Dana,
> During my Arecibo Observatory days we used NIST's TMAS service to keep
> our H-maser-based station clock synced with UTC.
And before that, the observatory used Tom Clark's Oncore & SHOWTIME and
later Rick Hambly's CNS clock & Tac32Plus, yes?
Rick continues to develop the CNS clock, having switched from the
Motorola and iLotus receivers to u-blox T receivers. His papers are on
cnssys.com or gpstime.com. Check out a recent one like:
"High-accuracy Time and Frequency in VLBI "
https://www.cnssys.com/files/TOW/High-accuracy_Time_and_Frequency_in_VLBI_2019_sem.pdf
Bonus: lots of graphs and photos, masers, receivers, etc.
Main publication page: https://www.cnssys.com/publications.php
Check out the performance he's getting. This is with a 6T. That's not
dual-frequency or multi-constellation. Just plain old L1 GPS. It's way
better than +/- 20 ns. So I'm really confused by what you're saying
below. Did Arecibo get rid of the CNS clocks?
/tvb
On 2/27/2021 8:18 AM, Dana Whitlow wrote:
Thanks, Bob.
It seems to me that, depending on the positions of sats visible to one's GPS
antenna and the spatial distribution of free electron density in the
ionosphere,
the ionospheric contribution to position errors could sometimes largely
cancel.
But that observation may (or may not) reflect strongly on one's ability to
get
accurate absolute time from GPS on "average" days.
During my Arecibo Observatory days we used NIST's TMAS service to keep
our H-maser-based station clock synced with UTC. Our user community
(mainly VLBI and pulsar timing people) seemed pretty satisfied with +/-
100ns
accuracy, so I tried to do better by keeping things well within +/- 50 ns
during
my reign. IIRC, NIST was claiming that TMAS could produce results mostly
within about +/- 20 ns.
To be honest I'm baffled by how time transfer much better than that could
be achieved in practice.
Regarding Q3, yes I'm aware that *some* GPS receivers do the estimation of
ionospheric delay. What I was asking was: Do any of the relatively
inexpensive
receivers to which we time-nuts have access do this? Here I'm speaking of
those being sold for no more than a few hundred USD.
Dana
On Sat, Feb 27, 2021 at 9:08 AM Bob kb8tq <[email protected]> wrote:
Hi
On Feb 27, 2021, at 9:41 AM, Dana Whitlow <[email protected]> wrote:
I've long understood that ionospheric delays and variations thereof lead
to
*position*
uncertainties in GPS navigation receivers, to the tune of perhaps 10m
(2DRMS IIRC).,
and that these are said to constitute the single largest GPS error
source.
Q1: Would this not imply timing errors of comparable magnitude (10's of
nsec)
for a single band GPS?
Once all the signals “hit” the antenna, the delays are mostly common mode.
Instead of showing up as a position error, they show up as an error in the
time estimate. Since time is one of the things you estimate in the
solution
(along with X,Y, and Z) it get’s it’s own independent solution.
Q2: Why have I not seen this issue raised in connection with the present
discussion
about achievable absolute timing accuracy?
GPS time transfer is often done to the sub-ns level. There are a number of
papers on this.
Q3: Do any of the "modern" timing GPS receivers available to civilians do
dual-band
reception in a way that includes estimation of (and correction for)
said delays and
their variations? I know that Garmin, for one, is now selling L1/L5
handheld GPS
receivers (GPSMAP66sr and GPSMAP65s), but I've seen no indication
that these
units make any attempt at doing such corrections.
Yes, some receivers do an estimate of ionospheric delay based on the
variation of that delay with frequency. This does not help with
tropospheric
delay or all of the various “common mode” issues we have been talking
about.
It is also unclear how the “unknown” timing variation between the bands
due to the antenna impacts these solutions…..
Bob
Dana
On Sat, Feb 27, 2021 at 7:43 AM Bob kb8tq <[email protected]> wrote:
Hi
The same 20 or so ns delay in a saw would also apply to the
saw (or tight filter) in some timing antennas. It also would apply
to the saw(s) in some modules. Even if the tolerance is “only”
a couple ns on each of them, you *could* have 3 or more in the
chain.
Lots of numbers to crunch to get to 5 ns “absolute”. One could go
grab a GPS simulator and start poking. First step would be to find
a simulator that is spec’d for a < 5 ns tolerance on the PPS into
GPS out. I do believe that rules out the eBay marvels that some
of us have lying around …..
Simpler answer would be a quick “clock trip” with your car full
of 5071’s …… hour drive over to NIST and then back home.
That sounds practical for most of us :) :)
Bob
On Feb 26, 2021, at 9:29 PM, John Ackermann N8UR <[email protected]> wrote:
A while ago I tried doing a decidedly non-anechoic measurement with a
VNA exciter going to a 1500 MHz ground plane and the receiver connected
to
the antenna (with a known delay cable) and I got a similar result, but
there was enough noise that I didn't think I could nail it down to
within
10 ns.
I've also measured GPS antenna splitters and they tend to have 20-ish
ns
delays, mainly due to the SAW filters. I did surgery on an HP splitter
to
remove the filters so it could be used for L1 and L2 and that dropped
the
delay down to only 1 or 2 ns.
So there's definitely lots of stuff to calibrate if you want to get
accurate time transfer.
John
----
On 2/26/21 8:02 PM, Michael Wouters wrote:
Typical L1 antenna delays range from 20 to 70 ns.
I know of only one antenna for which a delay is given by the vendor
and
the
technique used was just to measure the electronic delay ie by
injecting
a
signal into the circuit. To do it properly, you need a setup in a
microwave
anechoic chamber with transmitting antenna etc. The practical
difference
may be small though, 1 or 2 ns ( sample of one antenna!).
Cheers
Michael
On Sat, 27 Feb 2021 at 11:42 am, John Ackermann N8UR <[email protected]>
wrote:
They're claiming "even better than" 5 ns for relative time, which
given
the 4 ns jitter seems at least sort-of reasonable. But until someone
shows me otherwise, I'm still thinking that getting better than 25 ns
absolute accuracy is a pretty good day's work.
John
----
On 2/26/21 5:26 PM, Bob kb8tq wrote:
Hi
I can’t think of many antennas (multi band or single band) that
claim
to
know their
delay to < 5 ns. Simply having a *differential* delay spec of < 5 ns
is
quite good.
Same thing with delay ripple, you see specs out to around 15 ns on a
lot
of antennas.
None of this is getting you to the actual total delay of the
antenna.
It’s a pretty good
bet that number is a bit larger than either of these.
Some of the ripple probably comes out in the standard modeling. I’m
not
sure that
the differential delay is taken out that way. Total delay, not taken
out
in any obvious
fashion ( at least that I can see). If the F9 has a built in antenna
database, that’s not
mentioned in the doc’s. Any benefit from the corrections would have
to
be part of
post processing.
No, that’s not the same as talking about the F9 it’s self doing X
ns,
but it would be part
of any practical system trying to get close to 5 ns absolute
accuracy.
5 ns *relative* accuracy between two F9’s? I probably could buy that
if
the appropriate
one sigma / on a clear day / with the wind in the right direction
sort
of qualifiers are
attached.
Bob
On Feb 26, 2021, at 4:27 PM, John Ackermann N8UR <[email protected]>
wrote:
It's interesting that they talk about the F9 receivers offering 5
ns
absolute time accuracy. Does anyone know of tests confirming that,
and
what sort of care was required in the setup to get there?
John
----
On 2/26/21 9:34 AM, Robert LaJeunesse wrote:
FWIW. No detailed content, and a rather quick read. "Five key
trends
in GPS".
https://www.u-blox.com/en/blogs/insights/five-key-trends-gps
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