[time-nuts] ThunderBolt question

Sat Jun 6 13:54:25 UTC 2020

I wish I could give some numbers.  Sadly, I don't know how to calculate
them nor do I
have the equipment to do empirical measurements of that level of accuracy.

But I could do an arm-waving estimate.  Suppose that the delayed signal
component
is in phase with the direct signal, but 20 dB down, and is delayed by 100
nsec.  Then
I could see how the apparent centroid of a given chip could be displaced by
about
10 nsec, which would be enough to cause complaint from many time-nuts.

Personally I'm not into time keeping of that order of magnitude, but I'd
like my GPS-
disciplined oscillator to be phase stable to within 10's of ps over about a
1 minute
time frame.   At present I cannot seem to do this well unless I let my Rb
free-run.

Dana

On Sat, Jun 6, 2020 at 8:43 AM Tom Holmes <tholmes at woh.rr.com> wrote:

> No caveats required.
>
> Give some numbers on how loud that MP signal has to be to cause a problem.
> My original example of line losses for a relatively short cable still
> suggests to me that it is a minimal problem. If the S/N of a satellite is
> poor, the receiver algorithm is likely to not use it if there are better
> choices, and there often will be.
>
> Tom Holmes, N8ZM
>
> -----Original Message-----
> From: time-nuts <time-nuts-bounces at lists.febo.com> On Behalf Of Dana
> Whitlow
> Sent: Saturday, June 06, 2020 8:52 AM
> To: Discussion of precise time and frequency measurement <
> time-nuts at lists.febo.com>
> Subject: Re: [time-nuts] ThunderBolt question
>
> Basically true, Tom, but with some caveats:
>
> The GPS system was originally designed, as you say, to provide
> positions and velocity
> information to military users in the field.  Errors of a few meters were of
> little consequence.
>
> But now, we have the time-nuts, surveyors, etc, who are decidedly pushing
> the envelope
> of GPS accuracy.  Some users want sub-nanosecond timing accuracy, surveyors
> want
> mm-level position accuracy, etc, and doing these things indeed requires
> extreme attention
> to detail.
>
> As I understand it, multipath components that are delayed by about a few
> chips or more
> basically appear as a wee bit of added noise and have negligible
> consequence unless
> they are comparable to the direct signal in power.  However, MP components
> whose
> delay is less than about a chip time have the effect of tugging the
> apparent temporal
> location of the direct signal to some new value.  Note that this is in the
> range of practical
> round trip cable delays in at least some installations.  At Arecibo, for
> example, the one-way
> cable lengths from GPS antennas on the roof to their respective receivers
> was about
> 200 ft.  Round trip was thus 400 ft physical, hence up to about 600 ft
> depending on
> what the cables were using for the dielectric.   That's about 0.6 chip
> length for the C/A
> code GPS signal.
>
> Another factor which will influence the error introduced will be the RF
> phase relationship
> between the direct and delayed signal component.
>
> Gee, this stuff gets complicated ...
>
> Dana
>
>
> On Sat, Jun 6, 2020 at 7:30 AM Tom Holmes <tholmes at woh.rr.com> wrote:
>
> > Dana...
> > The question that comes to mind is just how much effect a weak a long
> > delayed reflection will have on overall system performance since it will
> > only matter to SV’s with poor S/N. The modulation scene which allows all
> > the SV's to transmit on the same frequency has to be pretty robust in the
> > face of both widely varying signal strengths and multiple signals
> arriving
> > at different times. It’s a similar scheme to CDMA cell phones, which
> > operate in a much more difficult environment with regard to signal
> > strengths, multi-path, and number of on channel signals. And those work
> > amazingly well.
> >
> >  Further, I am led to believe that once you have enough SV’s in view to
> > get a good set of ‘readings’, ionospheric effects are the limiting factor
> > until you go to a multi-band receiver.
> >
> > Yes, to wring the last ounce of performance out of GPS takes attention to
> > the details, but don’t lose sight of how it was designed to work for
> users
> > in less than optimum (military field operations) in the first place.
> > From Tom Holmes, N8ZM
> >
> > > On Jun 6, 2020, at 7:14 AM, Dana Whitlow <k8yumdoober at gmail.com>
> wrote:
> > >
> > > It's one thing to maintain lock in a multipath environment, quite
> > another
> > > thing
> > > to get "full" accuracy of GPS measurements of PVT.
> > >
> > > An interesting difference between my scenario of poorly matched
> > impedances
> > > and "ordinary" multipath is this:  In the poor matching scenario, all
> the
> > > received
> > > signals will be impaired identically, while in the ordinary multipath
> > > scenario,
> > > signals from different satellites will suffer different (and
> > time-varying)
> > > multipath
> > > impairments.  I'm not at all sure what effect this difference will have
> > on
> > > final
> > > outcome, but my gut feel is that the case where all signals are
> impaired
> > > identically
> > > could lead to worse effects.
> > >
> > > Dana
> > >
> > >
> > >> On Fri, Jun 5, 2020 at 8:43 PM Tom Holmes <tholmes at woh.rr.com> wrote:
> > >>
> > >> Dana...
> > >>
> > >> I think that you are neglecting two important mitigating factors.
> > >>
> > >> 1. the cable loss at 1575MHz, even for a 25' run of RG-6, reduces
> those
> > >> reflections quite a lot from one end to the other.     It amounts to 2
> > - 3
> > >> dB in 25', depending on cable quality.
> > >>
> > >> 2. a 1.5:1 SWR is not a very big reflection to begin with, on the
> order
> > of
> > >> 20% of the incident power, about  7 dB. I am rounding a lot here just
> to
> > >> keep the math easy...for me.
> > >>
> > >> By the time a reflection has made the round trip from the receiver
> back
> > to
> > >> the antenna and them back to the receiver, which is how the delay
> would
> > >> have
> > >> to manifest itself, it will be down at least 15 dB from its original
> > self,
> > >> and probably more. Given the coding of GPS signals which allows
> several
> > >> satellites to share a common frequency band, that is not going to be
> > much
> > >> of
> > >> a problem. And if only one end of the path actually is 75 ohms, then
> > there
> > >> won't be a delayed signal.
> > >>
> > >> Tom Holmes, N8ZM
> > >>
> > >> -----Original Message-----
> > >> From: time-nuts <time-nuts-bounces at lists.febo.com> On Behalf Of Dana
> > >> Whitlow
> > >> Sent: Friday, June 05, 2020 9:01 PM
> > >> To: Taka Kamiya <tkamiya9 at yahoo.com>; Discussion of precise time and
> > >> frequency measurement <time-nuts at lists.febo.com>
> > >> Subject: Re: [time-nuts] ThunderBolt question
> > >>
> > >> I'd like to point out that mismatches at the ends of an antenna cable
> > *can*
> > >> cause trouble.  When both ends are mismatched, each bit of detail in
> the
> > >> signal
> > >> gets partially reflected back and forth, each time delayed by the
> round
> > >> trip propagation
> > >> delay in the cable, and so you have something like multipath going on.
> > >> Fortunately the
> > >> successive reflections get weaker with time, generally quite rapidly.
> > >> Since many
> > >> GPS users seem very concerned about multipath resulting from poor
> > antenna
> > >> placement,
> > >> I think this factor should be considered as well and not just get
> swept
> > >> under the rug.
> > >>
> > >> The amplitude of the "multipath" resulting from cable mismatches
> > depends on
> > >> the product
> > >> of the voltage reflection coefficients at the two ends of the cable.
> If
> > >> either end is perfectly
> > >> matched, then the quality of the match at the other end is not
> > significant
> > >> vis-a-vis apparent
> > >> multipath problems and only affects transmission loss.
> > >>
> > >> But when there is a mismatch on both ends, then the length of the
> cable
> > >> comes into play
> > >> as well.  A longer cable means more delay between successive
> > reflections,
> > >> which is just
> > >> like multipath involving longer delays between the direct and the
> > reflected
> > >> signals.
> > >>
> > >> Cheers,
> > >>
> > >> Dana      (K8YUM)
> > >>
> > >> On Fri, Jun 5, 2020 at 7:13 PM Taka Kamiya via time-nuts <
> > >> time-nuts at lists.febo.com> wrote:
> > >>
> > >>> 50 ohm / 75 ohm question is really irrelevant in this kind of thing.
> > >>> Trmble itself says in manual, not to be concerned with this apparent
> > >>> mismatch.
> > >>> In my particular case, I have a home lab standard and existing
> > system.  I
> > >>> have an antenna and network of distribution amplifiers.  They are all
> > 50
> > >>> ohms and N connectors.  Some ports have BNC adapters attached.  I
> have
> > >>> pretty much standardized everything to SMA, N, or BNC.
> > >>>
> > >>> I boxed a power supply, T-bolt, and buffer amp in a metal case.  I
> > bought
> > >>> a short cable (RG58) that goes from F to BNC.  On back of the case, I
> > >> have
> > >>> BNC to N adapter.  I also have a few adapters that goes from F to BNC
> > for
> > >>> the test bench.  It really doesn't matter what you use, as long as it
> > >> makes
> > >>> a solid connection.
> > >>>
> > >>> Advantage of F connectors and RG6 are, cheap, abundant, and low loss
> > for
> > >>> the size.  Advantage of having house standard is, less adapters and
> > less
> > >>> headache.....
> > >>>
> > >>> ---------------------------------------
> > >>> (Mr.) Taka Kamiya
> > >>> KB4EMF / ex JF2DKG
> > >>>
> > >>>
> > >>>    On Friday, June 5, 2020, 7:22:33 PM EDT, Robert DiRosario <
> > >>> ka3zyx at comcast.net> wrote:
> > >>>
> > >>> I have a Trimble ThunderBolt GPSDO that I just received. It has an F
> > >>> connector for the antenna input, and BNC connectors for the 1 pps and
> > 10
> > >>> MHz outputs. Is the receiver input impedance really 75 Ohms, or is it
> > 50
> > >>> Ohms and they just used the F connector to distinguish it from the
> > >>> others? What do people do, just use a 50 Ohm antenna?
> > >>>
> > >>> Thanks
> > >>>
> > >>> Robert DiRosario
> > >>>
> > >>> KA3ZYX
> > >>>
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