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Old Jun 11, 2009, 08:39 AM
Registered User
Joined Jan 2008
3 Posts
Question
Unstable Locosys LS20033 GPS

Hi guys!
I’ve been working on a project including a Locosys 20033 GPS, and it’s just not stable enough for me. On the attachment you see data from a completely stationary position with a lock on 5-7 satellites. As shown the longitude deviation is close to 0.025 minutes -app. 46 meters, and the latitude deviation is app. 0.005 minutes app. 10 meters. Since the GPS is intended to be used for autonomous navigation and not just information, our project is not going to work.
SBAS is enabled.
The data is from GGA, all others are diabled.
The test has been performed several times with the same result, and the presented data is just from one of these.
Anyway my question is, do you also have the same deviation, or have I just been unlucky and bought one of the faulty ones?

Thanks
Kasper
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Old Jun 11, 2009, 01:51 PM
ehx
Registered User
Northern Minnesota
Joined Oct 2001
443 Posts
First off the longitude variation isn't 46 meters. It's more in the 25 meter range (assuming you are at ~56 degrees latitude).

Remember that these units are cheap in all senses of the word. They are optimized for high sensitivity and update rate and low cost and small size. Something has got to give which is accuracy and resistance to multipath.

In ideal receiving environments the Locosys might have typical horizontal position errors in the 6 meter range whereas some of the receivers that are better designed for accuracy like those from Ublox and Garmin will be 2 to 3 times better.

Five to seven sats doesn't mean anything if the geometry isn't any good. It's possible you had poor geometry primarily affecting the longitude, but multipath may be more likely the problem. The Locosys is quite susceptible to reflections off buildings.

So, tested in a fairly open area, but with buildings (especially metal or metal-sided) around I've seen worse from the Locosys. In an open field with no obstructions around (and good sat geometry) you should do better.
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Old Jun 11, 2009, 03:58 PM
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Netherlands, NH, Edam
Joined Jul 2004
5,535 Posts
Well, I'd have to agree with ehx here, you get what you pay for. Having said that there's a few important items to check off your todo list to make sure the GPS is working under optimal conditions, one of which is a clean power supply, another is keeping the GPS (antenna) clear of stray EM field or RF from external sources. Locosys has a document detailing which frequencies affect the GPS sensitivity and you'll be surprised to find a lot of out of (GPS)band frequencies in there.

Cheers,

Sander.
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Old Jun 12, 2009, 02:04 AM
Does it fly?
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Melbourne, Australia
Joined May 2007
844 Posts
If you want higher accuracy, you should really get something with a bigger antenna. The LS20033 has quite a small antenna. Take a look at it's bigger brother maybe, the LS20031.

I get much better results from my LS20033's, so like the others are saying, you probably have some interference, of the antenna doesn't have a clear enough view of the sky.
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