[time-nuts] ThunderBolt question

Tom Holmes tholmes at woh.rr.com
Sat Jun 6 14:26:39 UTC 2020 

As the chip is a message, not a time hack per se, all the delay can do is make it more difficult to decode the message. That's how these COFDM-type schemes work. It's why your digital TV signal (which in the US is not a COFDM signal, but still behaves similarly) falls off a cliff when things get bad rather than fade away into noise like analog does. With analog, you get ghosts, but in digital, you just get garbage which may or may not be decode-able into a picture and sound. 

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 9:54 AM
To: Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
Subject: Re: [time-nuts] ThunderBolt question

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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