1935 ARP

[GB119]

Hello,
I have been working on a boat with a 1935 and ARP11 board installed.. heading and position is from a Comnav sat compass. When locking up a target and using the TTM output to a plotter, the plotted target is not at the correct lat/long. At a distance of about 1.3 miles, the target is plotted about 300’ further away. I’m aware that heading can cause an error if it is not correct, but this is not the problem in this instance. I have tried adjusting the timing on the radar and this has helped, but the problem is still significant.
Position on plotter and radar is from the same source as is heading. Plotter is pseaWindplot

 

[Taniwani]

Are you saying that the radar display shows the target at a different range than the plotter?
Or is it that both have the target at wrong range?

 

[GB119]

The plotter plots the tracked target in a different position than the target on the radar.

 

[Johnny Electron]

Sounds like the radar timing is set improper or the position/heading data being fed to the unit has an issue. If you can't resolve, I recommend you contact your dealer.

 

[Taniwani]

In order to find out where your problem is coming from, it would be good to determine if range or bearing differ or both. And by how much. Position is a product of the two, but the radar actually only measures range and bearing. Radar is usually very good on range and the timing adjustment is for the range of cable lengths, so could introduce a maximum error of about 20m if set wrong.

For the radar bearing is relative to ship heading and depends on how well it is aligned. And then heading is added which may also need adjustment.

If your radar shows a target position in LAT/LON it needs to translate range and bearing and it will need a good heading reference to come up with a proper position.

When your external plotter shows a target position in LAT/LON it does so by taking range and bearing (relative) from the ARPA record and then applies heading from maybe the same reference that radar used.

When these results differ, most often the cause is bearing, rather than range. Because range will be the same if it comes from the radar. Confusion with bearing often is between magnetic, true and relative.

What's not clear from your note is, whether the position on the plotter, that you compare to the one on the radar, is derived by some entirely other means, like AIS or traget on the chart. Then, in theory the radar range could differ from the actual range and then there may be a problem with the radar.

If you let us know some more detail, we might be able to narrow in on what your problem is and possibly resolve it.

 

[GB119]

Right TTM is range and bearing. You are spot on with heading (true, magnetic or relative) and radar heading ... this target was being plotted further away, but the bearing was correct. I suspect that the radar was showing a contact echo..... as what was thought to be a particular landmass (rock) may not have been .....its a long story