Category Archives: SDR

Decoding ADS-B with free QIRX software

QIRX’ dashboard, decoding ADS-B: in the middle you see spectrum and spectrogram (“waterfall”) of the ADS-B signals. The window at the bottom lists alls received aircraft with additional data, whereas the top window places them onto a map.

In the last two blog entries, I took a look at the DAB capabilities of free software QIRX by Clem Schmidt, DF9GI, from Frankfurt. It directly works with RTL-SDR, Airspy and RSP2 SDRs. I tried this very smart software from my location near Hannover/North Germany now also with ADS-B, mostly with my RSP2.

ADS-B stands for “Automatic Dependant Surveillance – Broadcast” and is an automatic service where aircraft continuously transmits several vital data on around 1.090MHz. Most important part of these data is the 2D location of the aircraft which it gets by GPS plus height by a baromatric altimeter. From this position data, many other data are derived, e.g. climbing/sinking or speed. If matched to databases, you will also see type of aircraft, flight number and many other data.

“The internet” provides many services showing the results of ADS-B and other data, collected from receivers all over the world, among them Flightradar24, OpenSky, FlightAware and AirNavRadarbox. They each provide many additional data, somtimes available at different schemes. Most provide free access to much of their data, with some more specific data behind their paywall. OpenSky as a scientific and non-profit organization offers billions of datasets for free, see Scientific Datasets. QIRX uses an OpenSky data base with about 650’000 entries.

Backbone of all these services is a net of ADS-B receivers, connected via the internet and curated by each company.

QIRX shows some capabilities of such a receiving station, using a proper antenna and a simple SDR. It decodes the I/Q stream of it. ADS-B is transmitted via pulse-position modulation, or ppm. The system is explained in ICAO Annex 10 Volume IV [free download].

With QIRX, you must set the sampling rate of you SDR to 200000[Hz], as other sampling rates won’t work, see screenshot below.

To decode ADS-B, you must set the sample rate for your SDR to 2000000Hz.

After that, and having started QIRX in ADS-B mode, decoding is done automatically. Release your seatbelts, and simply relax by viewing the activities above your head. Coverage largely depends on the “view” of you antenna and a few other factors like te sensitivity of your SDR and the attenuation of your cable connecting your antenna with your SDR. Some web services, thanks to anticipatory obedience/security reasons/data protection etc., do mute some “special” flights . This is not the case, of course, with this setup. QIRX always provides stable decoding at even low SNRs – great!

Last, but not least, please find below a comparison of FlightRadar24 and QIRX setup with Flight Number TK1554/THY6KG, Hannover->Istanbul, starting from Hannover Airport. One difference between both screenshots is that at my location (Burgdorf), I got the Airbus only after it had climbed to an atlitude of 200m or so, whereas the FR24 receivers are placed at positions allowing for tracking the aircraft from even the runway.

Starting from Hannover to Istanbul: the airbus on track around Hannover. Top window shows the flight via FlightRadar24 web service, and even from the runway. Bottom window shows it received with QIRX from Burgdorf (red point in the northeast).

Also small aircraft is equipped with transponders, but not necessarily with ADS-B transponders, broadcasting the position, derived from their GPS. These small aircraft may haveonly Mode-S transponders on board, transmitting identification, height and squawk (transponder code) as assigned by their responsible ATC, or Air Traffic Control.

HF: Doppler, Signal Level and Time

Two views of the carrier of Sofia-Kostinbrod on 9400kHz from 15:30 to 18:30 UTC: On top the frequency within a window of 2Hz height only, at the bottom the synchronized HF level of this carrier; see text. [Click onto the picture for a better view.]

What you see in the picture at the top, is a mostly hidden gem of HF propagation. I took the carrier of Sofia-Kostinbrod transmitter form Bulgaria (250kW) on 9400kHz and observed it for three hours. In the upper window you see the frequency wihtin a window of 2Hz height only. You see two strong carriers: one nearly in parallel to the x-axis, the other snaking some fraction of one Hertz below it.

With one transmitter only on this frequency: How does this happen?

It’s multipath propagation. The signal takes one way via a groundwave-like way, the upper trace. It reveals a very slight drift downwards. As I use a GNSS-controlled receiver, the FDM-S3 from Elad, this miniscule drift should be happen within the transmitter, not the receiver.
The snaking trace stems from a second way, most likely via the F2 layer of the ionosphere. As the ionosphere is prone to winds and an ever dynamic change of its ionization, it is moving. And with all moving objects, also this causes a Doppler effect to waves. This is exactly what we see – the angular speed of the ionosphere, relative to the “groundwave-like” signal.
You may also see at least two weaker traces, caused by two further ways, hence showing other Doppler shift.

In the diagram at the bottom, you see the combined level of all traces. Because they reach the reeiver at different time and, hence, different phases, their addition leads to an ever changing signal level, called: fading.

I hope to continue this work with some other examples in the future, also taking fade-in and fade-out into account.

Doppler: Following Airplanes’ tracks

Carrier and Doppler trace (left), locations of transmitter, receiver and track of flight NH8406 – March 27, 2021, around 16:45 UTC [click onto the screenshot for richer detail]

Working on a project which will focus on Doppler spread of HF channels (see at the bottom) and other impairements, I also bumped into some more prominent Doppler catches, namely on the VHF aero band. I took the AM carrier of nearby Hannover VOLMET on 127.4 MHz and observed doppler traces about plus/minus 200Hz the carrier frequency. Following the acitvity in the airspace via Flightradar24 in parallel, it is easy to match traces and aircrafts. In this case, I nailed cargo flight NH8406 from Frankfurt to Narita/Tokyo. It is important to remember what is shown in left part of the screenshot: it is the signal of Hannover VOLMET, reflected by this moving Boeing 777-F. Thus, the reflected frequency shows a Doppler frequency shift – depending on the relative speed in respect to transmitter and receiver. A positive Doppler frequency signals that the aircraft is approaching my location. When it turns to the lower frequencies, I see the aircraft passing.

Things get more complex wen it comes to the Doppler shift at HF propagation. You will also see planes, but effects from high winds in the upper atmosphere, coming and fading of ionospheric layers and the influences of the geomagnetic field are prevailing. Due to the much lower frequencies, the effects are just about a tenth compared to thie above example on VHF.

See below a result from my observations on HF as a preview.

Carrier of TRT Emirler, Turkey, in the 19 meter band. Just after sunrise, the carrier splits into two, and you also see double lines due to magnetoionic effects. The window shos about 3Hz in the vertical, and about 40 minutes in the horizontal scale.

Medium Wave: Signals May tell sunris/Sunset at their transmitter’s site

The two stronger carriers (Romania left, Algeria right) exhibit Doppler-shifted scatter; see text for a more detailed explanation.

During my expeditions into the thicket of mediumwave offsets, I bumped into pictures like that at the top. In the lower part of the screenshot, you see two carriers mit seahorse-like structures looking to the right. In the evening, they look towards the West.

This is one of the several effects which can be seen at local sunrise/sunset. Here, the carrier gets “clouded” and show frequency changes. These effects are associated with Doppler shift (moving of ionospheric patches/layers) as well as scattering caused by irregularities of the ionosphere, most notably Travelling Ionospheric Disturbances, or TID. Whereas the Doppler shift, by vertical moving of reflecting layers like combining of F1- and F2-layer to one and lower F-layer when approaching darkness, is comparatively small, high wind speeds in these regions can cause a much faster horizontal movement of such regions. This, in turn, may cause a Doppler shift of about 1Hz or even higher in the medium wave range.

The Figure at the top demonstrates this effect at two transmitters on 1422kHz, namely SRR Radio România Actualități from Râmnicu Vâlcea/Olănești (sunrise 05:55 UTC/sunset 15:12 UTC; distance 1433km) and Radio Coran/Radio UFC/Radio Culture/Chaîne 3 from Ouled Fayet/Algeria (sunrise 06:58 UTC/sunset 17:00 UTC; distance 1840 km). Seen from midnight, sunrise first occurs at the Romanian transmitter, followed by the Algerian one with the seahorse-like pattern of the scatter towards the higher frequencies. Around each local sunset, first Romania sees darkness, followed by Algeria. Here, the scatter pattern turns towards the lower frequencies. In the insert at the right, contrast has been sharpened to additionally reveal a split-up of these carriers due to propagation into two paths.

This effect often helps to determine the local sunrise/sunset of a carrier. I marked what presumably is the carrier of MBC Radio 1 from Matiya/Malawi, sunrise 03:22 UTC; listed 02:00 to 22:00 UTC, but obviously on a 24 hours’ service this Tuesday.

Both Figures at the bottom try for some detective work without knowing specific offsets (because not available) but relying only on schedule and the above mentioned propagational effect. Crime scene takes place on 1233kHz, where we want to scrutinize two channels, one on 1232,9937 kHz, the other on 1232,9951kHz.

Distinctive scatter, associated with local sunrise at the transmitter, provides a strong hint towards the location.

The s/off- and the s/on pattern match that of Chinese National Radio #17’s Kazakh service. Incidentally, sunrise takes place in Qinghe at 01:42 UTC, and in Boertala at 02:04UTC – next Figure. Boertala is listed with 10kW (stronger signal), Qinghe with 1kW. Unfortunately, the f/out time of other CNR17 transmitters on this channel is mostly covered by phase noise from Rádio Dechovka in the Czech Republic and Absolute Radio in the United Kingdom.

Some CNR17 locations and the terminator during sunrise in Boertala, see text. Visualized with free Simon’s World Map.

Here I am indebted to Jens Mielich, Head of the ionosonde at Juliusruh/Germany, who was so kind to comment on this observation. According to him, the observed Doppler shift of 1Hz on 1422kHz should have been caused by a refracting medium, moving at an (angular) speed of roughly 105m/s. At Juliusruh, he observed e.g., an ionospheric drift of 311m/s±93m/s from East towards West on January 19, 2021 at 04:19 UTC: “You will get a positive Doppler shift during a West/North drift, and a negative one at East/South drift.” He adds that further investigations on a more longer time series are needed.


PSKOVNDB: An exciting new software for Mediumwave DXers

See the bunch of carriers on 590kHz at the left. PskovNDB shows at the right a diagram of noise, the combined signal strength of the 200Hz window and the signal strength of the carrier just picked.
Here the very carrier of VOCM/St. John’s had been clicked instead. You easily see that this signal is dominating the channel – only one of the many exciting features of free PskovNDB software!

Recently, I came across an upgraded version of Ivan Monogarov’s PskovNDB software, already having collected all laurels available as being the Gold Standard for chasing non-directional beacon, or NDBs. Recently, Ivan had expanded his tool with some as unique as exciting features for the avid medium wave DXer.

At a first view, it converts recorded WAV files (also: RF64 format, done with SDRC V3 software) into spectrograms of high resolution in which you can easily see the number of stations, measure their precise offset and see their signal strength.

A second view reveals the smart feature of producing diagrams of each signal – plus noise level and the combined power of the whole window. You can see both in the screenshots on top of this page.

A third view almost exactly helps to distinguish between signals where you can here music, listen at least to some words or phrases, or which do provide full audio.

Nothing more? Yes. Under the hood, there is much more. So, you can do automatically recordings each day and also automatically send them to PskovNDB software for showing the spectrograms, one after the other, like on a film roll. This enables you to pick the recording of the most promising day(s) for further inspection.

I wrote a short introduction to the beta version of this free software, and Ivan was so kind to add some most helping notes to this. You can download it here. It contains also some additional information, i.e. a link for downloading the software.

Spassiba, Ivan, for another software breakthrough!

Medium Wave: Offset Atlas – all 9 kHz channels Plus VLF & Longwave, 24 hours

The “Atlas” shows screenshots of all 9kHz channels on Medium Wave within a 50Hz window, sometimes better. It also shows some odd channels plus Time Signal Stations on VLF and all Broadcasting Longwave Channels. You can download it for free to determine accurate and stable offset readings over 24 hours (zoom in by e.g. 400%)

With the new Elad FDM-S3 and its OCXO/GNSS-stabilized clock, I did a 24h recording of the whole medium wave band on January 19, 2021 in Northern Germany; plus longwave on Januar 21, 2021. Free software SDRC V3 enabled me to make up a spectrogram of each channel within a window of 50Hz width, and at a frequency raster of 9kHz on medium wave. You can easily see:

  • sign-on/sign-off
  • fade-in/fade-out
  • accurate and stable frequency offset over full 24h down to a millihertz
  • frequency control of the transmitter’s oscillator (stable, drift, sinus, sawtooth …)
  • propagational effects (doppler, scatter …)

The format is PDF, DIN-A4, landscape, resolution 300dpi – see screenshot at the bottom. This allows you to zoom to a factor of about 400% to search for details and better read out of the time/frequency scale. It weighs 865MB. You can download it here, and open it with your PDF reader (you can also point your mouse cursor onto the link, click right mouse key, and choose “Save under …”). Leafing from one page to another gives an interesting overview.

A similar Atlas showing a raster of 10kHz is also available for free – just scroll to the previous post of this blog. It is also planned to publish a general article about the background, about what to do with such a tool, and how to do this by yourself.

I am sure that it will open some new horizons on Medium Wave DXing, including accurate offsets over up to 24h.

Medium Wave: Offset Atlas – all 10 kHz channels, 24 hours

The “Atlas” shows screenshots of all 10kHz channels on Medium Wave within a 50Hz window, sometimes better. You can download it for free to determine accurate and stable offset readings over 24 hours (zoom in by e.g. 400%)

With the new Elad FDM-S3 and its OCXO/GNSS-stabilized clock, I did a 24h recording of the whole medium wave band on January 19, 2021 in Northern Germany. Free software SDRC V3 enabled me to make up a spectrogram of each channel within a window of 50Hz width, and at a frequency raster of 10kHz. You can easily see:

  • sign-on/sign-off
  • fade-in/fade-out
  • accurate and stable frequency offset over full 24h down to a millihertz
  • frequency control of the transmitter’s oscillator (stable, drift, sinus, sawtooth …)
  • propagational effects (doppler, scatter …)

The format is PDF, DIN-A4, landscape, resolution 300dpi – see screenshot at the bottom. This allows you to zoom to a factor of about 400% to search for details and better read out of the time/frequency scale. It weighs 559MB. You can download it here, and open it with your PDF reader (you can also point your mouse cursor onto the link, click right mouse key, and choose “Save under …”). Leafing from one page to another gives an interesting overview.

Yes, a similar Atlas showing a raster of 9kHz is under way and will be published also here in due time. It is also planned to publish a general article about the background, about what to do with such a tool, and how to do this by yourself.

I am sure that it will open some new horizons on Medium Wave DXing, including accurate offsets over up to 24h.

Aloha: KUAU from Haiku/Hawaii, received on January 19, 2021 by DK8OK. Proofs are frequency, plus the rather unique fade-in/fade-out in the European afternoon.

Comments and suggestions are appreciated: dk8ok@gmx.net.

Millihertzing with Software “Carrier Sleuth”

24 hours on 590kHz on January 19, 2021 in Northern Germany, reveals a couple of North American signals with VOCM of St. John’s, Newfoundland being the strongest and KQNT Spokane on 590.002kHz/Washington State the most interesting with reception also in the afternoon.

“Millihertzing” seems to become “le must” of this season. The most recent software stems from smart software author Chris Smolinski, W3HFU, who over many years offers inspiring software,this new one dubbed Carrier Sleuth. It mainly analyzes I/Q-WAV files from software-defined radios at high resolution, being a perfect tool for measuring offset frequencies on mediumave. The screenshot at top shows such a spectrogram which covers 20Hz in width and 24h in length on 590kHz.

Why using “Carrier Sleuth”, when haveing SDRC V3 at hand? First, it works together with a multitude of WAV formats from many different SDR software (see Chris’ list, which is still expanding). Secondly, it let you hop from one channel (9kHz or 10kHz) to the next – if a proper part of the spectrum has already been converted from WAV to FFT. It also provides coverting spectrograms to CSV to apply some statistics on each signal. There are many more smart feature, and Chris will even add some exciting more, e.g. processing I/Q files in real time to save a lot of time.

With my bread-and-butter software being SDRC V3, recording in WAV RF64 one-file format (which sometime swells to nearly 10TB), “Carrier Sleuth” can even digest these recordings with a workaround: specify an interesting part of the medium wave, defined by upper and lower channel and time segment, and convert this into simple WAV. This is easily done with SDRC V3’s Data File Editor. It is also the way, Carrier Sleuth produced the screenshot at top of this page.

Chris published this software first on December 10, 2020. He eagerly looks for bug reports, applications and further suggestions form the users. Take a free test drive; registration code 19.99 US-$.

Magnificient FDM-S3: the Millihertz Magnifier

1340kHz, 25Hz window, resolution bandwidth 0.0061Hz: more than 100 U.S. AM stations are discernable by their frequency offset.Antenna: vertical active dipole MD300DX, 2 x 5m. Visualized with SDRC V3 software by Simon Brown, G4ELI.

With Elad’s FDM-S3 SDR now hitting the market, we have a receiver at hand which is supported by an OCXO/ GNSS frequency reference. This combines short-time accuracy with long-time stability and allows for precise frequency measurement in the millihertz range (under 30MHz). Exploiting this feature is as exciting as it is innovative. With this new tool, also a new kind of DXing is evolving. One example is propagation analysis. See below the 24h spectrogram of Radio Gotel from Jabura/Nigeria on its exclusive channel of 917kHz:

Radio Gotel transmits from 04:00 UTC to 23:00 UTC on 917kHz. In this spectrogram you clearly see sign-on, sign-off; fade-out, fade-in, plus some other feature like two short power breaks in the evening as well as some instabilities.

What surprises, is both, the late fade out at around 07:30UTC and the early fade-in as early as 15:20UTC. It is important to note that you here see the carrier with a resolution bandwidth of 0.0009Hz, roughly just one millihertz. The gain, compared to a listening bandwidth of 10 kHz, is a whopping 70dB, allowing extreme DX. Audio starts to emerge only from around 18:00UTC. As DX Atlas shows, the whole path between my location and Radio Gotel is under daylight at the palpable fade-in at around 15:20UTC, see screenshot below.

At the first visible trace of Radio Gotel at DK8OK’s location on 19JAN2021, with the whole path still is in daylight. Illustration with the help of DX Atlas software.

As with all new things: “We’ve only just begun”, Carpenters, 1970. To be continued.

RX-888: 32 MHz/16bit, 200 US-$ – pricks up your ears

The RX-888 covers 32MHz @16Bit in a row. Here it comes to life with Simon Brown’s unique and indispensable software SDRC V3 at an professional active dipole antenna MD300DX.

It is the (a few weeks) younger brother to the RX-666, a brainchild of Oscar Steila, IK1XPV. And it is one of the first palm-sized SDRs in the price class of 200 US-$ which covers the whole HF band for receiving, recording and playing with 16Bit resolution, resulting in a competitive dynamic range of about 100dB. I got one from China via eBay (there are numerous sellers) within a few days. Overnight, Simon, G4ELI, made his SDRC V3 software to match also to the RX-888 with excellence. You need a PC, an i5 should do it, with USB3.0 for data streaming, controlling and power supply. Yes, there is no need for a separate 5 or 12VDC!

Much had been speculated about one obvious fact: the price of the A/D chip is, if only a medium order is placed, the same or even higher than the price tag at the RX-888. How comes? One rumor with some substantial background results in this story: the chips had been desoldered from boards of other projects which didn’t pass the quality control. These boards had been sold at a low price as a bonanza to smart people who can use all the parts which on their own will have passed the quality control, most notably the pivotal A/D chip.

This blog should give you a first impression. The biggest difference between RX-666 and RX-888 seems to be that the latter is equipped with a permanent low-noise amplifier of +20dB which perfectly balances sensitivity and dynamic range for 90% of us DXers. Sensitivity on HF is nearly on par with FDM-S2.

Two antenna sockets – the impressive cooling fins on three sides of the box will be needed after a planned update of covering 64 MHz in a row with 16Bit, and up to 10MHz (now: 8MHz) above 64MHz.

I tested the RX-888 from 10kHz to 32MHz and had a look above 32MHz – see the two following screenshots.

RJH69 on VLF 25.0kHz. This time signal from Belarus was received at 07:06 UTC on 02SEP2020 in Northern Germany and read with CW decoder MRP40.
A look onto 8MHz of the FM broadcast band.

The RX-888 also worky nicely together with decoders like DRM or (other) data, see the two following screenshots.

The very weak DRM signal of China National Radio on 9655kHz [Urumqui, 30kW, antenna direction 98°!] is duly received by the RX-888 with the data decoded with free DREAM software.
US Air Force Diego Garcia [JDG] in the Indian Ocean calling their Lajes counterpart [PLA] in the Açores on 4721kHz at 17:18UTC in MIL-STD-188-141A.

PC power: Nowadays, a “receiver” is a system, consisting of an SDR (the box), software and the PC. While world-class software SDRC V3 is for free, and an top SDR costs just about 200 US-$, you should not forget an able PC. It must be an i5 and up if you want to digest bigger bandwidth like 8 MHz, 16 MHz or even 32MHz. Even for recording 32 MHz, there is no need for internal SSDs, a fast iron disk will do the job. Furthermore: 32MHz recording for 24 hours do expect a bit more than 11TB disk space. This calls for an external HD, and a second USB3.0 card (not: hub!) is a must. As external HD, I use the WD MyBook Duo, delivering 28TB at under 750 US-$. The combination of an desktop i7 and this HD ensures stutter-free recording and playing up to bandwidth of at least 32MHz. Here simply more is more …

Last, but not least, please find below a few audio examples of broadcast as well as utility stations. They proof that the RX-888 is a serious receiver at a ridiculous low price.

4712 kHz/USB: Russian Airports with radio checks in Russian: Kazan, Rostov (net control), Saratov, Samara, Novosibirsk, Chelayabinsk. They transmit with 1kW of power to a low-hanging dipole. 02SEP2020, 17:00 UTC.
4750kHz/AM-ECSS: Bangladesh Betar with ID over an obviously defective transmitter (nominal 100kW). 02SEP2020, 17:00UTC.
4800kHz/AM-SAM: Chinese National Radio Beijing I (Geermu, 100kW) ID in Mandarin, ID. 02SEP2020, 22:00 UTC.
4920kHz/AM: Tibet People’s Broadcasting Station (Lhasa, 100kW), ID in Tibetan. 02SEP2020, 21:00 UTC.
5000kHz/CW: Chinese Time Signal Station BPM (Sha’anxi/Pucheng, 5kW), ID in CW. 02SEP2020, 22:00UTC.
6676kHz/USB: Singapore VOLMET, 5kW, ID and weather in English. 02SEP2020, 17:20UTC.
6676kHz/USB: Bangkok VOLMET, 5kW, ID and weather in English. 02SEP2020, 18:10UTC.
7240kHz/AM: Tibet People’s Broadcasting Station (Lhasa, 100kW), ID in Mandarin. 02SEP2020, 21:00 UTC.
9275kHz/AM: FEBC Philippines/Bocaue (100kW), ID in Mandarin. 02SEP2020, 14:00 UTC.
9310kHz/AM: VoA Deewa Radio (Udon Thani/Thailand, 250kHz), ID in Pashtun/Urdu. 02SEP2020, 14:00UTC.
9664,77kHz/LSB: Radio Voz Missionaria (Camboriu/Brazil, 10kW), ID in Portuguese. 02SEP2020, 22:00UTC.
10’000kHz/CW: Chinese Time Signal Station BPM (Sha’anxi/Pucheng, 5kW), ID in CW. 02SEP2020, 17:00UTC.
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