Radar

[Phoenix of Hamble]

Another option is a cheap(ish) radar, and then at a later stage, rather than upgrade to a MARPA set, add AIS instead....

Upside:
removes plotting/tracking/acquiring errors completely

Downside:
requires a display capable of showing AIS data
not completely dependable until Summer '06 when all ships will be equipped by law

I suspect that various plotter manufacturers will add AIS support over the next 2 years, so my approach would be to buy a cheaper radar, learn how to plot manually, and then buy AIS in 24 or so months time to coincide with buying a new plotter.....

Good practice in my opinion to be able to plot manually anyway... means that you understand what the MARPA set is telling you, and its limitations... I suppose, a bit like buying plotter - you should still should know how to navigate on paper.....
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[Phoenix of Hamble]

Mind you.... what do I know....

last time I used a radar, it had a battleship gray casing, silver grab handles on front of display, weighed 40Kg, had a black rubber face shaped display surround to stick your head in, and benefited from a green line that span round in circles....... /forums/images/icons/crazy.gif

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[Talbot]

Thats roughly what I said (but explained better and with more detail) /forums/images/icons/smile.gif

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[chrisb]

I agree. for technical reasons i was unable to install the new gyro compass to my c120 untill late in the season. marpa was of little benefit in anything other than calm conditions when used with the autohelm compass.However now the unit is complete i find it functions well in all conditions. it would not be wise to budget for a marpa system without the gyro compass

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[tome]

<But the scanner rotates once every three seconds...>

That's 120 degrees per second, so even with a 10Hz update that only gives 12 degrees heading resolution - barely what you can achieve with a h/b compass.

It's important to appreciate this point, but harder to explain without diagrams. The job of the electronic compass is to provide a reference to convert radar relative bearings to true. Otherwise a target will appear on random headings as the vessel yaws and it is impossible for the MARPA to maintain lock except in a flat calm with a rock-steady heading.

GPS track (GPS cannot give heading) is less than useless for this and standard fluxgates tend to be 1 or even 0.5Hz, not 10Hz. I quoted figures earlier for CPA accuracy, but note that continuous lock is required for at least 1 and preferably 3 minutes to achieve these figures- impossible without a fast heading reference.

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[tome]

Chris

Spot on, and I suspect that you are far from alone in learning this the hard way. If you cannot afford a decent fast compass, don't bother with MARPA.

Tom

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[MainlySteam]

Tom

Both you and James have made some comments towards accuracy and Jimi started a thread some weeks ago on what separation from ship traffic to maintain when crossing the channel and I would be interested in your views on plotting accuracy (from the navigational point of view, not just the electronic point of view) when using pleasure sized radars - with and without MARPA.

You may have seen a recent post of mine where I had the opportunity of comparing almost side by side a 24 inch radome radar (the largest likely to be found on smaller pleasure boats) with a 3' 6 open array one (is said to be the smallest size useful for reliable navigation). Both sets were from the same manufacturer (Furuno) and were current models, were on professionally crewed vessels, and the 3' 6" array was used in poor visibility conditions on one of the occasions I was aboard. Neither had MARPA.

While I was aware of the compromises of the smaller radars (especially as most of my radar exposure has been in military and aviation applications), actually comparing them almost side by side it really struck me as to how inferior the smaller radar was and I would be very wary of using one for separation in traffic except in a very broad brush way. The 3' 6" array one gave a much greater feeling of comfort - almost like putting a good pair of spectacles on, in comparison.

Any comment would be appreciated

John

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[tome]

John

I don't have figures for small craft radar MARPA accuracy, and I don't really expect to see them quoted. What I do know is that we are hampered by power on a sailing boat. This means we cannot use large open array antennas which require significant reserves of power to take them through their windward sweep. Instead, we have to make do with tiny enclosed arrays inside a windproof enclosure.

Our antennas cannot provide the bearing discrimination of the order of professional sets, end of story. On range discrimination, they are similiar although range is limited by height above sea level.

To the point of your question, MARPA on a small vessel can only ever be a guide: perhaps a warning to the short-handed sailor that a situation is arising and that he'd better take care. I've quoted the IMO standards for CPA accuracy in a crossing situation and I would expect a small boat radar to be no better than twice the error, or 4 miles for a 1 minute plot (2 miles for 3 minutes). Lying in your bunk and watching a ship pass at a CPA of 1 mile is not an option, IMO.

Regards
Tom


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