[time-nuts] GPSDO+PC as a NTP server

[Attila Kinali]

On Wed, 23 Oct 2019 12:37:24 -0700
Fiorenzo Cattaneo <fio at cattaneo.us> wrote:

> GPS and GLONASS are the best GNSS around, they are operated by the
> military as a military operation and have the best reliability record.
> Beidu is fairly new, and Galileo is operated by civlian agencies.
> They've had major operational issues. So this leaves GPS and GLONASS.
> GLONASS tends to have better coverage on the polar regions, but
> otherwise I think GPS is better, and GPS has better time and space
> accuracy. Although I'm not that with the accuracy of RS232 interrupt
> handling it would be possible to even tell the time differenbce.
> between UTS(GPS) and UTC(GLONASS).

Some organization being military or not does not matter when it comes
service quality. Unless you had been in the military, then you would
distrust anything that says "military grade"... but that's a different
story.

As far as reliability goes, Galileo is more reliable then GPS had
been in its early days. Of course, they have an easier job with todays
technology and lots of know-how available from maintaining satellite
systems. But considering that they only had one major incident, which
immediately flagged all satellites as faulty, it is actually quite good.
Compare this to GPS' 2016 incident, where the satellites weren't flagged
as unhealthy. Not to mention that this was caused by a known bug in
the controll software. The worst performer in terms of reliability
is GLONASS. After the fall of the iron curtain, the GLONASS system
went from full constellation of 24 satellites (just achieved in 1995)
to only 12 operational in 2008 (i.e. they couldn't even provide basic
service within all Russia, which requires 18 satellites). And that
was after the decree of 2001 to restore full operational status
of GLONASS by 2009! GLONASS has restored full constellation in
December 2015 and has been operating decently well, with a few
glitches here and there.

I don't know enough about Beidu to comment on it.

So, which one would I trust?
None :-)
Mistakes and accidents will always happen. If you build your system
with blind trust in any of the GNSS systems, you are likely to have
your system fail at some point in time. As always: any external system
input should always be validated and cross-checked.


In terms of received ground performance, Galileo has been outperforming
GLONASS for a few years already. And that even though they only have 20
fully operational satellites (instead of the 24 of the full constellation).
Full constellation for Galileo is planned for somewhen in 2021, after
which it will be the one system that provides the highest location and
time accuracy to civilian users. In terms of theoretical performance,
it will beat even the GPS Block III signals, which, most likely, will
not be fully available within the next decade. Whether this performance
is actually deliverd to the receivers, is yet to be seen, though.
For all practical purposes, my guess would be that with an E5/L5
single-band receiver, Galileo would outperform GPS slightly, due to
better multi-path supression (the much wider E5a+b signal helps with
that tremendously). For multi-band receivers, the difference is probably
insignificant. On E1/L1-only, both should perform the same.

The major issue with GLONASS is that it uses FDMA. This gives slight
offsets in terms of atmospheric delay and delays within the receiver.
Thus is the biggest source of performance degradation for GLONASS
(after multi-path and ionospheric) and is the reason why Galileo
has been the major reason why Galileo has been outperforming GLONASS
so quickly. There have been plans to change the GLONASS frequency
plan such that they also operate as CDMA system, but sofar there
is only a test signal on L3 (on top of E5b) and only from the
two GLONAS-K satellites. There are plans to extend the CDMA signal
to L1 and L2 with satellites launched by 2023, but I would not
hold my breath for those.
 
> I'm not even sure than running GLONASS and GPS together will give you
> a better time solution, although it will definitely improve
> availability.

Combining different GNSS systems is known to improve performance.
But it is not easy to do it correctly.

			Attila Kinali

-- 
<JaberWorky>	The bad part of Zurich is where the degenerates
                throw DARK chocolate at you.