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Old 27-09-2013, 18:57   #31
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Re: Radar dome location??

Warning a bit of tread drift here!!! (and BTW, I'm with Paul on this topic!!)

I think you may be promoting this "dramatic" position because of some mistaken information you've been given??? (and I suspect that you have not actually used an AIS system on-board??? Nor maybe not MARPA??? but whatever the case, this should help clear things up for everyone!!)

And, maybe I can help illuminate things for you (and others here), so that everyone may come out with a better understanding of what AIS, Radar, and MARPA can do....and how they do it...
[Please note, that I am NOT advocating sailing without a proper watchstander/lookout, nor that AIS / MARPA, etc. are substitutes for a good pair of eyes (and/or a trained radar user, in fog)...
But, I'm just trying to educate / clarify what AIS and MARPA are, and what they do!!!]

Originally Posted by rocksculpter View Post
Radar really is only dependable way to keep track of traffic, being real time. Here in the Puget sound the AIS tracking seems to have a lag time that means they could run you over and be gone before got notice.
1) On a clear day, your eyes and a good sense of how to navigate near other vessels works great!!!
And, a good working radar unit, properly adjusted, AND used by a TRAINED / EXPERIECED operator, is a GREAT tool in reduced visibility situations, such as fog!!!
But, my guess is that you are not aware of how AIS works, and are probably referring to some website that does ship tracking (such as

2) AIS is, in fact, "real-time"!!!
Yes, vessels do transmit bursts of data, but all Class A AIS units (which is what most all commercial traffic would have, and ALL SOLAS-grade vessels, and ALL vessels controlled by a "Vessel Traffic System", MUST have), transmit the vessel's dynamic information (GPS position, speed, heading, course, rate of turn, etc.) EVERY 2 seconds to EVERY 10 seconds, depending on the vessel speed and ALL Class A AIS units, even those on vessels traveling at the slower speeds, will transmit more often than the "every 10 seconds", when they are turning, or have just completed a turn, or have recently made a course change.....

In actual practice most Class A AIS equipped vessels transmit their dynamic AIS data EVERY FEW SECONDS, and the exact time since this data was updated, is also displayed for you!!!
So, in all practical terms, you get these vessel's position, speed, heading, course, rate-of-turn, etc. almost instantaneously....and certainly in what I would call "real time"!!!

3) For those that are not fully aware of what AIS is, how AIS works, and what it can / cannot do.....I suggest you read over these pages...

AIS Frequently Asked Questions

There are other sources of info about AIS, but I thought getting the info straight from the USCG might be good, in that most will not quibble with that source....

4) As for Radar being "real time"....
Yes, it's "real time"...and in flat water with large targets, it does work VERY well....
But understand that radar "sees" the radar cross section of the target, and with our small boats moving around in heavy seas, and/or if you have odd shaped targets (such as tugs towing barges) you must understand that the "radar image" is not a "literal" visual image of what is out there....(and this is also a factor in MARPA effectiveness)

5) Thank you for mentioning MARPA....
Originally Posted by rocksculpter View Post
Marpa, I've heard sometimes will drop it's targets.
a) Just so we are all taking about the same thing, understand that MARPA is "Mini Automatic Radar Plotting Aid"....(note the words "mini" and "aid" it ain't perfect, and it is just an "aid"....)

b) One of often overlooked facts of how to get MARPA to work acceptable, is getting accurate and fast (rapidly sensed and updated) heading information into your radar!!!
Most heading sensors, autopilots, etc. do NOT provide such "fast heading data", and most users of MARPA find significant errors when attempting to use it in a heavy sea, where your vessel's heading might be changing drastically as you surf down a wave, etc...

Another is whether your course/heading and speed are "over ground" or "thru the water"....

c) I'd like to refer you a MUCH better written description of MARPA and its use, than I can do free-hand tonight....
Radar, MARPA and Radar Assisted Collisions | Play d'eau

And, I'd like to quote a few paragraphs...
My radar has MARPA – what is it?
MARPA is an aid to assessing collision risk that you’ll find on many leisure radars. MARPA (Mini Automatic Radar Plotting Aid) is the less capable version of ARPA (Automatic Radar Plotting Aid ) that’s been on commercial shipping for many years.
Can I rely on MARPA?
No. It’s only an aid and the information it provides can be wildly inaccurate since it’s fully dependant on the type of speed and heading inputs your radar utilises which in turn defines if it’s Sea Stabilised or Ground Stabilised.
So what’s the difference between Sea and Ground Stabilisation?
Given the effects of tide and/or wind, your boat’s speed through the water (STW) can differ significantly from your GPS speed over ground (SOG). For the same reason, your boat’s heading (BH) can differ significantly from your GPS course over ground (COG). These differences amplify the slower you travel and/or the stronger the tide and wind.
For example, moving at 6kts into a tide of 4kts shows a STW of 6kts but a SOG of 2kts.
Unfortunately, most leisure radars utilise GPS SOG and COG making it ‘Ground Stabilised’. Calculating collision risk in this way has been attributed as a significant factor in ‘radar assisted collisions’ on a number of occasions.
For MARPA to be of use and not offer potentially misleading and inaccurate information, your radar must use boat heading (BH) and speed through the water (STW). This is called being ‘Sea Stabilised’.

I really didn't want to drift this thread so far away from the topic of radar radome mounting sorry about that....
But, I did want to clear up some significant misunderstandings of AIS, etc..

So, I do hope this helps clear up things for you...

Fair winds..

s/v Annie Laurie

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Old 28-09-2013, 14:29   #32
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Re: Radar dome location??

Originally Posted by rocksculpter View Post
I guess a little dramatic but I've heard recent first hand accounts that showed a 20 minute delay and another that indicated a three mile error in location. Who you gonna trust? Your eyes. By the way 42 watts isn't much power which are the specs on my radar.
Eyes on the outside of the boat, not eyes on the Radar. As John said, no way AIS is delayed as you say or off by what you said. This must be info from people who don't understand the equipment they are using. Radar drops targets all the time -- that doesn't make it a bad thing.
42 watts is 3.36amps at 12.5volts. Over 24 hours that's 80 amp-hours. That is a ton for my battery bank -- and I bet it is a lot larger than yours.

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Old 29-09-2013, 17:30   #33
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Re: Radar dome location??

In "Modern Seamanship" by Don Dodds he recommends RADAR be mounted 16-32ft above the water, minimum 12' above the highest deck.

12' is so as not to cook anyone, or to inflict cumulative damage, especially corneal damage.

16' is 4' deck height + 12' and will give you a 16.8nm range to a 100' high return from a large ship or a 12.4nm range to a radar reflector on a 40' mast and 4.8nm to a return from the waterline.

32' max gives you a minimum range of about 150' which is the point at which things should be appearing visually out of the fog. 22' would give a minimum range of about 100'
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Old 30-09-2013, 07:09   #34
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Re: Radar dome location??

While I haven't read Don Dodds' book, I do have significant real world experience and I concur that small-boat marine radar scanners will provide the most versatility when mounted higher than the typical stern-pole or arch mount provides...

But, we all need to remember that everyone has the boat that they have....
And, they need to mount the radar where they can...
If, they do have a choice, then I recommend mounting on the mast / typically 16' to 25' - 30' high...lean to the higher end of that range....(too much higher won't buy you much more range, and lower will reduce your range AND usefulness when in a heavy sea..)

Please see my post #12 here...where I detailed all of this...
Radar dome location??

One other thing for everyone to remember, the range figures you use to determine where / how to mount your radar (and how you use the radar), should also take into account the "effective height" of the targets, which are NOT always the same as the target's actual height, as the "effective height" is where there is significant radar cross section (not a round exhaust stack or antenna mast) that can adequately reflect your radar signal....
So, as an example, while a vessel may have an "air draft" spec of 100', the top 25' of that may just be their stack or antenna mast....and/or a sailboat with a 65' tall mast may not a radar reflector up on the top spreader (at 50' high), but down on the lower spreader (maybe only 20' high), or even may not have a decent radar reflector at all...

So, understand that you "effective" radar range will, many times, be less than what the math says it will be!!!

I hope this helps clarify things...

Fair winds..

s/v Annie Laurie

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