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Radio Navigation > GNSS (25-05-18) > Flashcards

GNSS (25-05-18) Flashcards

1
Q

What is the US GPS system called?

A

NAVSTAR GPS

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2
Q

What is the Russian GPS system called?

A

GLONASS

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3
Q

What is the European GPS system called?

A

GALILEO

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4
Q

What is the Chinese GPS system called?

A

BEIDOU

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5
Q

What are the advantages of GPS?

A

Worldwide

Very accurate

24hr

4D monitoring (3D plus time)

Space based

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6
Q

What is the principle of operation with GPS?

A

The satellite and the A/C have clocks which are synced.

At a predetermined time a pulse is transmitted by the satellite and the A/C can calculate the distance from the satellite.

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7
Q

What number of GPS satellites provides a 3D fix?

A

4

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8
Q

What number of GPS satellites provide a position 2D fix?

A

3

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9
Q

What errors are encountered with a GPS system?

A

Propagation speed changing through the ionosphere

Errors in the accuracy of the transmission schedule (Einstein relatively)

Errors in the accuracy of the clocks (more likely in the A/C)

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10
Q

What are the 3 components of a GPS system?

A

Space segment

Control segment (ground facilities that track GPS sats, monitor Tx, perform analysis and send commands and data)

User segment

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11
Q

What frequency band do GPS satellites broadcast on?

A

UHF

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12
Q

What is the L1 signal of GPS?

A

1575.42MHz

contains C/A code: course acquisition

Contains P code: precision (friendly military)

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13
Q

What is the L2 code for GPS?

A

1227.6MHz

contains P code for owners military

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14
Q

How are each individual satellites signals differentiated between?

A

With a unique Pseudo Random Noise by binary phase shift keying

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15
Q

What allows GPS systems to be easily jammable?

A

Their signal is very weak

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16
Q

What is sent in the navigation message of a satellite in GPS?

A

Almanac - predicted satellite position

Ephemeris - correction data for error of satellite position

Conversion factors for GPS to UTC time

ionospheric model (every 12 hours)

satellite clock error

Satellite status

17
Q

How long is a satellite navigation message valid for?

A

4 hours

18
Q

How is a nav message from GPS sent and how long does it take for first position fix?

A

it is sent in 25 subframes each of which is sent in 30 seconds

in total taking 12.5mins

the sky search take 2.5mins

total time for first position fix is 15mins

19
Q

What does the control segment of GPS consist of?

A

The master control station

12 command and control groudn antenna

16 monitoring stations

20
Q

What is the main tasks of the control segment of GPS?

A

Manage space segment performance

nav data upload

monitoring satellites

21
Q

How is error in the user clock bias accounted for?

A

All satellite position ranges are reduced by the same amount until they intersect

due to the error being the same in all systems.

22
Q

How does a satellite provide a 3D fix in terms of latitude and longitude?

A

Calculates A/C position using Cartesian coordinates from semi minor and major axes.

This is then input into the WGS84 model to give lat, long and alt

23
Q

What can geometric dilution of position (GDOP) be subdivided into?

A

PDOP - position

HDOP - horizontal

VDOP - vertical

TDOP - time

24
Q

What is selective availability?

A

Where the atomic clock of a GPS satellite can be dithered to purposefully degrade accuracy

(turned off by presidential decree in May 2000)

25
Q

What are the 3 types of GPS receiver?

A

Sequential - checking one after the other

multichannel - checking all at once

multiplex - 2 channels which quickly cycle between satellites

26
Q

What are the 3 types of GPS augmentation system?

A

ABAS - air

GBAS - ground

SBAS - satellite

27
Q

How does ABAS work?

A

A fault in the position fix is checked for by using a 5th satellite - Receiver Autonomous Integrity Monitoring (RAIM)

Exclusion of fault is done with a 6th satellite. - Fault Detection and Exclusion (FDE)

BARONAV - uses internal barometer to reduce the satellite requirement by 1

Air Autonomous Integrity Monitoring (AAIM) - RAIM but uses internal A/C equipment instead of a 5th satellite

28
Q

How does GBAS work with GPS?

A

LADGPS - Local Area Differential GPS - uses a well surveyed position reference station

VDB - data broadcast sent through VOR, ILS or Mode S

Coverage is +/-10deg out to 20nm/ +/- 35deg out to 15nm

29
Q

How does SBAS work with GPS?

A

Measures the signal errors transmitted by GNSS on the ground and then transmits differential corrections and integrity messages through geostationary satellites.

30
Q

What are the different SBAS systems used around the world?

A

WAAS for North America

EGNOS for Europe

MSAS for Japan

GAGAN for India

SDCM for Russia

SNAS for China

Radio Navigation (17 decks)

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