GNSS Flashcards
Wherever you are on the planet there at least how many satellites visible
4
Principle of a GPS
Satellite transmits information about its position and current time at regular intervals at speed of light.
Intercepted by GPS receiver which calculates how far away each satellite is based on how long it took message to arrive.
Once info from three satellites GPS receiver can pinpoint location
Three segments of GPS
Space Segment - x24 satellites
Control segment - global stations controlling
User segment - gps receiver equipment (pilot)
Pseudolites
Ground based transmitters that send global navigation satellite system signals. Located near runway.
Ionospheric and tropospheric transmission paths identical eliminating errors
Acts like a satellite
Coverage of NAVSTAR
The coverage varies with time
Errors in satellite orbits are due to
Solar Winds
Gravitation of the sun and the moon
Other Planets
Airborne Based Augmentation System (ABAS)
Covers Aircraft Autonomous Integrity Monitoring and receiver autonomous integrity monitoring
Monitors integrity
Uses data from aircraft sensors (IRS/nav aids/barometric altitude)
Allows aircraft to notice any suspicious GNSS position data and notify crew
Receiver Autonomous Integrity Monitoring
Uses an extra GNSS satellite to verify the working order of the usual 4 satellites.
5 = fault detection
6 = detect and identify (fault detection and exclusion)
EGNOS use how many geo SVS
Up to 4 SVS - not necessarily all 4
GPS and Glonass
They are interoperable from a users perspective
Independent use of different data for navigation services
Nav Message
Sent from GNSS SVS
Information includes:
- Almanac - non precise data on location
- Ephemeris - precise data on exact location
- satellite clock correct - correction data
- UTC correction - time difference UTC and GPS
- ionospheric model - maths model to calculate errors
- Satellite health status
Ground Based Augmentation System
Reduce nautical errors within GNSS system - localised augmentation to the satellite signals via VHF data broadcast
- frequency band of VOR/ILS (108MHZ and 118MHZ)
Give integrity warning about faulty satellites.
+ Precision approach - GBAS landing system (GLS)
- very short range (30km)
Ionospheric/Tropospheric errors
Electrical charged ion where GNSS signal passes and interacts which reduces speed.
Depends on solar activity/time of year/time of day/location
Hard to predict delay
Reduced in standard positioning service received by using model of ion sphere transmitted by the satellites
Multi Channel Receiver
Monitors server SV at same time, using 1 channel for each SV.
Selects the 4 with the best geometry to perform a fix.
Used in aviation
Principals of L4
Used to determine ionospheric model to calculate the time delay of signals travelling through the ionosphere.
GNSS provides
PVT
Highly accurate position
Velocity
Time
L1
Link 1
Modulates with coarse/acquisition code for SPS and a precision code for PPS.
L2
Precision code only (not used for SPS)
L Signals contain
Pseudo random noise - generate a PRN code
Time of Transmission - time of signal transmission to allow calc
Nav message - data
Geometric Altitude
Height above the surface of the geoid - not the same as barometric
GNSS errors
Satellite clock error
Ephemeris error - errors in orbit due to solar winds & gravity from sun/moon
Ionospheric propagation delay - refracted and slowed in ionosphere
Tropospheric propagation error - variation in temp/density on propagation
Receiver noise - receivers cause error in time measurement - range errors
Multi path reception - reflecting of surfaces
Ac manoeuvres - line of sight obscuring
Geometric dilution of precision - poor triangulation of lines of position between SVS
Geometric Dilution of precision
Geometry + SVS in view
Poor triangulation of lines of position
Angles between SVS IS Small
Resolved: 4 SVS with best geometry - 120 degrees apart
Summary of errors
Satellite clock error - 1.5m
Ephemeris error - 2.5m
Ionospheric propagation delay - 5m (biggest error)
Troy’s-heroic propagation delay - 0.5m
Receiver noise - 0.3m
Multi path reception - 0.6m
Geometric dilution of precision - variable
Measured accuracy of NAVSTAR GPS
2.5 metres horizontally
4.7 metres vertically
Aircraft Autonomous Integrity Monitoring
GNSS receivers integrated with aircraft sensors to cross check
Barometric altitude/IRS data to fine tune position
Basic Receiver Autonomous Integrity Monitoring
Requires five SVS - provides pseudo range - integrity of the fix
If SV incompatible with range of others RAIM ALERT Issued
Just fault detection
Enhanced RAIM
Requires Six SVS - identify group of SVS responsible for fault and reject this SV for nav
Reverts to basic RAIM after exclusion and using five SVS
Ground Based Augmentation System (GBAS)
Local Area Augmentation System (LAAS)
Station transmit data to LAAS facility computer for correction value.
Corrects transmitted (pseudolites) using VHF data broadcast on ILS/VOR frequencies (108/118 MHz)
GBAS positioning service
Range - 20nm
Precision approach/less expensive than ILS
15nm - 25 degrees
20nm - 10 degrees
SBAS - Satellite Based Augmentation System
Wide Area augmentation system
Improves accuracy and integrity
Ground station fixes errors and send back to SV
SV then re transmit message to receivers
Identical to GPS signal
+ Alerts receiver of errors within 6 seconds
+ wide range of area
SBAS Compatibility
Interoperable - multi model receiver can benefit from same level of service and performance regardless of area of coverage
Compatible - do not interfere with each other
EGNOS - European geostationary overlay system - SBAS
Space segment
Ground segment - 4 controls centres/6 regional control centres
User segment
EGNOS can improve integrity monitoring by alerting users within 6 seconds of malfunction
GBAS Landing System
Enhances GBAS with additional data
Block of data that define final approach segment
Horizontal path over ground/glide path
Fed info to A/T A/P F/D systems
48 channels
15nm - 35 degrees
15-20nm - 10 degrees
What infomation is provided in a 3D position fix
Latitude
Longitude
Altitude
Time
Main difference between GBAS and SBAS
SBAS - uses a satellite to relay CORRECTION factors to ac
GBAS - transmit error data directly to AC
User Equivalent Range Errors are
Maximum position errors to be expected by the user.
Residual errors affecting receiver position
EGNOS stands for
European Geostationary navigation overlay service
SBAD service
Total position error can be derived from
Geometric dilution of precision and user equivalent range error
Range is calculated by GPS receiver is define by a
Sphere with its centre at the satellite
What are the main tasks of the control segment
Monitor/control satellite orbital parameters
Monitor health and status
Active spare satellites
Update nav messages
Resolve anomalies
Passive tracking
For a satellite to be visible by GPS receiver it needs to be
Higher elevation than 5 degree above the horizon
How many clocks in GPS and Galileo
GPS = 4 Cesium Atomic Clocks
Galileo = 2 atomic block, 1 rubidium atomic frequency standard and 1 passive hydrogen maser - synchronised
GPS uses what frequency
UHF
L1 - 1575 MHZ
L2 - 1227 MHZ
Galileo consists of
3 sets of 3
30 satellites
3 original planes
Original altitude of 23222km
GPS consists of
24 satellites
6 orbital planes
Altitude - 20,200km
FAS
Final approach segment data block - SBAS
Used to generate describe the final approach path
RAIM only works with a minimum of
Five satellites and sufficient geometry is visible
From biggest to small errors affecting accuracy of GNSS
Ionospheric propagation delay
Dilution of position
Satellite clock error
Satellite orbital variations
Multi path
Geometric Dilution of precision is reduced by
One satellite directly overhead the receiver and the other three close to the horizon and space 120 degrees apart in azimuth
RAIM prediction
Used to find out whether the current satellites around are expected to be enough to provide RAIM and a place and time.
If RAIM prediction shows error there is likely insufficient satellite within line of sight at expected time
Fault Detection - RAIM
Does not tell us which is faulty it just inform of a faulty satellite very quickly
Fault detection and exclusion
6 total satellites allows reliable calculation of the faulty satellite and remove it from usage
What is transmitted on both L1 and L2
Ranging signals - signals to determine the range
RAIM =
Redundancy - ensures integrity of the provided data by redundant measurements
GBAS provide aircraft with
Integrity information + approach data corrections
UERE =
Ionospheric propagation delay
Dilution of precision
Satellite clock error
Satellite orbital variations
Multi path
4 parameters contained in nav message
Satellite clock correction parameters
Universal time coordinate parameters
Ionospheric model
Satellite health data
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