On Tuesday, February 24, 2015 at 7:40:38 AM UTC-8, Bob Pasker wrote:
i should have also added:

with FLARM, the aircraft move slowly, and they are all VFR, so having relative position and altitude from a non-certified, low-res source is still fundamentally useful for improving see-and-avoid (although not necessarily for IFR separation)

On Tuesday, February 24, 2015 at 10:36:13 AM UTC-5, Bob Pasker wrote:
I wonder if the reason for requiring high-resolution TSO'd GPSs is that ADS-B will be used for IFR separation and for fast-moving aircraft, so high-resolution position and updates is an absolute requirement, a la fast-scan radar.

I believe Darryl. It's not the position resolution that is the issue. My consumer GPSs seem to do a pretty good job of resolution within a few meters. For ATC, when you are doing 5-mile separation, what's a few meters of resolution? It seems the TSO requirements that can't be met by consumer GPS is reliability (as in, is it producing a valid position at all) and error detection/reporting (can you tell when you are not producing a valid position at all and let the rest of us know).