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ATPL: RNAV > 11 Sat Nav Systems > Flashcards

11 Sat Nav Systems Flashcards

1
Q

The four main Global Navigation Satellite Systems (GNSS) are:

A
  • USA NAVSTAR GPS.
  • Russian GLONASS.
  • European Galileo (under construction).
  • Chinese BeiDou (under construction).
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2
Q

The two modes of operation are:

A
  • SPS – standard positioning system (civilian use).
  • PPS – precise positioning system (military use).
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3
Q

GNSS comprise three segments:

A

Space segment.
User segment.
Control segment.

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4
Q

Satellite transmission signals?

A

Each satellite transmits timing and ranging signals on two frequencies, L1 and L2.

L1 ranging signal: 1575Mhz - SPS+PPS - C/A and P code
L2 ranging signal: 1227Mhz - PPS - P code.

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5
Q

GNSS signal transmittion?

A

C/A or P code: The C/A PRN codes are Gold codes with a period of 1023 chips transmitted at 1.023 Mchip/s, causing the code to repeat every 1 millisecond.
Nav message

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6
Q

Each satellite transmits a Navigation Message, comprising:

A
  • Satellite health status.
  • Ionospheric model.
  • Almanac
  • Ephemeris (Sat orbital variations)
  • Satellite clock correction parameters.
  • UTC parameters.
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7
Q

Ionosphere?

A

The ionospheric model is used to calculate time delay of the signal passing through the ionosphere.

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8
Q

Atomic clock?

A

Each satellite is equipped with atomic clocks, to keep a very accurate time reference.

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9
Q

The control segment comprises:

A

A master control station.
A ground antenna.
Monitoring stations.

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10
Q

The control segment provides:

A
  • Monitoring of the constellation status.
  • Correction of orbital parameters.
  • Navigation data uploading.
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11
Q

How does the GNSS reciciver calcualte position?

A

The GNSS receiver converts the time a signal takes to reach the receiver from the satellite into a pseudo range.

The initial pseudo range is named such, because of the time reference difference between the satellite and receiver clocks.

Each range defines a sphere, with the satellite at its centre.

Using pseudo ranges from four satellites allows the receiver to calculate 3D position fixes, comprising latitude, longitude, altitude and a time reference.

The fourth satellite allows the receiver to synchronise to the correct time reference.

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12
Q

Types of GNSS reciever?

A
  • Single channel - picks up 1 satllite at a time
  • Multiplex reciever - Single channel which switches between satellites every 5ms
  • Multi-channel- 200 channels.
  • Multi-system - Navstar, Glonass, Compass, Galileo.
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13
Q

Nav star?

A

24 constellation (21+3) operational + 7 additonal satellites as spares or additions to the service.

  • Inclination 55 degrees to the plane of equator
  • Orbits altitude around 20200km
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14
Q

GNSS transmissions?

A

ID of SV
Transmission Time
Nav message
Ranging - PN code

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15
Q

Control segment

A

1 Master station in NORAD
Monitering station
Command and control ground antennas.

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16
Q

Reciever ground speed calculation?

A

The receiver is able to calculated groundspeed by measuring Doppler effect or change of receiver position.

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17
Q

RAIM

A

Receiver Autonomous Integrity Monitoring (RAIM) uses consistency checks of redundant pseudo ranges to ensure the integrity of the navigational solution.

Basic RAIM requires five satellites, and a sixth satellite allows for isolating a satellite from the navigational solution (RAIM + FDE) Fault detection and exclusion.

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18
Q

Reference systems?

A

Different GNSS use different data for reference systems, orbital data and navigation services.

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19
Q

Compatiblity?

A

NAVSTAR and GLONASS are compatible and interoperable.

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20
Q

The significant factors most affecting the size of the User UERE are:

A
  • Ionospheric propagation delay
  • Dilution of position
  • Satellite clock error
  • Satellite orbital variations
  • Multi path
21
Q

UERE:

A

UERE: User Equivalent Range Error.

UERE + GDOP = Estimate of position accuracy.

22
Q

Most significant source of UERE?

A

Ionospheric error is the most significant error in UERE

Dual frequency (L1 / L2) receivers can practically eliminate ionospheric errors.

23
Q

How are ionospheric error reduced?

A

A mathematical model of the ionosphere can be used to reduce ionospheric errors.

24
Q

PRN Code?

A

Each satelllite includes a unique Pseudo Random Noise signal repeatidly, featuring gold code and timing signal.

Modulated by BPSK

25
Q

Nav message?

A
  • Valid 4 hours
  • Transmitted in 25, 30 second frames over 12 min 30 seconds.
  • Without this it can take 15 minutes.
  • UTS parameters
  • Sat clock error corrections
  • Ionospheric model: Issued every 12 hours by the ground segment, accounts for 50% of errors.
  • Satellite health
26
Q

How does GNSS solve for position?

A

GNSS 4 unknown errors, X Y Z and Time

Requires 4 different statelites to solve for for 4 different variables in 3 different equations.

27
Q

Sat clocks?

A

Satellie has a very accurate atomic clock.

User and satellite clock are synchronised to provide accurate position

28
Q

User clock error?

A
  • Very small 1/100th, of a second is a distance of 3000km.
  • Clock error is the same for all the SV signals recicived.
  • Recicver uses as iterative process varying its internal clock time till all clocks are in error by teh same amount Oseudi Range are then all valiId.
  • If these reciciver timing chnages shrink the degree of time and position uncertaintly, thebn it must be correcting its clock in the right way.
29
Q

Satellte position

A

The number and geometry of satellites in view can contribute to geometric dilution of precision (GDOP)

30
Q

Deriving position?

A

Only gives a potition on a sphere
2 satelleites gives position anywahere on 2 spehere
3 1 position on a sphere.
4 3d position fix. No worse than 13m horizontal. 95% of the time.

Ground speed is measured by dopplar shift

31
Q

Sat GPS time

A

2-3 Cesium
1-2 Rbidium atomic clocks accurate to 1ns or 30cm of range.

Signals are transmitted with a GPS time stamp and converted to a UTS time at the reciciver.

Checked every 12 hours and updated, in the nav message.

32
Q

satellite clock error?

A

Relativity: They run 38us faster a day, they are then set to run 38us slower.
Recicver Error: Noise generated at the reciever.
Multipath: Refections of the ground, buldings.
Ionspheric propgation errors: Accounts of 50% o the errors.

33
Q

GDOP

A

The number and geometry of satellites in view can contribute to geometric dilution of precision (GDOP)

TDOP - Time
HDOP - Height
VDOP - Vertical
PDOP - Position

GDOP best when Mask angle is 7.5degrees abive the horizon, and 2 SVs are more then 120 degrees apart, and one directly above.

34
Q

GBAS

A

A form of DGPS.
Using accurately surveyed monitoring stations, a Ground Based Augmentation System (GBAS) measures errors in the navigation signals transmitted by the satellites, and relays them to users for correction via a VHF data broadcast (VDB) link on 108 – 118 MHz.

35
Q

GBAS minimum coverage?

A

The minimum coverage is 10° either side of the nominal centre line of an approach path out to 20 NM and 35° out to 15 NM.

36
Q

GBAS Range?

A

GBAS provides guidance information out to 20 NM in the terminal area, and 3D guidance in the Final Approach Segment by transmitting a FAS data block.

37
Q

GBAS infomation?

A

One GBAS can support aircraft with approach data, corrections and integrity information for all GNSS satellites in view.

38
Q

LAAS

A

GBAS based on GPS is referred to as Local Area Augmentation System (LAAS)

39
Q

GLS?

A

GBAS based approach to landing is referred to as GLS (GLS-GNSS Landing System).

Accurate to 1m. Can provide vertcical and horizontal guidence. This is considered a Precision Landing System.

40
Q

SBAS

A

Form of Wide Area DGPS (WADGPS)
A Satellite Based Augmentation System (SBAS) is designed to significantly improve system accuracy and integrity.

Using accurately surveyed monitoring stations, SBAS measures errors in the navigation signals transmitted by the satellites, and relays computed correction factors to users over very wide areas, using the same L1 frequency, via intermediate geo-stationary satellites.

SBAS signals transmit over the L1 Frequency, 1575.42 MHz, which is in the UHF band (300-3000MHz).

41
Q

Using geo satellites?

A

It is important to note that SBAS satellites also do the job of a standard GNSS satellite on top of their usual roles, in order to produce an extra pseudo-range measurement to receivers within their area of coverage.

42
Q

SBAS comprises two elements:

A
  • Monitoring and processing stations (on the ground).
  • Communication (geostationary) satellites.
43
Q

SBAS allows increased performance:

A
  • SBAS allows the implementation of 3D Type A and Type B approaches.
  • SBAS alerts users within six seconds if a GPS malfunction occurs.
44
Q

There are several worldwide SBAS in use:

A

EGNOS: Western Europe and the Mediterranean.
WAAS: USA.
MSAS: Japan.
GAGAN: India.

45
Q

ABAS?

A
  • An Airborne Based Augmentation System (ABAS) uses redundant GPS elements, or a combination of GNSS measurements with other navigation sensors, to develop integrity control.
  • RAIM is a type of ABAS using GNSS information only.
46
Q

AAIM

A

Aircraft Autonomous Integrity Monitoring (AAIM) uses additional onboard sensors, typically barometric altimeters and inertial reference systems (IRS).

47
Q

psudalite?

A

Used to give a very accuract altitude

48
Q

GBAS FAS?

A
  • Infomation on what is contained in the area on the final approach. (contains info on 20nm at +/- 10 degrees, and 15nm at +
  • Differential Data
49
Q

WAAS

A

Wide Area Augmentation System for the NAVSTAR GPS.

An alert will be initiated within 6 secs if a GPS malfuction occurs.

3 dimensional Type A, B and APV approaches.

Components
* WAMS - Wide area monitoring stations
* WARS - Wide area refernece signals.

ATPL: RNAV (14 decks)

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