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@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 _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.