[time-nuts] GPS L2 CNAV health reporting changes

[J. Grizzard]

I got a message from the Swiftnav support folks yesterday (I have a 
piksi multi amongst my little fleet of GPSen) because they had an 
emergency firmware patch to their kit because of a change in the L2C 
signal was giving them issues. (Props to them for getting a firmware 
update out in ~12 hour, too)

After a little bit of inquiring, it looks like what happened is that 
yesterday, the L2 CNAV message started marking all of the L1 signals as 
unhealthy. If a device actually listens to this health signal 
(apparently their GPS did), suddenly most GPS satellites become unusable 
(with the only remaining ones being ones that aren't transmitting an L2C 
signal).

(According to NANU 2014039, you shouldn't be using those health 
indicators, though it does also say that the L1 health indicators would 
all indicate "healthy", so this seems to be a departure from their 
stated plan.)

Anyhow, probably doesn't affect anyone here, but I thought it was 
interesting. Sadly, I can't see what's actually being transmitted, since 
my only pieces of kit that can decode L2C are my piksi (which won't give 
me raw NAV/CNAV, and they apparently have zero interest in adding that 
functionality), and the Novatel OEM628 that I picked up off eBay last 
year ... which crashes every 20 or 30 minutes if I ask it to decode L2C. 
Argh.

(Maybe u-blox's upcoming kit will be affordable(ish) and have good raw 
CNAV output... I can hope, right?)

-j