[time-nuts] Phase modulation detection/NIST plan

J. Forster jfor at quikus.com
Mon Jul 9 01:16:25 UTC 2012


If you have a deep fade every few hours or minutes, as is common, relock
time becomes an issue.

-John

==============


> Hi
>
> The clocks we would be using are *much* better than what most military
> systems use
.
>
> I also *assume* that an initial lock up that takes a hour is perfectly
> acceptable in this application. You will still need a lot of hours / days
> / what ever of data to get useful stability off of WWVB, spending an hour
> or more to acquire from a cold start will have little net impact.
>
> Bob
>
> On Jul 8, 2012, at 7:29 PM, J. Forster wrote:
>
>> A risky assumption, and a cold start could be tricky.
>>
>> Equatorial took many minutes to lock up, with a much higher data rate,
>> and
>> it did it by slowly sweeping the local clock.
>>
>> Aside: That's why military spread spectrum systems like good local
>> clocks.
>> They lock up a whole lot faster that way.
>>
>> -John
>>
>> ================
>>
>>
>>
>>> Hi
>>>
>>> In this case the data format and it's contents are highly "computable".
>>> If
>>> you have a good local clock *and* an initial lock, the rest of what
>>> follows is predictable. That of course assumes we know the real format
>>> 
.
>>>
>>> Bob
>>>
>>> On Jul 8, 2012, at 6:58 PM, J. Forster wrote:
>>>
>>>> Hi Peter,
>>>>
>>>> That's be the hard way, but yes, if the message BPSK coded is
>>>> computable
>>>> and of a known format. If the message contained more than time, like
>>>> solar
>>>> flux, it gets more complicated very rapidly.
>>>>
>>>> A similar thing was done with the Equatorial system 30+ years ago. In
>>>> that
>>>> case, each data bit was broken into something like 32 or 64 chips (I
>>>> don't
>>>> remember). There were two maximally distant, orthogonal chip patterns,
>>>> representing 1 and 0. The incoming BPSK message went through a 0 or
>>>> 180
>>>> degree switch, then the IF stages. The switch was driven from a local
>>>> (known pattern) chip generator, so that if everything was synced up
>>>> the
>>>> narrow band IF would put out the 0 or 1 that had been encoded. BTW,
>>>> this
>>>> trick vastly improved the system S/N becaust it narrowed the receiver
>>>> IF
>>>> bandwidth many times.
>>>>
>>>> If the chip pattern is not known (fixed) or computable (like a correct
>>>> TOD) things go to pot quickly.
>>>>
>>>> Rather than building such a kludge, it would be easier to use the
>>>> locked
>>>> clock in a newly designed receiver and phase compare that to your
>>>> local
>>>> standard directly.
>>>>
>>>> -John
>>>>
>>>> ==================
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>> Any possibility of using the decoded signal to un-do the modulation
>>>>> and
>>>>> feed the reconstituted signal to the older receiver?
>>>>>
>>>>>
>>>>>
>>>>> On 7/8/2012 12:56 PM, paul wrote:
>>>>>> Ei
>>>>>> Sorry if I have your name reversed. By taking this approach it
>>>>>> eliminates the ability to use wwvb as a frequency reference because
>>>>>> it
>>>>>> destroys that traceability.
>>>>>> Thats what we are trying to preserve. Or at least re-establish for
>>>>>> the
>>>>>> older phase measuring receivers.
>>>>>> Regards
>>>>>> Paul
>>>>>>
>>>>>> On 7/8/2012 12:10 PM, Tofurk Ei wrote:
>>>>>>> If the changeover you are talking about is this one:
>>>>>>> http://www.nist.gov/pml/newsletter/radio.cfm as a proof of concept
>>>>>>> a
>>>>>>> DVB-T
>>>>>>> dongle/upconverter combo could almost certainly handle PM easily to
>>>>>>> output
>>>>>>> whatever it encodes, when paired with gnuradio..
>>>>>>>
>>>>>>> The RTL2832U chip might also be able to handle some low band
>>>>>>> signals
>>>>>>> directly, using direct sampling. No upconverter.
>>>>>>>
>>>>>>> Regardless, then the data would be fed into gnuradio - the gnuradio
>>>>>>> developers GUI is called "gnuradio companion" It has a nifty way of
>>>>>>> doing
>>>>>>> this kind of thing, one builds a "flow graph" where the actual
>>>>>>> demodulation
>>>>>>> is simply laid out graphically and tested.
>>>>>>>
>>>>>>> When everything works to one's satisfaction the file is saved and
>>>>>>> it
>>>>>>> gets
>>>>>>> compiled - then it can run - its basically a python script.
>>>>>>>
>>>>>>> If the modulation scheme is public, I think you can be almost
>>>>>>> certain
>>>>>>> that
>>>>>>> gnuradio might be quite useful to rapidly design a tool to
>>>>>>> demodulate
>>>>>>> it.
>>>>>>> Perhaps very quickly.
>>>>>>>
>>>>>>> For the money, one really couldn't hope to beat the flexibility of
>>>>>>> this
>>>>>>> combination in any other manner. If I were interested in trying
>>>>>>> this
>>>>>>> I
>>>>>>> would join the gnuradio mailing list and ask there. Perhaps the
>>>>>>> answer is
>>>>>>> surprisingly simple.
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>>>>>>
>>>>>>
>>>>>>
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>>>>
>>>>
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