(GNSS) Global Navigation Satellite System

Question 1

A satellite Sideral day?

Answer 1

23hrs 56mins

Question 2

Sudo Range

Answer 2

Based on Time apposed to physical range

Question 3

No. Of GNSS Satellites & at what range

Answer 3

24 active satellites on 6 orbits (4 Satellites per orbit)

Range= 12,600 miles above Earth

Question 4

What is DGPS? Could also be asked as describe GPS Differential

Answer 4

• Differential GPS. This is where an augmentation system such as a ground-based augmentation system (GBAS) or a satellite-based augmentation system (SBAS) receives the signal from the GPS. Then compares the signal with its exact known position and corrects any errors in the pseudoranges, it then sends the corrections to these errors to ships within range. Increases accuracy from around 15m to within 3m.

Question 5

Explain and describe what is meant by DOP?

Answer 5

• Dilution of precision. This is with regard to the angle of cut of the position lines from received from the satellites. A good DOP has a lower number (1-3) and tight angle of cut and bad DOP has a higher number (6-8) and large angle of cut.

Question 6

Why is it important to check XTD when using GPS on a passage. Explain the importance of your cross track distance

Answer 6

• Because your GPS may fault and cause you to leave your track which would cause a cross track alarm. It is important for safety of navigation
• If you have left your track your ECDIS may give a bearing to next waypoint that has an obstruction between you and the waypoint, you therefore need an alarm to know when you’ve left your track

Question 7

State the most commonly used DOP value by a navigator

Answer 7

2

Question 8

Explain HDOP and is 8 a good or bad rating

Answer 8

• Horizontal dilution of precision refers to horizontal variables latitude and longitude and is used by mariners. A HDOP rating of 8 is very poor and could not be relied on for an accurate position /

Question 9

Explain range measurement

Answer 9

• Each satellite transmits a distinct code
• At the same time the GPS receiver generates the same code internally
• When the receiver receives the signal from the satellite it measures the time difference between them and then does a distance = speed x time calculation

Question 10

How would you put a satellite derived position (SDP) on a chart.

Answer 10

• Check if the chart is WGS84, if not then check for corrections on chart

Question 11

Explain GDOP

Answer 11

• Geometric dilution of precision. This applies to all 4 variables: Latitude, Longitude, Altitude and Time

Question 12

(a) Explain how the GPS receiver determines a range measurement between the
satellite and the receiver

Answer 12

a) Satellite transmits its code message.
b) At the same time, the receiver generates an identical code.
c) Code from Satellite is received at receiver and will be offset due to travel time.
d) The offset is the time delay and speed of propagation is known so range to
sat is calculated using
Range = speed x time. (12)

Question 13

(b) List three possible errors in a GPS range measurement

Answer 13

b) Any three from:
1.Ionospheric delay error
2.tropospheric delay error
3.satellite clock bias error
4.positional or ephemeris error
5.multipath error.
6.Receiver clock offset error. (3)

Question 14

(c) State the horizontal datum used by the GPS.

Answer 14

c) WGS 84 (1)

Question 15

(d) State what action the OOW should take prior to plotting the GPS position on a
coastal chart drawn to a local datum.

Answer 15

d) Extract datum shift from chart heading “sat derived positions”.
Apply differences to the GPS position either on paper or use the appropriate
page on GPS receiver.
Plot position on chart. (4)
If candidate states that he would choose the pre set datum in the receiver
then allow half marks

Question 16

1. Describe the difference between ground and sky wave propagation (10 marks)

Answer 16

• Ground wave direct ‘line of sight’ curvature of the earth between transmitter &
receiver. Shortest distance.
• Sky wave reflected from ionosphere. Bounced to receiver. Longer distance.
• Varies with changes to ionosphere, twilight and night effect.

Question 17

2. Describe 3 possible causes of errors in the LORAN C obtained position.(10 marks)

Answer 17

• Land refraction.
• Sky wave interference.
• Poor geometry.
• Precipitation.
• Clock error.

Question 18

Many GPS receivers offer the operator a choice of datum, for example
Tokyo, South American, European 1950 etc. Explain what is meant by
the term datum

Answer 18

A reference system for specifying positions on the earth’s surface. Or words
to that effect. (4)

Question 19

If the GPS receiver and the chart in use have different datums what
action should the OOW take to ensure that an accurate fix is placed on
the chart?

Answer 19

Obtain the shift in datum from the chart (satellite derived positions).
Apply the shift to the GPS fix either on paper or using appropriate page in
receiver.
Plot correct position on chart. (5)

Question 20

Some charts state that the datum of the chart can not be determined or
that it is “ill defined”. Describe how this might affect the navigational
strategy whilst navigating on such a chart.

Answer 20

The plotted GPS fix should be treated with caution. Efforts must be made to
check its accuracy by fixing the vessel relative to the land, e.g., visual brgs,
radar ranges and brgs. Additionally, use parallel index, Loran C if in
coverage and use echo sounder to check the depth at the plotted position.
(5)

Question 21

Describe the relationship between the Ground, Space and User segments in a
GPS system giving a basic outline of the operation of each.
(15)

Answer 21

Ground segment – contours and monitoring unit. Time and position
information to satellite. Corrects satellite information.
Space segment - Satellite transmits position information and time pulse to
receiver
User segment – Ship – GPS receiver – measures time difference hence range
circle from satellite.

Question 22

Give a brief outline of the method that is used in a differential GPS (DGPS)
system to provide greater accuracy? (5)

Answer 22

DGPS – Fixed receiver at a known position
Comparison of known position and GPS fix
Calculates correction
Transmits correction by VHF to ships in range

Question 23

Describe the purpose of each of the following in the GPS system:
a. The Ground Segment.

Answer 23

Tracking, correcting clock, updating ephemeris, (2 marks)

Question 24

Describe the purpose of each of the following in the GPS system:
b. The Space Segment.

Answer 24

24 satellites in 6 orbits.
Transmit position and time information and ephemeris + corrections.
(3 marks)

Question 25

Describe the purpose of each of the following in the GPS system:
c. The User segment.

Answer 25

Ship and receiver. (2 marks)

Question 26

Describe the causes of three errors in the GPS system.

Answer 26

Atmospheric – refraction, absorption (slows radio wave so time
incorrect)
Clock error.- transmitter & receiver clocks not synchronised.
Multipath –aerial screened by part of ships structure, signal reflected to
receiver therefore time error.
(9 marks)

Question 27

Explain the term HDOP and describe its’ significance to the accuracy of
the position obtained by GPS.
Horizontal dilution of precision.

Answer 27

Horizontal dilution of precision. Indicates the accuracy of a 2d fix.
Lower number the better. (4 marks)

Question 28

Many GPS receivers offer the operator a choice of datum, for example
Tokyo, South American, European 1950 etc,. Explain what is meant by
the term datum.
(3 marks)

Answer 28

Datum is the mathematical model of the earth

Question 29

Describe the dangers of using the wrong datum and list three methods
of determining the possible error. (4 marks)

Answer 29

Wrong position obtained if wrong datum used.
Visual fixes.
Radar overlay.
Cross reference with another system.