Seven Spades
Well-Known Member
Does AIS differ from the MARPA on Radar. As I understand it both with give a CPA, I know AIS will give the name to enable you to cal them on the radio, but apart from that is there any difference to the accuracy?
Marpa v AIS?
- Thread starter Seven Spades
- Start date
Twister_Ken
Well-Known Member
Radar will give you info on vessels that are not transmitting AIS data.
sailorman
Well-Known Member
marpa is not accurate in small boats without a gyro compass, the boat yaws about too much
AIS is course/distance/position every few minutes
AIS is course/distance/position every few minutes
Robin
Well-Known Member
Radar tells you where the target physically is right now, AIS tells you where the target says it thinks it is from it's gps , it should be the same but the AIS position data may have been delayed slightly in transmission from the target so an opportunity exist there for it to be inaccurate, plus where is the GPS position taken on the target, at the bow or the stern or in between and if it's position is corrected for position transmission, is any offset accurate? We are currently installing all new electronics with radar and AIS transmit as well as receive. Note also for Marpa to work accurately IMO there must be a fast reacting accurate heading reference input ( expensive but we are fitting one), not just a simple feed from say an autopilot fluxgate so there is again potential for the Marpa calculation to be in error as well especially on a boat that is swinging around a bit as they do and not maintaining a constant heading Radar combined with manual plotting either of the relative positions or even using the radar range and EBL info is probably most accurate if practiced beforehand enough to have confidence in it. Radar is providing basically the same information as a mark one eyeball can but it can continue to do it in bad vis when the eyes are not seeing.
THat is my take, others may well think differently. to my mind radar is the first priority to install and AIS is a useful add on to that but isnot a substitute for it.We have used radar for years on three different boats and are confident in it's use after years of practice in all conditions. For that reason I would believe my radar over AIS every time
THat is my take, others may well think differently. to my mind radar is the first priority to install and AIS is a useful add on to that but isnot a substitute for it.We have used radar for years on three different boats and are confident in it's use after years of practice in all conditions. For that reason I would believe my radar over AIS every time
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pvb
Well-Known Member
I have a Raymarine C120 radar/plotter, with a Smart Heading Sensor. The MARPA performance is really quite impressive for budget-priced kit and, as TwisterKen noted, it includes those vessels which aren't transmitting AIS. Radar is a first choice, AIS is a toy. And, no, I haven't bothered to fit an AIS receiver.
Seven Spades
Well-Known Member
I have a fluxgate compass but (RL70) and I have not heard that I need a better one for MARPA, but the MARPA CPA changes massively as the boat swings under sail. Generally if I see that CPA does not go below 1 mile I tend not to worry, but some times it will fluctuate from 0 to 2 miles +. It sounds like AIR will exhibit a similar problem, I guess I need a bigger boat for it to be more stable.
sailorman
Well-Known Member
See #3
not stable on a small craft with a fluxgate compass in anything but calm flat conditions
prv
Well-Known Member
AIS is more accurate, and simpler to use, but relies on the other vessel correctly transmitting its position. I prefer AIS to radar for crossing shipping lanes in clear weather (when you can see the ships are there and just want some help judging courses, speeds, and CPAs) but I wouldn't rely on it in fog.
Radar is self-contained, not relying on the other vessel (assuming it's big enough to reflect well) or on any other external system like GPS. This makes it safer to rely on in fog etc, but it does require skill to use - faulty interpretation has caused a number of radar-assisted collisions over the years where operators on yachts have misinterpreted the relative positions of themselves and a target and steered right into it. Radar also has navigational uses - I'd consider it unseamanlike to close a rocky coast in poor vis by GPS fixing alone (though that's not to say we've never done it...) but by cross-checking the GPS against radar the navigator's conscience can be clear.
In practice, the tenfold price differential means the decision is easy. If you can't afford radar, get AIS - even those who consider it a toy should grudgingly admit that it's better than nothing, and MES are selling a perfectly decent Digital Yacht AIS100 for £140 at the moment. If you can afford radar, the additional cost of AIS is practically a rounding error once you add up the scanner, the mounting, the cabling, perhaps a rigger to fit it if you're not confident with a drill halfway up the mast - and a big enough plotter to display the results. So again, you might as well add AIS - it'll cross-check your MARPA and help confirm targets, and might just highlight the odd ferry about to emerge from radar shadow or yacht with a crappy radar reflector but AIS transmit.
(On the slightly different question of making yourself visible, where the prices are more similar, I prefer a radar target enhancer to an AIS transmitter, but don't yet have either.)
Pete
prv
Well-Known Member
Nope, AIS doesn't care about (or know about) your heading. It works purely on the positions of you and the target - and the integral of those, COG and SOG, but these are averaged over some period so much more stable than a wildly-yawing radar beam.
MARPA on my C70 is basically useless, I assume because my boat lacks a rate gyro, so it's turned off. AIS is very useable.
Another benefit of AIS is that it can tell you the rate of turn direct from the ship's own gyro, so you know a ship is starting to manoeuvre some time before it's apparent visually and a long time before it's detectable on radar.
Pete
pvb
Well-Known Member
The Smart Heading Sensor makes a huge difference.
chubby
Well-Known Member
I have AIS linked to a plotter with a furuno radar that does not have MARPA and find the AIS serves me well for channel crossing ship avoidance although I take the points about non transmitting targets and radar navigation
prv
Well-Known Member
That's good to know. I found an article a while back about substituting the absurdly expensive Raymarine part with a device churned out in the millions for mobile phones and consequently cheap, while supposedly being equally effective. I plan to fit this at some point to improve autopilot performance; it'll be good if it also makes the MARPA useable which hadn't occurred to me.
Pete
pvb
Well-Known Member
Yes, there's a cheaper alternative, if you're handy with electronics. But perhaps the "absurdly expensive" Raymarine part interfaces seamlessly with the radar/plotter, which is my experience.
prv
Well-Known Member
Yep, I'm sure it does.
The French device is relatively simple to make, though, and interfaces in exactly the same way, for about £500 less.
Note that I'm not trying to suggest there's anything wrong with choosing the packaged part, it's quite understandable for a key piece of the steering and possibly radar systems. I just can't justify the cash myself when I know there's an alternative.
Pete
pvb
Well-Known Member
Wish I'd been aware of an alternative when I fitted mine about 10 years ago!
Robin
Well-Known Member
THat was my reasoning for going with the equally absurdly expensive Garmin transceiver to network into our all new Garmin setup using all NMEA 2000 stuff throughout which is about as plug and play as you can get I think, even though we are paying a plugger so we can play. our AIS TRAnsceiver was around $600 or 400 squids plus an extra 6.5% sales tax over here . We could have bought an AIS transceiver for half what we paid but only with NMEA 0183 connectivity and potential conflicts and yetmore beer tokens to a geek.
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oceanfroggie
Well-Known Member
The standard fluxgate compass used by older Raymarine autohelm course computers did not have a high speed rate gyro making MARPA's accuracy very fuzzy except in flat calm conditions. The rate gyro was an optional upgrade or add on.
oceanfroggie
Well-Known Member
Nowadays it is better to have both Radar and AIS. Most network display systems can intelligently combine and merge the display of real time radar targets with AIS targets. If I could only have one it would be radar as it sees everything in real time including non AIS targets such as small craft, buoys, land profile and significant physical hazards above the water line. Radar is a useful positional backup should gps signal degrade or die as it will show significant land mass outlines. MARPA is useless on leisure craft unless a high speed rate gyro is included in addition to the fluxgate compass. Some auto helm processor boards include a fast rate gyro which is used to dampen out changes in reported fluxgate heading caused by your vessels pitching and rolling motion. On iPhones this technology is commonly called an accelerometer.
prv
Well-Known Member
Even in flat calm conditions, in my case
. There are some piles in the river near my berth, with radar reflectors on top. When I had a go with MARPA, tied up alongside, these posts were reported at speeds of several knots at times, and closing dangerously on my position!Pete
oceanfroggie
Well-Known Member
Even in flat calm conditions, in my case
Pete
We used to have an RL70CRC, but the older auto helm did not have the fast rate gyro, so MARPA data was spurious except in calmish conditions when the boats relative motion was stable.
