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Radio Navigation > GNSS > Flashcards

GNSS Flashcards

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

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

A

2.5 metres horizontally
4.7 metres vertically

25
Q

Aircraft Autonomous Integrity Monitoring

A

GNSS receivers integrated with aircraft sensors to cross check
Barometric altitude/IRS data to fine tune position

26
Q

Basic Receiver Autonomous Integrity Monitoring

A

Requires five SVS - provides pseudo range - integrity of the fix

If SV incompatible with range of others RAIM ALERT Issued

Just fault detection

27
Q

Enhanced RAIM

A

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
Q

Ground Based Augmentation System (GBAS)
Local Area Augmentation System (LAAS)

A

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
Q

GBAS positioning service

A

Range - 20nm
Precision approach/less expensive than ILS
15nm - 25 degrees
20nm - 10 degrees

30
Q

SBAS - Satellite Based Augmentation System

Wide Area augmentation system

A

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
Q

SBAS Compatibility

A

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
Q

EGNOS - European geostationary overlay system - SBAS

A

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
Q

GBAS Landing System

A

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
Q

What infomation is provided in a 3D position fix

A

Latitude
Longitude
Altitude
Time

35
Q

Main difference between GBAS and SBAS

A

SBAS - uses a satellite to relay CORRECTION factors to ac

GBAS - transmit error data directly to AC

36
Q

User Equivalent Range Errors are

A

Maximum position errors to be expected by the user.
Residual errors affecting receiver position

37
Q

EGNOS stands for

A

European Geostationary navigation overlay service
SBAD service

38
Q

Total position error can be derived from

A

Geometric dilution of precision and user equivalent range error

39
Q

Range is calculated by GPS receiver is define by a

A

Sphere with its centre at the satellite

40
Q

What are the main tasks of the control segment

A

Monitor/control satellite orbital parameters
Monitor health and status
Active spare satellites
Update nav messages
Resolve anomalies
Passive tracking

41
Q

For a satellite to be visible by GPS receiver it needs to be

A

Higher elevation than 5 degree above the horizon

42
Q

How many clocks in GPS and Galileo

A

GPS = 4 Cesium Atomic Clocks

Galileo = 2 atomic block, 1 rubidium atomic frequency standard and 1 passive hydrogen maser - synchronised

43
Q

GPS uses what frequency

A

UHF

L1 - 1575 MHZ
L2 - 1227 MHZ

44
Q

Galileo consists of

A

3 sets of 3

30 satellites
3 original planes
Original altitude of 23222km

45
Q

GPS consists of

A

24 satellites
6 orbital planes
Altitude - 20,200km

46
Q

FAS

A

Final approach segment data block - SBAS
Used to generate describe the final approach path

47
Q

RAIM only works with a minimum of

A

Five satellites and sufficient geometry is visible

48
Q

From biggest to small errors affecting accuracy of GNSS

A

Ionospheric propagation delay
Dilution of position
Satellite clock error
Satellite orbital variations
Multi path

49
Q

Geometric Dilution of precision is reduced by

A

One satellite directly overhead the receiver and the other three close to the horizon and space 120 degrees apart in azimuth

50
Q

RAIM prediction

A

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
Q

Fault Detection - RAIM

A

Does not tell us which is faulty it just inform of a faulty satellite very quickly

52
Q

Fault detection and exclusion

A

6 total satellites allows reliable calculation of the faulty satellite and remove it from usage

53
Q

What is transmitted on both L1 and L2

A

Ranging signals - signals to determine the range

54
Q

RAIM =

A

Redundancy - ensures integrity of the provided data by redundant measurements

55
Q

GBAS provide aircraft with

A

Integrity information + approach data corrections

56
Q

UERE =

A

Ionospheric propagation delay
Dilution of precision
Satellite clock error
Satellite orbital variations
Multi path

57
Q

4 parameters contained in nav message

A

Satellite clock correction parameters
Universal time coordinate parameters
Ionospheric model
Satellite health data

SUIS

Radio Navigation (13 decks)

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