[time-nuts] Advantages of GNSS ???

[Attila Kinali]

On Fri, 5 Jul 2019 19:49:59 -0500
"Graham / KE9H" <ke9h.graham at gmail.com> wrote:

> Does using the signals from GPS (USA), plus GLONASS (Russia), plus Beidou
> (China), and plus Galileo (Europe), actually provide any improvement in
> time/frequency accuracy?

There are multiple angles to this:

1) Galileo offers the widest bandwidth signal (E5a+b) of all the currently
fully operational GNSS services. This brings huge advantages when it
comes to mitigating multi-path (the largest contributor to uncertainty,
even before ionospheric delay if you have a non-optimal antenna position).
GPS, as far as I am aware of, has no plans for any such wide bandwith signal.

2) Galileo is currently the only service that has full L1+L5 coverage.
It will take GPS a couple of years until there are enough block III
birds up there to offer a similar coverage on L5. Where a couple is probably
in the order of 10-20 years, if not longer (GPS satellites are too well
made and don't die quickly enough).
While L1+L2 works similarly well for ionospheric free solutions, L2
is not a protected GNSS band (it's even secondary user in some ITU regions)
and may get interference from radar systems and microwave links.
L5 is a protected GNSS band. Additionally, L2C (the civil L2 GPS signal)
is currently only broadcast from 19 satellites. While this should probably
be enough for a lot of cases, it does not provide full coverage of earth yet.
My guess for full coverage (ie 24 birds with L2C) is in the order of 3-8 years.

3) If you are doing common-view time transfer, then you want to have
as many visible satellites as possible to decrease errors due to noise 
and atmospheric disturbances. Thus having GPS+Galileo+Glonass+Beidu helps.
Similar things apply for all-in-view time transfer.

4) Only GPS and Glonass are sofar in the standard IGS products. Galileo
is only in the experimental MGES products. There aren't enough Beidu
receivers out there to reasonably use Beidu for IGS yet.

5) Galileo has hydrogen masers as clocks, which offer improved stability
compared to the rubidium masers (which all other GNSS use). But whether
this actually results in better performance, I do not know. I'm sure there
are studies on this out there, but I have not read any yet.

6) All but GPS and Glonass are fairly new systems and have not all kinks
ironed out yet. While the Galileo quaterly performance reports look good,
that does not mean that there aren't any hickups that might or might not
trip up your receiver. Similar concerns can be raised for Beidu, which
has the additional disadvantage, that it has been developed with a lot
of secrecy around it. Ie a lot of the potential system performance analysis
that scientists performed for GPS, Glonass and Galileo is missing for Beidu.

7) Local satellite based augmentation systems like QZSS, IRNSS, Beidu (the
China-only part) give additional satellites that are high up in the sky,
thus allowing to see additional satellites in urban areas (eg QZSS has
one satellite at elevations >60° at all times).

			Attila Kinali

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