Radar bearing question.

[Bav34]

Elaine and I have now reached the grand old age of 100 which hopefully explains some lack of mental dexterity /forums/images/graemlins/mad.gif.

We are practising hard on radar plots as in May we are zooming off for 5 months around the bumpy, foggy bits of France /forums/images/graemlins/ooo.gif.

Last Saturday the viz. was about a mile in the Eastern Solent and we were motoring … time for some practice!! We picked up an object, zapped it with the EBL and wanted to use a hand bearing compass to get a precise idea where it was from the helm.

We were steering 150 and our radar (see other JRC 1500 post) only wants to tell me the relative bearing of the target which was 325. What bugs me now is the only way that I can think of working out his direction is to take 325 from 360 (heads up only) then take the answer from the ships heading i.e. 360 – 325 (35), 150 – 35 = 125 which was indeed where we eventually SAW the boat.

This may be ok sitting in front of a computer but it worries me thinking about trying it for real with several targets in fog when I NEED to be right. Is there some quick trick that I am missing i.e. add the first number you thought of to the last ……….. ?

How quickly can you work it out if the course was the same but the object was 035 i.e the other bow or worse if you are doing 040 and the object appears 170 ??????

360 -170 = um 190 um we’re doing 040 um do I add on or take off 040 or 190 from um errrrr ?????

Aaarrgghh my brain is hurting.

What am I missing? (Apart from intelligence)

/forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif

 

[bbg]

[pedant]150-35=115, not 125 /[pedant]

I don't have radar (or even a boat at the moment), but I think I would work it out exactly as you did. It is really only two calculations
360-325=35
150-35=115

Only thing I would add is that the relative bearing gives you a very quick indication of where to look. Eg. 170? Look over your right shoulder.

If it really is critical, you could make up a table with Ship's Heading on one axis and Radar (Relative) Bearing on the other axis and keep the table to hand for ready reference.

 

[Bav34]

tipping orror /forums/images/graemlins/ooo.gif

 

[Skyva_2]

The usual method, and I don't know a slicker version, is to add your course to the relative bearing of the target. Subtract 360 if the answer is more than 360.

So if you are doing 040 and the target bears 170 relative, his true bearing is 210.

 

[SteveB_Sigma33]

Just done my radar course and agree, or you could do what I'm just about to do and fit a fluxgate compass. That way I'll not have to worry with number trickery and can start using MARPA on my raymarine set.

 

[gandy]

I don't have radar either, but I was thinking of similar issues with wind instruments (that I don't have either).

It would be better if the instrument could give the bearing as +/- 180 degrees, where +is to Starboard and - is to port. Then the reading could be added to the ships head to give the actual bearing.

 

[Cruiser2B]

You might look at the set-up menu for your radar and be able to change the relative readings a la Gandy's observation "It would be better if the instrument could give the bearing as +/- 180 degrees, where +is to Starboard and - is to port. Then the reading could be added to the ships head to give the actual bearing. " Some can.
Otherwise you can probably get or create a widget - fixed azimuth card with a rotating azimuth card in the middle. Turn the middle card so your course aligns with N on the fixed card and then you simply line up your radar's relative brgs from the outer ring and read off the true brg on the inner ring.

 

[brianrunyard]

Example 1
Ships Bearing = 40
Relative Bearing = 50
Bearing of object = 40 + 50 = 90

Example 2
Ships Bearing - 150
Relative Bearing = 325
Bearing of Object 150 + 325 = 475
Over 360, so subtract 360 475 - 360 = 115

 

[theotter]

Are you not over complicating things for yourself? In practice (in fog of course) it is sufficient to decide where a potential collision target lies in much broader terms i.e. ahead, astern, abeam, stern quater etc etc. You would be better off concentrating on plotting what course and speed the other vessel is doing so you can make the appropriate decision to avoid collision. Now when you have a couple of plots on the go, then you can say your brain 'urts!