uniquely identified for each airport but may be repeated for another airport (e.g., RW36 will be used at each airport with a runway 36 but will be at the same location for all approaches at a given airport)
  • The runway threshold waypoint, which is normally the MAWP, may have a five letter identifier (e.g., SNEEZ) or be coded as RW## (e.g., RW36, RW36L). Those thresholds which are coded as five letter identifiers are being changed to the RW## designation. This may cause the approach chart and database to differ until all changes are complete. The runway threshold waypoint is also used as the center of the Minimum Safe Altitude (MSA) on most GPS approaches. MAWPs not located at the threshold will have a five letter identifier

Position Orientation:

  • As with most RNAV systems, pilots should pay particular attention to position orientation while using GPS. Distance and track information are provided to the next active waypoint, not to a fixed navigation aid. Receivers may sequence when the pilot is not flying along an active route, such as when being vectored or deviating for weather, due to the proximity to another waypoint in the route. This can be prevented by placing the receiver in the non-sequencing mode. When the receiver is in the non-sequencing mode, bearing and distance are provided to the selected waypoint and the receiver will not sequence to the next waypoint in the route until placed back in the auto sequence mode or the pilot selects a different waypoint. On overlay approaches, the pilot may have to compute the along-track distance to step-down fixes and other points due to the receiver showing along-track distance to the next waypoint rather than DME to the VOR or ILS ground station

Departures and Instrument Departure Procedures (DPs):

  • The GPS receiver must be set to terminal (±1 NM) CDI sensitivity and the navigation routes contained in the database in order to fly published IFR charted departures and DPs
  • Terminal RAIM should be automatically provided by the receiver
    • Terminal RAIM for departure may not be available unless the waypoints are part of the active flight plan rather than proceeding direct to the first destination
  • Certain segments of a DP may require some manual intervention by the pilot, especially when radar vectored to a course or required to intercept a specific course to a waypoint
  • The database may not contain all of the transitions or departures from all runways and some GPS receivers do not contain DPs in the database
  • It is necessary that helicopter procedures be flown at 70 knots or less since helicopter departure procedures and missed approaches use a 20:1 obstacle clearance surface (OCS), which is double the fixed-wing OCS, and turning areas are based on this speed as well

Missed Approach:

  • A GPS missed approach requires pilot action to sequence the receiver past the MAWP to the missed approach portion of the procedure. The pilot must be thoroughly familiar with the activation procedure for the particular GPS receiver installed in the aircraft and must initiate appropriate action after the MAWP. Activating the missed approach prior to the MAWP will cause CDI sensitivity to immediately change to terminal (±1NM) sensitivity and the receiver will continue to navigate to the MAWP. The receiver will not sequence past the MAWP. Turns should not begin prior to the MAWP. If the missed approach is not activated, the GPS receiver will display an extension of the inbound final approach course and the ATD will increase from the MAWP until it is manually sequenced after crossing the MAWP
  • Missed approach routings in which the first track is via a course rather than direct to the next waypoint require additional action by the pilot to set the course. Being familiar with all of the inputs required is especially critical during this phase of flight

System Availability and Reliability:

  • The operational status of GNSS operations depends upon the type of equipment being used. For GPS−only equipment TSO−C129a, the operational status of non-precision approach capability for flight planning purposes is provided through a prediction program that is embedded in the receiver or provided separately

Special Category I Differential GPS (SCAT-I DGPS):

  • The SCAT-I DGPS is designed to provide approach guidance by broadcasting differential correction to GPS
  • SCAT-I DGPS procedures require aircraft equipment and pilot training
  • Ground equipment consists of GPS receivers and a VHF digital radio transmitter
    • The SCAT-I DGPS detects the position of GPS satellites relative to GPS receiver equipment and broadcasts differential corrections over the VHF digital radio
  • Category I Ground Based Augmentation System (GBAS) will displace SCAT-I DGPS as the public use service

Errors:

  • VHF "harmonic interference"
  • Atomic clock inaccuracies
  • Receiver equipment characteristics
  • Reflection from hard objects
  • Ionospheric and tropospheric delays

Conclusion:

  • U.S. civil operators may use approved GPS equipment in oceanic airspace, certain remote areas, the National Airspace System and other States as authorized (please consult the applicable Aeronautical Information Publication)
    • Equipage other than GPS may be required for the desired operation
    • GPS navigation is used for both Visual Flight Rules (VFR) and Instrument Flight Rules (IFR) operations
  • Although GPS has provided many benefits to the VFR pilot, care must be exercised to ensure that system capabilities are not exceeded
    • VFR pilots should integrate GPS navigation with electronic navigation (when possible), as well as pilotage and dead reckoning
  • Most receivers are not intuitive
    • The pilot must learn the various keystrokes, knob functions, and displays that are used in the operation of the receiver
    • Some manufacturers provide computer-based tutorials or simulations of their receivers that pilots can use to become familiar with operating the equipment

User Reports Requested on NAVAID or Global Navigation Satellite System (GNSS) Performance or Interference:

  • Users of the National Airspace System (NAS) can render valuable assistance in the early correction of NAVAID malfunctions or GNSS problems and are encouraged to report their observations of undesirable avionics performance
    • Although NAVAIDs are monitored by electronic detectors, adverse effects of electronic interference, new obstructions or changes in terrain near the NAVAID can exist without detection by the ground monitors
    • Some of the characteristics of malfunction or deteriorating performance which should be reported are: erratic course or bearing indications; intermittent, or full, flag alarm; garbled, missing or obviously improper coded identification; poor quality communications reception; or, in the case of frequency interference, an audible hum or tone accompanying radio communications or NAVAID identification
    • GNSS problems are often characterized by navigation degradation or service loss indications
  • Pilots conducting operations in areas where there is GNSS interference may be unable to use GPS for navigation, and ADS−B may be unavailable for surveillance
    • Radio frequency interference may affect both navigation for the pilot and surveillance by the air traffic controller
    • Depending on the equipment and integration, either an advisory light or message may alert the pilot
    • Air traffic controllers monitoring ADS−B reports may stop receiving ADS−B position messages and associated aircraft tracks
  • In addition, malfunctioning, faulty, inappropriately installed, operated, or modified GPS re-radiator systems, intended to be used for aircraft maintenance activities, have resulted in unintentional disruption of aviation GNSS receivers
    • This type of disruption could result in un-flagged, erroneous position information output to primary flight displays/indicators and to other aircraft and air traffic control systems
    • Since receiver autonomous integrity monitoring (RAIM) is only partially effective against this type of disruption (effectively a "signal spoofing"), the pilot may not be aware of any erroneous navigation indications; ATC may be the only means available for identification of these disruptions and detect unexpected aircraft position while monitoring aircraft for IFR separation
  • Pilots reporting potential interference should identify the NAVAID (for example, VOR) malfunction or GNSS problem, location of the aircraft (that is, latitude, longitude or bearing/distance from a reference NAVAID), magnetic heading, altitude, date and time of the observation, type of aircraft (make/model/call sign), and description of the condition observed, and the type of receivers in use (that is, make/model/software revision)
  • Reports should be made in any of the following ways:

    1. Immediately, by voice radio communication to the controlling ATC Facility or FSS
    2. By telephone to the nearest ATC facility controlling the airspace where the disruption was experienced
    3. Additionally, GNSS problems should be reported by Internet via the GPS Anomaly Reporting Form at http://www.faa.gov/air_traffic/nas/gps_reports/
  • In aircraft equipped with more than one avionics receiver, there are many combinations of potential interference between units that could cause erroneous navigation indications, or complete or partial blanking out of the display
    • GPS interference or outages associated with known testing NOTAMs should not be reported to ATC
  • GPS testing can be found here

References: