HOME

Flying

CFII
Instruments
GNS 530

CFI Instrument Practical Test Standards, FAA-S-8081-9B, June 2001

II. Technical Subject Areas

A. Aircraft Flight Instruments and Navigation Equipment

    2. Flight instrument systems and their operating characteristics

      f. Global Positioning System (GPS)
        1) Definition and description
          a) GPS is a spaced-based positioning, velocity and time system
          b) Department of Defense (DOD) developed and is responsible for monitoring the GPS satellite constellation to ensure proper operation
          c) Provides aircraft position referenced to DOD World Geodetic System of 1984 (WGS-84)
          d) Unaffected by weather
          e) Meets civil requirements for use as primary means of navigation in oceanic and certain remote areas
          f) Properly certified GPS equipment is approved for use as supplemental means of IFR navigation for
            i) Domestic enroute operation
            ii) Terminal operations
            iii) Certain IAPs
        2) GPS components
          a) Space elements
            i) Constellation of 24 Navstar satellites
            GPS Orbits
            ii) Four satellites in each of 6 orbital planes about 11,000 miles above the earth
            iii) At least 5 satellites are in view at all times
            iv) GPS constellation broadcasts pseudorandom code timing signal and data message that aircraft equipment processes to obtain satellite position and status
            v) Aircraft receiver/processor measures time each signal takes to arrive at receiver and, combining this with location data from each satellite, determines aircraft position
          b) Control elements
            i) Network of ground-based GPS monitoring and control stations
            • Five monitoring stations
            • Three ground antennas
            • Master control station
            ii) Ensure accuracy of satellite positions and clocks
          c) User elements
            i) Aircraft antennas and receiver/processor (e.g. Garmin GNS 530) that provide position, velocity, and timing information
            ii) Requirements for GPS equipment used under IFR
            • Meet standards in Technical Service Order (TSO) C-129
            • Meet airworthiness installation standards
            • Be "approved" for that type of IFR operation
            • Be operated in accordance with the applicable POH/AFM or supplement
            • Updatable GPS database that supports the appropriate operations
            iii) Equipment approved in accordance with TSO C-115a, VFR and hand-held systems
            • Do not meet TSO C-129 requirements and are not authorized for IFR navigation
            • May only be considered aids to situational awareness
        3) Function of GPS
          a) Operation based on concept of ranging and triangulation from a minimum of four satellites above the mask angle (lowest usable angle above horizon)
          b) Each satellite transmits a specific course/acquisition (CA) code containing
            i) Satellite's ephemeris (exact position in space)
            ii) GPS system time
            iii) Health and accuracy of the data
          c) Pseudo-range (distance determined by time measurement) is derived by receiver/processor
          d) Using pseudo-range and supplied position information from at least four satellites, GPS receiver/processor determines, by triangulation, a three-dimensional position (latitude, longitude, altitude) and time solution
          e) Navigational values are computed by the GPS receiver/processor using the position/time solution above and its built-in database
          f) Receiver autonomous integrity monitoring (RAIM)
            i) Verifies integrity (usability) of GPS signals
            ii) Needs at least five satellites in view (or four plus a barometric altimeter to provide baro-aiding) to detect an integrity anomaly
            iii) Some receivers, with six satellites in view (or five plus baro-aiding) can isolate a corrupt signal and remove it from the navigation solution
            iv) Two types of RAIM messages
            • Not enough satellites available (GNS 530: INTEG annunciation)
            • Potential error detected (GNS 530: WARN annunciation)
            v) Without RAIM, GPS accuracy is not ensured
            vi) Active monitoring of the required alternate means of navigation is not required if GPS receiver uses RAIM
            vii) Predicting loss of RAIM (other approved equipment must then be used)
          GPS RAIM
        4) GPS substitution
          a) IFR certified GPS system may substitute for ADF and DME when
            i) Determining position over DME fix (including operations above FL 240)
            ii) Flying a DME arc (GNS 530)
            iii) Navigating TO/FROM NDB/compass locator
            iv) Determining position over NDB/compass locator
            v) Determining position over fix defined by NDB/compass locator bearing crossing a VOR/LOC course
            vi) Holding over NDB/compass locator
          b) Requirements for using GPS to substitute for ADF or DME
            i) GPS meets airworthiness installation requirements and is operated in accordance with POH/AFM or supplement
            ii) At least en route RAIM or equivalent
            iii) Positions must be retrieved form current database
            iv) Be able to use alternate equipment when RAIM outages are predicted
            v) CDI set to terminal sensitivity (usually 1-1¼ NM) in terminal area
            vi) If alternate airport is required
            • A non-GPS approach must be available at alternate airport
            • If the non-GPS approach requires DME or ADF, aircraft must be equipped with DME or ADF avionics
            viii) Charted ADF and/or DME requirements can be met using GPS, except for use as principal instrument approach navigation source
          c) To determine position over DME fix
            i) Verify satisfactory GPS system integrity
            ii) Select as active GPS waypoint (WP) from GPS database either
            • Five-letter named fix, or
            • Facility establishing DME fix
            iii) Aircraft is over selected 5-letter named fix when GPS indicates aircraft is at active WP
            iv) If facility establishing DME fix selected as WP, aircraft is over fix when on course and GPS distance from WP equals charted DME value
          d) To fly DME arc (GNS 530)
            i) Verify (satisfactory GPS system) integrity
            ii) As active GPS WP, select, from database, facility on which DME arc is based
            iii) Maintain position on arc by reference to GPS distance
          e) To navigate TO or FROM an NDB/compass locator
            i) Verify integrity
            ii) As active WP, select, from database, NDB/compass locator or collocated fix of the same name
            iii) Select and navigate on course to or from WP
          f) To determine position over NDB/compass locator
            i) Verify integrity
            ii) Select NDB/compass locator from database
            iii) Aircraft is over NDB/compass locator when GPS indicates aircraft is at the active WP
          g) To determine position over fix when an NDB/compass locator bearing intersects a VOR/LOC course
            i) Verify integrity
            ii) As active WP, select from database either 5-letter named fix or NDB/compass locator
            iii) Aircraft is over fix when GPS indicates at 5-letter named fix WP, or when GPS bearing to NDB/compass locator WP is the same as that charted for fix as aircraft is flying along prescribed track
          h) To hold over NDB/compass locator
            i) Verify integrity
            ii) Select NDB/compass locator from database as active WP
            iii) Select nonsequencing mode and appropriate course according to POH or supplement
            iv) Hold using GPS according to POH (GNS 530)
        5) IFR flight using GPS
          a) Install, check current database
          b) Conduct operations in accordance with POH (GNS 530)
          c) Study to become comfortable with operation of specific receiver installed in aircraft
          d) Practice in VFR conditions first
          e) Check GPS NOTAMs
          f) Obtain GPS RAIM availability information
          g) Check required underlying ground-based navigation facilities and related aircraft equipment operational
          h) Enter flight plan into GPS receiver: departure WP, DP, enroute WPs, STAR, IAF, destination airport (GNS 530)
          i) Select, activate flight plan
          j) GPS provides course guidance between WPs, including desired direct track to WP and aircraft's actual track over ground
        6) GPS instrument approaches (GNS 530)
          a) Types
            i) GPS overlay approaches have "or GPS" as part of the approach name
            ii) GPS stand-alone approaches are named "GPS..." or "RNAV (GPS)..."
          b) Ground-based NAVAIDs and associated aircraft avionics are not required to be operational or monitored, but monitoring available backup navigation systems is always recommended
          c) Practice GPS IAPs under VMC first until thoroughly proficient
          d) Fly only those IAPs retrieved from current GPS database to assure compliance with published procedures and proper RAIM and CDI sensitivities
          e) Follow manufacturers recommended procedures (GNS 530)
          f) Fly full approach from initial approach WP (IAWP) or feeder fix unless specifically cleared otherwise
          g) Load and arm (activate) IAP beyond 30 NM from airport so receiver will change from enroute CDI (+/- 5 NM) and RAIM (+/- 2 NM) sensitivity to terminal sensitivity (+/- 1 NM) when within 30 NM of airport
          h) Follow manufacturers recommendations for holding pattern (GNS 530) and procedure turn (GNS 530) which may require pilot action to stop and later resume waypoint sequencing
          i) Follow receiver operating manual procedures when receiving vectors to final (GNS 530)
          j) Within 2 NM of final approach WP (FAWP), armed approach mode switches to active approach mode resulting in RAIM and CDI sensitivity changing to approach mode sensitivity, +/- 0.3 NM
          GPS CDI Scale
          k) To help maintain position orientation during CDI sensitivity ramp down
            i) Get established on final approach course beyond 2 NM from FAWP
            ii) Note crosstrack error (compare actual to desired/direct track)
          l) If approach mode is not armed by 2 NM prior to FAWP
            i) Approach mode will not become active at 2 NM from FAWP
            ii) GPS receiver will flag
            iii) Pilot should fly to MAWP and execute a missed approach (GNS 530)
          m) Check approach mode active prior to FAWP
          o) GPS missed approach
            i) Follow manufacturers recommendations (GNS 530)
            ii) Requires pilot action to sequence past MAWP to missed approach procedure
            iii) No turns prior to MAWP
            iv) Additional pilot action required if first tack of missed apporach is via a course rather than direct to next WP
          p) Flying to alternate airport after missed approach
            i) Avionics needed to receive all appropriate ground facilities for route to alternate must be installed and operational
            ii) Altenate airport must have an operational approach that is NOT based on GPS or LORAN C navigation; and aircraft must have the appropriate operational equipment to fly that approach
        7) GPS errors
          a) Whenever less than 24 satellites are operational, GPS navigational capabilities may be lost in certain areas
          b) Loss of signal in valleys surrounded by high terrain
          c) Loss of signal when aircraft's GPS antenna is "shadowed" by aircraft structure (e.g. when the aircraft is banked)
          d) Signal interference
            i) From certain receivers, transceivers, mobile radios, portable receivers; "harmonic interference" from some UHF transmissions
            ii) Isolate interference by moving or turning off suspected devices while monitoring GPS receiver's signal quality data page
          e) Equipment characteristic and geometric factors can cause small errors (typically less than 100 feet)
          f) Small position errors or momentary loss of signal
            i) Small atomic clock inaccuracies
            ii) Receiver/processor error
            iii) Mulitpath (signals reflected from hard objects)
            iv) Ionospheric and tropospheric delays
            v) Satellite data transmission error
          g) Selective availability (SA)
            i) Method by which DOD can create significant clock and ephemeris errors in the satellites
            ii) When SA active
            • Horizontal accuracy is within
                100 m 95% of time
                300 m 99.99% of time
            • Vertical accuracy is within
                156 m 95% of time
                500 m 99.99% of time
            • Time accurate within
                300 nsecs 95% of time
                900 nsecs 99.99% of time
          h) Do not rely on GPS altitude information (vertical error can be significant)

        References:
Greg Gordon MD, CFII
Updated: