GNSS

Question 1

3 main components of GNSS?

Answer 1

Space, user, control

Question 2

Describe the GNSS satellite constellation including:

(a) frequency used for transmissions;

Answer 2

1575. 42MHz (L1 band)

1227. 6 (L2 band)

Question 3

Describe the GNSS satellite constellation including:

(b) types of pseudo-random code and its content

Answer 3

Psuedo-random codes. L1 contains a coarse acquisition code and a P code. P only for military use.

Content:
Space vehicle number (SVN) identifies satellite
Time of transmission
Ephemeris data
Health of satellite
Almanac data

Question 4

Describe the GNSS satellite constellation including: GNSS time reference

Answer 4

GNSS rely on accurate measurement of time.

Atomic clocks are used in satellites (and control segments) keep an accurate track of time.

User segments (receivers) use quarts crystal clock.

Question 5

Describe the GNSS satellite constellation including: ephemeris

Answer 5

Ephemeris transmits it’s healthy and exact location Data. And uses data from each satellite to calculate its position using trilateration.

Question 6

Describe almanac

Answer 6

Provides data on which satellites will be visible at any given time and predict their estimated ranges.

Question 7

Elements of the control segment

Answer 7

In Colorado Springs

Master control station gets info from monitor stations and ground antennas around the world. Controls clock and orbit states of the satellites and the currency of their navigation messages.

Monitoring stations
Placed at various locations, equipped with precise clocks and GPS receivers, tracks all satellites in view checking altitude, position, speed and overall health of satellite. (Pretty much receives data)

Ground antennas -
Uplinks Data every day to satellites consisting of position and time and telemetry commands from master control station (automatic unlike monitoring)

Question 8

Describe the user segment, including the basic principle of satellite ranging.

Answer 8

Receivers can be anything that uses GPS e.g phones.

A GNSS unit receives satellite signals, decodes them, processes them, and then in some cases, displays these on a map.

For aviation use, receivers contain an almanac of navigation data (waypoints, obstacles, IFR charts, radio frequencies, etc) that requires updating regularly.

SATELLITE RANGING
GNSS works on the principle of trilateration, which is, roughly speaking, the process of finding out where you are by measuring distances with spheres.

Each satellite broadcasts a radio signal, which travels out in 360o in every direction (spherically) at the speed of light.

Question 9

Explain the principles of fixing position using the GNSS system; including:
(a) the number of satellites required for 2D and 3D fixing;

Answer 9

3 satellites for position and 4 for position and altitude or 3 and barometric aiding.

Question 10

Explain the principles of fixing position using the GNSS system; including:

(b) elimination of clock error;

Answer 10

Time synchronisation of receiver and satellite time has to be equal, the difference can cause errors. Atomic clock is most accurate by expensive, basically removes error

Clock error also removed by trimming, trims the time back and resets it back to corrected position fix. Receiver sees that theres a error and goes back to when it was all in synch and sets it back.

4 satellites for this to work.

Question 11

Explain the principles of fixing position using the GNSS system; including:
(c) the operation of RAIM;

Answer 11

Receiver function that analyses the signal integrity and relative positions of all satellites in view.

Select so not the best 4 or more satellites isolating & disregarding any other potentially incorrect satellites.

5 must be in view to have RAIM capability, and 6 to isolate unacceptable satellite.

Question 12

Explain the principles of fixing position using the GNSS system; including PDOP/GDOP

Answer 12

Dilution of precision defines how good a position fix is.

PDOP (Position dilution of precision); position accuracy; 3D coordinates

GDOP (Geometric dilution of precision); overall accuracy; 3D coordinates & time

When visible GPS satellites are close together, Geometry is said to be weak and PDOP value is high.

Far apart, geometry is strong and PDOP value is low.

GDOP = PDOP + untrained clock error

Question 13

Explain the principles of fixing position using the GNSS system; including barometric aiding and Baro-VNAV

Answer 13

Integrity augmentation that allows a GPS system to use a non-satellite input source to provide vertical reference. Requires for satellites and a barometric altimeter to detect integrity anomaly.

Basically adds redundancy to the RAIM.

Baro-VNAV
RNAV system which uses barometric altitude info from altimeter to compute vertical guidance for pilot during an approach.

Vertical path is computed between 2 waypoints or an angle from a single waypoint.

Question 14

Explain the principles of fixing position using the GNSS system including receiver masking function

Answer 14

The masking function in the GPS receiver software ensures that any satellites in view, which lie below a fixed angle of elevation relative to the receiver, are ignored.

Mask angle is 7.5º and anything below this angle it can’t see satellites

Question 15

Explain how the receiver predicts the position of various satellites.

Answer 15

RAIM uses multiple satellite signals to produce several GNSS position fixes and then compares them. An internal calculation determines whether or not a fault can be associated with any of the signals

Question 16

State the sources of GNSS error, and the maximum error which could be expected with and without selective availability applied. (Ephemeris, multipath, ionospheric, atmospheric error, receiver interface, clock error)

Answer 16

Ephemeris - any error inherent in the data that defines the satellites current position, which in turn is transmitted to the receiver. Up to 2.5m

Multipath - reflected and refracted signals bounce off terrain affecting timing of satellites. Up to 1m

Ionospheric propagation effect - due to activity within the ionosphere, up to 5m. Eliminated by the P(Y) L2 frequency

Atmospheric error - due to vapour and dust, slows down satellite signals and can be up to 1.5m. Minimised internally in the receiver.

Receiver interface error - due to low signal strength, random code < ambient noise.
High frequency - small antenna, susceptible to interference. Fuzzy correlation of receiver/satellite psuedo random codes. Up to 1m

Clock error - inaccuracies in satellite, receiver clock and relativity effects. Position error up to 2m.

Question 17

State the reasons for the display of a RAIM warning message, and the requirements under CAA rules for continued navigation

Answer 17

RAIM failure messages can appear due to not having enough satellites in view and or a potential error that exceeds the limit for the current phase of flight.

Requirements:
Obtain RAIM prior to departure for ETA

If RAIM warning for more than 10min or DR mode for more than 1min, advise ATC and verify position every 10min using other approved nav systems

Don’t commence an approach with a RAIM warning displayed.

Question 18

Explain the operation of the RAIM prediction service.

Answer 18

RAIM predictions obtained from the receiver are based on information uploaded onto the unit during the previous database update.

Predictions obtained online from an ANSP(air navigation service provider) contain the latest information available, including maintenance status of satellites, therefore are a more accurate source.

These predictions are valid for 72hrs.

Question 19

Describe the application of the WGS 84 datum, and the likely effects on the GPS display of using coordinates from another datum.

Answer 19

WGS84 = World Geodetic system. The international standard for cartography and satellite navigation. The current ICAO standard for GNSS navigation, with all procedures and waypoints reference to it.

The scaling of datum’s are different which can put you in a different position from what’s expected.

Question 20

Explain the principle of operation of Differential GPS (DGPS). Including what it is, how it works and where its used.

Answer 20

A form of ground based GNSS augmentation. Increases GNSS accuracy by providing a correction signal to the users GNSS receiver.

How it works:
GPS antenna place on a known surveyed point on ground and receives a signal from GPS satellites.

Ground station then compares position solution from GPS network to known point and a correction is calculated using the difference between the GPS solution and known position.

Aircraft receiver then applies correction to own GPS position increasing position accuracy of aircraft GPS system.

Where its used:

GBAS - located around airport. It supports all phases of approach, landing, departure and surface ops bit is primary used to support precision approach. Aircraft and airport needs to be equipped with GBAS.