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NAV&RADAR MCA > NAVIGATION AIDS > Flashcards

NAVIGATION AIDS Flashcards

1
Q

In the operation of a Global Navigation Satellite System what are the three distinct parts (In GPS known as segments) (3)

A

Space Segment
Control Segment
User Segment

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2
Q
  1. What is the minimum number of satellites that GPS is designed to operate with? (1)
A

24

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3
Q
  1. What is the purpose of monitoring stations? (2)
A

To track satellites, collect data and feed back to the master control station

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4
Q
  1. The receiver gives the user a position by calculating its ranges from satellites, these are called pseudo- ranges, why? (2)
A

Because the calculation includes a time value with an error in it, so the ranges are not accurate

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5
Q
  1. Explain how the range from a satellite is determined (6)
A

The satellite transmits a distinct code
At the same time the receiver generates the same code internally
The receiver receives the code from the satellite
The time difference between the two codes is measured
The range from the satellite is calculated using this time difference in the equation D = S x T where S is the speed of an electromagnetic wave

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6
Q
  1. List 4 errors which can affect range measurement (4)
A

Any of these six:
- Satellite clock error
- Satellite Orbital (Ephemeris) error
- Ionosphere Delay error
- Troposphere Delay error
- Receiver clock error
- Receiver noise Error
- Multipath error

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7
Q
  1. What must the OOW do when plotting a satellite position on a chart? (4)
A

Check whether satellite positions can be plotted directly on the chart
If not, then check the note that states the corrections must be applied
Correct the satellite position
Plot corrected position on the chart

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8
Q
  1. State GPS datum and describe what is meant by datum in this context
A

WGS84
This is a horizontal datum relating to latitude and longitude. lt can be thought of as a reference point or grid to which GPS positions can be related. This datum is based on a mathematical model to as far as possible resemble the shape of the earth

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9
Q

Define DOP (4)

A

Dilution of precision: The reliability of the data due to the geometry (azimut and elevation) of satellites in view of the receiver.

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10
Q
  1. State one example of a good DOP value and one example of a poor DOP value (2)
A

Good: 2
Poor: 8

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11
Q
  1. Which type of DOP is the OOW most interested in? (2)
A

HDOP - Horizontal Dilution of Precision

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12
Q
  1. What does the ‘D’ stand for in DGPS and what two augmentation systems are there? (5)
A

Differential 1
Ground based Augmentation 2 System (GBAS)
Satellite based Augmentation 2 System (SBAS)

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13
Q
  1. Explain how DGPS improves the accuracy of GPS (8)
A

A base station receives a signal from a GPS satellite.

The station knows its location on the Earth’s surface accurately.

It compares the calculated satellite range with the range calculated from its known position

The difference between the two allows the time error to be worked out

This error is transmitted to users within the area

The user’s receiver then corrects the GPS data to give a more accurate position

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14
Q

Explain, with the aid of a diagram, why a cross-track error alarm should be set when following a GPS route (4)

A

When following a GPS track the receiver gives the user an update on brg to next waypoint. If the vessel is off track there could be an obstruction between the (2) vessel and the waypoint. Therefore an alarm should be set to alert the user.

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15
Q
  1. What does LORAN stand for? (3)
A

Long Range Navigation

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16
Q
  1. It is a terrestrial hyperbolic navigation system, sketch the lines of hyperbolae between two transmitter (4)
A
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17
Q
  1. What do the hyperbolic lines represent about the signals received from the two stations? (3)
A

Lines of equal time difference between receiving the signals (2)
They are Lines of Position (1)

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18
Q
  1. What does ‘e’ in eLORAN stand for? (1)
A

Enhanced

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19
Q
  1. State three errors which affect eLORAN (6)
A

Atmospheric attenuation of ground waves (2)
Ionosphere effect (Skywave) (2)
Land effect (fixed errors) (2)

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20
Q
  1. Outline the echo-ranging principle (4)
A

A signal is transmitted
The signal hits an obstruction, sending an echo back
The time difference between sending and receiving is measured
The Range of the obstruction can be found by D=S x T divided by 2

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21
Q
  1. Explain how an echo sounder finds the depth, stating the basic formula and speed used (6)
A

The transducer transmits an acoustic pulse. The echo sounder measures the time between transmission and receiving the echo back from the seabed. It uses this time in the echo ranging calculation to find the depth below the transducer.

22
Q
  1. State two common frequencies used by an echo sounder and explain what each are used for (6)
A

50 kHz-
Used for deep soundings because the more powerful signal travels further. The cone is wide and the picture of the seabed is not very detailed
200 kHz=
Used for shallower soundings. The cone is narrower giving a more detailed picture of the seabed. It is less affected by aeration

23
Q
  1. Describe three sources of errors with an echo sounder (6)
A

Any three from:
Change in density of water can cause a change in speed of sound

Aeration can cause change in speed of sound, affecting indicated depth

Mechanical and electrical interference can cause errors

Multiple echoes due to signal bouncing between ship/surface and seabed

False echoes shown due to ‘false bottom’ or ‘second trace’

24
Q
  1. Explain the factors you would take into account when setting the alarm on an echo sounder for a coastal passage (4)
A

Draught in relation to available depth of water, considering whether squat is likely to be a factor due to depth under the keel and ship speed.
How close the depth contours are will also be a factor.
The alarm needs to sound to give the OOW enough time to react to it and keep the ship safe.

25
Q
  1. An echo sounder is giving a reading of 7.8m UKC. The draught of the vessel is 4.2m and the height of tide has been calculated to be 4.5m. What is the charted depth? (6)
A
26
Q
  1. Indicate next to each of the following speed logs whether they give Speed Through the Water {STW) or
    Speed Over the Ground (SOG): (7)
A
27
Q
  1. Describe with the aid of a diagram how an electromagnetic log calculates vessel speed (10)
A

The transducer houses a coil through which an electrical current is passed (2)
This sets up a magnetic field around it. As the vessel moves through the water an electro-motive force (e.m.f.) is created (2). Sensors (2) pick up this e.m.f. which increases with increase in vessel speed. The log measures the voltage which it converts to a speed and distance read out

28
Q
  1. Explain the Janus configuration with respect to a Doppler log (4)
A

One transducer faces in the opposite direction to the other so that any error due to change in trim or pitching can be illiminated

29
Q
  1. Describe with the aid of a diagram how a Doppler log calculates vessel speed
    (10)
A

The log transmits an acoustic signal from a transducer angled at 60° to the horizontal (1). The signal which is reflected back will have an increasingly higher frequency. The faster (2) the vessel is moving forward and an increasingly lower frequency the faster it goes astern (2). This ‘doppler shift’ can be measured and converted to a speed read out. (2)

30
Q
  1. What are two advantages of the Doppler log over the electromagnetic log? (4)
A

Speed over the ground reading is possible
Accurate in shallow water

31
Q
  1. Explain the dangers of using a Doppler log as a speed input to a radar (4)
A

If the radar is being sea stabilized by the log. It must be water tracking (2). If the seabed comes within range, the log may change to ground tracking automatically. The OOW may not be aware and the radar would then be ground stabilized. The correct aspect of targets may not be displayed for collision avoidance.(1)
Also, if the log is being used for sea stabilization of the radar, it is not certain which water layer is being used, therefore accurate aspect of targets will be affected.(1)

32
Q
  1. Briefly explain what ECDIS is (3)
A

A type approved Electronic Chart Display and Information System that complies with IMO performance standards.

33
Q
  1. Briefly explain what ECS is (2)
A

An electronic chart system which is unapproved. It cannot be used as a primary means of navigation.

34
Q
  1. Briefly explain what RCDS means (2)
A

A Raster Chart Display System is one displays raster charts. It cannot be used as a Primary means of navigation.

35
Q
  1. What are the chart carriage requirements under SOLAS V? {6)
A

ECDIS - ENCs required, no paper charts if authorised as primary means of navigation

ECS - Full paper chart coverage required for the voyage

RCDS - Paper chart coverage required

36
Q
  1. List seven advantages of Vector charts over Raster charts (7)
A

Seamless join between charts
Safety depth and safety contour can be set and will activate an alarm.
Chart features can be interrogated
Layers can be selected by the user
Charts can be rotated to any angle
Links are possible to extra information, such as from sailing directions
A choice of symbology for buoyage

37
Q
  1. What are the three mandatory inputs to ECDIS? (3)
A

Position (from EPFS)
Heading (eg. Gyro)
Speed (Log)

38
Q
  1. What is the S52 performance standard about? (2)
A

The standard of chart content and display on ECDIS

39
Q
  1. What is the S57 performance standard about? (2)
A

The standard to which vector charts must be produced

40
Q
  1. Explain what an ENC is (3)
A

An Electronic Navigation Chart is an S57 chart produced by or on the authority of a government authorized hydrographic office

41
Q
  1. What is meant by SENC? (2)
A

An ECDIS converts an ENC into a SENC (System Electronic Navigation Chart) for processing and display

42
Q
  1. There are mandatory ECDIS alarms and warnings (indications), list five of each (10)
A

Crossing safety contour
Deviation from route
Mandatory sensor failure
Approach to critical point
Different geodetic datum

Default safety contour
Information over-scale
Larger scale ENC available
No ENC available
System test failure

43
Q

Interpret the following electronic chart symbols:

A
44
Q
  1. List three factors to consider when setting a safety contour
A

l.Draught of vessel
2.Under Keel Clearance
3.Height of tide

45
Q
  1. Explain why draught, UKC and HOT should be considered when setting a safety contour (6)
A

1.This affects how far below the water line the seabed must be
2.This gives an additional margin of safety, squat should be considered
3.This gives additional water above Chart Datum that can be used

46
Q
  1. Show, using a sketch, how safety depth relates to safety contour if the safety depth has been calculated to be 8m and the safety contour is 10m (5)
A
47
Q
  1. How can the radar overlay confirm correct input information to the ECDIS? (3)
A

When switched on the overlay should match the charted features (1) underneath such as coastline and buoyage. An offset would indicate error in GPS position (1), a rotation would indicate error in heading input (1).

48
Q
  1. How can the radar overlay assist with pilotage navigation? (3)
A

This helps with easy identification of navigation marks for position fixing and for conning the vessel. It also makes it easy to see where radar targets are in relation to a channel and where they are heading for.

49
Q
  1. List two methods of correcting electronic charts up to date (2)
A

Downloading and transfer to ECDIS by USB
Receiving update CD by mail and putting into computer

50
Q
  1. Are Ts & Ps applied to electronic charts? Give reasons for your answer (3)
A

Some are, it depends on the country of origin. UKHO apply Ts & Ps as corrections. Other hydrographic offices apply them, some only apply a selection and some do not apply them at all. Details can be found online.

51
Q
  1. State how you can show Maritime Safety Information (MSI) on an ENC (2)
A

Enter the information on the chart in the appropriate place using a Mariner’s Note

NAV&RADAR MCA (11 decks)

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