HF propagation delay prediction

[Dave Typinski]

Hi all,

Is there a way to predict the real propagation delay between two HF band
stations? Thinking about stations separated by many thousands of km.

The naive way would be to use the great circle distance and divide by the speed
of light.

One can also use the multi-hop geometry to arrive at a better path length -- but
then one would have to know the reflection height (get that from ionosonde data
if any exists near the link path) and the number of hops. Determining the
number of hops is where I get stumped.

How does one know how many hops are involved for a link between two stations at
a given frequency on a given date and time? Simple geometric analysis yields
multiple solutions -- so how does one pick the correct solution?

I understand VOACAP will provide the likelihood of a successful link, but can it
provide the number of hops or the true path length (and hence a more realistic
propagation delay)?

Is there some other way to predict this?

Thanks for any help!

73,

Dave, AJ4CO

[David Eckhardt]

I'll throw my 2-cents on this.  I believe what you really need to both establish the path and the number of hops required is a 3-dimensional real time map of the ionosphere and the TEC.  The path-centered ionosonde may not be the "cloud" from which the signal is refracted.  What's more, once the wave is in the refractive portion of the ionosphere, the value of c in that medium is no longer the free-space c, but slower, whether it's the medium responsible for the O or X wave.  Most of us hams know that many times, the best azimuth for a given path is not the great circle path and can deviate greatly from that simple assumption.  Many times on 6-meters E and Es, it is anything but ray tracing and path-centric E or Es responsible for a given path.

Just my 2-cents

Dave - WØLEV 

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Dave - WØLEV

Just Let Darwin Work

[Dave Typinski]

Thanks for the input, David.

Any idea of the ballpark magnitude of the difference?

For example, if the great circle path is supposed to take 20 ms (6,000 km), what
range of real propagation times would one see in the middle of the night (both
stations in darkness) for a 10 MHz signal?

Are we talking about a 10% increase? 50% increase? 100% increase?
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[Robert McGwier]

Dave

Are you familiar with Pharlap?

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[David Eckhardt]

I am not the right person to give you quantitative answers to your question.  Possibly Dr. Terrance Bullett, WØASP, could shed more "scientific" input to your question.  However, I'll suggest a test you can run.  Three of us (Terry, WØASP, Jay, WØAIR, and myself, WØLEV) who are volunteers at the Little Thompson Observatory in Berthoud, Colorado, decided to exercise the "ECHO" application which is part of the WSJTX suite.  All three of us were able to detect our signal circumglobal propagating with a delay of nominally 135 ms.  This delay is what could be expected based on a slightly larger physical radius of the earth.  There was some "fuzz" in the echo delay, but it was real.   I first tried with 5-watts on 160-meters.  Then I reduced my power by 3 dB to 2.5-watts and then increased 3 dB to 10-watts.  The reported signal strength indicated through ECHO followed my reduction and increase in power lever to within 0.7 dB.  My tests were run during early evening hours here in N. Colorado.  Terry indicated there is no known propagation mode that might support this.  I tried it on both 160 and 80 meters with similar results.   Mind you, no kW++ involved which has become the "standard" for FT8 (in which I will not participate for that reason).

Dave - WØLEV  

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[David Eckhardt]

No, I am not, Robert.  Gotta look it up.....

Dave - WØLEV

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[Phil Erickson]

Hi Dave AJ4CO,

  I'll throw a few things in here.  

For example, if the great circle path is supposed to take 20 ms (6,000 km), what
range of real propagation times would one see in the middle of the night (both
stations in darkness) for a 10 MHz signal?

Are we talking about a 10% increase?  50% increase?  100% increase?

  The answer is: there is no way to answer that statement in the manner you are asking, without knowing what the ambient electron density is in 3 dimensions.  That's why HF propagation is interesting.  Bob's suggestion of PHaRLAP, a ray tracing program (https://www.dst.defence.gov.au/opportunity/pharlap-provision-high-frequency-raytracing-laboratory-propagation-studies), is how one would find out the question.  But such programs, including PHaRLAP, start out by assuming a given electron density distribution, in most cases through the monthly median ionospheric climate model IRI (International Reference Ionosphere).  That may or may not be right.

  Also regarding the other Dave W0LEV's comment:

What's more, once the wave is in the refractive portion of the ionosphere, the value of c in that medium is no longer the free-space c, but slower, whether it's the medium responsible for the O or X wave.

  I'd like to clarify that a bit.  c is c: the speed of light in a vacuum (~3E8 m/s).  Referring to the velocity in the ionosphere as "c" is potentially confusing.  The main point: there is a huge difference between the PHASE velocity (speed of the maxima of the carrier) of the EM wave and the GROUP velocity (speed at which information travels).  The phase velocity can indeed be greater than the speed of light without violating relativity, and this happens every time you send a wave up into the ionosphere, as its refractive index is less than 1.  (That's why rays bend back toward the ground in fact and not more toward vertical and out of the ionosphere.).  This fact perplexed everyone in the early days of radio, including most of the classical physics community.

  However, for the EM laws which govern electromagnetic propagation in the ionosphere as a medium containing electrons and ions, the GROUP velocity - that speed at which the wave envelope moves; think AM modulation for example - is always less than the speed of light.  The product of phase and group velocity must always = c**2.

  What confuses most people is that in free space, phase velocity = group velocity = c.  But not in a dispersive medium like the ionosphere, where different frequencies and spatial wavelengths travel at different rates.

  Some information on this is at this link (warning:math) -

https://farside.ph.utexas.edu/teaching/em/lectures/node100.html

  and discussion of group velocity is here (check the animated pictures for the "phase > group velocity" case):

https://en.wikipedia.org/wiki/Group_velocity

  Yes, it's complicated, but that's life isn't it?

73

Phil W1PJE

 

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[Ethan Miller K8GU]

Dave (AC4JO),

I composed a response and then realized that it might not be exactly what you needed.  It would be extremely helpful if we knew what you were attempting to do.  PHaRLaP is only as useful as the ionosphere model you use.  The built-in IRI is good enough for a lot of things, and certainly for demonstration purposes; but, it takes some expertise to interpret the results and tune the inputs correctly.  There's no one best ionospheric model; rather, you'll have to choose a model that represents the physics you care about.  "Consider a spherical cow," as the title of a book goes.  I, and I think others on the list, would be happy to assist in that.

There is "a ray-tracing method of progressively-less-naive assumptions starting with Great Circle propagation over a cue ball Earth surrounded by a partially-silvered mirror cavity ionosphere" that may be relevant here.  I discussed it a little bit in my 2018 HamSCI presentation on "The Ionosphere's Pocket Litter," but it could be pursued further.

73,

--Ethan, K8GU.

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[David Eckhardt]

Thanks, Phil for the needed clarification.  The phenomenon I believe is known as birefringence.  In optics (or mineralogy), the clear calcite crystal is the classic example of birefringence (long ago and far away, I first became hooked on mineralogy, then came electronics, then physics).  Properly oriented with a dark straight line passing through the crystal (or cleavage rhomb), both the O and X rays are evident in that the single line becomes two.  The stronger line is the O ray and the somewhat weaker line is the X ray.  This is a physical (optical) manifestation of what happens in the birefringent ionosphere.  In the ionosphere this phenomenon is caused by the free electrons (TEC) behaving in the ever-changing magnetic field of the earth.  Of course, both the TEC and mag field are continuously modulated by solar activity.  It is highly dynamic.  Yes, oh for a 3D realtime model of the ionosphere!!  Ray tracing (geometrical optics) does not serve the highly dynamic properties of the ionosphere well at all (my experience with HF propagation as a ham for some 62 years - yes, HF propagation is fascinating - wait until you experience the LDEs!!), but it's a good first approximation of a very dynamic medium.

Yes, c is c in free space and the phase velocity can exceed c (free space) while still keeping Herr Dr. Einstein happy.  But the ionosphere is not free space.  Nor is the propagation of energy through a coaxial cable.  The velocity of propagation in that case is "slowed" (with reference to free space c) by the [√(εr)]^-1.  We typically see this referred to as Vp, the velocity of propagation in the cable.   However, coaxial cable does not present a birefringent medium as does the ionosphere and the calcite rhomb. 

Just in case interested readers have never experienced the calcite rhomb birefringence,  I'll attempt to attach a picture of that.  See the attachment (I hope this group permits attachments).  View the picture as a static chunk of the ionosphere.     

Enough......

Dave - WØLEV

s 

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[Dave Typinski]

Hi Robert, ditto what Dave W0LEV said.
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[Ethan Miller K8GU]

hi again,

So, the biggest problems with PHaRLaP are the need 1) for the MATLAB license (solvable in many cases, but usually not free) and, 2) to have an appreciation for what inputs (and there are many) are appropriate.  While PHaRLaP can produce data for lovely plots, it's quite likely that it's overkill for your application.

I'm happy to walk you through setting it up...

73,

--Ethan, K8GU.

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[Leffke, Zachary]

Maybe a naïve question (still ingesting Phil’s response, thanks for the link!)…..But I’m also hyper interested in this topic area and measuring this sort of thing.

 

Would it be possible to use the WWV ‘time ticks’ (the metronome-like clicks when monitoring with AM or SSB) to get an estimate of the propagation delay, at least on that path between Ft. Collins and wherever the receiver is located?  Simple version of the idea is an SDR, synced with a GPSDO (including PPS), look for the rising edge of the ‘tick’ (or some equivalent but better version of a ‘tick detector’)……..count the number of samples since the PPS timestamp (or equivalent method for that) and then given the sample rate, compute the sample delay and thus delay in seconds?  Basically, I ‘know’ the tick start at time X (rollover of the UTC second, measured as a rising edge event of PPS tagged in the SDR sample stream) and I detect the tick at time Y (with Y being the delta from PPS), so group delay is thus Y-X?  Might be interesting with a wideband SDR that can channelize and run against all WWV emitters simultaneously to get a range of delays at a given time instant across the band.

 

I use WWV as the example above because I know they are using high precision/accuracy clock references, but if the method basically works it could be applied to things like WSPR signals (though the source time sync at NTP levels might not be accurate enough if theres milliseconds of slopt in the ‘start time’…..though maybe highly calibrated WSPR emitters could be put on the air that don’t use NTP but rather GPSDOs?).

 

And as I was typing this email……and googling WWV…..found this:  https://www.nist.gov/pml/time-and-frequency-division/time-services/wwvwwvh-scientific-modulation-working-group

 

So that explains the funky ‘swoops’ I’ve been seeing periodically ‘on and around’ the WWV signals (just got a remotely accessible dual pol HF receiver up and running on our Radio collection network)….instead of ‘tick detectors’ maybe matched filters on the modulations mentioned above, or some kind of autocorrelation method combined with knowledge of the transmit waveform and start time (guess…?) are a better why to get a more precise time of arrival measurement (maybe that’s part of the experiment!).

 

Also maybe of interest…if you can measure the group delay, then you can estimate elevation angle of arrival from a single antenna…(application of Breit-Tuve theorem and Martyns equivalent path theorem) from this paper: Separation of O/X Polarization Modes on Oblique Ionospheric Soundings (https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017RS006280) though azimuth might still be ambiguous.  But if you (maybe again naively assume great circle path for az estimate) have azimuth and elevation angle, you can infer the incidence angle on the antenna and the resulting expected phase differences between the propagating modes (if I’m understanding it, its only 180 degrees for X/O modes if the incidence angle is normal to the + sign of the crossed dipole, like with NVIS if the dipoles are ‘looking up’, and phase delta will decrease from 180 degrees as the angle of incidence drops towards the horizon)…….sorry all this is a bit of a distraction from the main question, but is why I’m interested in measuring group delay (to ultimately try to measure X/O mode separation with the crossed HF active dipole system).

 

Again, probably pretty naïve thought.  I need to start attending the HamSCI meetings more and stay on top of things (if I can ever get up for a breath of air from other work projects I’m drowning under).  Is the basic idea though of knowing the ‘start time’ of the tick (or similar waveform) and having something like a PPS reference on the receiving end to measure the delta (with detection via some kind of ‘tick detector’ or autocorrelation process) valid for obtaining group delay info?  Any significant gotchas (smearing of the ‘tick detection’ process maybe if multiple paths are present and relatively close to each other)?

 

Cool stuff.

-Zach, KJ4QLP

 

P.S.  IS there anyway to get info from the WWV Modulation Working group on what kind of modulations are transmitted when?  I know its on the 8 minute mark, but something like what kind of waveform at any given 8 minute mark?  Are the details of the waveforms and the schedule easily accessible and available to the public?  Thanks in advance!

 

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[David Eckhardt]

When I lived in Albuquerque, I used the WWV time 'ticks' for bounces off the ionosphere.  At present in N. Colorado near Berthoud and Lyons, I'm too close for that.  I'm too lazy and could use them, but have too many other projects of interest to presently pursue that.  The ticks are quite useful. 

Dave - WØLEV

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[MNaruta GMail]

A bit difficult to find from the main HamSci page, but here it it:

< https://hamsci.org/wwv >


The meetings are recorded Zach and available on YouTube:

< https://hamsci.org/telecons >



Michael Naruta - AA8K

[Dave Typinski]

Hi Ethan & all,

The project is, in part, to determine the range of the Moon using emission from HAARP. 

On Jan 14 at 0200 UTC, the HAARP beam was steered toward the Moon, emitting 90 minutes of 2 second long 30 kHz wide FMCW sweeps centered at 9.6 MHz.

The emission from HAARP and the lunar return was recorded at AJ4CO Observatory.

To calculate the average distance to the Moon using this data, we must account for the propagation delay between the transmitter (HAARP) and the receiver (AJ4CO).

A good thing to do would have been to simultaneously record some 1 PPS ticks from a GPSDO in another data channel and simply measure the delay.  And of course that didn't occur to me until after the fact.  Still, we'd have to assume that the HAARP sweep timing was very close to perfect.

So, we seek a realistic approximation for the HAARP to AJ4CO terrestrial propagation delay for said date, time, and radio frequency.

It may be that there is no good way to do that.  However...

Turning the problem on its head, we can use the measured lunar return delay and the known distance to the lunar surface to infer the station-to-station propagation delay.  In that case, the calculated terrestrial propagation time is 29 ms as opposed to the 19 ms given by the great circle route.

We are thus left to wonder, is that realistic?  How does that compare to what a propagation model predicts?


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Dave

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[Dave Typinski]

Oops, typo. Emission was on at 0600 UTC to 90 mins thereafter.
--
Dave
never fails...

On 1/18/22 21:04, Dave Typinski wrote:
> Hi Ethan & all,
>
> The project is, in part, to determine the range of the Moon using emission from
> HAARP.
>
> On Jan 14 at 0200 UTC, the HAARP beam was steered toward the Moon, emitting 90
> minutes of 2 second long 30 kHz wide FMCW sweeps centered at 9.6 MHz.
>
> The emission from HAARP and the lunar return was recorded at AJ4CO Observatory.
>
> To calculate the average distance to the Moon using this data, we must account
> for the propagation delay between the transmitter (HAARP) and the receiver (AJ4CO).
>
> A good thing to do would have been to simultaneously record some 1 PPS ticks
> from a GPSDO in another data channel and simply measure the delay. And of
> course that didn't occur to me until after the fact. Still, we'd have to assume
> that the HAARP sweep timing was very close to perfect.
>
> So, we seek a realistic approximation for the HAARP to AJ4CO terrestrial
> propagation delay for said date, time, and radio frequency.
>
> It may be that there is no good way to do that. However...
>
> Turning the problem on its head, we can use the measured lunar return delay and
> the known distance to the lunar surface to infer the station-to-station
> propagation delay. In that case, the calculated terrestrial propagation time is
> 29 ms as opposed to the 19 ms given by the great circle route.
>
> We are thus left to wonder, is that realistic? How does that compare to what a
> propagation model predicts?
>
>

>> > <mailto:hamsci%2Bunsu...@googlegroups.com
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>> Phil Erickson

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[Dave Typinski]

Hi Phil,

Thanks for the great explanation of group vs phase velocity!
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[Ethan Miller K8GU]

Dave,

OK.  Cool experiment!  The first cut is pretty straightforward to do with the technique I started describing.  Let's establish some assumptions:

1. It's night time along the entire (short) path; therefore, we are dealing with F-region propagation only.

2. Approximate the Earth as a circular cross section and ray propagation is a reflection at radius RE+h.  This lets us use the Law of Cosines to compute "rays" under the Martyn Equivalent Path (triangular) formulation for group delay.

3. Using a ray that takes off tangent to the surface (zero elevation) toward an assumed reflection height gives you the maximum single-hop distance.  For the F region, that's about 4200 km, give or take.

4. Your path is 5690 km...divide by that 4200 and round up...two F-region hops.  Now, you can work out the geometry of the length of that inverted-W for a couple of different F-region reflection heights using the Law of Cosines.  It turns out that it doesn't make much difference excess beyond the Great Circle path, maybe 21 ms vs 19 ms.

5. You can do the same thing for more hops, long-path, etc.  None of these "toy scenarios" seem to explain what you observed.  The long-path delay is on the order of 111+ ms, for example.  A bunch of hops would take this in the wrong direction.

There are a couple of additional interesting notes I'll make and then I must go to bed:

1. Few of the ionosondes for which I can easily pull data (DIDBase) near that (short) path estimate a MUF3000 that reaches 9.6 MHz at that time.  This suggests that your "direct path" could have come in via a different mechanism, possibly a skewed path from a patch out in the Pacific for which the MUF supported propagation to AC4JO and HAARP.

2. What I'd be really curious to know is how you computed the delays.  This seems like an excellent application for an auto-ambiguity function, although autocorrelation might be sufficient?  How many delay lag peaks do you have?  HAARP has sufficiently high ERP that the skewed path is a reasonable possibility.

3. There may be other sources of group delay in the lunar reflection and space plasma, but I don't think they're going to account for 10 ms.  I'd be happy to be proven wrong, though.

Hopefully, my "toy ray tracing model" at least gave you some appreciation for the scales of values involved.  The 29 ms is an outlier, but not unattributable with some digging, I suspect.

73,

--Ethan, K8GU.

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[Dave Typinski]

Hi Ethan,

Since HAARP was aimed at transit (due south), we were perhaps seeing emission
from a sidelobe. Their main beam was at about 45° elevation, but who knows what
the sidelobe(s) to the east were aimed at. Using simple geometric "cue ball"
analysis assuming a 400 km reflection height and a 45° takeoff angle from HAARP,
we get 8 hops and a 29 ms propagation time. Buuuuut, that's a /lot/ of
assuming! Sure would be nice if a model supported that.

We used autocorrelation (data folding / stacking) to increase the SNR. Each
FMCW sweep lasted 2 seconds. 90 minutes of observing gave us 2,700 sweeps to
stack. That increased the SNR by 17 dB which turned out to be enough to measure
the time difference between the terrestrial and lunar paths.

We are in the process of writing everything up. Will post a link here when it's
done.
--
Dave

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[David Kazdan]

I have an electrical engineering senior project group working on that "as we speak."  I recorded about a month's data the way you're describing in December and January on 15 MHz (that was convenient given available radios and antennas).  The graphs are indeed interesting and the project group is planning on improving the hardware, the statistical reduction of the data, and the visual presentation.  The plan is to have an inexpensive version ready for distribution in a year, as the group is doing a two-semester project.

Let me know if you want details.

David AD8Y

Case Western Reserve University.

[Leffke, Zachary]

All,

Thanks very much for the replies both on and off list on this topic…below is my attempt at concise responses to all.

 

Dave, WØLEV – thanks for the feedback!

Michael Naruta - AA8K, thanks for the links.  I’m very aware of the youtube channel (I often watch you guys in my garage when working on various projects in attempt to ‘keep up’ on things!).  For the WWV working group link, awesome, exactly the kind of info I was looking for!  I might follow up on the signal specifics after I ingest the material on that page, and some of the links mentioned in Kristina’s presentation (I think the matlab script on the zenodo page has the details I’m looking for).  

Ethan, K8GU - no worries, thanks for the reply.  Also thanks for the reference to Steve’s Presentation.

Larry N6NC, thanks also for pointing me towards Steve and the documents you sent.

Steve, WA5FRF – Thanks for reaching out, I’ll follow up shortly.

David, AD8Y – I’m definitely interested in more details.  What I’m personally focused on at the moment is attempting to ‘learn by doing’ in terms of getting the hardware up and running and developing GNU Radio flowgraphs (jn the 3.8 framework, possibly running on a Pi4) to perform the signal processing.

 

Alas though, as always, I am massively overloaded on other sponsored research programs at work.  So I feel bad taking time from folks who graciously offer their time to help me out, then its radio silence from me as I start to drown under the workload and have to shelve projects like this (I still have invites for the WWV group and the main HamSCI meetings on my calendar…).  My attempt to ‘contribute back’ is by posting everything on github (like my active antenna solution based on the LWA design…its all up on github), so if I ever get stuff working, and if it’s deemed worthy enough by the community, hopefully somebody can make use of it. 

 

For the moment, I think folks have given me plenty of resources to go off and read in what minutes I have here and there for this effort.

 

Thanks again for the awesome responses!  Super helpful!

 

-Zach, KJ4QLP

[James Secan]

Dave,

This might help a little with your HAARP moon experiment. These plots show foF2 (top panel, Boulder, CO) and GPS-derived equivalent-vertical TEC (bottom panel, Seattle, WA) for two stations near the center part of the path you’re interested in. The red curve in the top panel is from the URSI foF2 model (used in IRI) run using the effective sunspot number derived for the northern hemisphere (given for each day). The red curve in the lower panel is from the old Bent model, which also uses the URSI model for foF2, and the green curve is the AIUB CO-R GIM analysis of global GPS-derived TEC. The TEC observations are derived from RINEX files I download and process to make estimates of vTEC. The Bent model also needs an estimated 10.7cm solar flux, and I use the seven-day mean F10.7 (shown for each day).

The ionospheric response to the little geomagnetic disturbance on the 14th shows up nicely.

Jim
3222 NE 89th St
Seattle, WA 98115
(206) 430-0109

[Dave Typinski]

Thank you, Jim!
--
Dave