GPS Issues - Do I need a GPS Ground Plane & EMI Ring? - Discussions

So I read an interesting article about GPS antennas called "Adding a GPS Chipset To Your Next Design Is Easy".

A few points to bring up that I have concerns with dealing with my M8N antenna.

1. Active vs Passive Antennas. Two paragraphs within the article describes the difference between Active and Passive antennas. According to CSG Shop's specification for the NEO-M8N it comes with a low-noise regulator and RF filter built-in. So I'm assuming that it is a active antenna.

2. Antenna's requiring adequate plane. If I read that document correctly, these GPS modules may require a GPS plane as they are installed on a PCB that does NOT have 40mm of side to them.

Quote: "Generally, patch antennas in the 15- to 25-mm size range with a least a 40-mm (on a side) ground plane will give the best performance in portable equipment, but this may be too large for your application. This could force you to look at smaller antenna topologies such as linear chip antennas."

3. The next concern is to mitigate the noise interference from FC, ESCs, and PDB. Since my Y6B is set up with a clam shell cover and my M8N is attached under and close to the all the electronics, I may need to develop a shield "ring" connected to the shield can and then connect that ring to RF ground through an inductor at a single point.

Quote: It's common in VHF and UHF RF shielding to connect all points of the shield can to the PCB's ground plane. This can be a mistake at GPS frequencies, since the open-air wavelength of a GPS signal is so much shorter than UHF. Depending on the size of the shield can, if there is current flow across the can, the shield can will be able to resonate near GPS frequencies resulting in interference or de-tuning of the GPS RF.

By developing a shield "ring" connected the shield can and the inductor, the inductor will filter any EMI-induced current flow. The ring connected to the shield can will prevent any current flows or resonation issues.

I'm not an electrical engineer and need guidance from those out there who are. Did I interrupted this correctly? and if so I could use some help with developing the "ring".

Thoughts?

Hi Thorsten and all

Thanks Thorsten for your very interesting post

http://diydrones.com/forum/topics/gps-issues-do-i-need-a-gps-ground...

To recap , you are describing two "strange things" about your numerous flights with your GPS

1) timing jitter (missing samples)

Your guess is that this causes the reported INAVERR

You observed these timing jitters on both your 3 different M8N GPS

INAVERR have been reported by several using they 3DR LEA 6H, but INAVERR values were between 20 or 100 max (if I remenber well)

So not 255 max value as seen a lot of Time with M8N

So, there is probably "timing jitters" with 3DR LEA 6H GPS

You say also this timing jitter is creating twitching:

Is this twitching visible ( during loiter for example) od did you show it 's evidence by analysing post flight logs?

2)clock drift or communication latency between the GPS module and the Pixhawk.

this means that the clock given by the GPS differs from the PIxhawk on board clock

I have to read again in details the links you provided , but can I ask you if this latency is stable or in "within the same amont value" ?

3)"I think this has nothing to do with EMI"

What you mean is that INAVERR has nothing to do with EMI ?

Or is it about rhe two problems you are describing ?

4)Do you know why Performance Monitoring is checking that every 200ms , all GPS updates has been done ?

EKF is running at 400Hhz , ie 2.5ms: if no GPS updates are provided to EKF during 200ms , does this leads to the fact that Inertial Navigation results could be degraded , and is why this is logged as INAVERR ?

Regards,

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Replies

MarkM March 13, 2015 at 2:26pm

I don't know if my logs would help or hinder.

I have a Drotek M8N - http://www.drotek.fr/shop/en/home/512-ublox-neo-m8-gps-hmc5983-comp... - mounted on a 5" tower above my TBS Discovery.

The first flight included Circle mode, an auto mission and some general PosHold flying which after 12min still had a 0 inaverr.

The 2nd flight was pretty much the same but without the Circle mode and half the flight time showed a small error of 27 towards the end of the flight.

I have seen older logs with an inaverr of 255 but even with those the one constant is the quad always flies perfectly.

Noerror.bin

Smallerror.bin
- naitsuga > MarkM March 13, 2015 at 2:59pm
  
  Mark
  
  Do you have EKF enable on your FC for the logs above ?
  
  Thanks
  - MarkM > naitsuga March 13, 2015 at 3:45pm
    
    I do indeed, I've attached the parameter file as well.
    
    TBS Pixhawk 3.2.1 Parameter file currently loaded.param
    
    https://storage.ning.com/topology/rest/1.0/file/get/3702689521?profile=original
tshado March 13, 2015 at 6:21am

Hi Thorsten and all

Thanks Thorsten for your very interesting post

http://diydrones.com/forum/topics/gps-issues-do-i-need-a-gps-ground...

To recap , you are describing two "strange things" about your numerous flights with your GPS

1) timing jitter (missing samples)

Your guess is that this causes the reported INAVERR

You observed these timing jitters on both your 3 different M8N GPS

INAVERR have been reported by several using they 3DR LEA 6H, but INAVERR values were between 20 or 100 max (if I remenber well)

So not 255 max value as seen a lot of Time with M8N

So, there is probably "timing jitters" with 3DR LEA 6H GPS

You say also this timing jitter is creating twitching:

Is this twitching visible ( during loiter for example) od did you show it 's evidence by analysing post flight logs?

2)clock drift or communication latency between the GPS module and the Pixhawk.

this means that the clock given by the GPS differs from the PIxhawk on board clock

I have to read again in details the links you provided , but can I ask you if this latency is stable or in "within the same amont value" ?

3)"I think this has nothing to do with EMI"

What you mean is that INAVERR has nothing to do with EMI ?

Or is it about rhe two problems you are describing ?

4)Do you know why Performance Monitoring is checking that every 200ms , all GPS updates has been done ?

EKF is running at 400Hhz , ie 2.5ms: if no GPS updates are provided to EKF during 200ms , does this leads to the fact that Inertial Navigation results could be degraded , and is why this is logged as INAVERR ?

Regards,

JC
- DG > tshado March 15, 2015 at 9:29pm
  
  Well, now that is an interesting discussion there. Maybe I wasn't imagining the twitching (momentary loss of control actually) after all.
- T3
  
  Thorsten > tshado March 13, 2015 at 2:21pm
  
  Hi tshado,
  
  let me try to answer your questions:
  
  Your guess is that this causes the reported INAVERR
  
  Is this twitching visible ( during loiter for example) od did you show it 's evidence by analysing post flight logs?
  
  Yes: I found a log where you can clearly see that shortly after the visible twitching starts, the INARERR increases. And yes, the twitches are visible AND show up in the log. In the screenshot you see uncommanded spikes in DesPitch.
  
  INAVERR have been reported by several using they 3DR LEA 6H, but INAVERR values were between 20 or 100 max (if I remenber well). So not 255 max value as seen a lot of Time with M8N
  
  I had 255 with the 3DR LEA 6H as well
  
  Is this latency is stable or in "within the same amont value" ?
  
  The latency is increasing.
  
  What you mean is that INAVERR has nothing to do with EMI ?
  
  Well, this is probably not correct. I think that for sure a strong interference can cause high INAVERR values. What I meant is that there is more than interference. You cannot fly 26min on one spot and then out of a sudden the interference changes. That is why I think that it has something to do with the GPS and/or the communication between GPS and FC.
  
  Not really sure about 4). But most probably yes.
  
  I hope the devs will shed some light on all this.
- DG > tshado March 13, 2015 at 6:25am
  
  tshado,
  
  I don't see where you mentioned the info you posted at RCG here http://www.rcgroups.com/forums/showpost.php?p=31022593&postcoun...
  
  It is worthy of discussion.
  
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T3
Thorsten March 12, 2015 at 2:53pm
Hi Doug and all,

I have high INAVERR values no matter what GPS I use (3DR LEO 6H, as well as CSG, VR, and Drotek M8N) I am flying the M8N for 100+ hours with basically no problem and very high positional accuracy. But as mentioned at the end of the post I linked there are two strange things:
- There is some timing jitter (missing samples) which occurs at least with both M8N (not sure about the 6H) GPS modules. This jitter also results in some twitching. This is what leads to the INAVERR I guess. But I never saw a twitching with the 6H - which might also be related to different PID settings I was using over time.
- In addition there is some clock drift or communication latency between the GPS module and the Pixhawk.
I think this has nothing to do with EMI.

When it comes to shielding efficiency the UBX values like jamind and noiseperms are the ones to look at. But to interpret the results of any shielding effort it is important to know what values we are aiming at.

From some (statistically not valid) looks at some of my logs of different setups it seems (high degree of uncertainty) as if some aluminum foil placed between the Pixhawk and the GPS i) reduces the jamind and noiseperms values more than increasing the distance between the Pixhawk and the GPS up to about 10 cm and ii) further reduces the PDOP. However, Craig mentioned it would be "better trying to shield the noise sources".

Regards,

Thorsten
- Pavel Kirienko > Thorsten March 15, 2015 at 10:42am
  
  Thorsten,
  
  I want to quickly comment on the second item. This is a bona fide case of clock drift on Pixhawk, it has nothing to do with GNSS receiver performance and it shouldn't manifest into real issues for Pixhawk.
  
  While we're talking about RF shielding, may I casually suggest you (plural) to have a look at Zubax GNSS (yes, it works with ArduPlane 3.2.1, ArduCopter master and PX4 stack).
  
  Pavel.
- T3
  
  Thorsten > Thorsten March 13, 2015 at 3:24am
  
  Some more recent flights without Bluetooth.
  
  The first one has an INAVERR of 0, and in the second one (from just some min later) the INAVERR increases with no obvious relation to any GPS problems.

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