Radar/Chart Plotter - which way up?

[tome]

Very few, if any modern radars carry any video cables. The signal is processed in the scanner head and therefore cable lengths are inconsequential. Even in the older sets where video is conveyed via coax, the radar signal is generated in the magnetron within the scanner and the return is processed there also. The video cable length is insignificant.

Nothing wrong with having a Y cable to 2 locations.

 

[tome]

Re: Please explain

The only way to make the radar go north-up (or course-up, as opposed to head-up) is to fit a fast compass so that the radar can resolve true bearing of targets as they are acquired. GPS won't give this info- only historical cmg which is useless.

Plain old vanilla radars give relative bearings of targets only, so you need to keep a steady course or plot targets against boat headings to make sense of them.

 

[Vara]

Re: Turning maps round

Oh god I'm about to disagree with TCM.

Agree if your doing blind navigation from chart table and relaying instructions to hapless helm,north up is the way to go.
BUT if your using chart to relate to whats on the ground as you might do in a pilotage situation then orienting map/chart to route covered is sensible as then you can relate points on map directly without having to do an extra conversion in head.People who don't turn map to face the way they are going are very prone to getting left and right mixed up.
When I did my basic map reading a lot of time was spent orientating the map to the terrain.
Remind me not to navigate in a car for you "cos" on a winding road the map is going round like a demented carousel!

 

[ParaHandy]

Re: Please explain

worth reading this thread here and scroll down to PeterB's contribution about stabilised displays ...

as for t'other problem, unless you have a compass eg autopilot on the seatalk bus, the radar won't get heading data to go north up but that's too obvious init?

 

[pvb]

Stabilised EBLs...

The PeterB post you referred to seemed to suggest that all manufacturers' EBLs are fixed with respect to the ship's head. From my (limited) experience with the new C-series, I'd agree that whilst the default EBL setting is relative to the ship's head, it can be simply changed to a compass bearing if compass data is available to the unit.

 

[Oldhand]

It was professional advice from a Raymarine dealer/installer that a Raymarine radar installation required the cable lenths to be kept similar. My Raytheon/ Raymarine installation has a co-ax video cable, and my previous boat's Furuno installation had a co-ax video cable.

 

[Oldhand]

Re: Please explain

Raymarine radars can receive heading from either of 2 sources Seatalk or NMEA. Seatalk can only provide heading data updates at a maximum rate of once a second whereas NMEA heading data can be received 10 times a second or more with a suitable input. Seatalk should only be used if the compass output is directly in Seatalk (without conversion from MNEA) otherwise NMEA is the preferred source.

Not quite so obvious in respect to the source!

 

[tome]

Re: Please explain

To be honest, I think there was some misunderstanding here but didn't have time to respond to PeterB at the time.

Typical radar scanner rotates at 20 - 30 times per minute or around 180 degrees per second. A boat can turn 20 degrees per second. The scanner beamwidth (on the boats we sail) is typically 5 degrees.

In clear viz we'll use a handbearing compass and a visual alignment on the target to see if there's a risk of collision over time.

With radar, we have to convert a relative (ships head) radar target bearing to absolute target heading before we can make any decisions on collision risk. To do this we need a fast compass. Even at 10/sec there's a potential lag of 20 degrees between the radar target acquisition and related compass update.

For my peace of mind I'd want a 20Hz compass and a direct NMEA connection before I trusted it. Oldhand makes a good point about Seatalk. Don't feed your compass via a Raymarine NMEA/Seatalk bridge- go direct NMEA.

 

[ParaHandy]

Re: Stabilised EBLs...

hmm yes but, when you turn with the radar in north up mode does the EBL also turn whilst displaying, of course, the new bearing? To lay the EBL on a target you were looking at on your previous course, you have to turn the EBL to it.

 

[pvb]

Re: Stabilised EBLs...

If it's in default mode, the EBL is relative to the boat's heading, so if the boat turns, the EBL's real direction will change. Suppose the boat is moving north, and the EBL (relative) is set to 30 degrees; if the boat changes course to 20 degrees, the EBL will then be aligned to 50 degrees.

If you've selected the EBL to align to a compass heading, if the boat turns the EBL will maintain its same compass heading. So a target which was on the EBL immediately before the turn will (assuming a quick turn) still be on the EBL immediately after the turn.

 

[ParaHandy]

Re: Please explain

Doing this longhand!: if the radar "paints" a picture every 2 seconds, and if heading data is supplied once every second, the data presented by the screen is a maximum of a second old. If the heading is changing at a rate of 20 degrees per second, the positional error is a maximum of 20 degrees. With a heading update rate of 10hz, the positional error is 2 degrees? The error through the beam width will likely be constant? Hoo ahm ah daeing?

The mathematics are independent of the rotational speed of the radar which is fine if the target doesn't move or is far enough away that the change in bearing over 2 seconds is not great which is a reasonable supposition in practice.

Mind, I have to say that if the boat's turning at 20 degrees per sec, the puir fecker charged with looking at the screen is likely to be hinging oan fer dear life rather than recalling the finer points of the mathematics !!

 

[ChrisE]

Re: Please explain

You needn't worry about all this, in your tub (er yot) you'd be lucky to turn at 2 degrees a minute. Think of yourself as a liner then you'll be OK. Hope this helps.

 

[ParaHandy]

Re: Stabilised EBLs...

re last paragraph: you sure ? that directly contradicts Peter's point?

 

[ParaHandy]

Re: Please explain

thon's called directional stability which youse woofters without a long keel won't appreciate ... !!

 

[tome]

Re: Please explain

I think we need an animated rotating diagram to explain this. I'll have a go when I've a dull moment in the office, which is fairly frequent!

 

[pvb]

Re: Stabilised EBLs...

Yes, I was drawing attention to the fact that PeterB's assertion, based on talking to stand staff at LIBS, isn't necessarily true. As we all know, there's a massive lack of knowledge at shows - if you really need to find something out, your best hope is to check the owner's manual of the equipment. Stand staff often don't know, and many don't care.

 

[ParaHandy]

Re: Please explain

go on ... you'll just stitch a few bits together ... I used to use pipecleaners but nobody smokes the things these days ..

I nearly got stabbed by Jimi with his sail stitching needle as he set about his posh sail last weekend. he's full-time member of the solent seamstress and stichers WI nowadays ...

 

[ParaHandy]

Re: Stabilised EBLs...

oh ... you're right. What Peter was told was correct. its how he interpreted it that's wrong .....