Multi-band transmitter and monitoring system for Eclipse monitoring (Part 1)

It should not have escaped your attention - at least if you live in North America - there there have been/will be two significant solar eclipses occurring in recent/near times:  One that occurred on October 14, 2023 and another eclipse that will happen during April, 2024.  The path of "totality" of the October eclipse happened to pass through Utah (where I live) so it is no surprise that I went out of my way to see it - just as I did back in 2012:  You can read my blog entry about that here.

 Figure 1:
The eclipse in progress - a few minutes
before "annularity".
(Photo by C. L. Turner)

I will shortly produce a blog entry related to my activities around the October 14, 2023 eclipse as well.

The October eclipse was of the "annular" type meaning that the moon is near-ish apogee meaning that the subtended angle of its disk is insufficient to completely block the sun owing to the moon's greater-than-average distance from Earth:  Unlike a solar eclipse, there is no time during the eclipse where it is safe to look at the sun/moon directly, without eye protection.

The sun will be mostly blocked, however, meaning that those in the path of "totality" experienced a rather eerie local twilight with shadows casting images of the solar disk:  Around the periphery of the moon it was be possible to make out the outline of lunar mountains - and those unfortunate to stare at the sun during this time will receive a ring-shaped burn to their retina.

From the aspect of a radio amateur, however, the effects of a total and annular solar eclipse are largely identical:  The diminution of the "D" layer and partial recombination of the "F" layers of the ionosphere causing what are essentially nighttime propagation conditions during the daytime - geographically limited to those areas under the lunar shadow.

In an effort to help study these sort of effects - and to (hopefully) better-understand the propagation effects, a number of amateurs went (and are) going out into the field - in or near the path of "totality" - and setting up simultaneous, multi-band transmitters.

Producing usable data

Having "Eclipse QSO Parties" where amateur radio operators make contacts during the eclipse likely goes back nearly a century - the rarity of a solar eclipse making the event even more enigmatic.  In more recent years amateurs have been involved in "citizen science" where they make observations by monitoring signals - or facilitate the making of observations by transmitting them - and this happened during the October eclipse and should also happen during the April event as well.

While doing this sort of thing is just plain "fun", a subset of this group is of the metrological sort (that's "metrology", no "meteorology"!) and endeavor to impart on their transmissions - and observations of received signals - additional constraints that are intended to make this data useful in a scientific sense - specifically:

• Stable transmit frequencies.  During the event, the perturbations of the ionosphere will impart on propagated signals Doppler shift and spread:  Being able to measure this with accuracy and precision (which are NOT the same thing!) adds another layer of extractable information to the observations.
  

• Stable receivers.  As with the transmitters, having a stable receiver is imperative to allow accurate measurement of the Doppler shift and spread.  Additionally, being able to monitor the amplitude of a received signal can provide clues as to the nature of the changing conditions.
  

• Monitoring/transmitting at multiple frequencies.  As the ionospheric conditions change, its effects at different frequencies also changes.  In general, the loss of ionization (caused by darkness) reduces propagation at higher frequencies (e.g. >10 MHz) and with lessened "D" layer absorption lower frequencies (<10 MHz) the propagation at those frequencies is enhanced.  With the different effects at different frequencies, being able to simultaneously monitor multiple signals across the HF spectrum can provide additional insight as to the effects.
  

To this end, the transmission and monitoring of signals by this informal group have established the following:

• GPS-referenced transmitters.  The transmitters will be "locked" to GPS-referenced oscillators or atomic standards to keep the transmitted frequencies both stable, accurate - and known to within milliHertz.
  

• GPS referenced receivers.  As with the transmitters, the receivers will also be GPS-referenced or atomic-referenced to provide milliHertz accuracy and stability.

With this level of accuracy and precision the frequency uncertainties related to the receiver and transmitter can be removed from the Doppler data.  For generation of stable frequencies, a "GPS Disciplined Oscillator" is often used - but very good Rubidium-based references are also available, although unlike a GPS-based reference, the time-of-day cannot be obtained from them.

Why this is important:

Not to demean previous efforts in monitoring propagation - including that which occurs during an eclipse - but unless appropriate measures are taken, their contribution to "real" scientific analysis can be unwittingly diminished.  Here are a few points to consider:

• Receiver frequency stability.  One aspect of propagation on HF is that the signal paths between the receiver and transmitter change as the ionosphere itself changes.  These changes can be on the order of Hertz in some cases, but these changes are often measured in 10s of milliHertz.  Very few receivers have that sort of stability and the drift of such a receiver can make detection of these Doppler shifts impossible.
  

• Signal amplitude measurement.  HF signals change in amplitude constantly - and this can tell us something about the path.  Pretty much all modern receivers have some form of AGC (Automatic Gain Control) whose job it is to make sure that the speaker output is constant.  If you are trying to infer signal strength, however, making a recording with AGC active renders meaningful measurements of signal strength pretty much impossible.  Not often considered is the fact that such changes in propagation also affect the background noise - which is also important to be able to measure - and this, too, is impossible with AGC active.
  

• Time-stamping recordings.  Knowing when a recording starts and stops with precision allows correlation with other's efforts.  Fortunately this is likely the easiest aspect to manage as a computer with an accurate clock can automatically do so (provided that one takes care to preserve the time stamps of the file, or has file names that contain such information) - and it is particularly easy if one happens to be recording a time station like WWV, WWVH, WWVB or CHU.

In other words, the act of "holding a microphone up to a speaker" or simply recording the output of a receiver to a .wav file with little/no additional context makes for a curious keepsake, but it makes the challenge of gleaning useful data from it more difficult.

One of our challenges as "citizen scientists" is to make the data as useful as possible to us and others - and this task has been made far easier with inexpensive and very good hardware than it ever has been - provided we take care to do so.  What follows in this article - and subsequent parts - are my reflections on some possible ways to do this:  These are certainly not the only ways - or even the best ways - and even those considerations will change over time as more/different resources and gear become available to the average citizen scientist. 

* * *

How this is done - Receiver:

The frequency stability and accuracy of MOST amateur transceivers is nowhere near good enough to provide usable observations of Doppler shift on such signals - even if the transceiver is equipped with a TCXO or other high-stability oscillator:  Of the few radios that can do this "out of the box" are some of the Flex transceivers equipped with a GPS disciplined oscillator.

To a certain degree, an out-of-the-box KiwiSDR can do this if properly set-up:  With a good, reliable GPS signals and when placed within a temperature-stable environment (e.g. temperature change of 1 degree C or so during the time of the observation) they can be stable enough to provide useful data - but there is no guarantee of such.

To remove such uncertainty a GPS-based frequency reference is often applied to the KiwiSDR - often in the form of the Leo Bodnar GPS reference, producing a frequency of precisely 66.660 MHz.  This combination produces both stable and accurate results.  Unfortunately, if you don't already have a KiwiSDR, you probably aren't going to get one as the original version was discontinued in 2022:  A "KiwiSDR 2" is in the works, but there' no guarantee that it will make it into production, let alone be available in time for the April, 2024 eclipse. 

Figure 2:
The RX-888 (Mk2) - a simple and relatively inexpensive
box that is capable of "inhaling" all of HF at once.
Click on the image for a larger version.

The RX-888 (Mk2)

A suitable work-around has been found to be the RX-888 (Mk2) - a simple direct-sampling SDR - available for about $160 shipped (if you look around).  This device has the capability of accepting an external 27 MHz clock (if you add an external cable/connector to the internal U.FL connector provided for this purpose) in which it can become as stable and accurate as the external reference.

This SDR - unlike the KiwiSDR, the Red Pitaya and others - has no onboard processing capability as it is simply an analog-to-digital coupled with a USB3 interface so it takes a fairly powerful computer and special processing software to be able to handle a full-spectrum acquisition of HF frequencies.

Software that is particularly well-suited to this task is KA9Q-Radio (link).  Using the "overlap and save" technique, it is extraordinarily efficient in processing the 65 Megasamples-per-second of data needed to "inhale" the entire HF spectrum.  This software is efficient enough that a modest quad-core Intel i5 or i7 is more than up to the task - and such PCs can be had for well under $200 on the used market.

KA9Q-Radio can produce hundreds of simultaneous virtual receivers of arbitrary modes and bandwidths which means that one such virtual receiver can be produced for each WSPR frequency band:  Similar virtual receivers could be established for FT-8, FT-4, WWV/H and CHU frequencies.  The outputs of these receivers - which could be a simple, single-channel stream or a pair of audio in I/Q configuration - can be recorded for later analysis and/or sent to another program (such as the WSJT-X suite) for analysis.

Additionally, using the WSPRDaemon software, the multi-frequency capability of KA9Q-Radio can be further-leveraged to produce not only decodes of WSPR and FST4W data, but also make rotating, archival I/Q recordings around the WSPR frequency segments - or any other frequency segments (such as WWV, CHU, Mediumwave or Shortwave broadcast, etc.) that you wish.

Comment:  I have written about the RX-888 in previous blog posts:

• Improving the thermal management of the RX-888 (Mk 2) - link 
• Measuring signal dynamics of the RX-888 (Mk 2) - link
  

Full-Spectrum recording

Yet another capability possible with the RX-888 (Mk2) is the ability to make a "full spectrum" recording - that is, write the full sample rate (typically 64.8 Msps) to a storage device.  The result are files of about 7.7 gigabytes per minute of recording that contain everything that was received by the RX-888, with the same frequency accuracy and precision as the GPS reference used to clock the sample rate of the '888.  

What this means is that there is the potential that these recordings can be analyzed later to further divine aspects of the propagation changes that occurred during, before and after the eclipse - especially by observing signals or aspects of the RF environment itself that one may not have initially thought to consider:  This also can allow the monitoring of the overall background noise across the HF spectrum to see what changes during the eclipse, potentially filling in details that might have been missed on the narrowband recordings.

Because such a recording contains the recordings of time stations (WWV, WWVH, CHU and even WWVB) it may be possible to divine changes in propagation delay between those transmit sites and the receive sites.  If a similar GPS-based signal is injected locally, this, too, can form another data point - not only for the purposes of comparison of off-air signals, but also to help synchronize and validate the recording itself.

By observing such a local signal it would be possible to time the recording to within a few 10s of nanoseconds of GPS time - and it would also be practical to determine if the recording itself was "damaged" in some way (e.g. missed samples from the receiver):  Even if a recording is "flawed" in some way, knowing the precise location an duration of the missing data allows this to be taken into account and to a large extent, permit the data "around" it to still be useful.

Actually doing it:

Up to this point there has been a lot of "it's possible to" and "we have the capability of" mentioned - but pretty much everything mentioned so far was used during the October, 2023 eclipse.  To a degree, this eclipse is considered to be a rehearsal for the April 2024 event in that we would be using the same techniques - refined, of course, based on our experiences.

While this blog will mostly refer to my efforts (because I was there!) there were a number of similarly-equipped parties out in the fields and at home/fixed stations transmitting and receiving and it is the cumulative effort - and especially the discussions of what worked and what did not - that will be valuable in preparation for the April event.  Not to be overlooked, this also gives us valuable experience with propagation monitoring overall - an ongoing effort using WSPRDaemon - where we have been looking for/using other hardware/software to augment/improve our capabilities.

In Part 2 I'll talk about the receive hardware and techniques in more detail.

Stolen from ka7oei.blogspot.com

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