Tangerine SDR - Why is Chirp Decoding Important?

[Gary Mikitin, AF8A]

The following was a discussion point in the most recent Tangerine SDR telecom.   Being a newbie to space sciences, this went right over (many kilometers over) my head.

Is there a link, paper, etc. which explains why searching for and decoding ionosondes might be important to the PSWS project?  (I figured if I had this question, others might as well, so feel free to answer in the forum.)

73 de Gary, AF8A

“Discussion on chirp decoder algorithms for TangerineSDR. One issue is that some stations do not hold a rigorous schedule, another is that new stations will pop up unexpectedly with (previously) unknown chirp rates and from (previously) unknown locations. The search space is two or maybe three dimensional. Current solutions for searching for ionosondes are computationally expensive when done on the wideband data.”

[Ethan Miller K8GU]

Gary,

I think that's a great question---thank you for asking it. I'm
probably not the best person to answer this, but I'll give it a go.

The three aspects of this are purely practical:

There are really only about a dozen, maybe two dozen, if you include
space instruments, different types of ionospheric physics instruments
in practice. And more than 2/3 of them are unsuitable or impractical
for individual purchase or home use. So, that really leaves 3-4
instruments that can make meaningful measurements at reasonable cost:
a dual-frequency GPS receiver, a magnetometer, and an HF ionospheric
sounding receiver.

For most hams, if I had to sort those three, a real-time readout of
the data from the HF sounding receiver is probably most useful and
understandable for day-to-day operating. It will tell you the Maximum
Usable Frequency (among other parameters) on the circuit between the
sounder and the receiver. The magnetometer is probably second in the
usefulness of the real-time display, especially if you live at auroral
latitudes. The GPS won't show you much by itself; but, has scientific
value if you can aggregate the data.

As to the specific choice of chirp ionosondes, they have drawbacks
from a scientific perspective, but they produce very clean ionograms
and are pretty easy to process, especially if you don't know much
about the internal details of the transmitter. There are other, much
more sophisticated types, but these are easy to work with.

The fourth aspect is scientific:

The ionosphere, especially the ionosphere below the F-region peak down
to the D region (60-90 km), occupies a truly fascinating and
relatively poorly-understood part of the "space-atmosphere interaction
region" as it has been called. The behavior (delay, angle of arrival,
etc) of HF signals is particularly sensitive to structuring and
dynamic movement of the ionosphere in this region, especially between
the E region (90-120 km) and the peak of the F region density (around
300-350 km). The region between 100 and 250 km is particularly
challenging for most instruments due to the molecular makeup of the
atmosphere and relatively low ionospheric plasma densities at those
altitudes. It's also tough to maintain an orbit at these altitudes
for more than a few weeks or months. So, HF is potentially a great
diagnostic for this region. The advanced data processing techniques
needed to consistently combine data from more than a handful of
simultaneous HF soundings are current/recent research topics.

I'll stop there and see if it makes sense...

73,

--Ethan, K8GU.

> --
> Please follow the HamSCI Community Participation Guidelines at http://hamsci.org/hamsci-community-participation-guidelines.
> ---
> You received this message because you are subscribed to the Google Groups "HamSCI" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to hamsci+un...@googlegroups.com.
> To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/a2b7f2f5-1a78-4632-94f4-a88206c2ffean%40googlegroups.com.



--
http://www.k8gu.com/
Repair. Re-use. Re-purpose. Recycle.

[Gary Mikitin, AF8A]

Thanks, much, Ethan, that explains it well.  In layman’s terms:

If a given ionosonde, with known characteristics, does a sweep (chirps) across the bands...

And tens (or hundreds) of PSWS recorded their reception of the sweep...

With the reception information properly identified (who, what, when, amplitude, phase, etc.) and stored in a central database....

...a space researcher could analyze that data and do what scientists do....formulate, prove or disprove hypothesis.

Further, individual hams or spectrum users with the right skills, may be able to reach their own conclusions, based on their own short or long term experience.

Consider the light bulb lit!

Gary

[Ethan Miller K8GU]

Gary,

Yes, that's a great summary!

Just to amplify one point: One of the key things for hams is that it
will give some way to "validate" ("verify" might be a better term) the
VOACAP maps (e.g., those found on the HamClock and other places) and
other tools that we use for propagation forecasts/nowcasts. It might
even become an operating aid, if it picks up enough sounders (there
are not really enough of them to do a global specification; but, you
can learn about some specific paths of interest, e.g., to Europe,
Australia, etc).

On a personal note: I had a 75-meter QSO with Phil, W1PJE, on Sunday
night and heard several chirp soundings and a VIPIR during our
40ish-minute contact.

Thanks for asking that question...like you say, I'm sure others were
curious as well!

73,

--Ethan, K8GU.

On Tue, Dec 15, 2020 at 10:39 PM Gary Mikitin, AF8A

> To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/0b39f587-545e-45b5-8dfd-95dbfb705d07n%40googlegroups.com.

[Phil Erickson]

Hi Gary,

  Addition to additions:

  VOACAP is based on a monthly median view of the ionosphere.  Space weather happens on timescales down to seconds.  So real data beats a heavily smoothed model every time.  The research community is keenly aware of the problem.  So this is really valuable information wherever and whenever we can collect it.

73

Phil W1PJE

PS: Yes, K8GU and I had a nice conversation from Massachusetts to Ohio on Sunday, but hampered by S9+ band noise at least on my end - still have to investigate my local QRM environment.  Probably more of those pesky LEDs or grow lights out there; it's a jungle.

To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/CAFkosZLqJE%2B0w6nM7boFOF%2B6CiXNCYEhSZhgCXiJrVPOKK%2BaCA%40mail.gmail.com.

--

----
Phil Erickson
phil.e...@gmail.com

[Dev Joshi]

Thanks Gary, for a great question. I don't have to add anything new. But, to paraphrase -- it is the key diagnostic instrument  which will probe the ionosphere at finer resolution in time and space -- thereby 

providing an unprecedented opportunity to seek to understand the science behind many important phenomena in space. And, thereby be a valuable tool to make advances in space weather research -- modeling those phenomenon/predicting those phenomenon based on model results -- thereby saving many technological infrastructures on earth which could be impacted by adverse space weather impacts.

Just to share which we already know -- there is an institution Space Weather Prediction Center (https://www.swpc.noaa.gov/)  which does these jobs. The new research will further build upon the existing foundation ! That's the hope. Sorry, I couldn't add much than what is already known.

Thanks. 

Best,

Dev

To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/CAAZaqEsXBNuurf-ZE7Okcqc3oL6t8vYeGgK1icHQV_1SX0-cCw%40mail.gmail.com.

[Gary Mikitin, AF8A]

Once I understood the value of ionosonde reception to the PSWS project, I realized that I knew very little about them. So, in the off chance that future subscribers to this group search on ‘ionosonde’ or ‘ionogram’, looking for some education, I offer the following. Disclaimer: I’m no expert on these subjects, so before you take any of this as gospel, this famous maxim certainly applies: ‘Trust, but verify’ <grin>

Ionosonde / Ionogram Resources

Very basic, brief layman’s explanation of what these instruments are and why they are useful: https://en.wikipedia.org/wiki/Ionosonde

Somewhat better explanation, only slightly more technical, fills in some gaps other sites do not explain (ie what is an extraordinary wave?): https://www.hfunderground.com/wiki/Ionosonde

Very basic explanation of the curves seen on an ionograph (courtesy of web.archive.org, as this page is no longer available): http://web.archive.org/web/20030410232632/https://www.eiscat.rl.ac.uk/dynasonde/ionogram.html

Visual (waterfall displays) and audio clips of ionosondes on the HF bands: https://www.sigidwiki.com/wiki/Ionosonde

The mother of all ionosonde data collection sites, UMass Lowell’s Global Ionosonde Radio Observatory: A world map of ionosonde installations (real time, inactive and upcoming); links to millions of ionosonde records; ionosonde movies and more: http://giro.uml.edu

How can HF spectrum users, especially ham radio operators, use ionosonde data? Intriguing 2017 RSGB presentation (~1 hour long) by Jim Bacon, G3YLA: https://www.youtube.com/watch?v=u_ePpEZWE3c

Jim Bacon is the ham behind Propquest - a near real-time HF propagation tool (specific to the UK) which utilizes local ionosonde data: https://rsgb.org/main/technical/propagation/p-s-c-members-research/propquest-near-real-time-propagation-tool/

Bonus content: I’ve heard the term ‘SuperDARN’ (Super Duper Auroral Radar Network) come up in HamSci discussions. I can’t vouch for the accuracy of this (or any) Wikipedia article, but in the realm of ‘something is better than nothing’: https://en.wikipedia.org/wiki/Super_Dual_Auroral_Radar_Network

73 de Gary, AF8A
Cleveland, Ohio, USA

[Phil Erickson]

Hi Gary,

  Those are great links.  Very nice summary.

  One slight correction:   SuperDARN = Super Dual Auroral Radar Network (not "Duper").  The name came from the history of how this international network was constructed.  It was first DARN (Dual Auroral Radar Network), because the idea was to put two HF radars together with overlapping fields of view with enough angular separation that one could derive auroral E region (~100-110 km altitude) ionospheric velocities (two samples of an almost perfectly horizontal flow vector).  The pre-DARN system in Scandinavia called STARE did this.  The radar used scatter from decameter irregularities (10s of meters) that are embedded in this flow as tracers - hence the use of HF frequencies with wavelength of 10s of meters to match the scale size of the irregularities.  This was concentrated at high latitudes to study fast ionospheric velocities associated with the aurora borealis / northern lights.  As more stations were added beyond the two of "Dual", then it got morphed into SuperDARN - a network of paired radars with complementary fields of view.  Later, Antarctic radars were added to do the same sort of measurement at the South Pole.

  Early work comparing measured flows using different techniques (such as incoherent scatter radar) found that not only E region but F region ionospheric velocities could be measured, which opened another field of study.  The flows are not necessarily the same as a function of altitude, because in the E region ions are highly collisional and get bashed around by the neutrals for horizontal flow and large currents - the electrons are still stuck to the magnetic field lines.  In the F region, they do not do this because collision frequencies are much lower, and everything goes in directions perpendicular to B.

  The network expanded beyond a ring of radars viewing high latitude / auroral flows to then go to mid-latitudes, but the whole thing is still called SuperDARN.  It is a prime example of one of the best international coordinated observation networks in the world - many different countries under many different funding agencies, but with a common goal to measure flows of ionospheric plasma in the mid-latitude and high-latitude ionosphere on a wide scale.  Dr. Ray Greenwald (at Johns Hopkins Applied Physics Laboratory; AGU Fellow and now emeritus) was key in forging all the cooperative agreements needed to keep this very important measurement going.  Numberless scientific studies utterly rely on these flow measurements to explore the electrodynamics of the ionosphere.

  I recommend visiting the Virginia Tech SuperDARN site:

http://vt.superdarn.org/tiki-index.php

  The first thing you'll see is a map of stations and fields of view with international flags on it.  It's really impressive.  From VT, there is a great time lapse movie of networked radars from 1983-2016 here to show how the expansion has gone over the decades:

http://vt.superdarn.org/assets/img/fovs_m.gif

73

Phil W1PJE

To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/7ac37cb8-efa1-4ed8-be11-9fb00ffd44dan%40googlegroups.com.

[Gary Mikitin, AF8A]

Many thanks for the SuperDARN correction and the additional links, Phil.   Obviously, my mind was playing tricks on me after too many hours of web surfing!

Gary

[Gerry Creager]

Ethan, 

Thanks for the concise and cogent explanation. I learned something (again) tonight. The group's awesome in that regard.

I have a question regarding ionosonde transmitting antennas, with a specific example in mind.

One consideration for an ionosonde transmitter, as it sweeps across the spectrum (OK, steps...) is transmitting into a known resonant antenna. Of course, an antenna is resonant on one frequency primarily, and reasonably close on odd multiples of that frequency. However, there are a number of ways to "fool" the transmitter. Some are more practical than others. Most auto-tuners just don't move fast enough and have discrete channelized tuning points they're set up for. For instance, my SGC 230 and 237 tuners have a couple thousand memory points where they attempt to present a match to the transmitter, but are not continuous.

SGC, and others, offer a load-terminated folded dipole (SGC-104 90 ft folded dipole) that claims a vswr of better than 2:1 at all frequencies. 

Simple question: Would this be a good candidate antenna or reference design for an HF ionosonde antenna?

73

Gerry N5JXS

To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/CAFkosZ%2BXUiEuKuaMafBJNfL-jQ6wyaDgWJCagjtr30WhELmmOA%40mail.gmail.com.

--

Gerry Creager N5JXS

It's kind of fun to do the impossible. -- Walt Disney

[Phil Erickson]

Hi Gerry,

  Not all ionosondes use swept frequencies.  Some use pseudorandom noise sequences.  These are inherently more broadband than a narrow swept tone and act like a HF radar signal (because that is what they are) and you decode them accordingly.  See Hysell et al:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016RS005951

  Naively, I'd think the SGC-104 would be a fine ionosonde antenna, but perhaps Terry Bullitt can weigh in here.

73

Phil W1PJE

  

To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/CAMyi%2BGMYwk0L4zse_VwkvBSb7oyiUKDjN_O0G3CH7uEW6OmB4g%40mail.gmail.com.

[Tom Talley]

Could an advantage of a chirp be that it can be delt with by saw devices...if that would be an advantage here.

To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/CAAZaqEvn286rjOBnqKN8BXXDWMK7bDSR9XaOjnzOo3YG_WuqKw%40mail.gmail.com.

[Gerry Creager]

Thanks, Phil. Long day, I should have done my homework before I asked. I'll read Hysell tomorrow!

73

gerry N5JXS

To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/CAAZaqEvn286rjOBnqKN8BXXDWMK7bDSR9XaOjnzOo3YG_WuqKw%40mail.gmail.com.

[Terry Bullett]

Phil et al,

The PRN sequences are still limited bandwidth and are not a substitute for frequency sweeping.  They are an alternative modulation scheme with advantages and disadvantages compared to other modulation schemes.  You guys at Millstone Hill rote the book on ambiguity functions of various waveforms , but for the rest of the community I will try to explain. 
Frequency dispersion in the ionosphere plasma limits waveform bandwidth to about 10 of kHz,  Beyond this the channel scattering function exhibits too much time delay to decode the information in the transmit signal.
Frequency dispersion is a fundamental property of radio propagation in plasma.  Each Hz of your transmitted signal travels a sightly different path through the ionosphere on its way from the transmitter to the receiver, all arriving at different times with different phases and Doppler shifts.  This is determined by the ionosphere.  It turns out that about 10-20 kHz is about as much coherency bandwidth as you can get out of the ionosphere on a good day.  On a bad day the channel dispersion increases.  I see that as spread-F.  You hear it as fast fading or the funny sound of auroral propagation. 

All HF waveforms, radar or communication, have to deal with this and make tradeoffs.  In the HF modem world we would call this data rate vs bit error rate. The FT8 wave forms not only operate on very little signal but have narrow enough bandwith that  channel dispersion is minimal.  And you get very low data rates in return.  Increase the bandwidth to 1 MHz (slow by modern standards) and even with 1.21 Gigawatts  and  at 88 MPH in your DeLorian is not going to get a single bit of data through an HF channel on planet Earth  because your channel dispersion is too high.  :)

Ionosonde waveforms have similar issues.  For the PRN sequences, the received signal is cross-correlated with the transmit signal and when the pattern of phase changes matches there is a peak in the cross-correlation function we would call the time delay or range of that received signal.  But Doppler shift and Doppler clutter make this correlation fuzzy.  Worse (for science) is that the ionosphere reflections often have a very strong spectacular reflection embedded in a bunch of weak scattered signals.  The difference is often > 40 dB.  We want to pull out the weak scatter for plasma physics reasons but these correlation techniques have sidelobes just like antennas have sidelobes  and strong signal in a sidelobe will blind you to the weak scatter.  This is why I use a simple pulsed waveform with no pulse compression or correlation techniques involved.

Pulsed waveforms have their issues too, notably range aliasing.  If I transmit every 5 ms and a third reflection between the earth-ionosphere channel comes back at 7 ms, I think it came in at 2 ms.  This is quite common.  I have seen as many as 20 bounces between the ocean and the ionosphere at coastal stations.

Chirped or FM-CW signals have the infamous Range-Dopper ambiguity.  You are estimating range (time delay) by the frequency difference between the original signal and the received copy.  Any Doppler added by the ionosphere is incorrectly interpreted as a time delay or a range error.         


As for antennas, I am always asked "What is the smallest antenna I can use"  and the answer is always "a dummy load".  It work perfectly, fits under a desk, requires no installation, is HOA friendly, keeps the transmitter happy, and requires no maintenance.  But  you have no radiated signal.  Anything beyond this is a tradeoff between radiated signals and antenna size and complexity and involves having measurement criteria that you stick to.  In my day job we have a requirement to measure instantaneous phase differences to 1 degree of precision.  This requires 40 dB SNR, and thus ~12 kW ERP, obtained with 4kW peak power and +5 dBi antenna gain.    

There are some physical laws.  An antenna can be

1. Electrically small
2. Broadband
3. Efficient
   

Pick any 2. 

In HF sounding, we absolutely require broadband, a bare minimum of an octave, generally a decade and ideally 2-3 decades of antenna bandwidth.
And we never have the time to tune.  Modern instruments require frequency agility within 10 ms.  Tuning or impedance matching with frequency is not an option. 
So we must go big and high to obtain some degree of efficiency and broadband behaviour.  

The best solution is a log periodic design such as this monster:
https://www.sws.bom.gov.au/IPSHosted/INAG/web-69/2008/Transmit_Antenna_for_Ionospheric_Sounding_Applications.pdf

Not many Government projects can afford such a thing. 

Slightly smaller are the traditional  traveling wave designs of vertical rhombic (like at Millstone Hill) or delta (half rhombic) design, but these suffer from low frequency inefficiency and complex radiation patterns at higher frequencies.  But they are standard in the vertical incidence sounding world. 

The  folded terminated dipoles sold to hams, like the SGC-104 or the B&W  are just leaky dummy loads.  And with the fixed height above earth, the radiation pattern is all over the place with frequency. 

Back in the days of tube amplifiers, we could handle the high RF voltages of a bad SWR at a few frequencies.  
Modern MOSFET based transmitters can not handle an SWR > 3:1 at anywhere near full power.
Especially the way they have to be biased to keep the harmonics < -30 dBc.  So a good broadband match is quite critical and surely the hardest part of the antenna design. 
        


Probably most of the chirps you hear are coming from Navy AN/TRQ-35 chirp sounders. Some googling might reveal what kind of antennas they use. 

For reception, antenna situation is quite different.  With modern, low noise FET transistors, a preamplifier works rather well in conjunction with an electrically short receive dipole.  Or a small loop.
My Rx dipoles are 12 ft tip to tip and my loops are about 3 ft diameter
 

Share and Enjoy,

Terry
W0ASP

To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/CAAZaqEvn286rjOBnqKN8BXXDWMK7bDSR9XaOjnzOo3YG_WuqKw%40mail.gmail.com.

-- 
Dr. Terry Bullett          WØASP 
NOAA National Centers for Environmental Information (NOAA/NCEI)
Cooperative Institute for Research in Environmental Sciences (CIRES)
Terry....@noaa.gov    303-497-4788  
"Life is Complex.  It has a Real part and an Imaginary part."