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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Hi Doug,

Firstly, what GPS module do you have?  Is it one of these two?

Is the Taoglass patch antenna parts number one of these :

CGGBP-356A 02

CGGBP-184A 02. ]

If so, the the ground plane should be 70mm x 70mm.  This is quite clear in the Taoglass antenna specs. It certainly does not look like that in the images...  I think your second last sentence must be modified - I think the mnfr did NOT design as per the antenna requirement Specs...

Remember, that does not mean the GPS will not work - it means that the impedance match is wrong, so power is lost, which they make up for with the LNA, but the bad side is the loss of directivity and subsequent pickup of radiated noise from below.

I strongly suspect that radiated noise is the biggest culprit in this situation.  The best way to discover this would be to attach a similar antenna with ground plane to a spectrum analyzer and measure the detected emissions with the craft at full power.. not easy without the equipment.

Regarding your questions -

1. No there is no easy way to estimate this without measurement. The radiation decreases with the inverse square of distance ( a simplification, but good enough for this), but you have no idea how high the levels are to start with.

2. Yes - by shielding the noise generators, which is always the best approach anyway. There are some metallic sprays which can be used to create a 'shielded can' from your AP enclosure, etc - not totally effective as the cables still exit unfiltered, so the cables conduct the energy out from the enclosure. Cables can be fitted inside braided shields. ECS should be as close to the motors as possible - the 3 phase wires to the motors are the big culprit - hi current hi frequency square wave PWM signals with harmonics into the many hundreds of MHz. In essence, you want the electronics inside a shielded can, the same as that little can over the GPS chipset. That is very difficult, if not impossible, to achieve, but all the hi current, hi frequency cables should be shielded and grounded at a common point - place a disc of copper foil, maybe 60mm diameter, at the base or some convenient position in the craft, and connect all shield, braids and grounds to it. Braid shield all signal leads, servo leads, power cables. A lot of work, can add weight, but it does work. 

WRT your last sentence - I would recommend a ground plane ( with appropriate antenna of course) of at least 70mm x 70mm, placed maybe 200 to 300mm above the avionics. This is almost guaranteed to work in even the most stubborn cases. In less stubborn cases, both dimension can be reduced, eg, in a light craft with lower power to the motors in flight. I would not go less than 40mmx40mm antenna designs, maybe 100mm min above the craft. Any smaller and the antenna sees as much noise from below as GPS signal from above.

As an aside, you may be able to extend the ground plane on this module ( assuming it is one of the two in the pictures?)

Scrape the ground plane copper clean around the PCB periphery, and cut strips of double sided PCB , same thickness, and butt them up against the GPS PCB plane. Solder all along the seam, making sure to cover the joint. A thin copper foil over the joint would be great. The strip width should be such that the 'new' dimension ends up 70mm x 70mm. Don't leave big bumps of solder along the joint. Don't short out other signal tracks to the ground plane in this process..

Then raise the PCB to say 150mm above the avionics and see what improved..

Your question 3 I cannot answer - we have our own autopilot and software so it all works differently..However, with respect to the 'types of GPS on the market' - they are all pretty much the same - all have similar sensitivities, all have similar numbers of tracking channels, all use GPS engines from similar manufacturers, so there is not a lot of difference among them. Where the BIG difference lies is in their integration into a complete GPS unit, ie, when mated with an antenna, the 'correct' ground plane, etc.  

A simple check is to obtain the antenna mnfr and part number, get the data sheet, and see what the mnfr recommends as a ground plane. If the GPS unit you are looking at complies within say 10% of this size, go for it. Else avoid like the plague...Unless you are flying normal craft, or your lawnmower is not noisy...

I will be back next week to chat some more - on the way to the Namib Desert tomorrow - we will be darting 5 Desert Lions and fitting GPS SAT tracking collars for a conservation project. 

Have a look here just for fun:

http://desertlion.info/news.html

Cheers

The Nampilot..

Thx Jor for your excellent, as usual, explanations.

For the sake of simplicity and clarity, can you graphically point out, on the ceramic patch antenna pictures you posted, where is the "RF ground" : is it the top silver layer , is it the bottom pin ?

When people say to solder the RF ground to a ground plane extension, what part of the ceramic patch antenna must then be soldered to the ground plane extension ?

Hi Hughes,

The ceramic patch in the photo - the top shows the pin soldererd to the active element. The bottom shows the pin protruding - that is the RF connection to the antenna = goes to the GPS Chipset RF input, either directly, or via the center of your coax, or via the LNA. The underside of thes type patches, where the silvered 'ground' is, does not connect directly to your PCB copper plane - it connects capacitively, and the RF received 'sees' a ground due to the low impedance of that capacitor at those frequencies. So, your ground reference connection is your PCB copper plane, nowhere else.  

I do not understand the concept of a 'ground plane extension' - any connection to this ground plane other than to get power to the GPS from your battery source, serves no purpose at all. This ground plane is the GPS chipset ground, and the Patch antenna ground plane and reference only.

Sorry, back on Monday....

The Nampilot...

ok, if I understand correctly what you're saying, if I want to extend my M8N ground plane from the 35x35 size to a 70x70 size (what seems recommended in the specs of the manufacturer of the chipset), I just solder pieces of copper to extend the PCB size to a 70x70 rectangle (soldered on "scratched PCB" borders to reveal the ground copper layer).

I haven't had time to read all of this so I apologise if it's been said, but I'd like to share some thoughts/experiences.

The GPS signal is extremely weak. Things that jam or severely degrade gps in my experience include some cameras, Wii IR camera, Beaglebone black hdmi module, some SSD motherboards, and the internal oscillator on the stm32f4. Part of this is due to the sensitivity of ublox receivers, I didn't see some of this interference on the mediatek. I'm sure various EMI mitigation techniques have been already discussed, including a larger ground plane.

DOP is certainly an indicator of EMI issues but only when it's significant enough to lose satellites. A much better metric is the horizontal and vertical accuracy that ublox receivers output, but last I checked, the apm firmware didn't log or display it. It's a better indicator because it seems to be related to the signal to noise ratio so can fluctuate a lot while the number of sats doesn't change.

As a future point of reference for anyone who accesses this information, the best horizontal accuracy I have received is 0.6-0.8m inside out lab with an amplified GPS repeater. In the field, a good value is ~1.5m and anything greater than 3-4m is getting noticeably interfered with. Of course it doesn't necessarily mean you're getting 0.8m accuracy but it's an indication. For reference, I observed these results with both a 3dr ublox 6 and csgshop m8n. I'm using a stm32f4 based autopilot with Beaglebone and odroid u3 also onboard. There's also 2 xbee pro 900s, one of which is right next to the GPS antenna. Hope this helps.

I've been messing with my new M8N for a few days now, seriously driving me nuts! I hate this GPS so far!  Anyway, I have found that it does not like 200ms 5Hz. At that setting it drifts 100m, when I adjust it to 1000ms 1Hz its only drifting about 2-3m (in mission planner).

indoors, near a window. My Neo-6, does not drift. Its locked solid.

I have also tried various ground planes and ring, to some minor success.

getting 20 sats and .6 HDOP

my settings:

PRT (ports) UART 1  
  Protocol out UBX
  Baudrate 38400
   
RATE (Rates) Measurement Period 1000ms not 200ms
  Measurement Frequency 1Hz not 5 Hz
     
NAV5 (Navigation 5) Dynamic Model Airborn 4G
  Fix Mode 3 - Auto 2D/3D
  Min SV Elevation 10 Grad
  DGPS Timeout 120 sec
   

SBAS

IFTM (anti jamming) enabled, active antenna

Subsystem

Enabled, auto.

Go out in the open and keep the GPS running for some time and get a reliable fix, once you get this fix the next time you turn on the GPS the time to fix will be lesser and the readings more reliable, I experienced this drift also but it should resolve itself if you let the GPS get a reliable lock outside.  

Hi Dan,

I'm sorry for my question, what do you use the software on Beaglebone autopilot? Sorry for the question off topic.

Cheers,

Is there any tutorial step by step on how to create the ground plane for example CSG gps module size?
Sorry to ask this but english is not my native language and I'm some how lost in translation...

Forgive my ignorance but is it possible to graph INAVEER in apmplanner from the logs? I don't readily see how.

So far I've had good luck with my eBay M8N. No error messages. 

I hovered with poshold today while testing different props. Vibrations are very good, but the copter twitches ever so subtly. It doesn't do it in stab.

I'm wondering if EKF is causing this, meaning it is preventing an illogical move based on faulty gps data ala INAVErr, so it must constantly make minute adjustments which come out as "twitches".  I was going to disable EKF to see what happens, but maybe it's not a good idea.

Not much activity on the subject lately. 

I have had 3 wonderful 30 minute flights on my Hex using 2 M8N's..The INAVerr does go to 255 around the 2 min mark , but as ample posts here as well as on rcgroups have suggested this used to occur with the old Neo6m as well as the original 3DR GPSes, hence I am a bit less concerned while flying GPS modes...I do not see any twitching whatsoever in loiter..I have not tried Poshold but I will and get back if I see any twitching...I decided to ditch the GPS ground plane/shield idea coz' its not needed as I don't have anything interfering on the GPS band (I found this using a spectrum analyser) and also I don't think I am skilled enough to pull this off without making my GPS performance worse....

If anyone has already built a shield or a ground plane for this specific GPS+antenna config i.e CSGshop M8N then I guess all of use would be interested to see the difference is performance  

   

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