Cruiser2B
Well-Known Member
Re: Not necessarily....
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PLEASE PLEASE PLEASE read the powerpoint presentation imbedded in Piers previous post!!!
Also read the two MIAB reports referred to where one of the main reasons for the accidents was clearly identified as the vessel using ground based rather than water based (or stabilised) ARPA. You obviously have not absorbed any of this information before posting and are in danger of misleading others.
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You're correct in assuming I hadn't viewed the PPoint or the MAIB report(s). I have since read the presentation and the Vespucci/Wakhuna report - I saw another MAIB report mentioned but didn't see a link; frankly I didn't see the need to search for it, and am sure you'll let me know if it offers anything revelatory. Anyway, both items support what I said, so perhaps you need to have another go at "absorbing the information." I direct you to the bottom of MAIB report page 24 - "When radar is ground-stabilised, the output of data will relate to their ground track and, although accurate, can be highly misleading when assessing target aspect." If you need to see pictures, then slides 24 through 26 of the PPt presentation show this effect nicely.
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Let me try to explain where you are wrong.
You say
The only drawback to using ground-based 'ownship' data, is that the target's vector will also be ground-based.
Thats wrong
You have no way of getting a ground based track of the other vessel unless as in AIS BOTH vessels GPS position is known and constantly updated. That's the advantage of AIS in fact.
Your Radar can only give you the RELATIVE track of the other vessel, and as both you and he are affected by wind and tide thats RELATIVE to YOUR track THROUGH THE WATER! which your Radar doesn't know because it doesn't have any direct input to tell it.
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Where to start? Perhaps back at the aforementioned pg 24 and slides 24-26. Or just consider two ships on collision courses with each other - whether you plot their paths relative to the sea, or to the ground, they will collide in either case. Using ground-stabilised inputs however will not give you an accurate assessment of aspect, which plays in the application of the rules. But as the MAIB suggest, ground-stabilised is accurate - well, it's as accurate as the reference inputs allow; assuming you're using GPS for those inputs, then it should be quite accurate indeed.
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Honestly cruiser2b I know this is difficult to get your head around but it IS important. Go back and study the PowerPoint thing which explains it far better than I could with diagrams to show the danger of the wrong mindset.
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Honestly, I don't know what you're trying to illustrate with the PPt or MAIB report. The MAIB report stated quite clearly that the crews of both vessels were inept at using their radar systems. Capt Starkey should have included notes on the PPt, to better explain the diagrams. IMHO, using True vector leaders to determine risk of collision amounts to using scanty radar information; a radar system already has a level of inaccuracy, and a risk of failure. With True vectors, you now add the inherent inaccuracies and additional risks of failure of whatever systems supply your course and speed information. Add to that the difference in the two vessels' leeway and/or different current effects (eg. eddies localized to one vessel, or one is a deep-draught vessel affected by subsurface current) and those vectors are wildly inaccurate. They should only be used to help assess aspect, which should be confirmed visually, if possible. Only relative vectors should be used to determine CPA, and to monitor safe passing. The operator of any ARPA should be comfortable switching back and forth between relative and true, as the situation may dictate. Said operator should also be fully cognizant of the system, its inputs, and their limitations. If the operator is lucky enough to have a system that supports it, he should be comfortable switching between sea-stabilised, ground-stabilised and manually-input Co and Sp. There are occasions where each is superior.
[ QUOTE ]
PLEASE PLEASE PLEASE read the powerpoint presentation imbedded in Piers previous post!!!
Also read the two MIAB reports referred to where one of the main reasons for the accidents was clearly identified as the vessel using ground based rather than water based (or stabilised) ARPA. You obviously have not absorbed any of this information before posting and are in danger of misleading others.
[/ QUOTE ]
You're correct in assuming I hadn't viewed the PPoint or the MAIB report(s). I have since read the presentation and the Vespucci/Wakhuna report - I saw another MAIB report mentioned but didn't see a link; frankly I didn't see the need to search for it, and am sure you'll let me know if it offers anything revelatory. Anyway, both items support what I said, so perhaps you need to have another go at "absorbing the information." I direct you to the bottom of MAIB report page 24 - "When radar is ground-stabilised, the output of data will relate to their ground track and, although accurate, can be highly misleading when assessing target aspect." If you need to see pictures, then slides 24 through 26 of the PPt presentation show this effect nicely.
[ QUOTE ]
Let me try to explain where you are wrong.
You say
The only drawback to using ground-based 'ownship' data, is that the target's vector will also be ground-based.
Thats wrong
You have no way of getting a ground based track of the other vessel unless as in AIS BOTH vessels GPS position is known and constantly updated. That's the advantage of AIS in fact.
Your Radar can only give you the RELATIVE track of the other vessel, and as both you and he are affected by wind and tide thats RELATIVE to YOUR track THROUGH THE WATER! which your Radar doesn't know because it doesn't have any direct input to tell it.
[/ QUOTE ]
Where to start? Perhaps back at the aforementioned pg 24 and slides 24-26. Or just consider two ships on collision courses with each other - whether you plot their paths relative to the sea, or to the ground, they will collide in either case. Using ground-stabilised inputs however will not give you an accurate assessment of aspect, which plays in the application of the rules. But as the MAIB suggest, ground-stabilised is accurate - well, it's as accurate as the reference inputs allow; assuming you're using GPS for those inputs, then it should be quite accurate indeed.
[ QUOTE ]
Honestly cruiser2b I know this is difficult to get your head around but it IS important. Go back and study the PowerPoint thing which explains it far better than I could with diagrams to show the danger of the wrong mindset.
[/ QUOTE ]
Honestly, I don't know what you're trying to illustrate with the PPt or MAIB report. The MAIB report stated quite clearly that the crews of both vessels were inept at using their radar systems. Capt Starkey should have included notes on the PPt, to better explain the diagrams. IMHO, using True vector leaders to determine risk of collision amounts to using scanty radar information; a radar system already has a level of inaccuracy, and a risk of failure. With True vectors, you now add the inherent inaccuracies and additional risks of failure of whatever systems supply your course and speed information. Add to that the difference in the two vessels' leeway and/or different current effects (eg. eddies localized to one vessel, or one is a deep-draught vessel affected by subsurface current) and those vectors are wildly inaccurate. They should only be used to help assess aspect, which should be confirmed visually, if possible. Only relative vectors should be used to determine CPA, and to monitor safe passing. The operator of any ARPA should be comfortable switching back and forth between relative and true, as the situation may dictate. Said operator should also be fully cognizant of the system, its inputs, and their limitations. If the operator is lucky enough to have a system that supports it, he should be comfortable switching between sea-stabilised, ground-stabilised and manually-input Co and Sp. There are occasions where each is superior.
