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 :-)