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GPS Usage under IFR

Objective​

The student will have knowledge of GPS operation, including its principles, capabilities, limitations, and the necessary procedures for utilizing the GPS system for navigation and instrument procedures under IFR

Timing​

45 minutes

Format​

  • Garmin GTN Trainer

Overview​

  • GPS Principles of Operation
  • Receiver Autonomous Integrity Monitoring (RAIM)
  • GPS Errors & Irregularities
  • Wide Area Augmentation System (WAAS)
  • GPS Modes of Operation
  • GPS Use Under IFR
  • GPS CDI Scaling (En Route, Terminal, & Approach)
  • GPS Waypoints
  • GPS Direct-To Operations
  • GPS Flight Plan Operations
  • GPS Nearest Functions
  • Substitution of GPS for Other Navigation Radios Under IFR
  • Orientation, Position, and Waypoint Passage / Sequencing
  • GPS Course Intercepting and Tracking Procedures / Wind Correction Techniques
  • Computer / App Based GPS Procedures Simulator
  • Installed GPS Specific Procedures

Elements​

GPS Principles of Operation​

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  • GPS is a satellite-based radio navigation system
  • Space Element consists of over 30 Navstar satellites
    • The satellites broadcast a pseudo-random code timing signal and data message
    • Satellites transmit position, velocity, and precise timing
    • At least five satellites are designed to be in view to a user worldwide
  • Control Element is a network of ground-based monitoring and control stations
    • Monitoring stations ensure the accuracy of satellite positions and clocks
  • User Element consists of antennas and receiver/processors onboard the aircraft
    • The receiver requires a minimum of four satellites for a three-dimensional position solution (latitude, longitude, and altitude)
    • The receiver computes distance based on signal travel time (pseudo-range)
    • Position is determined by triangulation using the pseudo-range and position information supplied by the satellite
  • Other countries have their own satellite navigation systems, such as Russia's GLONASS, the European Union's Galileo, and China's BeiDou.

Receiver Autonomous Integrity Monitoring (RAIM)​

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  • RAIM is the capability of a GPS receiver to monitor its own integrity
    • It uses extra satellites to compute multiple positions
    • RAIM requires a minimum of five satellites in view, or four satellites and barometric altimeter input (baro-aiding), to detect an integrity anomaly
  • Some receivers are capable of fault detection and exclusion (FDE) that require 6 satellites in view (or 5 satellites with baro-aiding)
    • FDE allows the receiver to isolate a corrupt signal and remove it from the position solution, while still providing an integrity-assured position
  • The two general types of RAIM messages are:
    • Insufficient satellites to provide integrity monitoring
    • RAIM integrity monitor detects a potential error exceeding the limit for the phase of flight

GPS Errors & Irregularities​

  • GPS positioning data can be affected by various geometric factors and equipment characteristics, typically causing errors of less than 100 feet
  • Small position errors or momentary loss of signal can be caused by:
    • Satellite atomic clock inaccuracies
    • Receiver/processors
    • Signals reflected from hard objects (multi-path)
    • Ionospheric and tropospheric delays
    • Satellite data transmission errors
  • Loss of signals may occur in valleys surrounded by high terrain
  • Loss of satellite reception may occur due to aircraft dynamics (changes in pitch or bank angle)
  • Jamming (emissions that interfere with signal reception) and spoofing (emissions that mimic GNSS signals) are intentional unauthorized interference that can disrupt air navigation
  • The U.S. government regularly conducts GPS testing that interferes with GNSS signals, which are geographically limited, coordinated, scheduled, and advertised via NOTAMs

Wide Area Augmentation System (WAAS)​

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  • WAAS is designed to improve the accuracy, integrity, and availability of GPS signals
    • It is the U.S. version of a satellite-based augmentation system (SBAS)
    • Allows GPS to be used for takeoff through Category I precision approaches
  • A network of Wide-Area Reference Stations (WRS) on the ground monitors GPS signals for errors
    • WRS relays data to a Wide-Area Master Station (WMS)
    • The WMS calculates correction information and uplinks a message to a Geostationary Earth Orbit (GEO) satellite
  • The GEO satellite broadcasts the WAAS-corrected signal on the same frequency as GPS (L1, 1575.42 MHz) to WAAS receivers
    • The GEO satellite also provides an additional pseudo-range measurement, improving GPS availability
  • The accuracy is good enough to provide GPS/WAAS glide paths (vertical guidance)
    • LPV (Localizer Performance with Vertical Guidance) uses WAAS accuracy to provide an approach similar to a Category I ILS
      • LPV minimums are published as a Decision Altitude (DA)
    • LP (Localizer Performance) is a WAAS-based line of minima that provides angular lateral guidance and is published as a Minimum Descent Altitude (MDA)

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GPS Modes of Operation​

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GPS CDI sensitivity changes based on the phase of flight

  • En Route: Typically ±5 NM CDI sensitivity, but may be lower (see G1000 image above)
  • Terminal: ±1 NM CDI sensitivity
    • Terminal mode automatically activates when the aircraft is within 30 NM of the airport
  • Approach: ±0.3 NM CDI sensitivity
    • Approach mode becomes active when the aircraft is within 2 NM of the Final Approach Waypoint (FAWP)
    • CDI sensitivity smoothly changes (ramps down) from ±1 NM to ±0.3 NM at the FAWP
  • Missed Approach: CDI sensitivity immediately changes to terminal (±1 NM) sensitivity

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GPS Use Under IFR​

  • GPS must be TSO-certified (C129, C196, C145, or C146) and the installation approved per AC 20-138
    • GPS operations must be conducted in accordance with the AFM or flight manual supplement
  • Non-WAAS GPS (TSO-C129/C196) aircraft must be equipped with an alternate approved and operational means of navigation
    • GPS RAIM availability must be checked before IFR operations
    • If a continuous RAIM loss is predicted for more than 5 minutes, the flight must be delayed, canceled, or re-routed
  • WAAS (TSO-C145/C146) users do not need to perform RAIM prediction if WAAS coverage is confirmed along the entire route
    • The onboard navigation database must be current and appropriate for the operation
    • Procedures must be retrievable by name from the database; manual entry of waypoints is not permitted for approach procedures
  • Pilot should review appropriate NOTAMs before GPS IFR operation

GPS Waypoints​

  • Waypoint (WP) is a predetermined geographical position defined by latitude/longitude coordinates
  • WPs are used to indicate a change in direction, speed, or altitude
  • Fly-by (FB) Waypoints are used when a turn should begin prior to reaching the waypoint (turn anticipation)
    • Chart symbol: Four-pointed star
  • Fly-over (FO) Waypoints require the aircraft to fly over the point prior to starting a turn
    • Chart symbol: Four-pointed star enclosed in a circle
    • The MAWP is always a fly-over waypoint

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GPS Direct-To Operations​

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  • A Direct to Fix (DF) leg defines an unspecified track from an undefined position to a specific database fix
  • Generally there are two types of Direct-To:
    • On-route direct-to, where you navigate directly to a fix in your flight plan (the flight plan will then sequence from there)
    • Off-route direct-to, where you navigate to an arbitrary fix (flight plan sequencing will suspend when you arrive at the fix)
  • ATC may clear an aircraft direct to a fix when issuing approach clearance

GPS Flight Plan Operations​

GPS Nearest Functions​

  • GPS displays can provide information on nearest airports
    • The display can show information including runway length, surface, available approaches, and communication frequencies
    • The nearest airport function allows the pilot to activate a Direct-To course to the selected airport

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Substitution of GPS for Other Navigation Radios Under IFR​

AIM 1-2-3

  • IFR-certified GPS systems may be used as a substitute for traditional NAVAID navigation
    • This application requires the system to have en-route RAIM or WAAS
  • GPS can be substituted to:
    • Fly a DME arc
    • Navigate TO or FROM an NDB/compass locator
    • Hold over an NDB/compass locator
    • Determine position over a NAVAID
  • Pilots may not substitute for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach segment
    • Use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure based on a VOR, TACAN or NDB signal, is allowable, so long as the nav signal is monitored for final segment course alignment
    • Exception: GPS may be used on overlay approaches with "or GPS" in the title

Orientation, Position, Waypoint Sequencing​

  • GPS provides linear course guidance between waypoints
    • Position information
    • Along Track Distance (ATD) is displayed to the next active waypoint, not necessarily a ground-based NAVAID
  • The receiver sequences to the next waypoint when the current one is passed
    • This may be prevented by placing the receiver in non-sequencing mode (often referred to as OBS or HOLD)
    • SUS mode indicates that automatic sequencing is suspended
    • The Missed Approach Waypoint (MAWP) requires pilot action to sequence the receiver

GPS Course Intercepting, Tracking Procedures, Wind Correction Techniques​

  • Match your current track (TRK) with the desired track (DTK)
  • The difference between the aircraft’s heading and ground track visualizes wind drift
    • This information, often displayed on the MFD, simplifies crosswind corrections
    • To maintain the desired track (course), pilots must crab the aircraft into the wind
  • Garmin HSI's show ground track as a magenta diamond

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Computer / App Based GPS Procedures Simulator​

  • Computer-based tutorials or simulations are often provided by manufacturers to allow pilots to become familiar with the equipment’s operation
  • Practice should include:
    • Programming approach procedures (especially procedure turns and arcs)
    • Programming and flying holding patterns
    • Programming and flying missed approaches
    • Indicating actions required for RAIM failure

See the Avidyne IFD Trainer App and the Garmin GTN Trainer App

Installed GPS Specific Procedures​

  • Discuss specifics of Garmin GTN or Avidyne IFD
    • Garmin
      • "Activating" an approach just means going direct-to the initial approach fix (IAF)
      • On old units, when flying a vectors-to-final approach, it can be helpful to still load an approach with a transition (rather than the Vectors to Final Mode)
        • This loads all of the previous waypoints into the approach, in case you need to reference them
    • Avidyne

References​