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GNSS Flashcards

(57 cards)

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1
Q

Wherever you are on the planet there at least how many satellites visible

A

4

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2
Q

Principle of a GPS

A

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

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3
Q

Three segments of GPS

A

Space Segment - x24 satellites

Control segment - global stations controlling

User segment - gps receiver equipment (pilot)

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4
Q

Pseudolites

A

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

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5
Q

Coverage of NAVSTAR

A

The coverage varies with time

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6
Q

Errors in satellite orbits are due to

A

Solar Winds
Gravitation of the sun and the moon
Other Planets

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7
Q

Airborne Based Augmentation System (ABAS)

A

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

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8
Q

Receiver Autonomous Integrity Monitoring

A

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)

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9
Q

EGNOS use how many geo SVS

A

Up to 4 SVS - not necessarily all 4

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10
Q

GPS and Glonass

A

They are interoperable from a users perspective
Independent use of different data for navigation services

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11
Q

Nav Message

A

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
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12
Q

Ground Based Augmentation System

A

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)

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13
Q

Ionospheric/Tropospheric errors

A

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

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14
Q

Multi Channel Receiver

A

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

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15
Q

Principals of L4

A

Used to determine ionospheric model to calculate the time delay of signals travelling through the ionosphere.

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16
Q

GNSS provides

A

PVT

Highly accurate position
Velocity
Time

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17
Q

L1

A

Link 1
Modulates with coarse/acquisition code for SPS and a precision code for PPS.

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18
Q

L2

A

Precision code only (not used for SPS)

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19
Q

L Signals contain

A

Pseudo random noise - generate a PRN code
Time of Transmission - time of signal transmission to allow calc
Nav message - data

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20
Q

Geometric Altitude

A

Height above the surface of the geoid - not the same as barometric

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21
Q

GNSS errors

A

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

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22
Q

Geometric Dilution of precision

A

Geometry + SVS in view
Poor triangulation of lines of position
Angles between SVS IS Small

Resolved: 4 SVS with best geometry - 120 degrees apart

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23
Q

Summary of errors

Study These Flashcards
A

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

24
Q

Measured accuracy of NAVSTAR GPS

Study These Flashcards
A

2.5 metres horizontally
4.7 metres vertically

25
Aircraft Autonomous Integrity Monitoring
GNSS receivers integrated with aircraft sensors to cross check Barometric altitude/IRS data to fine tune position
26
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
27
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
28
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)
29
GBAS positioning service
Range - 20nm Precision approach/less expensive than ILS 15nm - 25 degrees 20nm - 10 degrees
30
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
31
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
32
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
33
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
34
What infomation is provided in a 3D position fix
Latitude Longitude Altitude Time
35
Main difference between GBAS and SBAS
SBAS - uses a satellite to relay CORRECTION factors to ac GBAS - transmit error data directly to AC
36
User Equivalent Range Errors are
Maximum position errors to be expected by the user. Residual errors affecting receiver position
37
EGNOS stands for
European Geostationary navigation overlay service SBAD service
38
Total position error can be derived from
Geometric dilution of precision and user equivalent range error
39
Range is calculated by GPS receiver is define by a
Sphere with its centre at the satellite
40
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
41
For a satellite to be visible by GPS receiver it needs to be
Higher elevation than 5 degree above the horizon
42
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
43
GPS uses what frequency
UHF L1 - 1575 MHZ L2 - 1227 MHZ
44
Galileo consists of
3 sets of 3 30 satellites 3 original planes Original altitude of 23222km
45
GPS consists of
24 satellites 6 orbital planes Altitude - 20,200km
46
FAS
Final approach segment data block - SBAS Used to generate describe the final approach path
47
RAIM only works with a minimum of
Five satellites and sufficient geometry is visible
48
From biggest to small errors affecting accuracy of GNSS
Ionospheric propagation delay Dilution of position Satellite clock error Satellite orbital variations Multi path
49
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
50
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
51
Fault Detection - RAIM
Does not tell us which is faulty it just inform of a faulty satellite very quickly
52
Fault detection and exclusion
6 total satellites allows reliable calculation of the faulty satellite and remove it from usage
53
What is transmitted on both L1 and L2
Ranging signals - signals to determine the range
54
RAIM =
Redundancy - ensures integrity of the provided data by redundant measurements
55
GBAS provide aircraft with
Integrity information + approach data corrections
56
UERE =
Ionospheric propagation delay Dilution of precision Satellite clock error Satellite orbital variations Multi path
57
4 parameters contained in nav message
Satellite clock correction parameters Universal time coordinate parameters Ionospheric model Satellite health data SUIS

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