Skip to main content

CORRECT CORRECTIONS


This week NAUTICAL LOG was asked a question about correcting GPS readings prior to plotting a position on the chart. Apparently what triggered the question was a post in another maritime themed blog about a ship that ran aground because of a GPS reading. Well NAUTICAL LOG did the research and found that the post was a copy from a MARINE LINK www.marinelink.com/ article. What caught our eye however was the comment to the post, we quote:
"I assume that the corrections they are referring to are related to the chart vs the GPS horizontal datum. Is that correct?"

Now NAUTICAL LOG thought it was an excellent question, worth further research and comment, so from MARINE LINK here is what happened. During a coastal passage a containership ran aground after the Watchofficer (OOW) altered course about half a mile before he reached the intended alter course position. An investigation suggested that the OOW was reading GPS alone to plot positions and was not aware that a correction needed to be applied to the readings prior to plotting on the chart. The investigation suggested that a "more detailed passage plan would have alerted the inexperienced officer to the danger and required him to cross-check his position by more than one method". All NAUTICAL LOG can say is - possibly - however a properly trained OOW is taught to confirm the position by at least two methods. On a coastal passage a bearing and range is quick and accurate, the range can be from radar or a sextant angle - remember those - which leads to a whole other question of how many ships even have a sextant on board these days. Certainly one does not see officers carrying their own anymore and few ships, including passenger ships, have one supplied as part of the Bridge navigational equipment.

Anyway the investigation went on to say that seafarers must be aware that on many charts still in use a correction has to be applied to satellite-derived positions before that position is plotted on the chart. To comply with current regulations all this data should be entered up in the Passage Plan and OOW's be made aware of the corrections. So in answer to that comment which started all this - correct the charts and correct the GPS - correctly !!

GPS Error Correction

The Pennsylvania State University has done work on the causes of GPS error. The sources of error includes the clocks in satellites and receivers, the atmosphere, satellite orbits, and reflective surfaces near the receiver. The arrangement of the satellites in the sky can also increase 'dilution of precision'. Many of these errors are correctable within the equipment however navigators should be aware of a correction needed to correct readings prior to plotting.

Good Watch.

Comments

Popular posts from this blog

PERIGEAN SPRING TIDES

It sometimes happens that one is going to write a Post on a subject when lo and behold there is already an excellent one.  Such is the case today; so rather than repeat everything let me refer you to the source of that Post

At present we are experiencing Perigean Spring Tides which occur when the Moon is at perigee on its oval path that is the closest point to Earth.  One of the principal results are higher than usual Spring Tides as against the Neap Tides.

Should you be interested in the full explanation of this phenomenon then you might like to reference "Old Salt Blog" which has an excellent explanation of this event and uses all the correct terms - quite unlike our Media here in South Florida.

Good Watch.

FAKE NEWS

In recent years there has been a steady decline in the professionalism, accuracy and quality of the various Media outlets and the social media.  In the rush to get there first with a story the shoddy Media - lets call it the really shoddy social media - have resorted to making up stories.  When growing up in the late 30's and 40's this was called lying and got one punished in our house.

Fake news includes inaccurate and unsupported stories all of which in the last year have been published without correction or apology.  Now it seems this attitude has spread to the maritime blogs.  One blog which was generally pretty good about its posts has slipped in accuracy lately including quoting from European tabloids.  A recent post was about H.M.S. Vengeance and its missile launches off the coast of Florida part of its programme to Certify boat and crew  The story was an ill advised, inaccurate choice in subject matter as to what happened and how the incident was reported to a Foreign N…

AN tSEIRBHIS CHABHLAIGH

This month saw the commissioning into the Irish Naval Service of a new Class of Irish Naval vessel more of the Frigate size than the previously Corvette size.  However they are all classed as Patrol vessels, the new vessel is LÉ Samuel Beckett P61.NAUTICAL LOG wishes her well and a successful service.


The older vessels saw unbelievable service and value for money the first being commissioned in 1979 and continued through the '80's and 90's into the 21st. Century.  During those years in addition to patrolling the stormy seas around the rugged Irish coast they made passages across the Western Ocean to the United States and Canada, south to South America as far as Argentina, and east to Asia as far as Korea.  Such passages are really remarkable for such small vessels and show the competence of Irish seafarers who as Naval Officers and Merchant Marine Officers train together.

Good Watch.