GNSS / GPS

Question 1

GNSS

Answer 1

Global Navigation Satellite System

Question 2

Principle of GPS

Answer 2

Continuously receive lines of sight from at least 6 Satellites

3 satellites give a 2D fix, 4 give a 3D fix

Question 3

HDOP

Answer 3

Horizontal Dilution of Precision

Mariners are concerned only with HDOP.

2-dimensional aspect of total error in terms of LAT and LONG

Occurs when satellites are too close together, too high or too low, giving a poor angle of cut

Question 4

HDOP values and their accuracy?

Answer 4

1-5 good for any nav ops

5-20 check against other navigational aids and bridge equipment

> 20 do not use for navigation

Question 5

Differential GPS

Answer 5

Utilises a GPS monitoring system to improve Standard Positioning Service (SPS) and negate GPS errors
Transfers range and range rate corrections to local users who use their Differential GPS to correct its own range instruments before establishing a fix

Question 6

Ionospheric and Tropospheric errors

Answer 6

Propagation

Ionospheric delays caused by solar activity in the ionosphere, will be worse during the day

Tropospheric delays in signal are due to weather and pressure systems in the troposphere

Question 7

Ephemeris error

Answer 7

Orbital error, caused due to slight variation in the satellites orbit

Question 8

Multipath error

Answer 8

Much like in RADAR, when signal bounces off an object such as a building or cliff, taking longer to reach the ship

Question 9

Receiver noise error

Answer 9

Resulting from electronic signals from within the receiver itself.

Can be combatted by higher quality equipment

Question 10

Control segment role:

Answer 10

Sends error calculations through calculated through GBAS, and sends these up to the satellites via uplink

Question 11

DGPS

Answer 11

GBAS/SBAS
Sends a signal to a station with a known location.
Station makes corrections for the errors in location and sends this to the user segment, giving a more accurate location (generally within 5m)

Question 12

In what publication do we find details of GPS/DGPS stations?

Answer 12

ALRS vol 2
Radio aids to Navigation

Question 13

What accuracy does GPS generally give?

Answer 13

95% of the time, up to 25m
DGPS, up to 5m

Question 14

How do we monitor the integrity of out GPS function at sea?

Answer 14

Verify # of satellites (more than 4, ideally over 6 for precision)

Check HDOP

Cross reference with other navigation methods

Observe for sudden accuracy changes

Use GPS with WAAS/EGNOS (SBAS) support

Update your GPS software

Understand GPS limitations