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Old 25-07-2011, 08:42   #1
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GPS Compass

When Manimal was developing Gippy he suggested that it shouldn't be too hard to implement a GPS compass using two receivers. I'm no computer maven but it seems to me that if one receiver served as the base station and the other showed up as a waypoint, the azimuth from the base to the waypoint would be an excellent true azimuth reference. It might even be good to 0.1 degrees, far better than we can steer and should be available on the NMEA 0183 sentences. Manimal even left some code for the same. It would be a lot cheaper than the OTC versions running $2k at the moment and would provide a redundant GPS besides. Has anybody tried to do this?
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Old 26-07-2011, 13:50   #2
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Re: GPS Compass

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When Manimal was developing Gippy he suggested that it shouldn't be too hard to implement a GPS compass using two receivers. I'm no computer maven but it seems to me that if one receiver served as the base station and the other showed up as a waypoint, the azimuth from the base to the waypoint would be an excellent true azimuth reference. It might even be good to 0.1 degrees, far better than we can steer and should be available on the NMEA 0183 sentences. Manimal even left some code for the same. It would be a lot cheaper than the OTC versions running $2k at the moment and would provide a redundant GPS besides. Has anybody tried to do this?
I don't know enough of the science to know if it would work. But here's my thoughts about why it's possible it would not...

If a GPS is accurate to 2 meters...
2 GPS is at either end of a 50 meter boat
total possible of 4 meters off

Someone better than me could do the math, but I don't see how it could be very accurate, especially on a 36' boat.

HOWEVER: Maybe the WAY a GPS could be off means they would be off the same direction and same amount for GPSs in the same general location.

Would they need to be the same brand, model? What would all the caveats be?

Basically, what are all the variables and what would the accuracy be for different conditions?

I don't know. To me it's a very interesting theoretical problem.

-dan
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Old 26-07-2011, 14:00   #3
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Re: GPS Compass

there are several commercial companies...Furuno and Simrad have been making them for almost a decade...
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Old 26-07-2011, 14:29   #4
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Re: GPS Compass

I would be hesitant to rely solely on a GPS compass. During an overcast night passage headed south along Everglades National Park, I tried to turn east into Ponce DeLeon Bay.

Long story short- between outgoing tide, no stars, no shore lights, fatigue of going 30 hours non-stop in human-powered craft, the fact that there is a 3-4 second delay on a GPS compass when you change headings made me become disorientated. Fortunately, a hole appeared in the clouds (Diving Intervention?)- I saw a star and got re-orientated.

Since then I always have an old fashioned compass. Would the delay be as noticable on a 30+ foot boat? Maybe not, but I will not take that chance.
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Old 26-07-2011, 15:28   #5
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Re: GPS Compass

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I would be hesitant to rely solely on a GPS compass. During an overcast night passage headed south along Everglades National Park, I tried to turn east into Ponce DeLeon Bay.

Long story short- between outgoing tide, no stars, no shore lights, fatigue of going 30 hours non-stop in human-powered craft, the fact that there is a 3-4 second delay on a GPS compass when you change headings made me become disorientated. Fortunately, a hole appeared in the clouds (Diving Intervention?)- I saw a star and got re-orientated.

Since then I always have an old fashioned compass. Would the delay be as noticable on a 30+ foot boat? Maybe not, but I will not take that chance.
a gps compass is different than using the compass mode of a single gps
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Old 26-07-2011, 16:06   #6
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Re: GPS Compass

A GPS can determine direction by comparing locations. That is how it knows your course and speed, by comparing multiple positions as you sail, and doing some averaging to reduce errors. As dacust pointed out, using two recievers only a boat length apart would likely be subject to big errors due to the position uncertainty of the two points that are relatively close together.

Many GPS devices have a built in electronic compass. However, as I understand it, this is actually a separate device (a fluxgate compass). It reads the earth's magnetic field just like a conventional compass does, only with electronics. My little Garmin hand held GPS that I use in the back country has one, and it seems about as accurate as an ordinary handheld compass. Larger and more elaborate electronic compasses are no doubt more accurate.
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Old 26-07-2011, 16:15   #7
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Re: GPS Compass

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there are several commercial companies...Furuno and Simrad have been making them for almost a decade...
(Disclaimer: This information comes mainly from an article from 2009.)

Yes, but they don't work by taking the position information from two receivers and deriving the heading.

They work by timing the radio waves from multiple satellites and calculate them that way. This is information not available from any standard GPS on the market. For them, it's the length of the radio wave itself that determines accuracy. And since the radio wave is only about 7.5", the commercial GPS compasses have antennas as close as 20" apart.

Given the technology they use, I still don't see how you I could take two standard GPS receivers and write computer code to get a compass heading that is very accurate.

Factoids: the commercial units promise about .8 degree accuracy and go for about $2k.

-dan
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Old 26-07-2011, 16:22   #8
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Re: GPS Compass

read here... http://www.psicompany.com/man-prod-i...av/sc110br.pdf

extremely accurate and fast
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Old 26-07-2011, 16:24   #9
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Re: GPS Compass

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Originally Posted by dacust View Post
(Disclaimer: This information comes mainly from an article from 2009.)

Yes, but they don't work by taking the position information from two receivers and deriving the heading.

They work by timing the radio waves from multiple satellites and calculate them that way. This is information not available from any standard GPS on the market. For them, it's the length of the radio wave itself that determines accuracy. And since the radio wave is only about 7.5", the commercial GPS compasses have antennas as close as 20" apart.

Given the technology they use, I still don't see how you I could take two standard GPS receivers and write computer code to get a compass heading that is very accurate.

Factoids: the commercial units promise about .8 degree accuracy and go for about $2k.

-dan
fair enough...guess skipped over the last sentence of the OP....
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Old 26-07-2011, 17:57   #10
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Quote:
Originally Posted by psneeld
a gps compass is different than using the compass mode of a single gps
My bad.... Thanks for the education!
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Old 27-07-2011, 09:35   #11
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Re: GPS Compass

FWIW:
The 3011 is a low cost GPS compass suitable for many dredging / construction / offshore / hydrography applications. The unit provides accurate position and heading data, as well as providing pitch or roll information and accurate velocity computation ...
The 3011 GPS Compass measures the vessel's heading with a precision of 0.5 with the standard antenna pod ...
To accommodate specific installation the 3011 base line (distance between the antennas) is variable as
the two antennas can be set apart up to 2 meters
The 3011 GPS Compass measures the vessel's heading with a precision of 0.5 with the standard antenna pod ...
http://www.canalgeomatics.com/produc...011_EN_l_1.pdf
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Old 27-07-2011, 10:30   #12
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Re: GPS Compass

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...two antennas can be set apart up to 2 meters
The 3011 GPS Compass measures the vessel's heading with a precision of 0.5
Kinda what I figured. If the designer has control of the two receivers she would probably not simply calculate two positions and do the trigonometry, but rather compare the phases of signals from the same satellites on the two antennas...and then do some horrendously complicated calculation to get a very accurate heading independent of position errors. Actually vaguely similar to the old RDF.
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Old 27-07-2011, 13:26   #13
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Re: GPS Compass

Wow Gord - inexpensive??? The thing is a Magellan 31000 which retails for about $7500.00! Thery make them a lot cheaper but still around $2k.
The real question here is where do the GPS errors come from? If they come from all the rigamarole of transmission, atmospherics etc., then they should be the same in both receivers and the track from one to the other should be good even if the positions are off (by the same amount).
I am aware that the highest quality comes from interferometric processing of two or more signals. I saw the Ashtech system they put on a DC3 about 15 years ago that was good in pitch roll and yaw to a milliradian. The very fact that the systems can do that shows that the errors are the same for receivers that are close to each other and that a simple two hockey puck system with a little software help should work.
I just need one of the OpenCPN geniuses to build a little program that can take the forward GPS signal and enter it into the program as a way point then take the track calculated by openCPN from one to the next as a heading and show it on the screen. The only problem I can see is whether openCPN can calculate positions down to about 1 cm without rounding errors. If they can then a simple compass should work. An error of 1 cm in a length of one meter is an angle of about half a degree. Good enough for sailboats.
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Old 27-07-2011, 13:36   #14
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Re: GPS Compass

Just another quick comment. It may be necessary to synchronise the two GPS receivers so that they take their fixes at the same time. If they are a half second apart they probably will see different errors.
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Old 27-07-2011, 14:06   #15
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Re: GPS Compass

Usually, GPS receivers take their fixes precisely on the second: they are synchronised by the signals they receive from the satellites. Two identical receivers should take exactly the same time to compute a fix.

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