[time-nuts] Substitute GPS in a Z3801A

Robert Benward rbenward at verizon.net
Thu Jun 24 12:30:31 EDT 2010


Excellent explanation.  Is it possible the phase shift is caused by the 
doppler of the satellite?  And the instantaneous change of the ramp slope 
(on boundries) caused by the receiver changing the PPS tracking from one 
satellite to another?  As the satellite passes overhead the spacing of the 
timing pulses will change, closely space on the horizon, far apart overhead. 
As the satellite drops below the horizon, the RX changes PPS tracking to a 
bird higher in the sky, accounting for the apparent change in phase slope. 
Approaching birds will slant the sawtooth one way, and departing birds will 
slant the other way.  Does any of this sound plausible?

Bob


----- Original Message ----- 
From: <bg at lysator.liu.se>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Thursday, June 24, 2010 12:02 PM
Subject: Re: [time-nuts] Substitute GPS in a Z3801A


Hi Bob,

Old GPS receivers had bad resolution with regard to when they could flipp
the 1PPS signal. Think along these lines... you have a digital system,
clock running at say 10MHz. You can only invoke the 1PPS at a clock edge,
this gives you 100ns resolution. The receiver then needs to calculate
GPS/UTC time and use the clock edge closest to its solution. With the
current non SA  GPS system the accuracy of the calculation is in the
10-30ns-ish region. This means the receiver internally have much better
clock information than it have means to export via the 1PPS signal. It can
however send its estimate on how wrong the 1PPS signal is via the serial
port.

Newer receivers have hardware with better 1PPS resolution - and better
tracking/position etc.

Tom probably has a much better explaination...

    http://www.leapsecond.com/pages/vp/sawtooth.htm

    http://www.leapsecond.com/pages/vp/heater.htm

    http://www.leapsecond.com/pages/m12/sawtooth.htm

--

   Björn
> So there is no point in substituting another GPS RX?   What is the
> sawtooth
> compensation is see mentioned? What about the antenna cable length
> compensation?  How much does that affect the AVAR?
>
> Bob
>
>
> ----- Original Message -----
> From: "Steve Rooke" <sar10538 at gmail.com>
> To: "Discussion of precise time and frequency measurement"
> <time-nuts at febo.com>
> Sent: Thursday, June 24, 2010 11:11 AM
> Subject: Re: [time-nuts] Substitute GPS in a Z3801A
>
>
> Sorry to butt in on this thread but, although you need a number of
> birds to get an accurate position,  you really only need one to get
> timing. If you have a six sat tracking receiver there should be enough
> to give accurate time. You can always set the elevation to limit what
> sats you can see so to screen out multipath reflections and it can
> still track six birds. OK, my Z3805A is currently tracking 9 birds but
> 3 of those are quite low horizon, although I really don't have
> anything to cause multipath from where I'm situated. The minimum
> number of sats I track is 4 and the max is 12 on a day, night cycle
> and the average is about 6.
>
> Steve
>
> On 25 June 2010 02:01, Robert Benward <rbenward at verizon.net> wrote:
>> Mike,
>> Thanks for the info!. Does anyone know for sure if the timimg improves
>> with more satellites tracked? I don't know how they derive the 1PPS, is
>> it
>> a blend or one satellite at a time? Maybe I will leave well enough
>> alone.
>>
>> Bob
>>
>>
>>
>> ----- Original Message ----- From: "Mike S" <mikes at flatsurface.com>
>> To: "Discussion of precise time and frequency measurement"
>> <time-nuts at febo.com>
>> Sent: Thursday, June 24, 2010 9:01 AM
>> Subject: Re: [time-nuts] Substitute GPS in a Z3801A
>>
>>
>>> At 10:48 PM 6/23/2010, Robert Benward wrote...
>>>>
>>>> Has anyone ever put another GPS in a Z3801A? Will the software
>>>> accommodate
>>>> another GPS that tracks say, 12 channels?
>>>
>>> Possibly 8 channels. You must use a Motorola Oncore VP. A 6 channel one
>>> was what HP used. There are mixed reports on using an 8 channel VP -
>>> some say it works fine, others say you must configure it in 6 channel
>>> mode (but you do pick up improved reception). Whether the different
>>> reports are due to changes in configuration or firmware of the z3801a,
>>> the Oncore, or both - I have no idea.
>>>
>>> The commands/responses used by the z3801a prevent it from working
>>> directly with newer GPS modules, like the GTs, UTs or M12s (even if you
>>> adapted them physically).
>>>
>>> If you want to try and get better performance with a newer GPS
>>> receiver, you should be able to leave the existing board in place, and
>>> bypass the 1PPS input with the signal from the newer receiver. You'd
>>> only get in trouble in exceptional conditions (new receiver loses sync,
>>> but existing one is reporting good satellite lock, etc.). Of course,
>>> the z3801a satellite reporting wouldn't be worth much.
>>>
>>>
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>
>
>
> --
> Steve Rooke - ZL3TUV & G8KVD
> The only reason for time is so that everything doesn't happen at once.
> - Einstein
>
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