Electronic Nav Aids & Position Fixing Methods

Question 1

Limitations of Celestial Observation?

Answer 1

1. Availabilty of Stars
2. Refraction of lower alt
3. Weather ( Cloud Cover )
4. Accuracy of chronometer
5. Sextent Errors
6. Human error
7. Time in between obs

Question 2

Accuracy Of Celestial Fixes ?

Answer 2

1 to 2 Miles

Question 3

Limitations Of Horizontal Sextant Angles?

Answer 3

Need 3 or more Suitably Placed Objects.

Question 4

How do you perform a Horizontal Sextant Angle?

Answer 4

Take a bearing of 3 minimum or if possible different objects with a good angle of cut between them. Then plot these on the chart where they cross is your Position.

Question 5

Accuracy of Horizontal Sextant Angles?

Answer 5

Used when great position accuracy is required.
Accuracy depends on the user and the objects used.

Question 6

Vertical Sextent Angles are used for…

Answer 6

Determining distances of know objects of height.

Question 7

How do you take a Vertical Sextant Angle?

Answer 7

Measure The height of the object from the sea, Note that the Height of the object is taken from MHWS AND SHOULD BE adjusted for the height of tide which you are in. You know a angle and a side or a right angle triangle so use Pythagorus Theorum to get the distance.

Question 8

What Technique can be used with Vertical Sextant Angle

Answer 8

Danger Angle.

Question 9

RADAR - what are the limitations of this piece of equipment for position fixing?

Answer 9

• Quality of radar returns
• Accuracy of radar returns
• Range in use
• Radar horizon
• False echoes (particularly from power cables)
• Super refraction & Sub refraction

Question 10

RADAR - what are the methods used for this piece of equipment for position fixing?

Answer 10

• MULTIPLE (at least 3) ranges and bearings
• Parallel indexing
• Clearing ranges

Question 11

RADAR - what is the accuracy of this piece of equipment for position fixing?

Answer 11

ON A 6nm RANGE:
- If bearing of targets are approx 60 degrees apart, accuracy = 0.1nm / 1 cable

ON A 12nm RANGE:
- If bearing of targets are approx 60 degrees apart, accuracy = 0.2nm / 2 cables

Question 12

GNSS - what are the errors of this piece of equipment for position fixing?

Answer 12

• Solar effects
• Tropospheric errors
• Ionospheric errors
• Multipath errors
• Clock errors
• Jamming
• Spoofing

Question 13

GNSS - what are the methods used for this piece of equipment for position fixing?

Answer 13

Measuring ranges from selected satellites. Range is determined by propagation time of satellite to receiver. Since there is a clock offset between each satellite due to variables, this calculates pseudorange.

Question 14

GNSS - what is the accuracy of this piece of equipment for position fixing?

Answer 14

If in GNSS = 10 - 20 metres
If in DGNSS = 0.5 - 3 metres

Question 15

AIS - what is the objective of this piece of equipment?

Answer 15

As per IMO Resoultion A. 1106(29):
- Enhance safety of life at sea
- Enhance safety and efficiency of navigation
- Enhance protection of the marine environment

Question 16

AIS - what are the functions / is the purpose of this equipment?

Answer 16

As per IMO Resoultion A. 1106(29):
- Used for COLLISION AVOIDANCE: identify and assist target tracking
- Used for SAR: assist operations or identify craft in distress
- Used as an AID FOR NAVIGATION: additional information to assist situational awareness
- Used for VTS: supplementary information in conjunction with radar, to enhance situational awareness

Question 17

RADAR - describe the safe use of this piece of equipment

Answer 17

You should keep in mind of the following:
- Quality of radar picture
- Heading marker is aligned correctly; incorrectly could lead to misleading information
- Small vessels, ice and other floating objects may not be detected from radar
- Sea or rain clutter; this can be improved through careful use of clutter controls
- Reminder of the radar’s shadow or blind sectors due to obstructions like masts
- Clear weather practice to check radar target detection
- Parallel indexing techniques should be practiced regularly during coastal navigation

Question 18

RADAR - state the checks that need to be carried out before the radar can be used

Answer 18

• Performance Monitor (before sailing or at least every 4 hours)
• Heading marker is in alignment