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ATPL - Radio Navigation > Satellite Navigation > Flashcards

Satellite Navigation Flashcards

1
Q

NAVSTAR GPS is broken down into … circular orbital planes each at … degrees to the equator.

Each plane consists of … to … satellites which orbit the earth at height of … km (… NM) once every … hours

A

6

55

4 to 6

20 200km

10 900 NM

12 hours

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2
Q

How many NAVSTAR satellites will always be in line of sight an any one time?

A

5 - 8

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3
Q

Satellite not considered visible until it is more than … degrees above horizon, this is called the mask angle

A

5

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4
Q

NAVSTAR GPS satellites transmit ranging signals on two frequencies in … band, usually referred to as L band

L1 frequency=

L2 frequency=

A

UHF

  1. 42 MHz
  2. 6 MHz
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5
Q

NAVSTAR GPS master control stations upload nav data to the satellites and they are distributed around the world near…

Known collectively as the …

A

the equator

control segment

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6
Q

NAVSTAR GPS standard position service is transmitted on L1 or L2 frequency?

A

L1

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7
Q

Precise (P) modulation or P (Y) code is only used for military and survey purposes and is transmitted on …

A

both L1 and L2 frequencies

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8
Q

What is pseudo random noise?

A

Orbiting satellites transmit accurately timed radio signals modulated on the L1 frequency and digital signals contain a unique satellite identifier and a timing message. This unique navigational signal repeats every millisecond

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9
Q

How does GPS receiving equipment calculate position?

A

uses its internal clock to measure how long the PSR signal has been in transit and converts the time delay in to range from the satellite (a sphere of range)

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10
Q

What is receiver clock bias?

A

As the clock in the receiver is less accurate than its atomic counterpart the initial range, called pseudo range, is inaccurate, an error known as receiver clock bias. Pseudo range from several satellites would produce a cocked hat instead of a precise fix

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11
Q

What is GPS receiver clock correction?

A

Receiver changes the ranges all by the same amount at the same time until a near pinpoint fix is achieved

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12
Q

For GPS system to work the receiver must be aware of the satellite positions, called their …

Expected positions stored in receiver memory as an …

A

ephimeris

almanac

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13
Q

What is GPS ephimeris error?

A

satellite not where it should be

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14
Q

GPS raw signal fixing accuracy?

Control of position is so stringent that errors caused are in the region of …

A

+/- 13 meters on 95% of occasions

+/- 0.5m

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15
Q

What are the two largest individual errors of GPS?

A

variations in ionosphere density and atmospheric conditions

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16
Q

Delay in transmission time caused by the ionosphere and the atmosphere is proportional to …

A

1/f(squared)

17
Q

If the satellites are close together, the angle of cut between the range lines will be shallow and the fix is liable to be less accurate. Called Geometric Dilution Of Precision (GDOP) and errors up to 70m can occur.

Optimum geometry for four satellites to obtain fix is to have …

A

three separated by 120 degrees in azimuth and one directly overhead

18
Q

GLONASS has 24 satellites orbiting the earth in … orbital planes, with … satellites in each orbit equally displaced by … degrees of latitude

A

3

8

45

19
Q

GLONASS orbits inclined at an angle of 64.8 degrees to the equator with the satellites flying at a height of approx. …. km, higher/lower than GPS, so orbital time is …

A

19 100

lower

less at 11 hours 15 mins

20
Q

What is GPS RAIM stand for?

A

Reduced Autonomous Integrity Monitoring

21
Q

How many satellites are required over and above minimum needed for a 2D and 3D fix for RAIM?

If receiver is fed barometric altitude this effectively creates another sphere of position and reduces the number of satellites required for each function by …

A

1 for RAIM monitoring, 2 for RAIM with one redundant satellite

one

(Receiver normally tracks four satellites for a 3d fix and every 20 seconds data from one of the four is replaced by data from a fifth. If the position changes either the one that was rejected or the new one is in error. By continuously sampling the satellites a faulty one can be identified and this means the where a 2d fix normally requires 3 satellites and a 3d fix requires four, when RAIM is monitoring the nav solution, an extra satellite is need for each fix. If an error is found a further satellite must be available for RAIM to continue to function so the ideal is to have 2 spare over and above minimum required for a fix)

22
Q

What is Aircraft Autonomous Integrity Monitoring?

A

Uses a mixture of GPS, IRS and radio navigation equipment to create redundancy and the capability to switch between system should an error occur in another

23
Q

What is GPS ground based augmentation system?

A

Makes ground measurements of the signal errors transmitted by satellites and relays those measured errors to various users to enable raw GPS data to be corrected

24
Q

ICAO standards provide possibility to interconnect GBAS stations to form a network broadcasting large-scale differential corrections – referred to as …

A

Ground Regional Augmentation Systems (GRAS)

25
Q

GBAS ground subsystem provides two services, precision approach and GBAS positioning service. Precision approach provides deviation guidance for final approach, whilst the positioning service provides horizontal position information to support RNAV operations in terminal areas.

Minimum GBAS coverage is …NM from landing threshold out to … degrees either side of centre line, with a further extension out to …NM and … degrees either side of centre line

A

15

35

20

10

26
Q

What is differential GPS?

A

GPS signals received on ground by a continuous receiver at an accurately surveyed location

Ground installation computes the difference between its known position and that from the GPS and sends the correction to any aircraft within 30km using a datalink

27
Q

What are pseudolites?

A

Includes another ground station that acts like another satellite

Pseudolite sends satellite type signals to the receiver and sends dGPS correction. Benefit is greater than raw dGPS because the pseudolite gives another range input to increase the accuracy of the fix, particularly in vertical plane reducing VDOP

Short range devices intended to increase GPS accuracy on approach but because they are placed on the ground can have problems with the aircraft fuse screening the receiver aerial and for this reason tend to be placed off to the side of the approach path

28
Q

What is GNSS?

A

Based on a combination of GPS and GLONASS. The additional accuracy and redundancy available from the two systems would allow GNSS to be used as a sole means of navigation

29
Q

What is the approximate range of a GBAS augmentation system in NM and KM?

A

19nm

30km

30
Q

The GPS SPS position errors shall not exceed … in horizontal and … in vertical

A

13m

22m

31
Q
A

ATPL - Radio Navigation (13 decks)

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