[time-nuts] Re: 100 MHz Low Phase Noise Mobile GPSDO/GNSSDO?

[Markus Kleinhenz]

Hi Carsten, Jim,

Am 19.04.2022 um 18:51 schrieb Carsten Andrich:
> 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).

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 at 1s. That in turn would mean long time constants for the PLL and
your receivers wouldn't track the same disturbances.

>> 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.

>
> 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?


>> 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
>
Best Regards,

Markus