GPS Lock being Lost due to Radio TX

[AlanK]

Since upgrading our classic to a Rosetta we have found it prone to losing GPS lock mid flight despite the improved chip, but have linked this to being at the same time as transmitting on the radio.  We have moved the location of the PAW unit as much as possible away from the radio aerial (limited as open cockpit) which has improved things substantially, but still get occasional drop out when transmitting.


Just wondering if anyone else had experience anything simmilar?

Also if using a USB Extension lead would have an adverse impact if we stuck the GPS 2m away on the end of a cable.

[Vic]

Alan

Does the GPS dropout occur irrespective of what frequency you are transmitting on?

[AlanK]

I have suspicions around this being a factor but when it happened last time it was about 0.2 MHz difference to my local field  (121.2 vs 121.080)

[Vic]

I would , as a starter check your transmitting equipment. The tenth harmonic (natural resonant frequency) of the channels you specified does fall in a part of the GPS band.

Most likely cause if so could be bad connections in any RF cabling from the transmitter to the antenna or corrosion leading to a poor VSWR which could result in the generation of harmonics.  Check no cabling is pinched or distorted from round. Also check all earth connections including any groundplane of the antenna.

Engine suppression could also be a possible factor, plug leads etc.

The newer GPS dongle in the Rosetta does have a much larger antenna giving it a better sensitivity, not only to GPS signals but possibly to likely interfering factors too.

[Admin]

Alan, can you confirm the symptom you are seeing.
I will make an assumption you are using SkyDemon, please correct me if I am wrong

Do you geta banner regarding lost satellites, which eventually recovers

or do you get a message about no gps data, which requires you to select 'go flying - Pilotaware' again ?

Thx
Lee

[AlanK]

Good to know there may actually be some science behind the issues we have as it was not making a lot of sense.

I will double check cables but its open cockpit and exceptionally hard to work in the nose cone where the aerial is.  It is at least a pretty short run but if anything its more than likely to be the ground plane which is effectively a small metal plate and some metallic tape.
 
(Its a banner pop up on Sky Demon that disapears as soon as I stop transmitting, sitting on the ground in the hanger I could see it drop several satellites on the PAW config screen)

[Admin]

(Its a banner pop up on Sky Demon that disapears as soon as I stop transmitting, sitting on the ground in the hanger I could see it drop several satellites on the PAW config screen)

Thanks, it does sound like interference on the GPS frequency, I thought it may have been a power problem - but unlikely.
One last question I would ask, a reduction in power does seem to make the GPS less sensitive, can I ask how you are powering your PilotAware, and is it sharing any power connections (multiple USB output etc)

thx
Lee

[AlanK]

Hi Lee - Its the recommended Anker battery pack - think we have spoken about this before via Facebook after the last PAW was ran over (just checked, looks like it was Ash)

Nothing else is being ran of the pack as last time I had a 0x500 error I started to suspect charging a very flat Ipad as the same time was too much for it.

[Admin]

http://www.vansairforce.com/community/archive/index.php?t-37374.html

Our KX-155 comm interferes with GPS in our RV-6. When transmitting on frequencies near 121.175 MHz, which has its 13th harmonic pretty much on the main GPS satellite carrier frequency, it takes out satellite reception both in the panel mount GX60 and the GPS 496 handheld. The effect is not subtle.

[AlanK]

http://www.vansairforce.com/community/archive/index.php?t-37374.html

Our KX-155 comm interferes with GPS in our RV-6. When transmitting on frequencies near 121.175 MHz, which has its 13th harmonic pretty much on the main GPS satellite carrier frequency, it takes out satellite reception both in the panel mount GX60 and the GPS 496 handheld. The effect is not subtle.

Ill try a quick TX on that freq to see if the effect is any more pronounced.

[exfirepro]

Alan,

Sorry I meant to phone you this morning but got caught up collecting a plane from the Borders and bringing it back to East Fortune. I can certainly lend you a ‘mouse style’ GPS on a USB cable if you want to give that a try to see if it helps. I have sent you an e-mail with my phone number.

Regards

Peter

[BPCLARKE]

I recently upgraded my Rosetta in my CZAW Sportcruiser to have an external aerial kit with the gps mouse positioned close to the front wind shield and back up compass . All was was well with my SkyDemon - pilotaware system when I was using a hand held 8.33 kHz radio and using the aircraft’s aerial on the top of the fuselage. Then I installed a panel mounted Icom 8.33 radio (more power) and immediately found that when transmitting on certain frequencies I lost the gps lock for a brief period. The frequencies in question were near Rochester airport, those being Southend 130.78 and Andrewsfield 130.555. Rochester’s 122.255 was ok. After consultation with others and Ashley Vinning at pilotaware it was suggested that the problem might be due to harmonics of the gps frequency combined with a critical  distance(~2m in my case) between the radio’s aerial and the gps mouse that was causing the problem? The mouse and Aerial were also line of site. It was suggested to reposition the gps mouse to the rear of the cabin closer to the radio aerial. This I did to just behind the pilot’s seat with a good view of the sky. I repeated the same flight track using the same frequencies and this time there were no gps disconnections . Fingers crossed this and cured the problem

[AlanK]

I still get GPS drop out but usual just Edinburgh approach on 121.205.  I moved the paw as far back as I could but not many options in an open cockpit gyro so just live with it now.

[Admin]

I think we worked out that at some of these RT frequencies there were possibilities of harmonic distortions at the GPS frequencies

This is interesting
https://support.garmin.com/en-GB/?faq=Xmp4TSuZR01KG64m0uUE08

[exfirepro]

Hi Lee /All,

Interesting article.

Like Alan and others, I have as you know also experienced this phenomenon - especially on Edinburgh Radar (121.205MHz), though I have only seen it on one recent flight since changing my radio from a ‘handheld’ Icom to a higher powered panel-mount Trig a year ago.

Interesting counter-intuitive suggestion to move the GPS antenna closer to the radio antenna - though I can see how that would work. Glad to hear it has for you BP. Another thing is to ensure that the antenna connections are sound and that the antenna is properly grounded to minimise radiation from the coax outer shield.

Best Regards

Peter