LC Nav Theory Flashcards
How many yards are there in 6 Nautical Miles?
1NM = 2000 yds (2025.4) 6 x 2000 = 12000
How Many cables in 4 Nautical Miles
1NM = 10 Cables, 4NM = 40 Cables
How many minutes are there in 2 Nautical Miles?
1 Minute = 1 Nautical Mile 2 = 2 Mins
if you travel directly north from posn 50 38.20N 002 12.4W to 51 6.7N 002 12.4W how far have you gone
28.5NM
If travelling AT 15 kts how far will you travel in 6 Minutes
1.5 NM
if travelling at 18 knots how far will you travel in 3 minutes
0.9NM
What is meant by the term Great Circle and how is it different from a small circle
A Great Circle is a plane that passes through the centre of the earth. (e.g. Equator and all Meridians). The shortest distance between any two points on the earth’s surface is attained along the arc of a Great Circle. Small Circles are lines that do not pass through the centre of the earth.
Why might you use a great circle route for a long distance passage between 2 points?
Since the Earth is approximately spherical, a great circle route offers the shortest distance between two destinations, which is of paramount importance for flight planning and navigation. This principle ensures that aircraft can fly the most direct and, thus, the most fuel- and time-efficient routes possible.
Where abouts would you look to get an indication as to the reliability of the data included on a chart, Give 2 examples of information that can be obtained from this reference
Source Data Diagram
Date of survey, Surveying organisation, survey scale, Seafloor Coverage /method, Zone of confidence ZCC.
Give 2 pieces of information provided in the source data diagram and explain why it is important?
Date of survey, Surveying organisation, survey scale, Seafloor Coverage /method, Zone of confidence ZCC.
The diagram is important because it provides a indication of reliability of the information provided on the chart.
What method of surveying can you expect from a chart with survey data of:
1. 1805?
2. 1940?
3. 2019?
1 Leadline
2 SB single lines of coverage
3 MBES Multibeam echo sounder - full seafloor coverage
What types of chart folio would you use for a LCVP and A LCU
VP Leisure or small boat folio, Standard Nav Charts LCU
Give 2 sources of information we can use to obtain tidal stream predictions?
Admiralty tidal stream atlases, Admiralty total tide, Admiralty charts (Tidal stream diamonds and TS symbology), Reid’s Almanac, NP167
What type of bearing does a gyro compass give you
If a gyro is reading 3 deg high what does this mean and what correction should be applied to the gyro compass
If A gyro reads 2 deg low what does this mean
Give us 2 methods to determine gyro compass error?
A Gyro Compass gives True Bearings
If it reads 2 deg low, or 3 High than a true bearing. The value should be added for low or subtracted to high to all visual bearings.
The following methods can determine Gyro error
Observation of a charted transit.
Observation of a line of jetty.
By comparison to a charted bearing of an object when the position is known to a high degree of accuracy.
By observing the bearing of the sun at sunrise/sunset (Amplitude)
By observation of the bearing of a heavenly body.
By reduction of a cocked hat fix.
The following levels on the chart are given to which fixed datums
1) Bridge Clearances
2) Charted elevations
3) Drying Heights
HAT
MHWS
CD
With respect to the Magnetic Compass explain Variation and Deviation.
And where would they be found
Variation The difference between true N and Mag N comes from the difference in posn of both poles T and MN
Deviation objects nearby affecting the magnetic field, ie electromagnetic systems, anything magnetic, or even big bits of metal
Deviation: Found on a COMPASS DEVIATION CARD or DEVIATION CURVE
Variation: From the COMPASS ROSE on the chart
Explain with diagrams how the moon and the sun affect spring tides and neap tides over a lunar month.
Spring tides occur approximately 2 days after after full and new moons. The lunar and solar tides work in conjunction to produce High HWs and LOW LWS (On diagram moon and sun in line)
Neap Tides Occur approx. 2 days after 1st and last 1/4. Lunar and solar tides work in opposition, producing LOW HWS and HIGH LWS ( Diagram of moon and sun at right angles around the earth.)
Name 3 Types of chart projection in use and when each would be used?
Mercator Projection
Introduced by Gerardus Mercator in 1569, the Mercator projection is a cylindrical projection that preserves local angles and shapes, making it valuable for navigation purposes.
The Mercator map projection significantly distorts the size of landmasses near the poles, leading to misconceptions about the relative sizes of continents and countries.
Transverse Mercator Projection
A variation of the Mercator projection, the Transverse Mercator projection, involves rotating the cylinder 90 degrees.
Gnomonic chart
A chart which is very useful in great circle sailing based on the gnomonic projection. This is a perspective projection in which part of a spherical surface is projected from the centre of the sphere onto a plane surface tangential to the sphere’s surface. The principal property of this projection is that great circle arcs are projected as straight lines.
In order to draw a great circle on a Mercator chart—the projection being a relatively complex curve always concave to the equator—the route is first drawn on a gnomonic chart by connecting the plotted positions of the places of departure and destination with a straight line. Positions of a series of points on this line are taken from the gnomonic chart and marked on the Mercator chart. A fair curve is then drawn through these points, which is the required projection of the great circle route on the Mercator chart.
Briefly explain how a GPS works
Made up of 3 sections Space, User, Control
Space fires a signal
Control, Stations that talk to the satelites and update their time
User, recives a signal and convrts it into a range 3 or more ranges gives a position.
A constellation of a minimum of 24 satellites is arranged in orbit such that the user on the ground always has 4 satellites visible from their posn.
Each transmits a code that travels at a known speed, the receiver generates identical codes and the times between each can be used to range the satellites,
These ranges can be used to obtain the receivers posn the more satellites the more accuracy.
Explain How GPS differs from differential GPS
Differential GPS uses ground stations whose posns are known to a high degree of accuracy. These ground stations have a GPS receiver. The errors in the ranges obtained from the gps satellites can be determined by the ground station as its at a high deg of accuracy.
Corrections addressing the errors can be calculated and are subsequently broadcast to users in the vicinity so that they too can apply the corrections
This can increase accuracy but relies on assumptions IE the errors are the same at the user as the base. This gets worse with distance from base station.
Other than GPS name 2 SAT NAV systems available for use,
GLONASS, Galileo and BeiDou
as well as two regional systems QZSS and IRNSS – and each are managed by a different country.
What error might you experience with your GNSS receiver when operating in an error such as a port or harbour which has lots of high buildings and cranes in your vicinity.
Multipath Errors These occur when satellite signals reflect off surfaces (such as buildings or cranes) before reaching the receiver antenna. The reflected signals interfere with the direct signals, causing inaccuracies.
Signal Obstruction: Tall buildings and cranes can block direct line-of-sight to satellites, resulting in signal loss or degradation. This can lead to position jumps or temporary loss of fix
How Many Satellites do you need for a 2d and 3d fix?
3 = 2d
4 = 3d
To what datum is NAVSTAR GPS referred?
WGS 84 – World Geodetic System of 1984
How would you tell whether positions from GPS can be plotted directly onto the chart that you are working off?
Check the Chart Datum of the chart/ Source data Diagram normally WGS 84 WRITTEN IN PURPLE
