Global Navigation Satellite System (GNSS) Flashcards
GNSS segments
- Space segment - satellites
- Control segments - ground based stations
- User segment - recievers
Space segment - satellites
- As of 2021 GPS has approx 21 satellites in orbit
- Minimum 24 operational satellites
- Each satellite orbits every 12 hours - pass over same locations in every orbit
- Always at least six visible satellites
Pseudo-random code
- Broadcasts a signal that carries a navigation message and a unique code know as pseudo-random code.
Contains:
- Space Vehicle Number (SVN) and identifies the satellite
- Time the message was transmitted
- Ephemeris data - current position of the satellite
- Health of satellite
- Almanac data - information of the entire satellite constellation
Control segments
Maintained by US Space Force.
Uses:
- 1 Master Control Station (MCS)
- 1 backup MCS
- 16 monitoring stations located around the world
- 7 remote tracking stations
- 4 dedicated ground antennas
User segment
- Civilian, military, and scientific users with receivers including specially built-for-purpose devices through to a mobile phone.
- GNEE unit receives satellite signals, decodes them, and processes them.
Position fixing
- GNSS works on the principle of trilateration - measures distances with spheres
- Each satellite broadcasts a radio signal - travels out in 360 in every direction at the speed of light
- 2D position fix requires a receiver to see three satellites
- 3D position fix requires a receiver to see at least four satellites.
The more satellites the better the accuracy.
Barometric aiding
- Alternative to the fourth satellite in 3D position fix.
- Allows a GNSS system to use a non-satellite input source to provide a vertical reference
Satellite clock error
- For a receiver to calculate the time a satellite signal took to reach it and obtain a position fix, the receiver needs to be synchronised with the satellite clock.
- If a receiver’s clock is even a fraction of a second ahead of the satellites’ clocks, then the period between transmission and reception will initially be sensed as taking longer
- Quartz crystal clock is less accurate and reliable than an atomic clock.
Ephermeris error
- Position error caused by the gravitational effects of the sun, planets and solar radiation.
- To minimise this, the satellite position is reviewed every 12hrs and where necessary, updated.
Ionospheric delay
- Ionosphere can slow down radio waves form satellites causing an “ionospheric delay”.
- Higher frequencies face less delay, while increased ionisation causes more delay.
- Most significant error in GNSS
Multipath effect
- Satellite signals reflect off buildings and terrain.
- Causes signal to be received at the wrong time
Interference error
- Signals are affected by other signals on the same or a similar frequency
Receiver error
any issues with the receiver, including placement of aerial.
Dilution of Precision (DOP)
- Defines how good a position fix is.
- Categorised into:
PDOP (Position dilution of precision): position accuracy of 3D coordinates
GDOP (geometric dilution of precision): overall accuracy of 3D coordinates and time.
PDOP = The closer the satellites are, the greater the error
GDOP = clock error which has not been trimmed out and is therefore related to PDOP like:
GDOP = PDOP + (untrimmed) clock error.
Datum/Coordinate system
Datum = reference point form which measurements are made.
Geodetic datum (or system) = a coordinate system and a set of reference points
Need to know - WGS84
NZ currently uses NZ Geodetic Datum 2000 (NZGD2000)
Receiver Autonomously Integrity Monitoring (RAIM)
- Inbuilt algorithm in the receiver unit to check the integrity of the information broadcasted by a satellite.
- Compares its position to at least five satellites
RAIM warning
Will display a RAIM warning it receiver detects an issue with satellite data.
- Calculated at different stages of flight
RAIM prediction
- RAIM holes can be predicted
- If there is a RAIM hole detected in the area of your destination at your ETA, you cannot carry out a GPS approach, you cannot carry out a GPS approach
How can RAIM prediction be obtained?
- Onboard GNSS receiver
- An air navigation service provider
Predictions online are valid for 72hrs.
Performance Based Navigation (PBN)
- Area navigation (RNV)
- Required Navigation Performance (RNP)
Area Navigation (RNAV)
Uses GNSS and the inertial reference systems on board the aircraft to create additional waypoints that an aircraft can fly to.
Give greater selection of routes and thus more efficiency.
Required Navigation Performance (RNP)
RNP is like a tube - the aircraft is allowed to be anywhere within the diameter of that tube - the margins of error for the aircraft’s track.
- Useful when operating around terrain and in low visibility conditions
RNP defined by ICAO
A statement of the navigation performance necessary from operation within a defined airspace.
- Refers to the accuracy of the navigation required for a specific procedure - usually an approach to an airport.
Advantages of RNP
- Greater navigational accuracy
- Reduces step-down and circling approaches
- Saves fuel and time
- Enables curved approach’s and arrivals which increases airspace capacity
- Reduces the number of missed apporaches
