SPOT bread crumb interference with GPS position

https://www.faa.gov/other_visit/avia.../InFO19006.pdf

My Spot has been in a drawer for a couple of years, and there it will stay.

On 5/9/2019 7:14 AM, son_of_flubber wrote:
https://www.faa.gov/other_visit/avia.../InFO19006.pdf

--
Dan, 5J

Cell phones can be noisy little devices too. Wouldn't it be funny if
your backup navigation device was causing interference with your primary?


On 5/9/19 8:47 AM, Dan Marotta wrote:
My Spot has been in a drawer for a couple of years, and there it will stay.

On 5/9/2019 7:14 AM, son_of_flubber wrote:
https://www.faa.gov/other_visit/avia.../InFO19006.pdf

On Thursday, May 9, 2019 at 9:14:45 AM UTC-4, son_of_flubber wrote:
https://www.faa.gov/other_visit/avia.../InFO19006.pdf

That PDF is short on details, but seems to imply that the transmissions from the SPOT device cause a nearby GPS receiver to have problems. Since the SPOT only transmits once in several minutes, do the observed GPS position losses only occur at those times and briefly? I wonder what the power and frequency of SPOT's transmissions are. We use many other devices in our cockpits that transmit periodically in VHF or UHF, e.g., transponders (high power!), FLARM (low power), cellphones (medium power), Bluetooth (low power), VHF radio (medium power), goTenna (medium power), switching power supplies (low power in any given frequency). Do any of these interfere with GPS reception? Actually I am amazed how well GPS receivers do in listening to very faint signals from the satellites, in the face of all the electric noise, and often poor "view of the sky" (e.g. the pilot's body nearby).

Very interesting point!Â* I'll try turning my phone off before flight or,
as a minimum, set it to airplane mode.

On 5/9/2019 8:59 AM, kinsell wrote:
Cell phones can be noisy little devices too.Â* Wouldn't it be funny if
your backup navigation device was causing interference with your primary?


On 5/9/19 8:47 AM, Dan Marotta wrote:
My Spot has been in a drawer for a couple of years, and there it will
stay.

On 5/9/2019 7:14 AM, son_of_flubber wrote:
https://www.faa.gov/other_visit/avia.../InFO19006.pdf




--
Dan, 5J

On Thursday, May 9, 2019 at 9:46:32 AM UTC-7, wrote:
On Thursday, May 9, 2019 at 9:14:45 AM UTC-4, son_of_flubber wrote:
https://www.faa.gov/other_visit/avia.../InFO19006.pdf

That PDF is short on details, but seems to imply that the transmissions from the SPOT device cause a nearby GPS receiver to have problems. Since the SPOT only transmits once in several minutes, do the observed GPS position losses only occur at those times and briefly? I wonder what the power and frequency of SPOT's transmissions are. We use many other devices in our cockpits that transmit periodically in VHF or UHF, e.g., transponders (high power!), FLARM (low power), cellphones (medium power), Bluetooth (low power), VHF radio (medium power), goTenna (medium power), switching power supplies (low power in any given frequency). Do any of these interfere with GPS reception? Actually I am amazed how well GPS receivers do in listening to very faint signals from the satellites, in the face of all the electric noise, and often poor "view of the sky" (e.g. the pilot's body nearby).

The Globalstar uplink (what SPOT uses) is L-Band. 1,610-1,618.725 MHz (had to Google the exact frequencies) so in the ballpark of bunch of GNSS signal frequencies (see https://gssc.esa.int/navipedia/index.php/GNSS_signal). The Globalstar STX transmitter chips used are putting out something in the ballpark of 100mW when they transmit. BTW lots of details on Globalstar STX inside these devices are in FCC device approvals. So very significant compared to the received power of a GNSS signal. And as mentioned, your transpoder is putting out hundreds of watts at 1090 MHz as well, and in busy airspace doing that hundreds or more times per second.

Electronics at these frequencies can be pretty magic in how precise their signals are, and how much they can reject close by signals. (I used to work on very exotic signal sources that ran at ~10,000 MHz and had an incredibly precise ~1 Hz signal bandwidth :-)). More details of the issues would be interesting to hear.

I expect Garmin to do a good job engineering stuff, and betcha they are double checking InReach with nearby GPS receivers now :-)

On Thursday, May 9, 2019 at 12:46:32 PM UTC-4, wrote:
... seems to imply that the transmissions from the SPOT device cause a nearby GPS receiver to have problems.

The FAA doc says that what matters is the proximity of the SPOT to the antenna of the GPS device (not the GPS device itself).

It also does not mention what model of Spot, there are several different models.
Chris

On 5/9/19 5:24 PM, wrote:
It also does not mention what model of Spot, there are several different models.
Chris


That's trying to put too fine a point on it. EMI issues in a cockpit
aren't limited to SPOT's in general, much less one particular model.

Radios can have interference from various sources, and people sometimes
have success sprinkling toroids in the wiring harness. With the antenna
traditionally located in the tail fin, most of the problems are caused
by conducted interference in the wiring.

GPS's are so sensitive that having two active gps antennas next to each
other can sometimes cause problems. Flarm says to keep all antennas at
least a foot away from any other antenna, but good luck with that in a
typical cockpit.

-Dave