Originally Posted by hasse_A
Hold your horses!
...To get marpa or even mina arpa working a couple of things must be in place.
I would need a high end plotter. The 721 I got does not support it and without I would have no way of R/E the thing. I have never used a radar with marpa so there would be a steep learning
curve. If I were to do it, a simulator may be needed.
I've got a 10Hz fluxgate compass
. I think that would be good enough but I'm not sure. Would you really need turn rate info ?
This page might give you the answer.
Your doing a great job, and that of course goes for Dave et al aswell.
I was just considering how it could work with marpa, I am not saying you or anyone in particular should do it. It is still interesting to explore the possibility.
Anyway, there seem to me there could be two ways. Either the target tracking is made by the hardware
in the scanner or by the control display.
If it is done by the scanner it would probably make sense to reverse engineer
it by getting a display with marpa support. That would probably be most economical in term of calculation economy and possibly also programming effort. I can't really judge whether reverse engineering or programming marpa from start is easiest, I would guess that reverse engineering it is easier for you guys that know how to do it.
But, if marpa is done by the control display I see no real reason it could not be made in a software
plug-in. It seem to me that this is (just) plane geometry. We are only talking about a 50 NM radius, so we can discard the curvature of the earth.
Marpa is about manually marking a target and let the system track it and record
its position over time. When this is done it is trivial to calculate its speed and course.
I dont think this is a very hard problem in principle even if it would take a bit of an effort to program it. It would be something like:
1. Mark target. Calculate its position.
2. Wait a sensible amount of time (1/48th of a second or longer...).
3. Check a circle around the targets position for the target. (The radius being determined by the expected speed of the target)
4. Calculate target position and log it.
The problem may be the accuracy of the target positioning. It will be greatly influenced by how much your ownship is moving and how well you can detect it. This is where a fast compass and/or rategyro is helpful.
We could make an estimate if we new the specs for your compass and compared it with the specs for Garmins rategyro. It seems it is manufactured by a subsupplier.
Airmar H2183 Heading Sensor
I have an old Silva=Nexus compass transducer, it is still very good, but quite slow since it is in essence a magnet floating in a liquid that is monitored by sensors on the outside of the glass bulb.
What is the model of your fluxgate? Specs?
I think the below specs are valid for the Garmin rategyro.
Airmar H2183 Heading Sensor - Solid State - 10 Hz
Static Compass Accuracy: 1° RMS when level
Dynamic Compass Accuracy: 2° RMS (Best-in-Class)
Heading Display Resolution: 0.1°
Settling Time: 1 second (adjustable)
Heading Data Output Update Rate:—10 Hz—NMEA 0183—Adjustable up to 20 Hz—NMEA 2000®
Rate-of-Turn Range: 0° to 70° per second
Rate-of-Turn Accuracy: 1° per second
Rate-of-Turn Data Output Update Rate:—2 Hz—NMEA 0183 (Adjustable up to 10 Hz)—Adjustable up to 20 Hz—NMEA 2000®
Pitch and Roll Range: ±50°
Static Pitch and Roll Accuracy: <1°
Dynamic Pitch and Roll Accuracy: <3°
Pitch and Roll Display Resolution: 0.1°
Pitch and Roll Boat Alignment: Yes with software
Pitch and Roll Data Output Update Rate:—2 Hz—NMEA 0183 (Adjustable up to 10 Hz)—Adjustable up to 20 Hz—NMEA 2000®
Supply Voltage: 9 VDC to 16 VDC
Supply Current: <140 mA
NMEA 2000® Load Equivalency Number (LEN): 3
Certifications and Standards: CE, IPX6, RoHS, IEC60945
NMEA 0183 Sentence Structure
$HCHDG ............ Heading, Deviation, and Variation
$HCHDT ............ Heading Relative to True North
$TIROT ............... Rate of Turn
$YXXDR ............. Transducer Measurements: Vessel Attitude
NMEA 2000® Supported PGNs
127250 .......Vessel Heading
127251 ....... Rate of Turn
127257 ....... Attitude