In late 2019 we successfully employed 4 SRS FS740 (with Rb option; GNSS
receiver: Trimble RES SMT 360) to synchronize spatially distributed SDR
receivers over distances of 1~2.5 km for the purpose of locating RF
emitters via time difference of arrival. As a side product of that
measurement campaign we managed to assess the accuracy of the time
synchronization of the FS740, which was about 5 ns over the course of 4
hours with a little post-processing relying on log data from the FS740.
We circumvented the issues mentioned by Bob via a reference measurement
with known emitter location and line of sight to all receivers.
If you're interested in details, I'd refer you to a paper we published
on the measurement [1].
Best regards,
Carsten
[1] https://ieeexplore.ieee.org/document/9128562 (behind IEEE paywall; I
can send you a personal copy of the paper upon request)
On 01.05.22 02:50, Bob kb8tq wrote:
Hi
If you are looking at time ( = the absolute offset from GPS’s version of UTC)
then
there are a number of issues.
The antenna you use will have a delay and it may well vary more than a bit. The
cable to the antenna is in the same category. If both your modules run off a
power
splitter then those will not show up in an A-B comparison.
The modules themselves likely have SAW filters in them. These have group delay
just like any other bandpass filter. The tolerance on this is likely in the
10’s of ns
module to module. There are other bit and pieces that can contribute at the
“nanoseconds”
level.
A typical set of modules from a good supplier should come in to a +/- 20 ns
sort of
window for 2/3 of the parts ( = 1 sigma).
Geometry errors are fairly simple. A meter is 3 ns in free space. Each meter you
are off from “correct” will add 3 ns of “wobble” in the results. Just how this
shows
up is very dependent on the direction of the error an what sort of satellite
view you
happen to have.
Ionosphere can ( in high sunspot years) contribute 50 ns or more to timing
errors.
Tropospheric issues can also get into the mix at a bit lower level. You might
think
these would wash out on co-located units. Unfortunately they are not going to
start /stop using this or that sat at exactly the same time.
Lots of fun stuff to look for ….
Bob
On Apr 30, 2022, at 6:41 AM, Erik Kaashoek <e...@kaashoek.com> wrote:
Some more info
The two GPS do keep their phase stable vs a Rb within +/-10 ns. But the
absolute time difference of their PPS pulses was, after a cold start,
stable within +/- 20ns but the average value could be up to 100ns and
differed after every cold start.
The two GPS antenna cables had a length difference of 1 meter, but that
should cater for only 5 ns (?) One module is connected to the antenna with
only a C, the other has a 1 GHz CLC high pass filter between antenna and
module
Erik
Op za 30 apr. 2022 om 12:32 schreef Erik Kaashoek <e...@kaashoek.com>:
The PPS jitter of a cheap Chinese GPS module was measured at about +/-
10 ns.
But the phase of the PPS compared to a Rb varied substantial more.
To verify if this was possibly due to ionospheric or atmospheric
conditions the time difference between the PPS of two identical modules
using two identical rooftop antenna was measured. Both only used the GPS
constellation.
This showed difference of up to 100 ns. Switching to GPS+GLN did not
make a visible difference.
It was tried to set both GPS modules into fixed position mode but the
reported position still kept moving a bit (within 3 m) and the fixed
mode did not have a visible impact on the time difference variations.
Is a time difference of up to 100 ns to be expected when using two GPS
receivers or is this difference possibly due to bad application or
performance of the cheap Chinese GPS modules
Erik.
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--
M.Sc. Carsten Andrich
Technische Universität Ilmenau
Fachgebiet Elektronische Messtechnik und Signalverarbeitung (EMS)
Helmholtzplatz 2
98693 Ilmenau
T +49 3677 69-4269
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