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DougLorenz

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Questions about GPS/GNSS units

by DougLorenz » Sat Aug 22, 2015 5:15 pm

I've had bad luck with GPS/GNSS units in the past, with slow satellite lock, losing a lock, etc. There are some questions that I've come up with that I can't find definitive answers for anywhere.

1) Is there a "best" GPS/GNSS unit to use? Fastest lock, most accurate, etc...

2) Is there a real benefit to newer units that use other GNSS systems like GLONASS as well as GPS?

3) Is there a "best" way to mount a GPS/GNSS unit for most accurate results?

4) Has anyone tried redundant GPS/GNSS units to improve results?
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pug398

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Re: Questions about GPS/GNSS units

by pug398 » Sat Aug 22, 2015 7:10 pm

Everyone gets different mileage from their devices. I am not one to recommend a particular unit but with that said I have tried quite a few different gps modules and currently one stands out in the low/mid price range for me. If you were to have a problem with it I am not sure you would ever get a reply but the "mini8" from RTFQ has shown very good performance for $35. The ext mag on it is fairly quiet as well if you should need to make use of it. To answer your second question, yesterday my LEA-6H with large high gain active antenna would barely get enough GPS sats to navigate (6-7 of them). The mini8 set for GPS/GLONASS navigated fine with 16. From what I have seen so far it is better to have high sat count/low nav rate than low sat count/high nav rate. GPS/GLONASS at 5hz works well for me and there are days when it sees over 30 sats and navigates using over 20 of them. Also I have recently sworn off using modules that have ant cables on them. I have had too many troubles with them. For mounting make sure it is highest point and nothing block view of sky to the horizon. If using ext mag it has to be away from any interference. As far as I know TL does not make use of redundant GPS yet. As far as accuracy another would not improve the position but it would be a backup in case of failure. This is just my experience and others may have different opinion.
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cedric

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Re: Questions about GPS/GNSS units

by cedric » Sun Aug 23, 2015 5:13 am

I don't have much to add to what pug said, except I'm pretty sure more sats is much better than update rate. Good data is better than fast garbage. (I'm still on my first quad, but I made a arduino self driving monster truck). So for affordable units, the Neo-m8 GPS/GLONASS should be a notable improvement, I would think.

The compass can be a problem too. I have tested the FC internal mag at all four compass points(comparing to readings from an old fashioned compass) in my quad and the calibration in the firmware works extremely well for "static" field distortions like iron and battery. I have a 4200mah battery half an inch away from sparky in my setup and I was amazed the calibration compensated for the egg shaped field. I tested this with a arduino and same compass breakout board w/o calibration and got up to 60 degrees of error between some compass points.

But there is till the field that comes from power wires to worry about, and maybe EMI too.
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DougLorenz

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Re: Questions about GPS/GNSS units

by DougLorenz » Sat Oct 03, 2015 5:46 pm

Well, it's been a few weeks since I first opened this thread. I purchased a RTFQ Neo8 module, and have not had a lot of success with it. I can get a few satellites, but not enough for a fix. I'm about to start hitting it with a rock, an act which relieves frustration, but rarely solves the underlying problem.

An overview of my GPS mounting... The electronics are inside Darth Vader's head...
20151003_103322.jpg
Darth Hex
20151003_103322.jpg (59.65 KiB) Viewed 3521 times


The top part is removable...
20151003_103501.jpg
Darth's Skull Cap
20151003_103501.jpg (48.73 KiB) Viewed 3521 times


With the GPS unit mounted to the very top...
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A view of the GNSS unit
20151003_103445.jpg (56.55 KiB) Viewed 3521 times


The head is a lightweight plastic popcorn bucket, I assume that it's pretty transparent to the radio signal.

Any ideas about this? Should I place some aluminum foil shielding below the GPS unit inside the skull cap? Don't know if that would have any effect, I really don't know why I'm not getting more satellites.

Any other recommendations for GNSS units out there?
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icee

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Re: Questions about GPS/GNSS units

by icee » Sat Oct 03, 2015 5:59 pm

If you're using uBlox + our module autoconfiguration, there's a thing in, modulesettings, which is "gpsconstellation". You can set it to both GPS and Glonass-- we only use GPS by default for now (maybe will change the default this release.)

GPS+Glonass navigation is less variable than only GPS. The best and average results with just GPS are a bit better, but the elimination of variability / worst-case behavior is nice.

Your GPS will perform better with a crude ground plane under it; and shielding from other junk is nice especially if you have a camera or VTX. Just make sure any foil you add cannot short anything.

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