Ground vs Sea Stabilized Radar Policy

Just curious as to how radars are being used on other ships. Specifically with regards to sea and ground stabilization. Do you keep your radars in sea or ground stabilized mode? What is the most often used mode on your ship and why? Do you switch them for certain situations or does your ship always keep them in the same mode? What do you as captain or the captain of your ship prefer mates use when standing watch?

For collision avoidance it is recommended that the radar remain in sea stabilized mode. IMO requires it to be available on the radar. Many seafarers will use GPS input (ground stabilized) and this can be deceptive when in areas such as the Florida Capes where ships crossing the Gulf Stream will not have the same visual course as the vector shown on the radar CAS. I have witnessed this phenomenon and it will confuse a watch officer quickly. In many areas Sea Stabilized is required to be used. The only reason to use ground stabilized is when your speed log fails.

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I was always taught the col regs are based on ships conduct through the water.
They didnt change once GPS turned up.
There is a very good MAIB investigation on a daylight collision between 2 ships due to the argument, your passing, no its a port stb.
Radar was on bottom lock and the punter didnt realize


Really? You can’t think of any others?

In a river would be another place

Anywhere around fixed objects of any kind but especialy when there’s a current.


Yes I was taught the same thing. Some others have not been taught very well. There are many non SOLAS vessels using GPS. The CFR took out speed through the water if I remember correctly.

Not in the gulf stream when using collision avoidance system. True vectors on the radar will mislead the navigator.

This isn’t about true vs relative vectors, it’s about ground vs water stabilization.


The projected motion of AIS targets are ground-stabilized by virtue of GPS whether the radar is using STW or SOG as the speed input. It is true that in water stabilized mode, the true vector of a radar-tracked target (ARPA/TT) will be a better representation of aspect compared tp ground stabilized, but only in areas of faster currents.

yes, so you actually get the wrong data in high current low speed incidents with AIS
Hence that great MAIB investigation where it was an AIS assisted crash.
Trust the radar when its on sea stabilized mode
Thats how I was taught

It’s poor practice to rely upon the AIS for close in work but isn’t the source of the ship’s heading data the gyro?


I always ask each company the policy on the AIS they always reiterate the IMO statement:

“AIS might be used in the future for collision avoidance”

My experience is the radars are left ground stabilized. Most watch-standing officers would likely assume that a difference between the heading vector and the COG vector is total leeway.

The ideal situation might be to switch to sea stabilized when appropriate and then back to ground stabilized but that’s another opening for Murphy’s Law.

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But your log with switch to the thermocline layer in the sea once its deep enough unless its fixed to bottom lock.
I cant see why you would lock radar to bottom lock unless you want it to agree with your ais?

Because I want fixed objects to show on the radar as stationary and that only happens when ground stabilized.

I have seen AIS targets close aboard to port when the actual target was off to starboard, so it really doesn’t work when you get that close.

It’s dependent on the other vessel’s AIS GPS position being accurate and the transmission rate being fast enough. Especially on private vessels with class B AIS units that don’t refresh data as fast that can be an issue.

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yes when close to shore you go ground stabilized
What do your ISO company documents say for operating in a port?

typically a pleasure-boat with ais B wont make much of a scratch on a ship so you dont have to worry.