Target bearing

[Taniwani]

When I look at a target (AIS or ARPA) it gives me its bearing relative to my boat's heading, how do I set it to display the bearing as a true compass bearing?
When I want to visually spot a target, I usually use the bins with compass. I don't have a pelorus and I think most boats actually don't.

 

[Pthein]

So are we talking about the difference between bearing TO and bearing FROM a waypoint?

 

[Taniwani]

No, I didn‘t mean TO or FROM. I‘m talking about what is given to you, when you click on an AIS or ARPA Target as range and bearing. Say I‘m heading south 180 degrees with my boat and the target is exactly west of me. If I looked through a bearing compass, I would see 270. If I click the target on the display, it will tell me the bearing is 90 R.

That is because the display shows it RELATIVE to my heading and not the cardinal direction. It is only the TZT2 that does it this way, if I click the same target in MaxSea I get 270. So I think there might be a setting that I haven’t which would change it to the more common way of a compass bearing, independent of my own heading.

 

[Pthein]

That is the difference in bearing TO a target and a bearing FROM a target. We are saying the same thing.

Heading south a target due west of you would be a 270 bearing TO and a 90 bearing FROM.

I have the exact same question as my TZT2 is displaying bearings FROM targets and waypoint but I’d like them in bearings TO the target or waypoint.

Phil

 

[Taniwani]

In the example I gave, it indeed seems like TO and FROM are reversed, but let me take this a bit further to explain the problem I have:

So we are heading exact south and the target is exact west of us. We see the taget at 270, and the target would indeed see us bearing 90.

Now, let both targets remain at the same position, but we are changing our heading from 180 to 170. Means our ship is now pointing 170. If I take a bearing compass and take a bearing of the target in the west, it would still show 270. But the TZT2 would show the target bearing 100.

Somebody at the target, taking a bearing compass would still see us bearing 90.

You see what I mean. The displayed bearing changes with the orientation of my boat, and as such is always displayed relative to my boat. The reading starts with zero at the bows and increases towards starboard, showing 180 for targets that are straight astern and 270 for targets that are sure off to port.

It's an old way to call out targets relative to the own ships orientation and works on boats that have a fix mounted bearing disk. It is entirely unrelated to any compass direction.

The interesting thing is, that with AIS, you get the position of the target and from GPS you get yours and from that a compass bearing is calculated, (which is what most systems show), but here the MFD takes the heading information to calculate how the target bears relative to your boats heading. An extra step that is not wanted.

It feels like there must be a way to select how you want it displayed, but I have not found it.

 

[C-Bass]

Good morning Taniwani,

I believe there are a couple of things you can try.

First the Bearing scale on the radar display. You can change the bearing scale on the range ring that shows the bearing readouts around the radar. To change from Relative to True: Home Button, Settings, Radar, Bearing Scale Mode: True or Relative. Please reference 6.8.3 in the operators manual.

Next the EBL (electronics bearing line) can also have a Relative or True Reference: Relative referencing the heading of your ship. True referencing North. To change: Home button, Settings, Radar, EBL Reference: Relative or True.

You may also want to change the way all the other bearings are referenced: Magnetic or True: To change: Home Button, Settings, Units, Bearing Display: Magnetic or True.


Please keep in mind that the bearing range ring and the EBL reference to True require a heading sensor.


C-Bass

 

[Taniwani]

Thank you for your input C-Bass! I have again checked the settings you mention and I have all three set to TRUE as you suggest.

I have attached two screen shots to show the discrepancy. I have taken the radar screen so things are more clear to read, but I'm using an AIS target some 30 miles away to show the effect. (Radar targets are obstructed by hills and walls as I'm in harbour right now).

In the first picture you can see a north up radar bearing scale and my heading line going through 228 true. The EBL is set to 302.5 true and covers a target some 30 miles away. So that seems all fine and as expected.

In the second picture I tap the target to get its details including bearing and the inbox shows the bearing as 73.5 (no qualification like TRUE MAG or REL). If you add my heading 228+73.5 = 302.5, you get the true bearing of the target. So it is clearly still a relative bearing, despite all settings that I know of, being set to TRUE.

I understand that for a radar target (ARPA) to be shown with true bearing, the heading information is needed, and without that, it would obviously have to be relative.

But for an AIS target the situation is just the opposite. Without a heading information, you would only have your position and the target position, and the only bearing you can derive from that would be a true bearing.

I haven't tried to see what it does if I take away the heading info, as it only allows me to choose one of my two heading sensors, but not none. I would have to unplug them from the backbone to try it.

Thanks again for your help.

 

[colemj]

The bearing data transmitted by an AIS target is bearing at own position, which has nothing to do wth any ship other than the transmitting one. Whatever bearing you are seeing from an AIS target is that target's absolute bearing.

 

[Taniwani]

I think you are confusing bearing with heading or COG. Has nothing to do with what I’m looking for.

 

[C-Bass]

Taniwani,

Thank you for your inquiry. Please allow our team a little time for research. May I ask what software version you are running and what software version the scanner is running? If you go to the menu, settings, Initial Setup, Sensor list it should provide a software revision number for the scanner.


Kind Regards,
C-Bass

 

[colemj]

Then I got confused by your wording. You stated that when you look at a target, the bearing information it gives you isn't what you expected. The bearing data sent out by a target's AIS data stream is that target's bearing at own position. It is an absolute value that has nothing to do with other ships.

If this isn't what you meant, then continue...

 

[Taniwani]

Thank you for your interest C-Bass,

Software version of the TZT2s is 6.21

For the DRS6A-NXT scanner, it shows 01.01:01.01:01.02:01.01

Regards

Harald aka Taniwani

 

[Taniwani]

Hi colemj,

thanks for trying to help, but I think AIS stations do not send out any bearing, just LAT LON and COG SOG and possibly heading. A bearing is nothing that would be specific to a single point, like the position of an AIS target. It is the angle of a line between two points and a reference line. The reference may be a line to true north, for true bearing, or magnetic north for magnetic bearings, or relative to the ships heading line for a relative bearing.

 

[C-Bass]

Good afternoon Taniwani,

It appears this topic has been brought up before. I will post a link to a similiar forum topic. Please have a look at your convenience.

https://www.furunousaforum.com/viewtopic.php?f=26&t=3664&hilit=target+bearing

C-Bass

 

[Taniwani]

Thank you C-Bass, I hadn't spotted that post. And so the answer is that I cannot be changed.
The justification of course seems a bit stretched calling this a safety feature. I for example like to look for targets with my Steiner bins and find them turning t to the magnetic bearing. But I guess doing some mental arithmetic should keep us young...