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Old 07-03-2015, 07:55   #1
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Furuno ARPA

I am researching a new radar set up. It seems that only Furuno provide an economical ARPA feature as opposed to MARPA, which is provided on most mainstream radar systems from Garmin, Navico and Raymarine.

Why I think I would benefit from it is I am hoping my new radar system will improve my watch keeping. I sometimes find it hard to see visually small boats, sometimes not so small boats, fishing boats, yachts and skiffs/pangas, especially if they are partially hidden by a large sea. Often these are the very boats that don't have AIS.

It would be nice if the Radar will help by alarming me only if there is a collision danger. I realise any radar system's MARPA will let me know if there is someone in the guard zone, but if I have say a 5 mile guard zone I am going to be alarmed by a lot of boats I don't need to know about. I like the idea that the ARPA system should only trigger an alarm if the CPA/TCPA threshold is breached and save you the trouble of plotting every MARPA alarm to see if it is a problem and should in theory reduce work load quite a bit.

If you have a Furuno ARPA radar do you use that feature? If so, how good is the Furuno ARPA system? For example is it good enough to run the ship and allow you to leave the helm for a short while? Does it miss targets altogether or does it miss calculate CPA/TCPA data? If it misses targets is that a problem resulting from a low spec antenna and should I look into an open array?
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Old 07-03-2015, 09:59   #2
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Re: Furuno ARPA

I've found the ARPA works fine on my DRS6A but when it gets rough it tends to acquire waves and boats. Not a big deal but just an observation.
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Old 07-03-2015, 10:00   #3
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Re: Furuno ARPA

I am also interested to hear to hear concrete experiences from Furuno owners. I'm curious.

I posted this on the other thread: Well functioning MARPA or ARPA would be nice to have, but I have come around to the conclusion that it's not really very important if you have a well-functioning AIS transponders and have decent radar skills.

I'm not sure you can really calculate a meaningful CPA etc. for a small boat not carrying AIS, no matter how good your radar is. I do not treat small boats like ships, what regards collision avoidance. I actually find normal radar to be more useful than AIS in these cases -- put the EBL on the target and you very quickly see if it's passing ahead, behind, or collision course, and you just know what to do. Set 10 minute trails, and you've got TCPA at a glancec. It's very different from a ship encounter, where the ship may be more than a cable long, and you need to work it out many miles ahead of time. It's been my experience that only a minority of sailors actually know how to use their radars -- this is a waste, because good old fashion radar is a tremendous tool.

Now it just became even better after Tanglewood tipped me off about a feature in the Navico plotters which I didn't know about -- you can set up relative motion extension lines. This is t*ts for understanding AIS targets -- thanks Tanglewood.

As to alarms -- I wouldn't trust an ARPA alarm in any case, and I think that Tanglewood has not played around enough with radar guard zones. I find these absolutely satisfactory offshore with very, very few false alarms -- this is the killer app for the problem Tanglewood stated. Not only would I not trust an ARPA alarm, I don't think it gives a useful warning in any case. Sailing offshore I do not want to be unaware of an object off my bow two or three miles away, no matter what direction it's moving. I do not want to be unaware of any object within a mile of me. An ideally functioning ARPA alarm would not inform you of such objects unless or until you're on a collision course. But we're not talking about ships -- we're talking about small boats which don't have AIS. Such vessels, especially fishing boats, but also recreational motor boats, etc., are given to sudden course changes. I don't want the alarm only right at the moment when the vessel turns into me -- I want to know about it two or three miles off my bow, and within a mile in any case, so that I can keep an eye on it.

In more crowded waters, you tighten up the guard zones -- you could have one mile and a sector off your bow, and a half mile or quarter mile circle. You can narrow the sector alarm, too. Any more crowded than that and anyway you need to turn off your alarms and just keep a constant, intense watch -- electronics are already not going to help you.

That's how I look at it. For well funtioning guard zones, you need good target discrimination and good signal processing -- the 4G radars excel at this. For well-functioning MARPA, though, you need accurate radar bearings, which no small domes provide (you also need accurate heading data; we've talked about that). This is to a great extent a function of the beam angle, so the smaller the dome, the worse. If you want this to work really well, you need a 48" open array on a radar pole -- drool drool. I've spent some time on big yachts with such radars. All I can say is there's no substitute for size. I would absolutely love to have a radar like that.

That's my point of view -- YMMV.
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Old 07-03-2015, 12:58   #4
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Re: Furuno ARPA

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I've found the ARPA works fine on my DRS6A but when it gets rough it tends to acquire waves and boats. Not a big deal but just an observation.
Thanks.

Would you say it identifies conservatively therefore? i.e. Alarming too much if anything, but not missing all the true alarm situations?
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Old 07-03-2015, 13:08   #5
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Re: Furuno ARPA

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I do not want to be unaware of any object within a mile of me. An ideally functioning ARPA alarm would not inform you of such objects unless or until you're on a collision course. ..big SNIP..
Indeed,there are plenty of hazards you may encounter, many of which are also stationary. Uncharted buoys, wind farms, islands you failed to see on the chart etc. I wonder if the radar will pick them up. i.e. If the CPA alarm doesn't just look for moving targets. I would hope it would work? If it doesn't will a guard zone work with ARPA?
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Old 07-03-2015, 14:07   #6
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Re: Furuno ARPA

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Thanks.



Would you say it identifies conservatively therefore? i.e. Alarming too much if anything, but not missing all the true alarm situations?

Couldn't really tell you yet as I just recently installed it and haven't had a lot of time on it.


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Old 07-03-2015, 16:49   #7
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Re: Furuno ARPA

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Originally Posted by poiu View Post
Indeed,there are plenty of hazards you may encounter, many of which are also stationary. Uncharted buoys, wind farms, islands you failed to see on the chart etc. I wonder if the radar will pick them up. i.e. If the CPA alarm doesn't just look for moving targets. I would hope it would work? If it doesn't will a guard zone work with ARPA?
That's what guard zones are for. They are exactly for the case when you are offshore where there's nothing but water around you and you need to know when suddenly you're not alone. A MARPA/ARPA alarm will alert you to a stationary object IF you are on a collision course with it. But that's only IF it's picked up on a number of consecutive sweeps with enough heading accuracy to make an accurate calculation.

Let's take a fishing boat without a radar reflector fishing at night in an erratic pattern -- an actually very typical, and dangerous situation. ARPA is designed for ships holding steady course and speed, not this. First of all, you need a series of reasonably strong returns to get an ARPA solution, AND the solution must indicate a collision course.

Guard zone alarm will go off at any level of sensitivity you set -- and only needs a single faint return to work, not a series of strong ones. AND guard zone alarm doesn't attempt to solve course and speed -- just that there is some target in the area which you set.

In this case, a guard zone alarm will alert you to the presence of the fishing vessel. An ARPA alarm will most likely not, at least, not until the last moment after the fishing boat has already turned right into your path, because all hands are hauling nets and no one is in the wheelhouse.

You seem to want to use an ARPA alarm for the purposes for which guard zone alarms were designed -- but it's not a substitute.


I've been in real life surprised by a wind farm, off Borkum Riff in the North Sea, which was not on my up-to-the-minute updated chart. It was the middle of the night and I was on watch. The alarm system which got it was not AIS, not ARPA, not even the guard zone, but the Mark 10 eyeball, which was keeping an eye on the radar screen, so I saw it some miles before the guard zone. It looked like a flotilla of UFO's on the radar screen -- gave me quite a jolt.
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Old 07-03-2015, 23:24   #8
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Re: Furuno ARPA

Just to be clear, guard zones and ARPA are really two different things. You can have guard zones which will alarm based on anything in the zone, but not acquire any of those targets on ARPA. And just because a radar has the ability to acquire targets automatically, doesn't mean you can't turn that off and acquire them manually. The two only come together when you want ARPA to acquire everything that appears in the guard zone.

Personally I have only used guard zones a little bit. The issues I ran into is that in rough water I was getting lots of false alarms. And if I turned up the sea state filtering enough to not get false alarms, I was concerned that I wouldn't see boats that were actually out there. In calmer water I could see it being very useful.

I do, however, have a very different experience with ARPA accuracy with Furuno radars, in particular with a 24" 4kw dome, so nothing huge. It tracked targets very well, calculated a consistent and accurate CPA and TCPA, and tracked boats that were not making steady courses. And this was with a rate compass, not a sat compass. In fact, I have found it very useful for identifying lobster boats in the fog because they move around like drunken sailors racing from one pot string to the next, then circling around, stopping, then racing off again. It tracks exactly what they are doing. I can also say that the Simrad/Navico radar does not track well at all, but won't drag us down that path again. I can't speak for Garmin or Raymarine.

As for the automatic part of ARPA, I actually haven't used it. I acquire targets manually, mostly becuase up until recently I didn't know the Furuno radar could do it automatically. I though my little one was MARPA, not ARPA. Silly me. There have been some situations where I would have welcomed automatic acquisition, particularly in dense fog. I'm looking forward to trying it on my new 4kw and 12 kw radars as soon as I get them running.
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Old 08-03-2015, 07:12   #9
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Re: Furuno ARPA

Dockhead: I think your point about MARPA being the right system to use does rely on ARPA not working well. If it doesn't then it has no place in a boat.

In my previous post I meant to ask if you could have both alarms in the Furuno i.e. to have an ARPA alarm to let you know if say a target came within half a mile with less than a TCPA of 10 minutes and have a MARPA alarm to alert you to any target in an alarm zone say 2 miles in front of you and 15 degrees of either side of your heading. I guess I didn't make that clear. I was also asking in a round about way if the ARPA system had to have moving targets to work.

To put it another way can you and do you simultaneously run an ARPA alarm and a MARPA alarm?

Tanglewood: Why do you have a 4Kw and a 12Kw radar? Don't modern systems allow you to run two ranges on split or separate screens (with a black box system)?
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Old 08-03-2015, 08:59   #10
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Re: Furuno ARPA

Poiu, I think you are mixing up terminology a bit. A radar has either ARPA or MARPA, but not both. It stands for "Automatic Radar Plotting Aid", and "Mini Automated Radar Plotting Aid". The main difference is that ARPA has the option to automatically acquire targets, where MARPA requires that you manually select any target that you want to acquire.

When you "acquire" a target with either ARPA or MARPA, the radar keeps track of the vessels movement relative to you, and from that it is able to calculate the vessels true course and speed, relative course and speed, CPA, and TCPA. You can do this manually by plotting the target's position over time, but it's a great job for a computer and is why the feature is called "automatic radar plotting aid"

Guard zones, or Target Alarms (different companies use different terminology) are adjustable ring segments around your boat, and an alarm is generated when a target enters the ring. On at least some Furuno radars, you can also alarm if a boat leaves the ring. Anything entering or leaving will alarm, be it a boat, channel market, or land mass. And if you aren't tuned properly, waves and sea chop. Note that none of the targets that trip this alarm get tracked by ARPA, so you know the target it there and you can watch it on the screen, but you have no calculated course, speed, CPA, or TCPA.

Automatic target acquisition with ARPA uses an adjustable ring or a segment, and anything entering the zone gets acquired as an ARPA target. Once acquired, the targets will have speed/direction vectors just like an AIS target, and will have calculated CPA and TCPA, also like an AIS target. The difference between AIS and ARPA is that with AIS, the target has told you over the VHF what it's Position, COG, SOG are. With ARPA, the radar observes and records the targets movement, and from that calculates it's position, COG, SOG. In both cases, you chart plotter (or whatever) then calculates CPA and TCPA.

With ARPA and with AIS, alarms are based on CPA and TCPA, not just on the existence of the target. These alarms are independent of guard zone alarms, so for any given radar return, you could get alarms both when it enters your guard zone, and again if it has a CPA that is too close.

On my Furuno radars, the guard zones and ARPA acquisition zones are different. I don't know if that's true for all of their models, but it is for the 1835 family, and for the FAR2xx7 family.

The manuals for all these products are on the manufacturer's web sites, so I'd suggest downloading them and giving them a read to get more familiar with what each vendor can and can't do. Also, there are a number of good books on radar operation. I have one by Monahan that is very good.

Getting back to what you originally said you wanted to do, I think it would work in theory, but I would still want to keep a full-time watch. I think the biggest issue is the risk of tuning out small returns in an effort to prevent false alarms. You really want a small amount of sea clutter to know you are right at the edge for picking things up, and that risks a lot of false alarms. The other issue is that all radars have some limit on the number of targets they can track, and MARPA devices have fewer than ARPA devices. For example, the Simrad radars which are MARPA can track 10 targets. The Furuno 1835 is ARPA and can also track 10 targets. The Furuno NN3D and TZTouch radars are ARPA and can track 30 targets, and the IMO FAR2xx7 can track 100. The issues is that once you have used up all your targets, it can't acquire any more. So you could have 10 targets locked on, none posing a risk, and not acquire an 11th target that is on a collision course. The good news is that if you have 10 targets acquired, you will likely be paying close attention :-)

Oh, and you asked why two radars. It's mostly about redundancy. The main radar is the 12kw open array. The 4kw dome is primarily a backup in case the 12kw craps out in a far away place.

Hope this helps
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Old 08-03-2015, 09:39   #11
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Re: Furuno ARPA

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Dockhead: I think your point about MARPA being the right system to use does rely on ARPA not working well. If it doesn't then it has no place in a boat.

In my previous post I meant to ask if you could have both alarms in the Furuno i.e. to have an ARPA alarm to let you know if say a target came within half a mile with less than a TCPA of 10 minutes and have a MARPA alarm to alert you to any target in an alarm zone say 2 miles in front of you and 15 degrees of either side of your heading. I guess I didn't make that clear. I was also asking in a round about way if the ARPA system had to have moving targets to work.

To put it another way can you and do you simultaneously run an ARPA alarm and a MARPA alarm?

Tanglewood: Why do you have a 4Kw and a 12Kw radar? Don't modern systems allow you to run two ranges on split or separate screens (with a black box system)?
Tanglewood gave a good answer to most of these questions.

Yes, an ARPA/MARPA alarm will alert you to a stationary target -- if your relative courses and speeds compute to give a CPA and TCPA within your alarm parameters.

But remember what I wrote about guard zones vs. MARPA/ARPA alarms -- why would you want to rely on a computed CPA and TCPA in order to even find out about a target in the first place? Even if the system (say, Furuno) works much better than the ones I've used? Even with an ideally functioning MARPA/ARPA system, the radar needs a series of returns strong enough to give a threshold bearing -- that will not work with weak returns. And why would you want to ignore nearby targets, just because there's apparently not a collision course -- yet?

Would you really want to be making a cup of coffee, or watching the stars, and look over to see a fishing boat passing close by the starboard bow, with you unaware of it because your MARPA/ARPA never figured there was a collision course?

I like the way Tanglewood uses his ARPA -- and I use mine the same way. Once he's detected a target, he manually acquires it and keeps track of it in ARPA. This is really useful also because ARPA/MARPA will show a course/speed vector extension line -- so you see quickly (not instantly because of the way ARPA/MARPA work) if the target has turned into you. But he's not using it to alert him to the existence of the target in the first place, and I wouldn't recommend using it that way.
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Old 08-03-2015, 10:36   #12
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Re: Furuno ARPA

It think the underlying message here is that neither guard bands nor ARPA is a substitute for keeping a proper watch. I wouldn't rely on either to go take a nap, or a shower, or otherwise be away for more than a minute or two. Both are useful tools to aid in your watch, but you still need to stand watch. I know lots of people run single handed and/or nap on watch, but we don't.

As for ARPA tracking boats that are maneuvering vs keeping a steady course, in my experience ARPA is never more than a few seconds behind, and certainly no more than 5. In that sense it's significantly better than Class B AIS which is a 30 second update. Yes, you need to have enough of a return to reliably lock on, but I've never had that be a problem once a target is within 1-2 miles. Further out it depends on the size and quality of the target return. But it's small boats in that 1-2 mile range where I think the automatic part of ARPA is really attractive. I've been in fog with 3-6 boats at a time close enough in that I want to know what each is doing. It can be both time consuming and distracting jumping around with the cross hairs trying to acquire them all. It's like a game of whack-a-mole. As I said earlier I haven't use the auto-acquire feature yet, but am looking forward to trying it under these circumstances. That's where I see it's value, not out in the open ocean.
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Old 08-03-2015, 11:02   #13
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Re: Furuno ARPA

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It think the underlying message here is that neither guard bands nor ARPA is a substitute for keeping a proper watch. I wouldn't rely on either to go take a nap, or a shower, or otherwise be away for more than a minute or two. Both are useful tools to aid in your watch, but you still need to stand watch. I know lots of people run single handed and/or nap on watch, but we don't.
This is already yet another conversation, but an interesting one.

I don't know anyone who stares out over the bow continuously at sea offshore in open water. Proper watch keeping on a recreational vessel offshore means, probably, scanning the horizon on a regular, disciplined schedule. And that is usually much less often than once every minute or two.

Radar guard zones enormously enhance watchkeeping offshore, because the radar IS continuously staring over the bow, as it were, which no human is able to do without going crazy.

So you are right -- the radar is not actually standing watch for you -- it is an enhancement to watchkeeping. But oh, what an enhancement.

I think the OP was talking about this situation -- using alarms to make you aware of objects, of which you were not aware before the alarm sounded. For this, the radar guard zone is the killer app, at least what concerns vessels not carrying AIS.

And by the way -- how about AIS alarms? Offshore I also set these to mimic a radar guard zone, only you can set the radius to be much wider since you have orders of magnitude fewer false alarms. I set mine to alert me of a vessel which is anywhere within x miles of me ("x" depending on how far offhore and how crowded), and CPA less than a mile (or two if far offshore), and CPA less than 30 minutes (to filter accidental, hypothetical CPA matches for vessels too far away for this calculation to be meaningful). This is t*ts and covers 90% of what you're interested in offshore.

Even keeping a pretty sharp human watch in decent vis, you still find out about most contacts from the alarms.
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Old 08-03-2015, 12:30   #14
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Re: Furuno ARPA

I looked at the manual as Tanglewood suggested. RTFM! I should have looked earlier. It clarifies a few issues for me. According to the NN3D manual the system will give you two guard zones. One guard zone can apparently be used as an ARPA guard zone. It will alert you only for moving objects, which have a CPA/TCPA of less than your threshold. You can additionally acquire a target manually. The other guard zone can be a conventional guard zone as would be used in other MARPA only featured radars and will alert you if there is any target in that guard zone, stationary or moving. So, it seems you can have what I am looking for. A guard zone ahead of the boat to pick up everything you might crash into. Boats, buoys and islands and another guard zone, which can be set all around the boat to pick up anything that moving and which is coming too close for comfort.

I hear what Dockhead is saying about wanting to know every boat that is near, but I am not sure I do want the thing alarming unless it is a real likely problem. A fishing boat half a mile away on a parallel course is not of interest, as is an island I am passing maybe much nearer and which is not likely to change position. Most targets will have been picked up visually or by AIS far out after all, so this is a refinement and fills in the gaps nicely.

Whilst fog is not at the front of my mind right now, I can see it would be a huge help in having automatic calculation of targets done for you. Say when you are entering a busy estuary in fog. Workload is high and you need all the help you can get.
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