GNSS Flashcards
RAIM (receiver autonomous integrity monitoring)
Provides a warning that GPS can no longer be relied on, in the process it ensures that a potential error, known as the position dilution of precision (PDOP) is minimised. The PDOP depends on the position of the satellites relative to the fix, the further the satellites are apart(but still in view) the lower the value of the PDOP and thus the greater the accuracy
RAIM warning displayed for more than 10 minutes en route
Notify ATC and use an alternative means of navigation to confirm you position every 10 minutes
When you can you not do a GPS approach
If RAIM is lost because the position error limits cannot be guaranteed
GPS/GNSS limitations
Human error (Data entry error, Mode error, control knob/switch non-standardisation, placement of GPS, collision risk) and system error (multi path error, Ionospheric propagation effect, tropospheric propagation effect, receiver error, interference)
GPS/GNSS coordinate systems
GPS is based on WGS84 and relies on the UHF signals received from a constellation of 32 satellites which orbit the earth. The reception of 3 or more signals is used to provide accurate position and ground speed information (plus a variety of other navigation data) although the reception of 4 satellites is needed to give accurate altitude info.
Data entry error
Insertion of incorrect data into the GS computer (entering wrong coordinates of way points, made worse by small size of some GPS units)
Mode error
Incorrect mode selection
Control knob and switch non-standardisation
Lack of standardisation between manufactures of GPS units in terms of control knobs and switches
Placement of GPS
Issue with instrument scanning, the dividing of attention between flying the a/c visually and trying to manage and interpret a poorly placed GPS can cause problems
Collision risk
GPS accuracy when in poor VFR conditions means that multiple pilots are likely to fly to the same reporting point using GPS so there is a high risk of collision
Multi path error
Satellite signals bouncing off the earth surface result in signals arriving at the GPS receiver from different directions and therefore different times
Ionospheric propagation effect
Charged particles in the ionosphere interfere with the propagation speed of satellite signals
Tropospheric propagation effect
Water vapour in the troposphere slow down satellite signals
Receiver error
Difficulty of the receiver in precisely matching the internal pseudo-random code with that of the satellite
Interference
Insufficient shielding from the VHF transmitters, radar, TV and FM stations in the vicinity of the GPS receiver can degrade equipment accuracy
