Hi Markus,
On 20.04.22 08:24, Markus Kleinhenz via time-nuts wrote:
Am 19.04.2022 um 18:51 schrieb Carsten Andrich:
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).
The concept of most GNSSDOs is to "flywheel" the 1PPS generation using a
stable local OCXO for tau <100s. This would allow for your spec of ADEV
<1e-10@1s. That in turn would mean long time constants for the PLL and
your receivers wouldn't track the same disturbances.
That's also what I had in mind for generating a stable 1PPS output. For
first locking the OCXO to the GNSS' jittery 1PPS, a high resolution time
interval counter is required, so the measurement jitter is significantly
below the 1PPS jitter. Certainly, finding the right tradeoff for the
choice of PLL time constant will not be trivial. A dynamic solution that
responds faster to local disturbances would be desirable.
On 20.04.22 08:24, Markus Kleinhenz via time-nuts wrote:
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).
I would love to read that Paper, that sounds impressive.
As Jim wrote, I think it hinges on the fact that all receivers see the
same sats. Dual-band mainly eliminates ionospheric errors but those are
pretty much the same if all receivers are in the same area.
I've sent you the paper privately. Unfortunately, even for the
stationary use case, a common satellite view for all GNSSDOs cannot be
guaranteed. It only gets worse for mobile applications where the
obstructions and therefore satellites in view change rapidly. This is
where I'd expect dual-band to excel, because I'd assume ionospheric
conditions differ substantially when the receivers view a disjoint set
of set of satellites.
On 20.04.22 08:24, Markus Kleinhenz via time-nuts wrote:
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.
Does the RTCM feature expect one fixed base station?
I think so. The ZED-F9T's datasheet specifies an RTCM 1005 message as
the base station information [1], which AFAIK does not include the
information required for moving baseline corrections [2]. However, you
wouldn't necessarily have to operate a separate base station, as regular
RTK correction services should also work (haven't verified with the
ZED-F9T). The German state of Thuringia (and some others), provide these
RTCM streams via Ntrip free of charge [3].
Best regards,
Carsten
[1]
https://content.u-blox.com/sites/default/files/ZED-F9T-00B_DataSheet_UBX-18053713.pdf#page=6
[2]
https://content.u-blox.com/sites/default/files/ZED-F9P-MovingBase_AppNote_(UBX-19009093).pdf#page=10
[3] https://sapos.thueringen.de/
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