MARPA

[jimh]

Anybody know if I need to have a 'gyro compass' in order for MARPA to work properly? - I have a Raymarine SL70 CRC with GPS receiver and whenever I acquire a MARPA target the course and speed of the target seems to continually fluctuate??

 

[jimg]

You need to be on a constant heading and speed and the target needs to do the same for it to work well. I find the same to a certain degree, it is a problem in a small boat when you do not have a stable platform. I find on a target of a large ship that it does change quite a bit and does not give anywhere near a sensible reading for a several seconds.

 

[hlb]

What the hell is MARPA and why should one need it??

 

[jimg]

Luddite!
It is a fancy gizmo on modern radars that tell you the speed, course of targets and how close to you them may come. Stands for Mini- summat!

 

[tome]

Short answer yes.

MARPA cannot maintain lock without a fast compass so will be next to useless with a standard compass which only outputs once per sec. You need closer to 10/sec which is why you need to invest a good few more sheckels.

 

[jimh]

so the GPS heading and speed data isn't enough? - even if I try to steer a straight course?

 

[tome]

No, gps only gives historical course over the ground, not actual present heading. MARPA needs to know precise heading as it acquires each target on each scanner rotation which requires fast compass. Even when you try to steer a straight course you will wander enough to throw MARPA off.

 

[hlb]

So it's utter crap, thort so. Can I go back to sleep now!! /forums/images/graemlins/smirk.gif

 

[tome]

No, stay awake.

It's only utter crap if you don't fit a fast compass. Smacked wrists. Now you can go back to sleep.

 

[hlb]

But my radar already tells me where everything is and if it's going to hit me. But I usualy just go the other way anyway!!

 

[Oldhand]

Without decent radar heading stabilisation your radar is not going to tell you "if it's going to hit me" to any degree of certainty at all. Have you tried manually plotting targets on a radar plotting sheet while you are sailing (the recognised alternative to having ARPA)? Due to the variation in bearings resulting from the changes in your own heading in a seaway, even manual plotting is compromised. Anyone who is seriously going to use a radar for collision aviodance (isn't that what you have the radar for in the first place?) should make the effort to have it heading stabilised, now it is relatively easily achieved.

The Raymarine Fast Heading Sensor is a logical choice for a vessel already equipped with a Raymarine fluxgate compass and autopilot. However, Furuno also produce a well thought of high speed rate enhanced compass and Simrad's excellent RC25 rate enhanced comapss will provide fast heading output if used with a suitable Simrad autopilot box.

I strongly suggest that having raised the subject you should give it more serious thought. /forums/images/graemlins/tongue.gif

 

[smurfer]

Marpa is very easy to use and will overlay the target with a heading vector and leave a ghost image of the old position from 3mins to 6mins before. So you can really see whats going on.

This system has got me out of trouble countless times and enabled me to travel faster than I would otherwise, if you can get it working it's well worth the effort.

 

[tome]

Also worth considering the Honeywell HMR3000 Digital compass module which we've been evaluating. Very stable, even at large angles of pitch and roll with output up to 20Hz. Uses 3-axis magnetometer in place of normal fluxgate technology. It knocks the spots off the fluxgate we supply and once we can waterproof it we'll make this our new standard.

Available in UK from TDC at about 500 squids.

 

[jimh]

to be honest - i haven't needed to use MARPA (or the manual paper based ARPA you mention) in the last 2 years since I took ownership of my present boat. Exceptions are our March 2003 delivery trip from Hamble to Ipswich - very poor vis (un-forcasted) off Brighton (wound down the speed and kept radar lookout) and a couple of summer afternoons last year (same as previous).

Our first trip this year - Ipswich to Dunkerque (last weekend) had pretty bad vis over the TSS off Ramsgate and I tried using the MARPA - however this gave target courses which I knew were inaccurate - so now I understand that to use MARPA effectively would need me to invest in the gyro rate stabilised compass and sensor (£500ish)

so to answer your question - I now (thanks to the forum as ever) know the answer to my post - but maybe am not convinced that I need to rush out and spend more money to 'fix' this problem.

 

[mjf]

You can of course use relative motion and plot the target trails.

Those on a steady bearing are 'the issues'.