[time-nuts] KD2BD WWVB receiver/decoder in QEX

Bob Camp kb8tq at n1k.org
Sun Nov 22 21:47:42 EST 2015


It’s not to hard to detect the ionosphere changing and impacting the 60 KHz signals. If you run one of the old 
style strip chart systems it goes nuts as the sunrise or sunset point passes between you and the transmitter. 
At some point the signal may even drop by enough for the receiver to loose lock. The mechanism is the 
changing height of the layers as they enter or leave the sunlit region. 

There are indeed interesting data plots generated on the low frequency stations. Some use fancy modulation
to help things out. We have yet to see how well the new modulation format on WWVB helps. 

One thing to consider: The distance from London to Anthorn isn’t all that far in it’s east-west component. It’s also a lot
shorter distance in absolute terms than the route signals take to get to the east coast of the US from WWVB. 

When I go to the NPL site, 


the most recent data that I see is from 2011 for MSF. 


The data is in microseconds. One us would be 1.16 x 10^-11 at one day. You don’t have to dig to deep to 
find days with half us deltas. Comparable data on GPS from the same source shows 3 to 4 ns as about the 
biggest delta on a day to day basis. Based on that, you have about a 100:1 advantage with a GPS system. 


> On Nov 22, 2015, at 5:16 PM, Alan Melia <alan.melia at btinternet.com> wrote:
> Hi Bob I have just realised that MSF may work diffently??  The Anthorn signal is monitored by NPL at Teddington, West of London and frequency off-sets twice a day are published in parts in 10^12 on their web-site.....involving  lot of averaging I think.  They do not recommend using the signal after dark.
> You certainly could predict roughly the the skywave phase change during the day and the variation as the sun angle changes on the path over the year. It gets more difficult if there are flares or geomagnetic storms. If two independent stations at slightly different distances collect information it could be corrected even more accurately.
> The same ionospheric problem occurs of course with Loran, and the now closed Decca system..
> Alan
> ----- Original Message ----- From: "Bob Camp" <kb8tq at n1k.org>
> To: "Nick Sayer" <nsayer at kfu.com>; "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Sent: Sunday, November 22, 2015 7:34 PM
> Subject: Re: [time-nuts] KD2BD WWVB receiver/decoder in QEX
>> Hi
>> The most basic gotcha with WWVB is that propagation can (and does) shift the
>> carrier more than a full cycle over the course of a day. That’s at 60 KHz, so one
>> cycle is a lot (as in 16.666 ppm). Even at one second with a not so great receiver
>> and a poor antenna, GPS should give you ~0.01 ppm. Right up front, you have a bit of a problem.
>> (Yes I’m mixing measurements in that comparison, but the point is still valid).
>> What I keep wondering is - There is no big mystery about the WWVB transmitter's
>> location. You likely know your own location as well. Part of demodulating the data
>> gives you day of the year. From that you can figure out some of the basics of the
>> propagation effects (sunrise is at X:XX sunset is at … etc). You also could grab stuff
>> like weather data fairly easily (no idea if that actually helps). If you fit out the basic
>> propagation impacts, WWVB could get a lot better. At the very least, you would know
>> when to ignore the signal.
>> So yes, you could do better today than they could back in the good old days in terms of
>> the propagation coarse effects.
>> Unfortunately, there also is data on 24 hour comparisons of WWVB carrier (same time of
>> day, one day apart). If you pick your time right (noon or midnight), the variable propagation
>> can be reduced quite a bit. Based on that data, you are doing well at 100 ppt over
>> 24 hours. Might the new modulation help that by 10X? ..maybe.  GPS over a 24 hour
>> period should be giving you something in the 0.1 to 0.01 ppt range (same sort of pick a likely
>> stable ionosphere time slot and compare).
>> Does that make a WWVB device un-interesting? Not by any means. If you stretch out the
>> time, both systems get down into the “I have nothing else that good” range. Checking one
>> against the other is indeed an interesting thing to do. You just need a *lot* of time to do it.
>> Bob
>>> On Nov 22, 2015, at 12:41 PM, Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
>>>> On Nov 22, 2015, at 7:47 AM, paul swed <paulswedb at gmail.com> wrote:
>>>> As mentioned a nice answer to the wwvb modulation change.
>>>> I looked up the parts and it seems that they have gone into the NOS state.
>>>> Though you can get some from digikey and such especially in the SOIC
>>>> package. Also the VCO isn't available.
>>>> It appears that the Chinese sight has the lmc6484 and LM387n at reasonable
>>>> prices for small quantities. Most likely will order from there.
>>>> Have not checked out the PIC chip yet.
>>>> The 74HCXX are common and reasonable.
>>> That’s kind of a shame. I’m sure a redesign with modern SMD parts could be accomplished.
>>> The big question is how the stability of a wwvb disciplined oscillator would compare to a GPS disciplined one (all other things being equal). Well, it’s a big question for me, since I have no idea, but I imagine simply asking here will give an immediate answer. :)
>>> I’d have to guess that the PLL would behave better given a 60 kHz reference rather than a 1 Hz one. But how stable is that 60 kHz reference after going through, what, a thousand miles of ionosphere or so?
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