SPOT locator interferes with GPS

Just thought I would update all on a problem that I have finally solved.

I recently had an ADS-B system installed in my Cessna 180. I installed a Garmin GTX-345 and connected it to my Garmin GNS-650.

During testing to meet the compliance requirements and get my $500 rebate I had a problem keeping the 650 from losing satellites. It only occurred in flight and it was intermittent. All the satellites would drop out and the 650 would then re-acquire in about 1 minute. I couldn’t pass the compliance test.

After several more flights, switching antennas, turning off various other devices, swapping out the 650 and replacing the 650’s antenna, I finally figured out what the problem was.

The culprit turned out to be my SPOT Gen3 locator.

The reason I didn’t think it was the SPOT was that it has caused no problems in the last 3 years that I have had it in the same location, on top of the panel’s sunshield. Apparently, the added 345 and associated wiring influenced the RF absorption from the SPOT that it blanked out the GPS signal to the 650 when the SPOT would transmit position or messaging information. Since the SPOT had resided in the same location for the last three years it was the last thing I suspected when I first encountered the problem. It was only after I changed the batteries in the SPOT that I had a clue to what the problem was. Obviously, I had already eliminated other devices that may cause the problem, cell phone, transponder, com radio, etc. After I put fresh batteries in the SPOT it started to occur more frequently, every five minutes, which is the refresh period for the SPOT. The SPOT is apparently sensitive to battery voltage, and while it still blinks properly, the signal strength diminishes with battery power. Maybe that’s why they tell you to use lithium batteries.

I went to the SPOT support page and on page 16,384 , it does warn against placing the SPOT less than 12 inches from a GPS. The sun shield in the 180 is aluminum, so one would believe it to provide some RF shielding and like I said, it had been coexisting with the 650 for three years without a problem. An Internet search only revealed one incident of a SPOT causing this problem. That was with a GTN-750, the big brother to my 650. I use the same SPOT device in my experimental SuperCub variant, but it resides up on the wing root, plus the avionics in that airplane are much simpler, transponder, comm radio and Garmin 560.

I went to the SPOT support page and on page 16,384 :smile:, it does warn against placing the SPOT less than 12 inches from a GPS

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OH! Maybe that's why Garmin Pilot refreshes the screen on my tablet periodically. Maybe about 5 minute intervals? I will look at that more closely.

Thank you!

LOL
BTW: I tried moving my SPOT as far as I could while still giving a clear view of the sky (26 inches) and it still kills my GPS every 5 minutes.

Interesting. All the rules about antennas and distances that are hard to meet some installations. I’ll look for the references I just read a few days ago in reference to gps


Sent from my iPhone using SuperCub.Org

23.3.2 GPS/XM Antenna Mounting LocationThe GPS antenna is a key element in the overall system performance and integrity for a GPS navigation system. The mounting location, geometry, and surroundings of the antenna can affect the system performance and/or availability. The following guidance provides information to aid the installer in ensuring that the optimum location is selected for the installation of the GPS antenna. The installation guidelines presented here meet the intent of AC 20-138A section 16. The greater the variance from these guidelines, the greater the chance of decreased availability. Because meeting all of these installations guidelines may not be possible on all aircraft, these guidelines are listed in order of importance to achieve optimum performance. Items 4a - 4c below are of equal importance, and their significance may depend on the aircraft installation. The installer should use their best judgment to balance the installation guidelines.


1. Mount the antenna on top of the aircraft in a location with an unobstructed view of the sky, as close to level as possible with respect to the normal cruise flight attitude of the aircraft. If the normal flight attitude is not known, substitute the waterline, which is typically referenced as level while performing a weight and balance check.


2. The GPS antenna should be mounted in a location to minimize the effects of airframe shadowing during typical maneuvers. Typically mounting farther away from the tail section reduces signal blockage seen by the GPS antenna.


3. The GPS antenna should ideally be located at the opposite end of the aircraft from the COM unit in order to make the GPS less vulnerable to harmonics radiated from the COM itself.


4a. The GPS antenna should be mounted no closer than two feet (edge to edge) and ideally three feet from any VHF COM antenna or any other antenna which may emit harmonic (or other) interference at the L1 frequency of 1575.42 MHz. An aircraft EMC (Electromagnetic Compatibility) check (Section 34.4.15.1 for GDU 37X systems or Section 35.4.21.1 for


GDU 4XX systems) can verify the degradation of GPS in the presence of interference signals. If an EMC check reveals unacceptable interference, insert a GPS notch filter in line with the offending VHF COM or the (re-radiating) ELT transmitter.


NOTE


The separation requirement does not apply to GPS and COM combination antennas, provided the antenna has been tested to meet Garmin’s minimum performance standards. The separating requirement includes the combination with an XM antenna element as well.


4b. The GPS antenna should be mounted no closer than two feet (edge to edge) and ideally three feet from any antennas emitting more than 25 watts of power. An aircraft EMC check can verify the degradation of GPS in the presence of interference signals.


4c. To minimize the effects of shadowing at 5° elevation angles, the GPS antenna should be mounted no closer than 6 inches (edge to edge) from other antennas, including passive antennas such as another GPS antenna or XM antenna.


5. To maintain a constant gain pattern and limit degradation by the windscreen, avoid mounting the antenna closer than 3 inches from the windscreen.




6. For multiple GPS installations, the antennas should not be mounted in a straight line from the front to the rear of the fuselage. Also varying the mounting location will help minimize any aircraft shading by the wings or tail section (in a particular azimuth, when one antenna is blocked the other antenna may have a clear view).

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this is just the first page, read the next few pages also...
starting on figure/page 23-5 from G3X Touch Installation Manual - Garmin Supporthttps://support.garmin.com/support/manuals/manuals.htm?partNo=010-00G3X-00

Thanks Mike. Good info. Both of my comm radios are a lot less than 25 watts. Garmin 650 and KX-155. Never had a problem with them and the antenna for the 650 GPS is mounted close to the two comm antennas. The SPOT never interfered with the GPS until the GTX-345 was installed.

scaleram said:

..The SPOT never interfered with the GPS until the GTX-345 was installed.

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and THAT'S a HUGE hint!!! probably not the SPOT...

mike mcs repair said:

and THAT'S a HUGE hint!!! probably not the SPOT...

Click to expand...

It has been my experience as an electrical engineer, electromagnetic interference in the radio frequency spectrum is rarely due to a single element. However, minute changes in component layout, or changes in the configuration of components will affect the disturbance generated by an external source within an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction. The disturbance may degrade the performance of the circuit or even stop it from functioning. This is not to say that the SPOT, solely and of itself is causing the malfunction. It is just the final trigger to the degradation of the GPS signal presented to, and processed by the GNS-650 receiver. Additional equipment, likely the GTX-345, changed the way that the energy emitted by the SPOT propagated amongst the avionics in the airplane. Not to mention that the Globalstar network, that the SPOT uses for messaging, and GPS signals are close in frequency, further exacerbating the problem.

However; the final nail in the coffin is the period of disruption in the GPS. Exactly 5 minutes. That is the position refresh period of the SPOT locator. To further verify that the SPOT is the trigger, I queued an “OK” message from the SPOT and it too caused the GPS failure in the interim between the 5 minute SPOT scheduled time. All of these tests were repeated several times to verify the source of the problem. I had hoped that moving the SPOT would eliminate the problem, but it did not. I do intend to do some additional testing using a Garmin inReach. It also operates in the L band, but I’m hoping that there will be enough difference that it will not corrupt the 650. It will be a few weeks before I can do that and I will report on the forum.

Thx scaleram. Any reason to believe this couldn't happen with an InReach device as well? I've noticed some intermittent loss of GPS signal on my 796 that sounds similar.

Every device is different. Even a different SPOT may not cause the problem. The SPOT that I have may have a transmitter section that is emitting a spectrum of RF energy that is excessively noisy. Meaning it is scattering the spectrum with RF that is outside the desired frequency. There are numerous friends here that have both the SPOT and the inReach. When I get a chance, I intend to borrow some of them and try them out.