GPS Flashcards
Abbreviations:
- HDOP
- GNSS
- XTE
- SOG
- COG
- TTG
- VMG
HDOP - Horizontal dilution of Precision. Higher the HDOP the less confidence in quality of position. HDOP below 2 indicates excellent precision.
GNSS - Global Navigation Satellite Systems. Uses multiple satelites to generate 3D fix. A GNSS using less than four satelites is likely to have a very imprecise position.
XTE - Cross Track Error
SOG - Speed Over the Ground
COG - Course Over the Ground
TTG - Time to Go
VMG - Velocity Made Good
What is XTE?
Works out if vessel is off course. An alarm can be set at set distance off ground track.
Normally gives new course to steer but will not take into account tide or wind so best to calculate corrections manually.
What is SOG?
Works out previous and current fix and time tanek to calculate distance travelled and therefore a speed in real terms. Takes account for leeway and tide.
I.e Speed 5 knots with a 4 knot favourable tide means vesel will do:
Speed through the water = 5 knots
Speed over ground = 9 knots
What is COG?
Works out previous and current position to calculate course steered in real terms. Takes account of tide and wind. Differs from the course through the water which doesn’t.
What is TTG?
Uses SOG to calculate how far to waypoint in distance and time.
Time To Go.
TTG in minutes: Distance / Speed x 60
What is VMG?
Gives actual speed and time towards destination whether tacking or not.
What general rules should you follow to monitor your position?
- Use traditional navigation & piloting to confirm GPS
- Maritime Law - GPS can only used as an aid not a complete replacement.
- Plotting positions on chart helps avoid hazards
- Keeping log book helps retrace and go back to basics if GPS breaks down.
- Plot waypoints on chart to ensure positions keyed are correct. Incorrect entries are common and has resulted in many groundings.
How does a Navigation Web work?
Useful for fast vessels or quick fixes in crowded area or navigating through busy port or channel.
Use compass to draw curves 1 mile apart.
Can easily see what he needs to do when his vessel reaches a certain distance and course and can get fix in seconds as GPS gives bearing and Distance to waypoint.
E.g if GPS gives 320 T at 4.5 miles then the position can be plotted as A.
What is a Raster Chart? What are some potential drawbacks?
Scanned bitmap images of paper charts so identical.
Cost effective but carry along any errors in paper charts or scanning errors. Zooming in will distort image slightly.
Does not have additional data like electronic charts such as alarms or hazards.
What are Vector Charts and what are the potential drawbacks?
Constructed of lines, points or areas which show charted features but additional data and information. Can zoom in without image distorting.
Organised as layers of data that can be read by Electronic Chart Data Information System ECDIS.
Can set alarms to warn of hazards and navigatione etc
They are however expensive and time consuming to construct. Magnification beyond scale can give false impression of reliability of position information.
Data is largely dependent on the manufacturer.
