Heading Sensor for MARPA

[Capn Morgan]

Originally Posted by bazzer

I don't have a Raymarine autopilot, have a very old, but working alpha system. I do have a fluxgate that i purchased used. But I obviously need some other hardware to connect it to my E80 system to make Marpa work

Sorry bazzer, i reached the end of my expertise on the subject. I just know these things because I installed a fully integrated Raymarine system on my boat, if not I probably would not even know them...

However, I believe many of the sentences that these electronics send can be read by different brands. I would ask someone if the alpha system could be connected via NMEA to your E80 just to filter out the heading signal if you were to connect the fluxgate to the autopilot. Both my old Autohelm (which gave service until the last day) and the new one (faultless so far) had the input from the fluxgate directly into the autopilot systems. I don't think you can connect it directly to the E80. There are multiplexers in the market that may be able to isolate the sentences and translate them for the E80 but this is way, way over my head.

[goboatingnow]

Originally Posted by Capn Morgan

MARPA on the Raymarine E Series setup requires a heading sensor. The fluxgate is that sensor. If you are using the X series autopilot, the heading is stabilized by a laser gyro (inside the autopilot, not the fluxgate). I believe this also to be the case on the S3G systems. The more stable the heading source, the more accurate it can be. The narrower the radar beam, ditto.

However, the results you will get from MARPA require that you understand some of its limitations. When we used to plot targets on the radar screen with grease pencils, we used to put a cross over the target every three minutes. After six minutes we would draw a line joining the crosses and figure out if we were far enough at CPA. MARPA on this system tries to give you the answer much quicker. It takes into account every time the radar beam hits the target and the rate of change. This time is just simply too short, at least in the beginning, so you have to wait quite a bit to get a fairly decent vector. Feeding it anything else, like course, would be using incorrect information. I'm not sure you could anyway.

I hope I'm not preaching to the converted here....sorry if I do.

Raymarines heading sensor is " stabilised" by solid state " gyro " which is nothing of the sort ( nor is it laser) it's simply a 3-axis accelerometer and it's outputs are used to electronically dampen the fluxgate compass.

Contary to what you say unlike your manual calulations, raymarine marpa vectors are not integrated or averaged over time. ( not to any significant effect) Hence the accuracy you get on a small yacht is directly a function of two things ( a) the ability or inability of the heading source to keep up with the constant often abrupt heading changes ( and the tx time) and (b) the processing power of the MFD to perform the required calculations and display the real Marpa vectors of the target. If these are above the required response threshold, then there's no need to perform any equivalent 6 minutes averaging as the vector would be calculated and displayed in " real time"

In reality I've always wondered if heading is really the right "metric" to use for small boats. COG is more highly damped though less accurate and there are some situations where COG is wrong. ( you and the target are experiencing significant different tidal streams). But given that small boat MARPA is very poor anyway I suspect there would be an improvement.

Better heading ( ie GPS RTK compasses), faster information flow and quicker processing will eventually improve the lot of small boat MARPA

Dave

[s/v Jedi]

For MARPA (and ARPA) and radar over chart views you need a "gyro" stabilized heading sensor with minimum 10Hz updates (10x per second). The Raymarine fluxgate that comes with the auto-pilots will not do. Raymarine is one of the few that even allows MARPA to work with the standard fluxgate; other plotters refuse this as the results are just not accurate enough.

Good sensor: Maretron, Airmar. The Simrad-Robertson auto-pilots come with a Maretron and I'm sure some other AP's do too. I think Raymarine will sell a suitable sensor too but it was too expensive when I looked at it.

cheers,
Nick.

[goboatingnow]

Originally Posted by s/v Jedi

For MARPA (and ARPA) and radar over chart views you need a "gyro" stabilized heading sensor with minimum 10Hz updates (10x per second). The Raymarine fluxgate that comes with the auto-pilots will not do. Raymarine is one of the few that even allows MARPA to work with the standard fluxgate; other plotters refuse this as the results are just not accurate enough.

Good sensor: Maretron, Airmar. The Simrad-Robertson auto-pilots come with a Maretron and I'm sure some other AP's do too. I think Raymarine will sell a suitable sensor too but it was too expensive when I looked at it.

cheers,
Nick.

Don't agree nick. The ray pathfinder smart heading sensor does 10 hz 2 degree accuracy and 0.1 resolution. It's as good as any of the leisure brands

The heading system in the smart pilots is " gyro " stabilised and the nmea output is 5hz . The seatalk connection is 10 hz.

Hence the rate gyro autopilot outputs are as good as anything else.

In my experience most MfD will compute MARPa once a " HDG" is available the better it is the better marpa. I have a satellite compass and that provided very good heading info indeed; better then any fluxgate). The processing power of the MFD still generates significant lag though.

I've always found MARPA to be almost unusable on the very conditions that I needed it in.

Dave

[s/v Jedi]

Originally Posted by goboatingnow

Don't agree nick. The ray pathfinder smart heading sensor does 10 hz 2 degree accuracy and 0.1 resolution. It's as good as any of the leisure brands

Yes, you're talking about the extra box added to the standard fluxgate... total combined price close to $1,000.- right? Somebody recently confirmed you still only get the little round black fluxgate thinghy ($170 item) with most autopilots, which is what I meant and what 99% of the owners have...

Hence the rate gyro autopilot outputs are as good as anything else.

Well, that is something a test/review would show... but the ones I've seen all put Maretron and Airmar at the top.

In my experience most MfD will compute MARPa once a " HDG" is available the better it is the better marpa.

All Raymarine units do that yes. Units like Furuno really require 10Hz sensor input before ARPA is enabled.

I have a satellite compass and that provided very good heading info indeed; better then any fluxgate). The processing power of the MFD still generates significant lag though.

I've always found MARPA to be almost unusable on the very conditions that I needed it in.

I have a 10Hz gyro-stabilized heading input from a Simrad Robertson AP into the old Raytheon C80 radar/plotter and both ARPA and radar overlay on chart both work good enough. But I know what you mean with the lag you mention.

Very soon I will be able to report how the Furuno MFD-12 will do. As it has so much more processing power, plus true ARPA, I think it'll be a significant improvement. I will feed it data from the Maretron sensor.

ciao!
Nick.

[Capn Morgan]

Originally Posted by goboatingnow

Raymarines heading sensor is " stabilised" by solid state " gyro " which is nothing of the sort ( nor is it laser) it's simply a 3-axis accelerometer and it's outputs are used to electronically dampen the fluxgate compass.

Contary to what you say unlike your manual calulations, raymarine marpa vectors are not integrated or averaged over time. ( not to any significant effect) Hence the accuracy you get on a small yacht is directly a function of two things ( a) the ability or inability of the heading source to keep up with the constant often abrupt heading changes ( and the tx time) and (b) the processing power of the MFD to perform the required calculations and display the real Marpa vectors of the target. If these are above the required response threshold, then there's no need to perform any equivalent 6 minutes averaging as the vector would be calculated and displayed in " real time"

Dave

Hi Dave

I may be using the wrong terms here, the solid state accelerometers are, for lack of other terms perhaps, called gyros, mainly because they are strapped platforms that measure those accelerations in all three axis. They used to be called laser gyros though in reality they were diodes in the early days. My terminology may be quite out of date.

However, two comments on the vectors. One is they do improve in quality over time after the target capture. The other is that a relative vector is necessarily calculated as a function of time. I do not know what time they use, probably quite short. But it needs to be averaged over time to smooth out errors in bearings or the fact that in some turns the radar may not pick up the target at all.

I agree that it is not as useful as I first thought. Mark I eyeball is my preferred method.

Cheers

[goboatingnow]

I may be using the wrong terms here, the solid state accelerometers are, for lack of other terms perhaps, called gyros, mainly because they are strapped platforms that measure those accelerations in all three axis. They used to be called laser gyros though in reality they were diodes in the early days. My terminology may be quite out of date.

However, two comments on the vectors. One is they do improve in quality over time after the target capture. The other is that a relative vector is necessarily calculated as a function of time. I do not know what time they use, probably quite short. But it needs to be averaged over time to smooth out errors in bearings or the fact that in some turns the radar may not pick up the target at

Solid sate 3 axis are not gyros in the true sense of the term,( ie not true north seeking) Laser Ring gyros are very different devices more akin to the solid state accelerometer.

My experience with MArpa on Simrad and Garmin and Ray is that there is no averaging over any significant time. The vector doesnt stabilise over time.

In you think of it, it cant, The time constant of the calulations has to be short enough to handle real yaw in both the target and you. ( worst case marpa tracking a small boat on a small boat). If the averaging is too long the vectors could be completely misleading. Remember teh manual grease pen approach ( I am a RYA radar instructor) effectively uses the human mind to do some serious averaging.

APRA or MARpa performs many more calulations than the human CPA/TCA calcs, as it attempts to plot "real time" vectors.

To compare the two , try comparing the Marpa computations with teh AIS vector calulations. Without a fast processor and a good solid HDG value, teh Marpa vectors are often complete fabrication.


BTW Nick, I think Nobeltec is going the same version as Maretron and seems to be consistently cheaper. Raymaines smart heading sensor is about $700 addon. The new Ray autopilots have it all built in now. The SMart heading box, can be added to old autopilots to rate stabilise the sensor.

Equally I wouldnt overstate the effect of "gyro" stabalising on fluxgates.

The Honeywell HMR3600 stabalised compass module , is about $350 and gives 0.5 degreee accuracy at up to 25hz. Interesting to mount in in a box and test it.


Dave

[Capn Morgan]

Originally Posted by goboatingnow

Solid sate 3 axis are not gyros in the true sense of the term,( ie not true north seeking) Laser Ring gyros are very different devices more akin to the solid state accelerometer.

My experience with MArpa on Simrad and Garmin and Ray is that there is no averaging over any significant time. The vector doesnt stabilise over time.

In you think of it, it cant, The time constant of the calulations has to be short enough to handle real yaw in both the target and you. ( worst case marpa tracking a small boat on a small boat). If the averaging is too long the vectors could be completely misleading. Remember teh manual grease pen approach ( I am a RYA radar instructor) effectively uses the human mind to do some serious averaging.

APRA or MARpa performs many more calulations than the human CPA/TCA calcs, as it attempts to plot "real time" vectors.

To compare the two , try comparing the Marpa computations with teh AIS vector calulations. Without a fast processor and a good solid HDG value, teh Marpa vectors are often complete fabrication.

Dave

There you are. The gyros are not really gyros, we just call them thus probably because of the old gyros used on ships. It would be interesting to know why the manufactures do not derive gyro headings from the accelerometers themselves. It is done, just not for this market.

Still, I don't really get your point on MARPA. When the radar sweeps a target two consecutive times, it calculates a vector based in those two sweeps. It needs a time interval between two sweeps to calculate anything. If that time is longish the vector is bound to be more precise up to a point. I just have no idea how close in time are the sweeps that it uses for the calculation. When the interval is really short the error is quite high. What the system cannot do is to calculate a vector from just one sweep of the radar, so it cannot be instantaneous, it is just one bearing and one distance.

Looking at how my system works, it takes (at a bit of a guess, I have not timed it) about 40 or 50 seconds to come up with anything resembling a final vector, but once it does it gives at least a fair approximation of what is happening. Nothing like the precission of AIS, not the same league really.

In any case, while my radar paid for itself several times since I bought it, I can still live without MARPA. Where I am, I regularly get the bows cut by fishing boats with no lights in the middle of the darkest nights. It is sometimes nearly impossible to see them with the naked eye.

Above all, thank you for sharing your invaluable knowledge.

[rcdelude]

From what I understand Marpa relys on fast heading data (bearing,magnetic fluxuations etc.) 10HZ or better. The RM fluxgate compass does provide some data but is not a true heading sensor. I finally picked up a used KVH Autocomp 1000 heading sensor on ebay for $100.00 (usually under $400.00 new). I've been doing a lot of research on this and was reassured that this unit would work w/my Raymarine Pathfinder set-up. Will be installing (hopefully) this week. This unit should interface w/ the E80 via NMEA 0183 input. PS.anyone interested in a used Raymarine/Autohelm M81190 fluxgate compass (bench tested) $90.00 plus shipping.

[Capn Morgan]

Originally Posted by rcdelude

From what I understand Marpa relys on fast heading data (bearing,magnetic fluxuations etc.) 10HZ or better. The RM fluxgate compass does provide some data but is not a true heading sensor. I finally picked up a used KVH Autocomp 1000 heading sensor on ebay for $100.00 (usually under $400.00 new). I've been doing a lot of research on this and was reassured that this unit would work w/my Raymarine Pathfinder set-up. Will be installing (hopefully) this week. This unit should interface w/ the E80 via NMEA 0183 input. PS.anyone interested in a used Raymarine/Autohelm M81190 fluxgate compass (bench tested) $90.00 plus shipping.

Hi rcdelude

I'm always interested in these things so I would appreciate if you report further when you have installed it. I see no reason why it would not work so long as the E80 can read the sentences. You say the fluxgate is not a true heading sensor, would you explain why?

Cheers

[rcdelude]

Originally Posted by Capn Morgan

Hi rcdelude

I'm always interested in these things so I would appreciate if you report further when you have installed it. I see no reason why it would not work so long as the E80 can read the sentences. You say the fluxgate is not a true heading sensor, would you explain why?

Cheers

Hi Capn, sorry about the delay. I concede my statement "the fluxgate is not a true heading sensor" is not valid. The point that I was trying to make was that the units sold by the various brands, as heading sensors utilized stabilization, damping and data processing etc. as well as heading info. The KVH Autocomp works well, but their is a bearing difference of 3-4 degrees between the chartplotter and the radar display. Any thoughts would be appreciated. RM - L755 plotter, RL70 radar. Thanks, Bob

[bazzer]

I have a E80 and I wanted to interface a fluxgate compass, I managed to find a RM fast rate gyro for a $100 so wiring in the fluxgate was pretty simple, it gave me what I wanted which was both COG and True Heading as well as good sync with radar overlaid on the plotter chart. I've also installed a RM autopilot which is a none G model. I've still to complete the full setup, but considering I've interfaced it to my old drive unit I'm pretty happy.

[Capn Morgan]

Originally Posted by rcdelude

Hi Capn, sorry about the delay. I concede my statement "the fluxgate is not a true heading sensor" is not valid. The point that I was trying to make was that the units sold by the various brands, as heading sensors utilized stabilization, damping and data processing etc. as well as heading info. The KVH Autocomp works well, but their is a bearing difference of 3-4 degrees between the chartplotter and the radar display. Any thoughts would be appreciated. RM - L755 plotter, RL70 radar. Thanks, Bob

Hi Bob

There may be a way to align the radar bearing and even calibrate the heading. The E series have it but I don't know in your setup. I first did a deviation card for my magnetic compass and then one for the fluxgate, (which when first started was about 100 degrees out!). That way I can compare both curves which is useful if the electronics pack up.

Best

M

[bazzer]

You can't calibrate the heading on any of the E series RM plotters without a fluxgate or compass input of some kind. COG will be pretty accurate when underway, but not the heading. As I already wrote in my previous post, I did this on my E 80 with a RM fluxgate and a rate gyro. Ok it might not be a gyro but a accelerometer, but it's pretty damn nice. True gyro's are mechanical devices that are old hat and unreliable.

[rcdelude]

Originally Posted by Capn Morgan

Hi Bob

There may be a way to align the radar bearing and even calibrate the heading. The E series have it but I don't know in your setup. I first did a deviation card for my magnetic compass and then one for the fluxgate, (which when first started was about 100 degrees out!). That way I can compare both curves which is useful if the electronics pack up.

Best

M

Thanks for the reply Capn. I'm rarely out of sight of land, so the deviation isn't a big deal even in fog (radar, plotter). In the event of power loss I do have an Airguide magnetic compass and charts from Maptech Thanks again, Bob