GTA02: NavBoard V3 causing GPS interference

Hrabosh hrabosh136 at
Thu Feb 7 18:00:27 CET 2013

A few comments inside:

Pascal Gosselin píše v Čt 07. 02. 2013 v 08:36 -0500:
> Yesterday I was able to confirm that the installation of the Golden 
> Delicious NavBoard V3 very seriously degrades the performance of GPS 
> when using the built-in GPS antenna.  Getting a 3D fix is taking over 20 
> minutes on average using the built-in GPS antenna.  When using an 
> external GPS antenna, the issue goes away and an unaided GPS cold start 
> can be achieved in about 41 to 44 seconds.
> I suspect that the use of unshielded cabling between the GTA02 is a 
> likely cause.  Being in the Avionics business, I am familiar with the 
> use of shielded twisted pair wiring, but generally in large 22AWG (will 
> have to figure out where to source very thin STP cabling).  The wires we 
> use to perform this mod is solid core, probably 30 AWG.  In avionics, 
> when it is desired to keep a signal from radiating outside a cable (such 
> as Headphone/Microphone wiring), then the shield of only a single side 
> of the cable is terminated to a good grounding point.  When instead it 
> is desired that the signal inside a cable be protected from EMI/RFI from 
> the "outside", then the shields on both sides of the cable will be 
> terminated to ground

We are usually trying to refrain from connecting shileding to ground on
both sides as a prevention of grounding loop.

> .
> I theorize that only the SCL/SDA would need to be shielded (together in 
> a shielded twisted pair cable ?), Power and Ground may not need to be 
> shielded.

We have never twisted two differnet signals together. What about
cross-talks? The only type of signal we are twisting together are for
example positive and negative leg of RS422 bus. Or A429 (you may be
familliar with).

> This issue seems very similar to the SD Card access/GPS issue of the 
> earlier GTA02s, so I also wonder if a capacitor fix should be looked at 
> in this case as well.
> -Pascal


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