Ground vs Sea Stabilized Radar Policy

I have never seen radar settings mentioned in any company documents or standing orders.

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thats scary

Me either, but this is making me think we should!

I was wondering about this a while back when I saw on our type approved radar the option in the setup menu to combine AIS and radar targets into a single target, which I thought a bit odd since I have always been taught to favor radar data over AIS. So if you combine them, which data “wins”? As this was on a type approved radar (Furuno) it seems odd that they’d leave it up to the user.

Why? 90% of the people in the office have probably never been on a ship for more than a port visit and could probably only barely tell you which end is the front.

Items like this are better left to the Master’s standing orders where someone that actually knows how and is qualified to operate the ship is making the standards and rules.

This right here. And if that’s something that upsets you, then you’re probably violating the first part of rule 5 and keeping your nose stuck in the radar instead of using your MK1 mod 0 eyeballs. A good chunk of us sailing predate ARPA and properly relegate radar to being an aid to navigation rather than the main method of navigating.

AIS doesn’t always work… not every boat has it. Radar can glitch and have interference or be out of tune. Your eyes though… always work if you just use them.

A doppler log with move off the bottom once its deep enough.

So you dont have any fleet management in the office, not heard of that before?
Who is in charge of maintenance?

We do… it’s just that the ones that are in charge of making the ISM rules aren’t “boat trash.”There’s definitely fleet management… but again, very few of them have ever held a z-card or MMC, much less been in a senior officer position.

So once again, any operational ship specific rules like ground/sea stabilization are best left to the masters to establish. Or better yet, the mates to set the radars up the way they prefer for their watch. Some like ground/true vectors/North Up (like me), some prefer a different setting.

Personally, I prefer ground because I know that no matter what the current is doing, and what layer my POS Doppler is picking up, my speed over ground is always going to be my speed over ground. My radar targets will be indicating their real speed over ground, CPA, and TCPA to the best of the ARPA’s ability to calculate it.

underway, or making way in current, hmmmmm
Ground or water speed and arpa?

Dude, you’re missing the point of how ARPA works. If a target is bearing 010° at 3000 yards on one sweep, and at bearing 010° at 2850 yards shortly after, it’s going to be calculating that closing rate and all regardless of whether you’re sea or ground stabilized. It’s doing your popsicle sticks and grease pencils math for you.

Even if your radar isn’t properly aligned with the antenna, that target will hit you when it says it will hit you. It’s bearing may be a degree or two off, but it’s re-adjusting that vector with every 3 (usually) sweeps and fixing the data presented as it goes.

If you’re using speed through the water in your calculations and assuming that everyone around you is feeling that exact same current, and being effected the same way, you’re messing up right there. GPS stabilized doesn’t care about those variables.

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if your underway drifting and the another vessel is making way but ground speed zero who has right of way?
How will arpa show the 2 situations when on ground or water speed?

First off, you’re not giving enough information to determine that. Are you NUC? What are the vessel types? Meeting? Crossing? Overtaking?

ANDDDDDD Secondly, there is no such thing as one vessel having “right of way” over another. Even a stand-on vessel is required to take action to avoid collision if needed.

We’ll assume it’s a CBDR situation of two weekend warriors with their little 6-pack licenses for your example though. On ground stabilization it will show you closing on the other target, on water it will probably show the other target closing on you.

I say probably because, we’re not going to take into account that maybe there’s a countering wind or that your drifting boat may be getting pushed even faster by the wind than what your Doppler is saying. Regardless, the TCPA and CPA are going to be the same.

And that is when this comes into play: Risk of Collision “Rule 7(c)- Assumptions shall not be made on the basis of scanty information, especially scanty radar information.” And you always assume a risk of collision exists if in doubt.

That means, use your eyes and get your nose out of the radar.


Yes all true, and now lets do that in fog.
Its all the examples taught to me in school and explaining when radar and AIS disagree.

As I said before there is a great MAIB ( UK ) accident report on the exact issue we are discussing as the argument on converging ships was one said your overtaking the other said port stb due to one radar on bottom lock and the other on stw and they were in current.

The argument is that in strong currents own ship might not properly display the true vector of an other vessel which could result in an error in the aspect. Wrong aspect on the radar could possibly lead to misunderstanding the COLREGs situation.

If one radar could be reliably switched to ground stabilized and back to sea stabilized as appropriate then having one radar ground stabilized could reduce the possibility of confusion in special cases.


you got it

I haven’t read this report but the stabilization mode of the radar seems like a lame excuse.

First of all, aspect can and should be determined with visual means. If the vessels are not in sight of one another (ie, restricted vis) then aspect doesn’t matter - both are responsible for avoiding a collision.

Second, leaving AIS out of it for a minute, an ARPA target will always show the same CPA/TCPA regardless of stabilization mode since that is based on relative motion (and a relative vector will not mislead in this case).

Ground stabilization (preferably by GPS - Dopplers are very inconsistent) is much much preferred in a pilotage environment (or anywhere else where objects fixed to ground are plentiful). Anyone running a sea stabilized radar in that type of area is losing out on a very powerful tool in that of the ship’s own true vector.


Just saying I understand the argument. It makes sense by logic but in practice it’s another story. If it was the expected practice in some ports it might work, it does violate the KISS principle.

I vaguely recall that MAIB case but it could be the MAIB being too clever by half.

Furuno calls this “Association”. It’s merely a way to declutter the display. The target is still being tracked internally as both an ARPA and AIS target, although you (the user) can select which information to prioritize for display purposes. You also specify the tolerances/differences allowed in the AIS and ARPA target parameter differences: gap (position diff), range and bearing from own ship, speed and course. If any single one of those differences exceed those settings, the targets will disassociate and you’re left with a separate AIS and ARPA target.

BTW, most Furuno radars have multiple mode types (at the software level) that aren’t technically IMO compliant (A, B, C and W). While Target Association appears to be available in all modes, there are many other features that are native to the software but are blocked in IMO mode, such as the absence of some range scales (1, 2, 4, and 8 NM).


This is something that I always forget to bring up when having this discussion. Thanks for mentioning it.


the accident was totally lame, daylight clear weather.
It had some funny issues all too common:
The bridge crews on both vessels didnt have a common language ( OOW, helmsmen, lookout)
The 2 guys on lookout watched the ships hit each other but said they were not told to say anything when they saw anything so they didnt.

Yes. The interesting part to me is that with a type approved radar the operator is allowed to favor AIS over the ARPA, which seems like that could come back to bite you. I would prefer best practices to be baked into my radar.