[time-nuts] Russian GPSDO

Tom Van Baak tvb at LeapSecond.com
Tue Oct 18 17:21:31 EDT 2011

> So where do I get a cheap, used Russian GPSDO?
> I have not seen any on EBay
> Can I trade them my Tbolt?  :)
> per:
> http://tycho.usno.navy.mil/ptti/1998/Vol%2030_18.pdf

Note that's a rather old 1998 paper and a lot has changed
since then. Not the least of which is GPS S/A going away
back in 2000. In addition, we have more and better GPS
SV operating now, along with WAAS, and other real time
GPS signal correction methods. Read some recent GPS
papers for comparison. See also CORS and IGS.

There's no doubt that adding GLONASS or Galileo sats
into the mix will help a lot. Compared with an 8-channel
TBolt many of today's high-end receivers can how handle
36, 72, or 120 data channels (see ads in a GPS world

The nice (awkward) thing about GLONASS is that each
SV has its own frequency. So the antenna and receiver
architecture is more complicated. Have you seen any
portable car navigation or cell phones with GLONASS?
But as a side effect you get ionospheric corrections for
free; something you can only get in GPS with expensive
dual-frequency (L1/L2) GPS receivers. So GLONASS
is cool, even if they do land more of them in the ocean
than in space ;-)

I used a fancy (loaner) GPS+GLONASS Javad receiver
in 2000 which got me to the sub-cm and sub-ns level.
You can now find these old receivers on eBay for next
to nothing; mine looked like item 300611206685 (but I
can't vouch for seller or the firmware options of that, or
any similar unit). You never get precision for free; a lot
of work goes into it.

I am very eager to see lab reports of low-cost combined
GPS+GLONASS receivers. I bet Said has good info on
the u-blox 4T vs. 5T vs. 6T but now that he's selling to us
instead of sharing with us, you've noticed how quiet he
is about all this. (Hey, no harm meant, but don't forget
where you came from).

I suspect the u-blox units are more concerned with quick
acquisition time and urban coverage than they are with
nanoseconds, but if someone knows for sure, please tell.

In summary, my impression is that -15/day level of GPS
performance is now common in the precision time world.
The usual set of tricks is single-sat or all-in-view common
view, dual-frequency receivers, full GNSS coverage, and
some level of post-processing (or at least near real-time)
for clock, orbit, and ionospheric corrections. Independent
of GPS, the guys with serious clocks (and your money)
just use two-way commercial satellite time transfer (e.g.,
USNO, PTB, etc.).


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