Furuno SCX 20 Satellite Compass

[kokajambo]

Originally Posted by Tugwit

The SCX, however, measures heading without using the earth's magnetic field. Instead, it uses signals from GPS/GNSS satellites. As I understand, it effectively measures the subtle time difference between arrival of a GPS/GNSS signal at two independent receivers affixed to a boat but separated by some distance, and, through prodigious calculations, computes how the boat must be oriented (heading) to observe that time difference because it knows the location/direction of the satellite that sent the signal.

Thank you for an excellent explanation of the sattellite compass principle, it makes much more sense now. It actually triggered me to read more about them and I might want to make one of those myself...

Originally Posted by Tugwit

I also scrupulously followed Airmar's calibration instructions--many times, in fact. I've been out at 5 in the morning on high tides and low tides, not a breath of wind, and ran 180-degree/minute circles until my crew got dizzy and threatened to throw-up. I've run so many circles in the local bay that the boys in the club bar were taking bets as to whether I'd lost my rudder or lost my mind. To avoid any local magnetic anomaly, I've run calibration circles in half the bays between Olympia and Cape Scott. No joy. I've never been able to reduce the deviation to less than 15 degrees on some courses.

The above sounds similar to my experience with GH2183 calibration routine, except it did perform satisfactory in the end. Admittedly, I didn't measure exactly how many degrees of deviation off the known course it took to satisfy me, but 15 degrees sounds like it would've pissed me off big time. I'll go ahead and run some tests on my Airmar when I'm on the boat next time and will share the results.

[kokajambo]

I'll go ahead and run some tests on my Airmar when I'm on the boat next time and will share the results.

So, I've made it to the boat the other day and took a few readings from compasses. Here's my approach:

0. Get out of the marina.
1. Pick a landmark or a large aton about a mile away that's marked on the chart and is clearly visible.
2. Use tiller pilot to turn boat to point as close to that landmark/aton as I could. Record a reading from tiller pilot screen as TP.
3. Check the reading from a regular marine compass (the one floating in a ball of liquid installed in a bulkhead). Record as MC.
4. In OpenCPN, determine bearing to the said landmark (by pointing cursor at it on the chart and checking the reading in the right bottom of the screen). This one is based on my GPS position and known location of the landmark, not actual compass, so this is a reference reading. Record as BNG.
5. In OpenCPN, determine current heading: hover a cursor over the end of the red heading line (not the COG line!) that's sticking out of the boat icon. The boat is rolling a bit, so the heading line drifts left and right a couple degrees. I picked approximate average (while making sure that the bow of the boat still points at the landmark), check the reading in the right bottom on the screen and record as AM (Airmar).
6. Repeat 1-5 for each selected landmark/aton.

BNG AM TP MC
294 292 282 279
180 178 161 160
103 103 086 090
316 313 300 300
340 341 324 325
268 269 254 250

Airmar seems to be within 2-3 degrees from the reference, which is close enough that I never noticed the difference.

So, because I was taking this samples from a moving boat, I expected them to be all over the place, and was somewhat surprised by how consistent the difference between the readings turned out.

A bit more context on the setting:
- My home port is Edmonds, WA and that's where I calibrated Airmar after installing it, and that's where I took the above samples.
- For my regular marine compass I never bothered to make a correction table because I hardly need it here in Puget Sound.
- My Tiller-pilot is a Raymarine ST2000 and I did calibration near Point Roberts before I sailed from there about a year ago. Clearly, it needs to be done again. I haven't noticed the problem before because again in familiar waters I navigate by line of sight anyways, and it keeps the direction okay regardless of what the numbers say.
Now that I've seen the deviation though, I might not be able to unsee it again, so you can probably catch me running circles in the ferry lane sometime next week...

[Artemon]

Originally Posted by Tugwit

I just installed an SCX20 a couple of weeks ago to replace an infuriatingly inaccurate Aimar H2183 (dunno why the otherwise well-regarded 2183 never worked properly on my boat--maybe the 2 ton cast-iron keel ).

Due to an ongoing hatch replacement project, I haven't been away from the dock to perform a sea-trial yet, but the heading data provided by the SCX20 is spot on for the position of my boat in its slip. Not so for the 2183.

After some initial difficulty, I used my Actisense NGT gateway to run the Furuno SC Setting Tool to provide the necessary configuration coordinates for the SCX20. While I don't have a particularly large N2K network, it was necessary to disconnect many of the devices from the network to provide sufficient bandwidth for the Setting Tool to work. Once that was done, however, the tool worked well. (I've never experienced a similar problem with configuration software from other vendors, e.g. Airmar Weathercaster, Maretron N2KAnalyzer, etc, but there you have it...)

Hopefully, I'll be out on the water in the next couple of weeks and can provide a more comprehensive report later. I expect to use the heading data for better autopilot control and sailing performance data--not so much MARPA or fathometer corrections....

Thank you for the valuable input
I just purchased the SCX-20 and I'm getting ready to dive into the setting tool.
Yianni

[liberated]

Hoping Tugwit can provide us with more color on his experience with the SCX20.

I plan to install the system on my cat - coupled with a B&G Zeus3 system.

I see this as a huge benefit in certain cruising situations - like reef fishing.

In my former life as a powerboater/fisherman I found the compass in my Garmin7612 to be maddening - the heading shown on the chartplotter drifted randomly - making it difficult to hover over a spot, or to attempt to anchor proximal to a reef. The idea of having a true heading indicator seems like a great value. Particularly if you are motivated by reef fish.

[s/v Jedi]

I have both the Airmar H2183 and the Maretron SSC300. No problems with calibration of either one and biggest difference between them is 1 degree.

If you have a 15 degree deviation then obviously the calibration wasn’t done properly or the unit is malfunctioning.

Even though these units work great for me and are more accurate than any of my systems need, I wouldn’t hesitate to buy a $1,000.- satellite compass instead. The only reason I don’t have one is because they were much more expensive at the time.

[Tugwit]

Originally Posted by liberated

Hoping Tugwit can provide us with more color on his experience with the SCX20....

The SCX20 continues to perform very well, providing very accurate headings. In my cruising area, Puget Sound, it gives a much more reliable indicator of current set when transiting narrow passages during flood and ebb.

A secondary benefit I hope to experience sometime is additional positional accuracy due to its reception of GLONASS satellites, something my venerable Maretron GPS does not provide. Unfortunately, plans to cruise the northern British Columbia fjords have been postponed due to the pandemic. That said, the SCX20 has demonstrated the best positional accuracy of all the GPS receivers on Grendel, and it is now configured to be the highest priority source for that data on my network.

Again, I currently fly without radar so I can't speak to its improving MARPA performance, nor do I have a sophisticated sounder, so cannot assess the benefit of heave on sounder performance.

Overall, however, I'm delighted with the installation!