Hi Tim, thanks for the reply.
Sad to say, though, that I have to disagree with you on this. I've captured NMEA output from both my old and new GPS units, and run both of them through several different mapping packages and GNSS positional analysers.
For the sake of example, one such package was VisualGPSView, available for free at
http://www.visualgps.net/VisualGPSView/default.htm.
The NMEA capture tool I used shows perfectly valid data from both units, complete with accuracy messages. VisualGPSView showed an accuracy for my older, GPS-only receiver as 2.0 PDOP, 1.4 HDOP and 1.5 VDOP.
Over the same time period, the newer GNSS unit detected usable signals from GPS, WAAS and GLONASS and produced a solution with 1.1 PDOP, 0.7 HDOP and 0.9 VDOP, both tracking and viewing more satellites. This would seem somewhat difficult to achieve if, by your suggestion, it wasn't outputting valid NMEA messages including accuracy.
The
apparent conclusion is that SkyDemon either isn't NMEA-compliant, in that it either doesn't understand messages from anything other than the $GP talkers, or it filters out non-$GP messages by design - meaning that it doesn't support GLONASS or SBAS. Something that those using such receivers - or about to select one for use with SD - perhaps ought to consider.
I'm willing to stand corrected but certainly, on the basis of my own findings, SD is the only one of several packages I've used that won't garner a valid GPS position from my new receiver. Pity.