Furuno SCX20 & SCX21 satellite compasses, the impressive demo at NMEA

Ben Ellison

Ben Ellison

Panbo editor, publisher & chief bottlewasher from 4/2005 until 8/2018, and now pleased to have Ben Stein as a very able publisher, webmaster, and editing colleague. Please don't regard him as an "expert"; he's getting quite old and thinks that "fadiddling fumble-putz" is a more accurate description.

26 Responses

  1. Sweeeeet. This looks really good.

    I learned the hard way that not all satellite compasses are equally good in multipath performance, and these new Furuno ones look like they are going to be best-in-class.

    This joins the Merrimac wishlist, although the Cortex VHF still tops that list!

  2. John Sherman says:

    Yes, the SCX2x heading sensors do look good. I was just about to place an order for the Hemisphere V200n (which appears to be a fine product, too), but the SCX20 offers both heave (PGN 127252) as well as 3-axis speed (PGN 130578) which appear to be absent from the V200n. Moreover, if/when the SCX20 may be configured/updated with an Actisense NGT, that capability will facilitate maintenance, as well.
    Kees–have you shared your “hard-won” experience with other satellite compasses somewhere on-line? I’d be very interested in hearing about your experience–the good, the bad, and the ugly 😉

  3. Dan Corcoran Dan Corcoran says:

    Is a 3-axis speed of any special use measuring the performance of sailboats?

    I am confused why gps compasses look to put all the receivers in the same housing. Why not split in two, put on each end of the boat, allow the user to enter information on how far each sensor is off the centerline? The separation between receivers should make COG extremely accurate.

    • Dan Corcoran Dan Corcoran says:

      What is the compass accuracy of the DIY version? Is multipath performance an issue that is dealt with in packaging, software, other?

      Is multipath an issue for DIY users to work out, or is it already solved at the sensor or software level of the Arduino components?

      • Dan Corcoran Dan Corcoran says:

        Does DIY require that the sensors are in line with each other, or can you provide an offset between where each sensor is installed and the centerline of the object (boat) you want to determine the compass heading of?

        • Andy says:

          It does not.

          You can align sensor freely. Then you end up with baseline length and direction between antennas, or antenna phase centers to be exact, and can deduce vessel true heading from this.

          Also being RTK units you can feed external RTCM stream for outside source, either your own RTK basestation or a public free/paid service, and end up with centimeter accurate position as well. But this is another topic in itself, but this hardware can do this also. Could be used for precision docking for example, could steer the boat to exact spot in full darkness and fog. Same techinique they use for offshore drilling for example.

      • andy says:

        I would not worry about multipath that much, it helps a lot when you have so many channels available on dual frequency. False fixes resulting from multipath simply don’t happen in practise.

        Accuracy is limited by baseline length, and even few meters is enough for sub 0.1 deg accuracy I think. I think this is a non-issue.

        • Ben Ellison Ben Ellison says:

          Well, I’d like to see some evidence from real ocean use. But I’d also like to see this discussion move to the Forum where I just created a “DIY Satellite Compass” topic. Let’s please keep this entry about the SCX. There much to discuss just about this design — like the fact that it is dual-frequency — and units will soon be in real use.

        • Andy says:

          Makes sense and I agree, lets keep this on SCX.

          It seems from material this receiver is Furuno’s own design – correct?

          Datasheet states:

          1575.42 MHz (GPS/Galileo/QZSS/SBAS)
          1602.5625 MHz (GLONASS)

          This is a little disappointing, as first it is lacking Beidou support altogether, and secondly it seems it is single frequency (L1) only.

    • Chriggel says:

      It seems that the DIY version has no gyro / accelerometer sensors as backup in case of no GNSS signals, or am I wrong?

    • Ben Ellison Ben Ellison says:

      “Is a 3-axis speed of any special use measuring the performance of sailboats?”

      Special I don’t know, but wonder: Is there any measurement of boat motion — especially if delivered in a standard N2K PGN — that could not be useful to thorough performance analysis, at least stabilization of other sensor info?

    • Ben Ellison Ben Ellison says:

      Answers to Dan’s second question became an interesting discussion about how accurate DIY satellite compasses are apparently possible these days. Check it out on the Panbo Forum:


  4. Ben Ellison Ben Ellison says:

    Dan, I don’t understand the problem you are trying to solve. Compact satellite compasses deliver very accurate COG, and quite accurate Heading. Hope you realize that they are not deriving Heading by comparing two or more GPS locations. The technology is quite different from that, as you can learn in this brochure for an older Furuno model:


    I doubt there’s a DIY version of carrier frequency phase decoding tech.

  5. John Sherman says:

    “Is a 3-axis speed of any special use measuring the performance of sailboats?”

    The SCX20 outputs PGN 130578, Vessel Speed Components, providing longitudinal speed (in line with the bow of the boat), transverse speed (speed at 90 degrees to the longitudinal speed), and stern speed (not much use, perhaps, unless you’re backing down to set an anchor).

    All three components may be ground-referenced and/or water-referenced. As the SCX20 is a GNSS device (with COG as well as heading), I assume that the values we see would be ground-referenced. Compare to the Airmar DX900+ Multilog sensor which also outputs PGN 130578 but with water-referenced component values. Note that the DX900+ also outputs PGN 128000, Nautical Leeway Angle, which is water-referenced only and does not account for slip due to current.

    It seems to me that transverse speed would provide an indication of leeway. And assuming the SCX20 outputs ground-based component values, it would also seem to account for leeway due to current, as well as leeway resulting from being blown sideways. Effectively, the vector sum of the two, I would assume. Teasing them apart is another story, however, and would seem to require both ground-referenced and water-referenced component values (i.e. both devices installed).

    In earlier Panbo posts, I seem to recall an extended and animated conversation about “True wind” speed & direction–water-referenced, and “True wind” speed & direction–ground-referenced. I suspect many of the same issues would apply here, but I’m not going anywhere near that subject. 🙂

    • Ben Ellison Ben Ellison says:

      Thanks, John! It sounds like you have read the details of PGN 130578, which I have not. (And I sure wish NMEA would make detailed PGN descriptions public.)

      However, the Furuno SCX and Setting Tool manuals indicate a lot about 3-axis speed and I think you misunderstand the aft component. It too measures transverse speed, just like the forward component. And the installer is not required to configure the two tranverse speeds to bow and stern, though that’s suggested.

      In other words, if you’re backing straight down on an anchor, with no side current or wind-driven leeway, the only speed might be longitudinal (but with a down arrow). But if you apply some stern thruster, you should see the results in the aft 3-axis speed plus maybe a bit in the forward x-axis, though in the opposite direction. Conversely, if you get catawampus to the anchor when it fetches up, you might see some serious action on the forward x axis.

      Check the SCX21 3-axis speed in the demo photo near the top of this entry. The bow and stern tranverse speeds of 0.7 knot left indicate the current drift as we crossed the Elizabeth River at 5 knots longitudinal speed. I could sort of see that relative to shore objects and the situation is also verified by COG 347 and Heading 354. The boat was going left over the ground compared to where the bow was headed relative to Truth North.

      Had we also been turning a bit, the X-axis speeds would be different, maybe still the same direction, maybe opposite depending on rate of turn (also measured by the SCX).

      Frankly, I think I’m still getting my head around 3-axis speed, but I suspect that it will be very useful once a skipper gets used to it (and assuming it’s accurate).

    • Dan Corcoran Dan Corcoran says:

      Is it a fair guess that the SCX20 & SCX21 don’t agree with each other in the photo at the top of the page, because one or both compasses are pointed slightly out of alignment with the bow? What would it take, a 1 degree error at the displayed SOG of 5 kt? When working with a compass of this accuracy, is it a challenge to get the SCX20/21 aligned?

      As we know, at sailboat speeds, small changes in STW or compass error can have large impacts in calculating set & drift, before raising the sails. Once the sails go up, the numbers provided by our MFD’s become junk.

      As I think about the approaching Newport-Bermuda race, I wonder if the traverse speed could help detect subtle changes in current alone, or if that would require nautical leeway angle from the 900+. Perhaps the leeway component can be calculated (during slack tide) on any given boat and then programmed as a correction, much like we put in a correction for our wind instruments.

      • Ben Ellison Ben Ellison says:

        As I wrote, Dan, the two demo SCX sensors were prototypes, and as evident from the cabin top photo, it was a temporary install. So maybe sensor offset was the problem, but I don’t know.

        But I do know that correcting the problem of an install that is not well aligned with the boat’s bow — that is, not square to the x and y axis — is pretty easy with most any electronic Heading device. You identify the error by comparing COG and Heading — preferably at speed on flat water with no current to minimize sideways motion, averaging reciprocal runs if necessary — and apply an Offset correction. (Heading should equal COG in perfect calibrations conditions.)

        The SCX supports angular Offset corrections for Heading, Pitch, and Roll, as well as Offsets for SOG/3-Axis, air pressure, and air temp. Also Smoothing of SOG/COG, 3-Axis, and RoT.

        The 3-Axis speed is all SOG, so I don’t think that the SCX can distinguish transverse speed through the water — which I think of as a navigator’s leeway, and apart from current set and drift — from overall transverse speed relative to earth and a boat’s bow.

        I don’t know if there is any instrumentation or performance software able to work with SCX type sensor output and the transverse STW measured by an Airmar DX900+ (supposedly), but the results might be interesting!

    • Cameron Sawyer says:

      Just for clarity — there is no “leeway” resulting from current. Leeway is strictly water-referenced. Current produces set and drift, which are ground-referenced concepts. Three axis SOG from this device and three axis STW from the DX900+, if that data is reasonably accurate, would be enough to allow us to finally untangle set, drift, and leeway. Would be fantastic to have this information. I’d really love to have an instrument which could display all three of these values — would be of immense help for sailing.

  6. Ben Ellison Ben Ellison says:

    Great that Andy has continued to discuss DIY satellite compass tech over on the Forum and also that the Forum software allows commenters to put up screenshots and photos:


  7. Furuno Light Marine Product Department says:

    Actisense Driver Update: The SCX20 Software Configurator named “SC Tool” is downloadable from our www site and now includes the Actisense DLL in the download which now provides full use of the software configuration tool with an Actisense NGT1-USB Converter. Of course, the SC Tool also works with the Furuno IF-NMEA2K2 “NMEA-to-Serial” Converter if you prefer a serial connection to your Windows PC. NOTE that both the NGT1 and the IF-NMEA2K2 need to be updated to the latest software revision before using the SC Tool!!! Here is the link to the Furuno SCX20 Product Page for more information…https://www.furunousa.com/en/support/SCX20

  8. Mike Morgan says:

    Can the SCX21 be used successfully with a Comnav Commander P-2 autopilot? The G-2 sat compass from Comnav has given me nothing but trouble.

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