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

Posted by Doug Walmsley on March 11, 2015 at 8:32am

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?

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Replies

    • tshado > DG March 12, 2015 at 6:10am

      Hello DG,

      Your post is summurazing very well the situation :

      today, after almost 1 month and a half you discovered this INAVERR problem (only reported in arducopter logs),

      it is not possible to say is these INAVERR errors are pointing to a real functionnal problem that could cause crash or fly away during flights

      With M8N GPS

      Sometime INAVERR goes up to  a max value of 255 (probably a 32 bits counter)

      Sometimes goes up around 20

      With 3DR GPS

      This INAVERR has also been reported by several users:

      http://www.rcgroups.com/forums/showpost.php?p=31022247&postcoun...

      Quote:
      Originally Posted by jdennings View Post
      Absolutely.

      I've started looking back at my logs, just looked at 20 of them for now, with flights > 15 minutes. Here's what I've found so far. Most have INAVerr > 0, with InavErr varying from 1 to 40, straight horizontal line suggesting the error happens before flying or right at the beginning. 19 logs out of 20.

      *BUT* ...

      I've got one scary one. Inaverr starts at 0, then goes to 40 about 30 seconds in the flight. Stays there for 5 minutes or so, then ramps up again to 100 right during the flight.

      This was a while back and I don't remember the flight, except I know I had no issue. Go figure. This is quite disturbing.



      I'd fall in that category. Great pos hold yet some errors.
      Going to look into this more carefully from now on, + got lots more logs to look at.

      I looked at many logs from the Y6B that had several >0 INAVErr but no indication of flight problems in missions.

      Ha! My INAVErr is 255 start to finish with the current setup using the M8N, so I'd be in the category of "I'd be concerned". Of what I don't know, but those that say they've been using them without issues either aren't pegging at 255 or are playing Russian Roulette.

      I looked at earlier logs with the 3DR GPS and it averages around 30 or so. Some have 0.

      CSGShop advertised the M8N as the greatest thing since sliced bread and probably sold a large number of units. As they say Caveat Emptor.

      What I'd like to see now is if and how people are making them work without high INAVErr. If not, then I'll just go back to the 3DR unit and live with not flying on some days.

      3701956300?profile=original

      ----------------------------------------------------------------------------------------------------------------------------

      I would suggest to open a problem in Github for arducopter code ( if not already done):

      * Randy says it is a problem

      * Last sentence below  seems to point to an arducopter code problem

      * As this INAVERR is discussed on several thread on DIYDrones and RCGroups, it will allow to gather all informations in one place and allow everyone  to better foollow this INAVERR problem investigation

      http://diydrones.com/forum/topics/csgshop-neo-m8n-is-giving-pm-erro...

      Reply by Randy on February 11, 2015 at 9:44pm

      DG,
      I should be more clear.  The PM message's long-loops error is a red-herring.  The inav errors are likely a real problem. I don't know why in your logs it keeps having problems.  Rob apparently looked at your logs and the GPS seems to be updating at the correct rate
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      • tshado > tshado March 12, 2015 at 6:46am

        an important reason to open a problem in Arducopter code is that INAVERR are also reported  with 3DR GPS

        • Rob_Lefebvre > tshado March 12, 2015 at 7:34am

          If INAV Err is occuring with 3DR GPS, that does not necessarily mean there is a problem in Arducopter code.  It's more likely to indicate the code is doing exactly what it is supposed to, which is filtering out implausible GPS data before it upsets the inertial navigation.  Those errors are likely happening to EMI on the GPS.

          • tshado > Rob_Lefebvre March 12, 2015 at 8:34am
            Randy who was named  on another thread to be the person knowing about INAVERR is speaking about GPS update at the correct rate.
            Performance monitoring, PM , as far as I understand, is used in log analysis to check that the ebvery 200ms , arducopter code should get updated values from GPS
            If after this 200ms period, arducopter code doen't get updated GPS values , then INAVERR counter is incremented
            This is decribed in detail and analysed here
            http://diydrones.com/forum/topics/inaverr-is-it-the-gps-or-fc-or-ne...
            Are you the Rob, that Randy mentionned in the post above who has analysed the logs for correct GPS update rate ?

            http://diydrones.com/forum/topics/csgshop-neo-m8n-is-giving-pm-erro...

            Reply by Randy on February 11, 2015 at 9:44pm

            DG,
            I should be more clear.  The PM message's long-loops error is a red-herring.  The inav errors are likely a real problem. I don't know why in your logs it keeps having problems.  Rob apparently looked at your logs and the GPS seems to be updating at the correct rate
            • naitsuga > tshado March 12, 2015 at 9:58am

              tshado

              Your link didn't represent your statement that some problem with 3DR GPS , INAVERR,  how high? and how low.? 3DR hasn't released the M8 series .. yet... even Craig gave discouraging statement to apply 8 series.

              what for some as high as 255 INAVERR  (the link you represent is about average 30 or so.and some have 0)

              we don't know if 30 is big problem and can cause crash.

              people still flying and landed safely without realizing that their INAVERR is 255.

              I don't have Pixhawk.(I will have soon),  I am just spectator.

              Why don't we give better reporting about more detail when the INAVERR is high

              what mode we are flying ?

              The firmware at the time? For some dare  flying with master beta too.

              The least we know already  that most of time  the problem of INAVERR related to  GPS (M8N) Module

              I haven't seen that INAVERR reported by the one using Neo 6

              I don't know,  starting AC 3.2 firmware that EKF can be checked or unchecked as is only applied to PIXHAWK (again as I stated I currently don't have PH). this question is very important related to DCM to EKF.

              with the code I saw on the current library.(master) I don't see the config of Ublox 8 series was created ( the latest was Neo7 by Craig about 5 months ago)

              and I will encourage you to help the developer to trace the problem easier. by tabulating the data who have concern about high INAVERR.

              Neither I am developer nor coder or programmer.

              I have strong feeling  that the message parsers from Ublox M8 are sending differently compare to Ublox 6/7 since the Ublox  M8 was configured with current AC firmware without specific HW code for M8 even they are  using  same Ublox protocol. hopefully my feeling is incorrect

              at the end I want to see the solution too for all, but I realize we may not push developer as they are working under our payroll :)

              • John Dennings > naitsuga March 12, 2015 at 11:48am

                I've opened up a github code enhancement ticket here: https://github.com/diydrones/ardupilot/issues/1972

                This could help with further testing if implemented. There's also more information on INAVerr errors both with m8 and 3dr gps  in the pixhawk thread on rcgroups .

                • tshado > John Dennings March 12, 2015 at 12:54pm

                  @John,

                  sorry , but as English is not my native language, what means the title of 1972 ticked ?

                  Report un-truncated INAVerr as uint

          • Doug Walmsley > Rob_Lefebvre March 12, 2015 at 8:31am

            @Rob,

            You bring up a valid point.  It's quite possible that the EMI from the FC processor and possibly ESC frequencies could be interfering with the data communicated back to the FC thus it's possible that one is causing a cascading increase in iNav errors.

            @DG,

            Could you relocate your GPS unit to a tall mast at least 50cm from FC and retest.  It would be a temporary but could help us identify if the relocation far away from the airframe electronics helped to reduce the iNav errors?  You may have to build a longer than usual cable.
            Also may consider different lengths as well to show the different iNav errors. 

            The hope is to:
            1. Identify if in fact an EMI issue is the culprit.
            2. If so, at what distance away from airframe is optimal and practical.
            3. And, if it's true this will help me with my project to develop a shield to ground out EMI issues in hopes that we all can keep the GPS close to the frame if possible.

            • DG > Doug Walmsley March 13, 2015 at 9:21am

              I had a long mast, probably close to 50cm, when first observing the high INAVErr.

              However, I did shorten it at some point because well, it looked nicer and didn't affect compassmot much.

              I'll see if I have an extra mast and raise it back up.

              All antennae are point down and are below the frame.

        • Doug Walmsley > tshado March 12, 2015 at 6:52am

          I concur. However I believe your comments and concerns should be posted in DG's post here.

           

          I set this discussion up to address EMI and RF reception issues.

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