I was setting up my new airplane with me new APM2 and noticed that I had a very unsteady display on the Mission Planner. The artificial horizon and other parameters were jumping around a little. This was with both USB and telemetry connection.

 

I had the GPS mounted on top of the APM2. I found that if I unplugged the GPS that everything was steady. I hooked up my old Media Tech GPS and all was well. Has anyone else had this problem?

 

Next order I will have to get a longer GPS cable and relocate the new GPS to see if that solves the problem. The problem is that the new 3DR/uBlox GPS very HUGE and it’s not easy to find room for it.

 

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I have the same setup, the uBlox and 3DR on my Hexacopter. I found this little sherbet icecream container which I used to store both my u-BLOX GPS and 3DR. Looks great too! All I did was to cut a piece of EPP foam and stuffed it between the GPS and 3DR Radio. At the bottom of the foam I stuck some aluminum tape to reduce any radio interference from the 3DR  . During my Battery test, my Hexa was in loiter mode and it seems to hold its position very well. I haven't had a chance to bring my laptop to the field but at home during stationary tests with GPS lock, I don't notice any deviation in the Artificial Horizon. 

Mike, please don't take this as me trying to argue. I'm just stating some facts to help you and others. I spent a lifetime in RF and Satellite communications and just received my engineering degree. These are things I know from the field, the lab, and texbooks are true.

I measured the voltage and it was 5v

Measuring voltage only with a multimeter will not show noise. Meters have filters (especially digital ones) to average the inputs anyway. It's just not the right tool for the job.

The only way to see noise is with an o-scope that is fast enough to capture the frequency of the noise.

 I also tried adding a large filter capacitor to APM power and that made no difference.

A capacitor is not by any means a good noise filter alone. It can smooth out some low frequency power issues, but real radiated RFI on a power cable goes right past the capacitor. You need a more active noise approach and an inductor in series is key along with the capacitor. It doesn't take much, and that's why ferrite beads can work against very high frequency, but in the low kilohertz ranges, a good inductor with a few mH is what it takes.

That said, I think we all should be using a circuit like this on the power going into the APM, even if you don't think you have a noise problem.

Great page about removing noise, decoupling, and all those other things.

http://williamson-labs.com/480_dec.htm

You can't do any filtering on the GPS velocity, it would delay the readings too much. We are only getting 5 Hz updates. In fact the gps velocity vector is estemated in between gps updates at 50 Hz or what ever the AHRS refresh rate is. Just taking my unit out of the house made the problem totaly go away. BTW it was Tridge who came up with this expaination.

That's not the method used for GPS trilateration in the units we use.  That's only on $1000 commercial units.

I sure hope they're not munging good internal AHRS data with questionable, externally obtained data from the GPS. 

In fact they are. The AHRS in Arduplane 2.5 (not sure about Arducopter) uses the velocity and heading from the GPS to calculate east / west velocity vectors.They are using the baro to get vertical velocity. The 3d velocity vector is used in the DCM drift correction algorithm. I don't think there is a fall back function if you loose gps, yet anyway. It just won't drift correct without gps. Correction: It does do drift correction with out GPS, it gust freezes the last velocity vector. It must work better for taking accelerations out of the gravity vector, but it seems risky to me.

That sure seems like a bad idea.  You have to get your centripetal correction somehow, but it's scary that they're getting this problem with near zero velocity.

For what it's worth - I have same problems with my APM2.5 + LEA6H's.

Tested (outdoor) USB powered - 8GPS, very solid fix - still the AHI jumps around a little.

Tried a GPS cable of twice the length of the original/included one - no big improvement.

Tried a Mediatek GPS - to my big surprise, same problem.

Veryfied that my APM1's AHI is 100% stable with GPS

Connected LEA6H to APM1  - and AHI on APM1 was as bad as APM2 !

how can this be explained ? - LEA6H may cause more noise/interference or harmonics for AHI than Mediatek, - and APM2 (it's sensor) is more sensitive to noise than APM1 was ?

Anyway - with jumps up to 10% on roll, pitch, yaw axis - no kind of stable flight can be expected unless the aircraft is really heavy/sluggish.

With all the information people wish, we must admit that the MAX7456 is not good enough in the long run - the more you display, the more cluttered the screen gets - and - we are completely missing analog indicators, they are considered much more readable.

For really cool graphics, and audio warnings - take a look at this project;

http://forums.openpilot.org/topic/6736-openpilot-osd-prototyping/

Fortunately it's open source too - it would be sweet to make that read mavlink...

(the post above is in wrong thread, it was for MinimOSD Extras)

Another interesting observation - AHI goes bananas only when GPS gets fix - not while it's searching for sats.

is it even possible that this is some bizzare software bug related to decoding the ublox protocol ?

I've been getting a regular twitch in my copter since installing the ublox GPS. Seem to be at about 2hz. If I unplug the gps, the twitch is greatly reduced. I tried unplugging all of my other components (bec, video tx, moving my radio rx far away etc) but unplugging the GPS is the only thing that seems to really make a difference to the twitch.

Is the APM using the ublox binary protocol or NMEA?

You could be on to something.

Funny none of the uBlox advocates mentioned these problems before.  Is it only affecting the 3DR uBlox unit and not the others?  People have been using uBlox units with the APM for quite a while before the 3DR unit came out.

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