[time-nuts] Three-cornered hat on timelab?
jimlux at earthlink.net
Thu Apr 27 16:37:27 EDT 2017
On 4/27/17 12:10 PM, Bob Stewart wrote:
> Hi Bob,
> "You have roughly 25 ns p-p in the data you show."
> OK, here's a misunderstanding on my part right off the bat. You see the swing as a p-p value, when I've been looking at it as only +/- 12.5ns from the trendline.
> said some time ago:
> "Now toss in the basics of GPS. Depending on the day, you will get <10 ns to >100 ns swing over a 24 hour period. Today may or may not be the same as tomorrow."
> So maybe I'm thinking too much about the >100ns figure, and not so much about the <10ns figure you mentioned. The average doesn't seem to do much for me, either. So, is the probability curve between 10ns and 100ns, where 100ns is least probable, of the type y=2^-x? IOW, in a year, I might see one 100ns swing, I would probably see at least one or two 50ns swings, and will probably see anything less than that multiple times, with the probably increasing as the value gets lower.
> Not trying to crucify you with your own words, Bob. Like many of the time-nuts who don't post, I'm just trying to make some sense of this in terms I can deal with.
> From: Bob kb8tq <kb8tq at n1k.org>
> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com>
> Cc: Magnus Danielson <magnus at rubidium.se>
> Sent: Thursday, April 27, 2017 1:18 PM
> Subject: Re: [time-nuts] Three-cornered hat on timelab?
> You have roughly 25 ns p-p in the data you show. There are a number of 10 ns “cycles” in the data.
> Any of this *may* be due to ionosphere. They also could be due to other issues. With ~4.4 days of noisy
> data, it may be tough to spot a trend. Since the ionosphere is a bit random, there is no guarantee that
> you *will* always see a pretty sinusoidal trend line through the data. It’s a good bet that things quiet down
> around midnight. There is no guarantee that they always go nuts (or go nuts to the same degree) around noon.
I don't think it necessarily has a nice distribution. precise phase
measurement with GPS has lots of effects that are in the "about a meter"
Ionosphere is one - if you're near a ionosonde you might be able to get
a data set to correlate against. It's not necessarily a "worst at noon,
best at midnight" thing - some of the satellites contributing to your
fix will be on slant paths, so the solar ionization is not uniform.
Solid earth tides are another effect in that general magnitude (10s of
cm) - see Wikipedia or:
Thermal expansion and contraction of a variety of things:
1) the structure supporting your antenna
2) the earth's surface
3) the coax from antenna to receiver
4) any filters
5) the antenna
For most of these, they *are* periodic in some sense: solid earth tides
are affected by moon and sun, which have reasonably well known periods.
Thermal effects typically have a strong 24 hour cycle (although I've
seen some weird ones caused by shadows intermittently falling on stuff,
but still, it has a 24 hour periodicity)
Then there's various geometry/multipath things - choke rings help if
you're in a wide open area.
More information about the time-nuts