CPA on Radar

[KREW2]

I am updating my radar and plotter. I have removed the older one a Raymarine C80 mfd and pathfinder radome.
Someone on ebay is interested and has asked me if it has CPA function. I know it does, but will it work without a heading sensor?

 

[srm]

I am not familiar with your specific instrument models. However, as a general principle CPA can be calculated on a radar system without a heading sensor. All the heading sensor does is give a bearing value to the heading marker. Without it the heading marker is zero and usually displayed to top centre of the screen. All bearings are then relative to the ships head.

 

[ithet]

I assume you are referring to a MARPA track? I am not sure if my C70/Pathfinder combination gave a CPA for MARPA derived tracks but in any case they are are not available without a heading sensor.

Or are you referring to AIS targets which are nothing to do with the radar and C series does show the CPA for those.

 

[srm]

I assume you are referring to a MARPA track? I am not sure if my C70/Pathfinder combination gave a CPA for MARPA derived tracks but in any case they are are not available without a heading sensor.

MARPA stands for "Mini Automatic Radar Ploting Aid", as opposed to full ARPA used on commercial sets that can track a higher number of targets. The whole purpose of a radar plotting aid is to give CPA and TPCA of a target without the need for a manual plot.

You are correct that automatic plotting aids need a heading input to be fully functional. However, if no heading (or speed) input is available the display reverts to relative motion and projecting the relative track of a target towards the centre of the screen (own ship) gives CPA. Just as in the visual collision regs a constant bearing and decreasing range indicates a risk of collision. The problem with interpreting a relative motion display is that the relative bearings on the screen appear to change as the vessel's head changes, but the heading marker remains in the same position on the screen.