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DIY Satellite Compass???
Based on a prototype demo and Furuno claims, I believe that a compact marine satellite compass like the new SCX design can accurately and quickly measure all sorts of boat motion besides compass-type Heading. Can this also be done at much less expensive with modern DIY electronics? Is there real-world on-the-ocean evidence?
Switching this discussion over got a little confusing so I'm reposting Andy's latest here:
"Anyways, I have thought about this more, settling for single frequency and sourcing GPS units with raw timing aka carrier phase measurements from Aliexpress (many options):
And using Raspberry Pi with rtklib results in satellite compass with BOM less than 50 USD and with long enough baseline unlimited accuracy - well dimensions of boat do limit this somehow, but sub 0.1 deg is achievable in practise. As a bonus, doing this type of project teaches you how these things _really_ work and that is priceless in my opinion.
Options for DIY GNSS compass:
1. cheapest possible, external calculations:
- 2 x aliexpress sourced ublox 8T receivers with integrated antenna, sub 15 usd each
- Raspberry Pi running Rasbian linux, around 20 usd
- RTKLIB - doing the RTK calculations, free
- optional Arduino with can-shield or Yacht Devices NMEA2000 gateway or Actisense NGT-1 to interface this to NMEA 2000 -network
2. mid range, onboard calculations
- preconfigured kit with (nearly) everything
<a href=" removed link " rel="nofollow ugc"> removed link
Ublox F9 receiver inside, absolutely the best receiver available today, does all the constellations and on dual bands as well.
- optional Arduino with can-shield to interface this to NMEA 2000 -network
- or Raspberry Pi (required for interfacing) and Yacht Devices NMEA2000 gateway or Actisense NGT-1 to interface this to NMEA 2000 -network
I have had both types running, first type for a couple of years and second type now for almost a year. The dual frequency beats single frequency every way. Time to fix is seconds instead of sometimes minutes, solution is rock stable and I have yet to see a false fix. With single frequency it is a bit flaky, but works fine once you get it working.
Easiest way to try this out:
- get two of of these 12 usd unit from Aliexpress, 25 usd total including the shipping
The unit can be any model, but it has to have ublox 8T or similar receiver that outputs raw GNSS messages, these are required for carrier phase measurements
- connect them to any laptop via two usb ports or get an usb hub
- set them say 1-2 meters apart. More is better. Sky has to be visible 360 degrees, boat is an ideal place to try, backyard of a house not so ideal.
- configure them to output raw gnss messages with free Ublox U-Center sowtware. Also select constellations to be used
The relevant messages are called UBX-RXM-SFRBX and UBX-RXM-RAWX. Good value for testing is say 1 Hz repeation rate. It can be set higher later on when everything works
GPS + Glonass + Galileo is a good starting point. 8T can only do three out of four at one time, 9T/9P can do all four + on dual frequency also - but on higher price
- Open the connection to the receivers with Rtknavi, which is included in RTKLIB package
- Select one USB/Com-port as base and another as rover
- Select constellations and set mode to moving base
- Wait till it shows first "float" (partial solution) and then "fix" (full RTK solution)
- Now the baseline heading indicated is true heading between the units
- From Rtknavi you can set to output this data as NMEA 0183. No N2K support unfortunatelly, that has to be dealt seperately
Anyway, here is a walkthrough for 30 minute and 25 usd satellite compass. As a bonus you will learn endless amount of new things about GNSS once you dig into the details.
ps. Laptop can well be a Windows laptop for trying and evaluating, though for long term use Raspberry with linux is cheaper, more stable and uses less power
Something like a precision9 compass uses magnetic compass to determine heading which is prone to interference in normal ways that GPS compass are not affected.
The mems sensor for 3-axis orientation supplements the data derived from the GPS antennas moving in 3d space. This is useful to autopilots for rate-of-turn and 3d systems like DFF3D, timezero professional, etc.. Some sounders can use it to remove boat motion from the depth finder picture.. Going over waves makes it so you can't see if the bottom is bumpy or smooth, etc... Sometimes autopilots get misbehaving because of magnetic interference.
DFF3D has a mems type of motion sensor IN the transducer which is useful but not perfect. They will recommend a sat compass for better detail. If you're in 100' of water and your boat rocks 1 degree without the computer knowing about it, you can do the high school geometry and figure out the distance change between 60 and 61 degrees with the vertical leg being 100' ... It does make a difference.
It's a good project I suppose.. It also has to output data in a format useful... Furuno uses different sentences than Si-Tex for example for heave/roll data. So keep that compatibility in mind. But since these devices have come down so much in cost, it's attractive to just buy the new antenna.