GTA02: NavBoard V3 causing GPS interference

Jiří Pinkava j-pi at
Thu Feb 7 18:18:30 CET 2013

Dne 7.2.2013 18:00, Hrabosh napsal(a):
> 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).
Just ground betwen all data lines is common solution to this.


Emited radiation is equal of surface of closed loop betwen GND and 
signal line.

>> 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
> Zbynek
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