Hi Markus, Jim,
thank you for your responses!
On 19.04.22 11:53, Markus Kleinhenz via time-nuts wrote:
I believe you will have a hard time finding a COTS solution that fits
most of your requirements.
You will likely have to split your signal chain into multiple devices:
1. UBlox ZED-F9T based GPS-Receiver with 1PPS out
2. Something like a JacksonLabs RCM (a ULN Oscillator that locks to a
externally provided 1PPS)
3. Distribution Unit
So you can spread your requirements.
Thanks for the suggestion. I've had a quick look at the RCM.
Unfortunately, its "1PPS synchronization counter resolution" is ±2.5ns,
which is below what a ZED-F9T can do with pulse quantization correction
(~4e-10 ADEV @ 1s) [1]. I presume a short PLL time constant and
therefore high resolution are key here to keep the OCXO within 1 ns of
the GNSS despite environmental disturbances (temperature, acceleration,
vibration). I think I'll have to realize the 1PPS tracking and OCXO
disciplining myself with a sufficiently accurate time interval counter
(e.g., TI TDC7200, which has ~50 ps resolution).
Another big challenge will be the 1ns relative accuracy between multiple
(moving!) devices. Without communication between devices this should not
be possible.
We've conducted RF measurements with a bunch of stationary(!), but
spatially distributed (1~2 km) SRS FS740 GNSSDOs (with Rb option) back
in 2019 and have achieved better than 5 ns synchronization accuracy
(verified using line-of-sight RF measurements) without communication
between the FS70 [2], simply because the FS740 does not support that. Of
course that's not really comparable to 1 ns with moving GNSSDOs, but is
already pretty decent for the FS740s' single-band GNSS receiver with 15
ns 1PPS timing accuracy (likely Trimble RES SMT 360).
On 19.04.22 15:49, Lux, Jim wrote:
I think that 1 ns might be achievable, if the receivers are in the
same general area, so they see the same propagation, but perhaps not
as an "off the shelf" device.
That's where the ZED-F9T's dual-band reception and differential timing
feature come into play, which improve the time pulse accuracy to 2.5 ns.
It's similar to real-time kinematic positioning (DGNSS on steroids) by
distributing a correction signal to multiple receivers.
We are getting <1ns with post processing using GIPSYx in a weak signal
environment with generally poor geometry from GNSS standpoint (we're
going to be in GEO, above the constellation, so the signals we see are
on the other side of Earth).
Unfortunately, post-processing is not really an option for me, as long
as I can avoid it. I think the ZED-F9T is as good as it gets in terms of
value-for-money. I'll keep you updated when I make any progress on
wiring up a proof-of-concept.
Best regards,
Carsten
[1]
https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/N8UR_GPS_Evaluation_August2020.pdf#page=25
[2] https://ieeexplore.ieee.org/document/9128562 <--- Behind IEEE
"paywall"; I can send you a personal copy of the paper upon request
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