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Adventures in Electronics and Radio
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WWV Short Term
or "How far away is Ft. Collins CO?"
I've written about time variation of HF signals at my
Signal Statistics page. An alternative view
of the vagaries of HF radio propagation is to examine time shifts in signal
arrival. If the path length changes, either due to changes in ionospheric height
or changed reflection zones, the travel time from transmitter to receiver will
In most cases, of course, it's not easy to determine how
many milliseconds or microseconds it takes for the signal to travel from the
transmitter to your receiver, or how this changes from moment to moment.
However, it is possible to measure changes in transit time for signals from WWV.
And, of course, there are a few special cases, such as LORAN-C, where the
purpose of the station is to measure transit time. (If we can monitor the
received signal phase, and if the transmitted signal is stable, we can determine
timing changes by looking at phase shift, but this requires more work than the
methodology I've used. WWV also transmits a 100 Hz binary encoded time signal
that can be used to derive a 1 pulse-per-second signal with suitable
November-December 1959, HP Journal, I found an interesting article on
comparing local crystal oscillators with WWV, using a method I had not
previously read. Anyone who has listened to WWV is aware of the time "ticks."
These ticks consist of 5 cycles of 1000 Hz tone (6 cycles of 1200 Hz for WWVH).
Interesting, but how is that relevant to transit time? The key is that these
ticks occur at a very stable interval, tied to the NIST's national time base, as
is WWV's carrier frequency.
The figure below shows the tick modulation details.
The HP Journal article measures the long term (hours or days or months) time
base stability by comparing the arrival time of the first half-cycle of the tick
tone against the time base. The time base under study is divided down and
triggers an oscilloscope at a one pulse-per-second (PPS) rate so that drift in
the time base causes the ticks to arrive early or late. As the HP Journal notes,
in any given second, changes in propagation may cause the tick to arrive a few
hundred microseconds early or late, but if observed over a long period, these
second-to-second changes wash out.
changed in the precision timing world since 1959, and in 2008 the methodology
laid out in the HP Journal can be used in reverse—if we have a sufficiently
stable and accurate 1 PPS time source, we can see how the propagation path
changes second-to-second by watching the tick arrival time advance and retard.
(It might be better to decode WWV's 100 Hz sub-carrier time data but that
requires more than just general purpose test gear.)
For the precision 1 PPS timing reference, I use a Trimble
Thunderbolt GPS disciplined crystal oscillator. The 1 PPS output is maintained
with 20 ns or so of UTC time, and is automatically corrected for the
latitude/longitude and elevation of my GPS receiver. Hence, changes in tick
arrival time from second to second reflect changes in the ionospheric path, not
changes in the local clock, with a high degree of precision.
The figure below shows my test setup. The TDS430A digital
oscilloscope triggers on the GPS 1 PPS waveform. Not shown is a
Prologix GPIB-USB adapter and a
PC running KE5FX's 7470
plotter emulation program.
The figure below shows tick arrivals with about 300 µs propagation jitter as
received 50 years ago by HP. In 1959, WWV was located in suburban Washington DC,
so HP's data reflects a coast-to-coast path.
WWV Tick Arrival as Illustrated in the Nov/Dec 1959 HP
The image below shows WWV (5 MHz) ticks as received in my test setup, with a
slight difference. In order to show the ticks cleanly, I've enabled averaging on
the TDS430A oscilloscope, in this case 32 sweeps. Hence, second-to-second jitter
is suppressed by the averaging.
Now, let's look at second-to-second variation. To do this,
I've switched the TDS-430A's display mode to "variable persistence" which
displays the last 10 sweeps. The most recent sweep is shown as a solid line with
older traces changing to dots, with the oldest sweep having the fewest dots.
Hence, this is an image of the last 10 seconds of propagation delay data. (This
is the digital version of HP's multiple sweep photo exposure in HP Journal's
The image below shows quite stable
propagation from WWV at 10 MHz. The worst change in tick arrival time is around
100 µs but most of the sweeps show close to zero arrival jitter.
The figure below shows results closer to those seen in HP's Figure 6; there's
about 300 µs peak-to-peak jitter. However, the time ticks are still reasonably
well individually defined.
The image below shows an exceptionally stable period with little to no time
In order to show some not so good jitter, I switched to 5 MHz, at a time when
the path between Clifton VA and Ft. Collins CO was still being established. The
peak-to-peak jitter is close to 500 µs.
The image below shows a particularly disturbed period of 5 MHz propagation. It's
difficult to accurately assess the peak-to-peak jitter during this 10 second
period, but I think its around 700 to 800 µs.
How much of a change in propagation distance does a change of
a few hundred µs make? The speed of light in free space is 300x106
meters/second. Or, 300 meters per µs. Accordingly, a change of 100 µs
results from the path length from Ft. Collins, CO to Clifton, VA changing by 30
km, or about 18.5 miles. In the worse case I've illustrated, an 800 µs change is
a path difference of 240 km or 150 miles.
When the path length changes this much over the space of
10 seconds, it's unlikely to represent a single point of reflection physically
moving. Rather, it's almost certainly the result of a signal arriving in
multiple rays with rays from different physical reflection points predominating
during different sweeps.
This should also give us a crude view of how many paths
simultaneously exist. Looking at the best 10 MHz data, there's likely only one
predominant path during this 10 second interval. (It's possible that there are
multiple stable paths and the signal is the stable vector sum of those multiple
paths, but this seems unlikely.) In other cases—particularly at 5 MHz
during the day/night transition period—, it's apparent that there are multiple
paths, perhaps as many as five or six, with each path's dominant period being a
second or less. This implies that the signal statistics are more accurately
described by Rayleigh statistics. At other times, two-ray Rician fading may
predominate. And, at least for periods of 10 seconds, a single ray can
predominate. I've discussed Rayleigh fading at my
Signal Statistics page.
Is it possible to determine the distance from WWV's Ft.
Collins, CO transmitter to Clifton VA? If we know the relationship between the
1000 Hz ticks and UTC time, the answer is yes.
According to N. Hironaka and C. Trembath, The Use of
National Bureau of Standards High Frequency Broadcasts for Time and Frequency
Calibrations, Nat. Bur. Stand. (U.S.) Tech. Note 668, May 1975, 49 pp.
http://tf.nist.gov/general/pdf/453.pdf), the leading edge of time ticks
identify the start of each UTC second. Tech Note 668 recommends measuring to the
second zero crossing, however, as the leading edge of the first cycle is not
well defined. In this case, of course, the extra 1000 us (833 us in the case of
WWVH) must be subtracted.
In addition, there is some delay within the WWV audio
generation and processing chain that must be accounted for. This
information can be found in G. Nelson, M. Lombardi and D. Okayama, NIST Time
and Frequency Radio Stations: WWV, WWVH, and WWVB, NIST Special Publication
250-67, January 2005 (available at
http://tf.nist.gov/timefreq/general/pdf/1969.pdf). Although only WWVH
measured data is provided, the report says WWV has similar delays. Since the
mean audio processing delay is on the order of 17 µs, we'll ignore it.
There's also some timing uncertainty within the GPS chain,
including the Trimble Thunderbolt. This error, however, is considerably less
than the other errors in the system, being well below 100 ns, so we'll ignore
that as well.
The last delay that must be considered is that within the
6790/GM receiver. I measured the RA6790/GM's signal delay as 646 µs. (I
used a signal generator AM modulated at about 500 Hz and measured the phase
shift between the 500 Hz modulating signal and the 500 Hz recovered audio
from the 6790. The phase shift is 119°and the actual modulating frequency is
511.5 Hz, as indicated in the image below. The modulating signal is on trace 1
and trace 2 has the recovered receiver audio.) Incidentally, the Publication
250-67 measured the delay of its test receiver, an Icom R10, as between 118 and
122 us, depending on frequency band selected. The 6790/GM has significantly more
filtering than the R10 which accounts for the extra delay. A DSP-based receiver
might be expected to have delay of several milliseconds or more, by the way.
(To verify that this shift is not actually N cycles plus
119° degrees, I started with a much lower modulating frequency.)
The plot below shows the measured delay to the first
full-cycle zero crossing of WWV at 5 MHz. To reduce propagation jitter, I've
enabled averaging on the oscilloscope, with 32 traces averaged. The zero cursor
reference is on the leading edge of the GPS's 1 PPS output, and the total time
delay measured is 9.81 ms, or 9810 µs.
We now adjust the measured delay to account for the known
1 tick cycle
Net Propagation Delay
8164 µs corresponds to 2449 km or 1522 miles assuming the
speed of light in the ionosphere is essentially that of free space.
Using the airline distance calculator at
the distance between Ft. Collins CO and Clifton VA (actually Centreville, VA,
about 5 miles from Clifton) is 1466.42 miles or 2359.47 kilometers. A radio
signal, of course, traverses a longer path as it is reflected from a vertical
point perhaps 50 to 100 miles above the earth's surface. And, the reflection
points do no necessarily occur on the great circle path, which also increases
the total distance traveled by the signal.
My "time of arrival" measurement yields a propagation
distance quite close to the real distance, considering the measurement method
used and the uncertainty in the reflection point.