Beacons Required?

Nonetheless, I had repeated ghost signals from my Tracker when I was well outside the range of a transmitting beacon.

And the repeated ghost signals then ceased entirely when the GPS was turned off?
If so, what generation was the Tracker? (Current housing shape with tethered pouch? Or previous oval with all straps? And if latter, were the straps elastic only in certain sections, or pretty much everywhere?)

Correct. Not sure what gen, but it was the older oval shape with no pouch. Elastic straps connecting the beacon to the waist belt, but the belt and the shoulder strap are webbing.

That's great.  Nonetheless, I had repeated ghost signals from my Tracker when I was well outside the range of a transmitting beacon.   I don't know how to reconcile that with ClimberDave's post or your findings, but that's what happened.

A GPS doesn't send a single as a cell phone does; it only records and calculates the single sent from the GPS satellite (at least 3 sats for 2D location 4 for 3D).  If your Tracker is getting a ghost single from a GPS it should, in theory, have that happen all the time since the constellation is always broadcasting.   I don't doubt your sincerity in what happened with your tracker, but I think it was coming from a different source than your GPS.  Flux lines are funny things!

I think the interference issue is related to the power draw?

Here are my latest results:

Tested beacons = Ortovox S1 (1.2.3074), Barryvox Pulse (3.0), Tracker (final version), Tracker 2 (November pre-release firmware), Ortovox M2 (old), Ortovox F1 (older)

Tested devices = CDMA phone (non-smart, i.e., just a phone), Garmin 60Cx gps, Spot 2, Petzl Zipka headlamp (4 LED)

Devices that caused no interference = CDMA phone not doing anything (i.e., on, but clamshell closed, and not making a call), Spot 2 on but not doing anything

Device that caused noticeable interference in all beacons = CDMA phone making a call

Device that caused minor interference in all beacons = CDMA phone not making a call but being flipped open

Devices that caused interference (loud static) in only the M2 and F1 = Garmin GPS (yes, I was surprised!), Petzl headlamp

Device that caused interference in some beacons = Spot transmitting a message

Based on all of this, seems possible that a gps might have been able to cause interference in an older version of the Tracker.

If your Tracker is getting a ghost signal from a GPS it should, in theory, have that happen all the time since the constellation is always broadcasting. 

It's true that the GPS satellites are continuously broadcasting, but the signals are extremely weak at the ground. The receivers almost certainly have a high frequency oscillator in them to mix the high frequency GPS signal down to easy signal processing frequencies (much like a an AM radio converts kHz frequencies down to audio), an on-board processor, and a bunch of other ancillary electronics that can generate noise in the beacon's signal band. If I recall correctly, in order for a beacon to run for the usual number of hours, it can only transmit, at most, a few tens of microwatts of signal power. It's easy for most handheld devices to produce intermittent interference at that level.

It wouldn't be hard to convince me that even some wristwatches and fancy calculators could interfere with beacon performance if placed close enough to the beacon.

Hummm - sounds plausible. Excellent tests!!!