From Board to Broadcast
When Alex first powered up the SDRplay RSPdx on their Linux laptop, the dark glow of the screen promised a world of waves that had always felt just out of reach. The RSPdx, with its 70 MHz to 2 GHz band and 100 kHz selectable bandwidth, was a gateway to the invisible currents that blanket our seas. Alex had heard tales of the NavTex service—those handwritten maritime messages that sailors relied on for weather, coast guard alerts, and navigational warnings—and was determined to listen in from their own living room.
Installing the Driver and Linux Air‑Setup
First, Alex downloaded the latest SDRplay 2.4 driver from the vendor’s website. Though the installer was packaged for Windows, the open‑source port for Linux allowed the user to run sudo ./install_linux.sh in a terminal. After a few brief recompilations, the /dev/sdrplay0 device appeared, and the dongle's ID 0x4566 was recognized.
With the driver in place, Alex launched gqrx, the graphical SDR receiver that ships well ahead of the MSRP of most radio‑software bundles. The interface is terse but elegant—individual knobs for Frequency and Bandwidth, a scrollable spectrum, and a quick‑click “Start” key that awakens the tuner. Alex entered 518000 to set the center frequency to the global NavTex VHF channel and slid the bandwidth knob to 5 kHz, isolating the narrow band where the maritime crew-to-crew and ship‑to‑port transmissions lurk.
Sensing the Midnight Transmissions
The sea of spectrum was quiet—a swan‑like hush that let the faint whine of distant ships break through the computer’s faint hum. Alex tuned the radio to the 518 kHz sweep and waited for the telltale “CAMARRA” ping that signals a NavTex start. When the signal arrived, a 10‑Hz burst of white‑noise spread over a small spike of signal that exhaled the chartered message.
To convert the audio stream into human‑readable text, Alex ran Fldigi, a powerful decoding engine loved for its ability to handle TTY‑4—the Morse‑like mode used by NavTex. Under Radio → Connect to Remote Radio, the soundcard was selected, and the Decode tab swirled into an automated pipeline: the sound intake, a 1‑kHz filter, and the Fsk2 algorithm that decoded the 300‑baud FSK stream into text.
Decoding the Message
The first line that emerged on the terminal read “*HELLO STP*” followed by a line of weather information. Alex leaned in, gripped a thermos of coffee, and watched the trailing stream of symbols—each a tiny wave that the decode process translated to letters. The messages, in a choppy but unmistakable rhythm, came from a fleet of ships datelining the eastern Atlantic at 03 UTC, rustling away announcements about a severe storm off the Azores and updated shipping lanes. The NavTex system’s brevity and timing—every 10 minutes on the hour—gave Alex a sense of disciplined rhythm, as if someone kept a pulse on the sea’s megaphone.
Tackling the Challenges
Earlier attempts had produced a garbled string of garbles, but Alex discovered that the SDRplay RSPdx required a proper Gain setting to avoid clipping the faint 518 kHz signal. By adjusting the gain knob in gqrx to 20 dB, the receiver stayed within range. When wind gusts from the laptop’s fan or interference from the room's
Setting the Stage
It was a quiet evening on the coast, and I tossed my notebook open, lit a cup of tea, and decided to explore the world of weather fax, a lingering technology from the golden age of NOAA radios. My tools? The SDRplay RSPdx, the latest 2.5 firmware, and a freshly installed GNU Radio 3.9 stack on my Linux machine. The goal was simple: tangle with the meteorological mysteries locked in the 162‑MHz weather fax signal and bring them alive in audio on my speakers.
Connecting the RSPdx
First, I plugged the RSPdx into USB-C, letting the libpsrdsp driver do its ceremony. The kernel logged the clear, green LED, and the RSPdx’s powerful 22‑channel tuner was ready to roam the spectrum. I opened a terminal and typed psrdumpstream -s 1.0 -f 0 -fp 162000000 -l 14400000 -t 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 | tee nare2.wav, a command that captures a 14‑MHz slice around 162 MHz into a WAV file. The raw data would be my first raw poem to decode.
Building the Flowgraph
With my raw stream on hand, I launched GNU Radio Companion and stitched together a flowgraph. The source block was osmosdr source (SDRplay), tuned to 162.550 MHz with a 100 kHz bandwidth—just wide enough to include the NOAA 163 MHz channel and, hopefully, the hidden ZIP codes. I poured the stream into a rational resampler set to 4×, then fed it into the WF block from the gr-wf GitHub repository. The new gr-wf package — updated early this year — now supports the 120‑bit CRC check that modern weather fax frames use, freeing me from the older 80‑bit bug in the old code.
On a crisp evening in spring 2025, I was standing in my studio, the SDRplay RSPdx perched on the desk like a gleaming microphone. The little white LED on the front blinked rhythmically, a ghost of the ocean’s pulse that waited just beyond the line of its antenna.
I had been following a thread on Reddit where an enthusiast had posted a video of NAVTEX arriving cleanly through an SDR. A year ago the kit was a solo hobbyist’s dream; now it was marrying to modern software in a way that seemed almost inevitable.
Preparing the stage: software and drivers
First, I opened SDR# (SDRSharp), the flagship SDR client on Windows. The RSPdx driver had already been installed as part of the SDRplay Atom package. Just a quick reboot after the driver install brought the device into the SDR# list, where I saw a fresh entry labeled “RSPdx”.
Next I downloaded the latest SDR# configuration bundle that included the NAVTEX demodulation plugin. The bundle’s readme informed me that the plugin would automatically spot the 518 kHz band and switch to narrow‑band SSB demodulation without any fiddling.
Turning the virtual tuner to sea
With the ship’s cloak of silence around me, I set the center frequency to 518 kHz, the international NAVTEX frequency that ships around the world send their weather bulletins over. The sample rate I selected was 4 kHz; that’s low enough to keep the process light, yet high enough to preserve the carrier’s subtle scintillation. The bandwidth knob lay at 3.3 kHz, just wide enough that minor GPS drift would not drown out the message.
When the plugin activated, a phantom column of spectral density lights flickered across the screen. The carrier blinked. I felt the vibration of a distant monitor, the same feeling a sailor has when the compass needle points true north.
Listening to the beating ocean
At the Satellite label, the SDR# software changed into a soft analog BCS display. The carrier, now decoded by the plugin, whispered the letters of the NAVA (or collate list). The letters that would later VHF decode into “METAR KAXT - wind 240 at 14 knots, visibility 12 miles,….”. The entire family of weather reports from **Seymour Island** or any station that feeds its data to NAVTEX wafted over, heard by my newly tuned antenna.
Writing a faithful log
While the signals arrived, I wrote down every adjustment to the settings. Those notes would become the recipe for others in 2026, ensuring that another curious mind could follow the same path from a blank SDR client to hearing a distant ship’s weather bulletin. In the final lines I added a gentle reminder that the world’s seas were still button‑armed: “Keep your software up to date, keep your antenna in the sun, and the sea will always have a story for you.”
Setting the Stage
Felix, an amateur meteorologist, always dreamed of pulling live weather images from the clouds. He had heard that the SDRplay RSPdx, a lightweight yet powerful SDR receiver, could bring him exactly that, but the doorway to the world of WEFAX seemed shrouded in jargon. One autumn evening, fueled by curiosity and a cup of strong coffee, Felix set out to demystify the process.
Getting the RSPdx up and running
Felix began by connecting the RSPdx to his Windows machine with the supplied USB cable. He made sure the tiny power indicator blinked steady, signalling the hardware was alive. Then he installed the latest SDRPlay SDK from the manufacturer’s website, which, as of June 2024, offered a unified API for all RSP devices and a dedicated Windows driver. The installation was straightforward; the installer asked for a location, simply chose the default, and ran the auto‑configurator. Within minutes, the system recognized the device and presented it as a new COM port.
Choosing the right software
With the hardware ready, Felix turned to software. For WEFAX decoding on Windows, he preferred the open‑source package SDRSharp (SDR#) because of its lightweight footprint and active plugin community. While downloading the latest release from the SDRSharp forums, he noted that the 2.5.6 build came with a compatible SDRPlay plugin that supports the RSPdx natively. After installing SDR#, Felix opened the application and saw the familiar menu bar populate with “Dx.” He then selected “RSPdx” under the “Add Source” dialog, confirming the correct sample rate of 2 MS/s.
Fine‑tuning for WEFAX
WEFAX operates on the 2.5 MHz to 3.5 MHz band using a 3‑kHz channel width modulated with DSB‑AM. Felix navigated SDR# to that frequency range and clicked on “FM” to ensure the radio was in the right mode. He then adjusted the RF Gain slider, highlighting the delicate balance: too high and the signal drowns in noise, too low and the faint fax becomes invisible. A light on the console turned green as he reached an optimal gain of about 30 dB.
Installing the WEFAX decoder
In the same folder where SDR# resides, Felix found a triumvirate of decoder utilities: Audiodownload, Peace Of Mind, and the WFaxDecode script. The community’s recommended way to run a WEFAX workflow on Windows is to pipe the SDR# audio stream into the decoder. Felix created a tiny batch file:
"sdra.wav" | wfaxdecode.exe -o weather.png
Here, “sdra.wav” is the recording inside SDR#, while wfaxdecode.exe is a lightweight, Python‑based decoder built from the wefax library. When executed, it automatically reconstructed the fax into a PNG image, complete with time stamps and a soft overlay of snow‑flake symbols that appeared in the original nebula.
Decoding the fax
Felix opened the resulting “weather.png” in his favorite viewer, and there it was: a crisp, accurately titled swath of cell‑map data stretching from the northern coast to the southern plains. It showed cloud cover, precipitation probabilities, and even a few extratropical front markers. He marveled at how a simple USB dongle and a few lines of code could bring satellite‑derived meteorology into his living room.
Capturing the images
To create a routine, Felix scheduled the batch file in Windows Task Scheduler to trigger every hour between 00:00 and 23:00 UTC. He also added a small logger so if the decoder failed, a notification would pop up on his desktop. Whenever the system pinged the batch, the console window flickered into existence, streamed the SDR# audio into the decoder, and produced a new weather map. Felix now saves the images in a dedicated folder and even tags them with the date and time, enabling him to build a personal archive of climate patterns.
Through patient reading, careful setup, and a twist of storytelling, Felix discovered that the SDRplay RSPdx turned a humble Windows PC into a portal to the sky. Each new WEFAX decode felt like flipping a page in the planet’s living weather report, and Felix was ready to keep reading, visualizing, and sharing.
When the dawn broke over the harbor, a thin line of light traced the horizon, and an eager radio hobbyist named Marla decided it was the perfect moment to crack open the world of digital and analog signals that had escaped the confines of her MacBook. The SDRplay RSPdx was already humming in the corner of her studio, a compact beast capable of listening to almost any radio frequency, but somewhere between the knobs and the serial port, a challenge still lingered: how would she finally tap into the NAVTEX broadcasts that shipmasters relied on to receive weather and navigational warnings?
Connecting the RSPdx to a Mac
Marla's first task was simply to bring the RSPdx on line. She had a newly released RSPdx firmware version 1.0.49, which the official SDRplay software package said had improved carrier‑suppression for the 420 MHz band. The tiny USB‑C cable slipped into the Mac’s USB‑C slot, and the tiny indicator LED glowed amber, a gentle acknowledgement that the SDR was awake. She pulled the quick‑start guide from the manufacturer’s website, but quickly realized she didn’t actually need step‑by‑step instructions—she just had to keep the RSPdx powered and ready, leaving the rest to her software.
Choosing the Right Software
Next, Marla let the world outside her screen unfurl with a few clicks. She launched SDR#, the most popular open‑source SDR software for macOS that had just received its April 2024 update, boasting a cleaner UI and enhanced gain controls. In the Device menu she selected RSPdx ‑ Dual 2‑Stream, a hint that both GPS‑based IF calibration and wide‑band signal fidelity were in order. The software asked for a Firmware revision check; after confirming RSPdx was indeed at 1.0.49, she pressed Apply and took a deep breath—a crucial moment for the stories that were about to begin.
Zeroing in on the NAVTEX Frequency
Once the software had oriented itself, Marla switched the frequency dial to 420.405 MHz, the same broadcast frequency used worldwide for NAVTEX. The autofocus of SDR# scanned the band with a luminous carrier detection** line bursting like a lightning strike, capturing the familiar 2.4 kHz pilot tone that all navigational radiobeacon stations use to identify themselves. The instrument panel displayed the signal strength as +45 dBm, a smoking gun that she had indeed captured a NAVTEX station in her area, and the screen also revealed a lot of wire‑tap PPP EP BKN 0… weather messages as short bursts of text, waiting to be decoded by the software.
Playback and Decoding
With the dial centered, Marla turned on the audio output to the Mac’s built‑in AirPods. A faint, but unmistakably human, voice began to rise in the left channel—a maritime radio operator speaking in a pattern of advisory labels ’MET’, ’WIND’, ’SEA STATE’ and embassy updates. She watched the waveform on the screen oscillate in sync with each spoken phrase. The FFT window sliced up the band, revealing the pilot tone and the recording of half‑second pulses. Some episodes had slight volume dips, a tell‑tale sign that the signal was battling atmospheric noise, but each phrase was clean enough to capture on the hard drive. After a small pause, she hit the record button to archive the next broadcast, each file labeled with date and time stamps—anything she might want to examine later.
Fine‑Tuning the Receiver
Marla noted that the RSPdx’s controller kit had an onset of improved Gain Control when armed on a low‑noise location. She adjusted the gain ladder upward to +30 dB, but only incrementally; just enough to offset fading channels during evening transmissions. The IQ offset correction controlled by the SpectrumView application allowed her to shape the waveform into a single, perfectly centered shape. She also tweaked the digital downsampling to 600 Hz, a bandwidth limit that removed spurious radio chatter from neighboring VHF lifeline services. All the while, the Mac’s Audio Engineer app recorded the output as a WAV for immediate playback on a playlist.
When the Waves Go Low
And yet no story is complete without a moment that almost stalls the plot. One evening, as Marla tuned the RSPdx back to 420.405 MHz, the spectrum suddenly blanketed all data with a *sudden* broadband hiss. The Fast-Scan on SDR# flickered in a 2
It was late at night when the faint hiss of a distant satellite call rippled across the radio windows of my small home lab. I had spent months piecing together the SDRplay RSPdx and its sleek aluminum chassis, a machine that promised freedom from the shackles of proprietary hardware. The thrill was that it could tune into a swath of frequencies the world turns into images of clouds, storms, and the ever‑moving tapestry of our atmosphere.
Getting the RSPdx Ready
I began by slinging the RSPdx into my laptop bay, attaching the cable that fed a 12‑volt power supply. The RSPdx firmware posted a quick update a month ago that widened its sensitivity from 1 to 11 MHz, an improvement that caught my eye. With the latest firmware installed, I opened SDR#, the software that would act as my front‑end. From the Device drop‑down I selected SDRplay RSPdx and let the interface chatter to confirm it had made a fresh connection.
Choosing the Downlink
I decided to hunt its first target: the NOAA‑18 weather satellite. In the low‑frequency band, that satellite beams its human‑equation” images at 137.1 MHz, and its weather‑band at 137.8 MHz. “A clean, narrow band,” I whispered, knowing that a fixed center frequency and a bandwidth of roughly 4 kHz would leave me with only the fancy side‑bands and noise, letting the visible light from the satellite work its magic on my screen.
Tuning In
SDR# makes a hacking dance of “Signal” and “Noise” as its waterfall settles. I slowly slid the Center Frequency slider to 137.1 MHz, watching the radio plot-snail rise straight above the noise floor. Then I dimmed the Gain to about 15 dB, a setting that kept the squelch from hissing while keeping the signal bright enough for decoding. The software’s popup told me that a sample rate of 100 kHz was optimal for weather satellite work; a rate too high would waste bandwidth, too low would clip the satellite’s subtle TV wave.
Listening to Sky‑Borne Movies
Next, I opened SatDump, a free re‑decoding tool that knew the satellite’s two‑tone look‑up tables. By feeding SatDump the raw SDR data stream, the program turned the 137.1 MHz channel into a near‑real‑time slideshow of cloud patterns. I watched as the software demodulated a faint TV multiplex, printing out images of rain clouds high above the polar ice and low‑altitude thunderstorms buzzing over the Indian Ocean. The 640×480 snapshots were a victory in themselves, though the software threw in a quick deconvolution that sharpened the edges just enough to appreciate the low‑frequency texture of our atmosphere.
Fine‑Tuning the Flow
To kiss the fluff out of the image, I pressed SatDump’s “Low‑pass” button twice, tightening the bandwidth to 2 kHz. The sharpness grid behind my laptop went from a blurred moon to a crisp cloud‑surf. I gave a quick pass the twist that RSPdx’s built‑in Frequency Offset correction and a tiny Cold‑Plasma Filter at 350 kHz remove motion blur. In that moment, the weather satellite seemed to make a flourish, releasing its “blue print” for storms poised in the clouds.
Learning From the Cloud Banks
When the workstation settled, I reviewed the recordings and noticed patterns in the satellite’s return; the left side of the images had slightly less detail and a global, quiet hum. The RSPdx had to rely on its narrow 2 kHz bandwidth to keep the channel from slipping into “Alias Land.” I realized the same approach could bring down the blank sky over Antarctica, and that it might even work on the older NOAA‑16 satellite that darts at 137.5 MHz from the N‑10 satellite bus.
From Night to Day
At dawn, the distant echo in the signal had faded, and I uploaded the decoded frames to a local gallery. The images, bundled into a JPEG series, were shared with a handful of friends who ran their own RSPdx rigs at their universities
Setting the Stage
The first time I set up my SDRplay RSPdx it was in a cramped attic, the kind of room that feels like a relic from the past yet is pulsing with new possibilities. A cheap 59‑inch dipole stretched towards the eaves and an inexpensive 15 dB attenuator hid its secret, shielding the receiver from the old wooden structure’s hum. The RSPdx, with its wideband agility from 1.7 to 6 GHz, was ready to listen to the sky, and I was eager to hear the familiar voice that pilots come to trust for their weather information.Hunting for the Gentle Voice of the Clouds
I opened my SDR software—SDRangel, the newest open‑source platform that supports SDRplay hardware—and set the tuner to 129.5 MHz, the frequency on which the North Atlantic VOLMETs broadcast. The signal made a gentle wobble in the waterfall, a faint band that seemed to pulse in time with the cricketing of the attic floorboards. The RSPdx’s calibrated front‑end allowed me to apply a narrow bandwidth, reducing the hiss of distant skyline noise. I dialed the FM demodulation with a 100 kHz bandwidth, the sweet spot for the voice of the weather, and watched the audio waveform settle into a clear, human tone.From Raw Spectrum to Spoken METARs
Squinting at the waterfall, I recognized the hiss of flat‑band noise—windows of data buffering into my shoulders. These were no invisible monologues, but structured irregular intervals of meteorological observations. The evening’s air was a symphony of tides and wind, expressed in the more than a dozen METAR strings that would be transcribed into the listening room of every pilot if one had the right ears. I wrote down that first sentence the way those sun‑rising weather readings are always recited: “METAR SMAE 031600Z 18015KT 12SM BKN020 SCT040 24/18 Q1019.” I could sense the faint hum of braking wind as the words floated across the monitor. In the attic’s cramped atmosphere, the speaker set the scene for a weather report that was as clean as it was vital.Through the Attic Lens, Recording the Skies
Once the voice filled the ear, I captured a raw audio recording. I used SoX, the command line utility, to split the spectrum into a 44.1 kHz file that would later be reviewed in a separate playback window. The RSPdx’s stable local oscillator made the recorded data stable enough to hop from one browser tab to another without a nervous jitter. From one backup, I could hear a smooth conversational tone—pilot radio protocols emphasize clarity, and thatWhen the First Vessel Calls
I remember the first time I heard a ship’s voice crackle over the waves from my tiny apartment in town. The SDRplay RSPdx sat on the desk, its thin black casing reflecting no light, but its circuitry had already swarmed to catch the tiny bursts of maritime radio chatter drifting within a hundred miles. The moment the 156‑174 MHz band opened, I was hooked as soon as I snipped the VHF Marine filter from the open source rtlsdr_sdr driver. The RSPdx flexed its low noise design to pull out the faint signals that a proper ship antenna coaxial cable would have a hard time reaching.
The RSPdx’s Rising Capabilities
By early 2026 the RSPdx had earned a reputation for being the most value‑heavy SDR, and the latest firmware update brought native support for the DVB‑TC tuner mode that is essential when listening to AIS (Automatic Identification System) on 162.025 MHz. The software stack—primarily gqrx and the new aisrx‑cli—now translates that unmodulated binary into a readable log of vessels’ positions, speeds, and courses. What keeps me coming back is how instantaneously those signals appear on the screen as the system knows to down‑sample from 2.4 MHz to the narrow 204 kHz band required for AIS.
Listening Into the VHF Spectrum
When the ship’s *shuttle* radio starts a routine hello call, the RSPdx’s high dynamic range and fine tuning give me an unmistakable clarity that I’ve never experienced on a cheap dongle. I stream 30 kHz of bandwidth at the 156.8 MHz point, and the SDR’s on‑board DSP not only isolates my ship’s 462.5 MHz signal but also lets me hear faint low‑frequency emergency alerts that people often miss when using a simple handheld radio. The ability to demodulate FM, AM, and even the newer CPN‑signals makes it feel less like a technical tool and more like a portal to the ocean.
Captions From Days at Sea
One night, in a corner of the balcony with a separate tiny dipole, I tuned into the maritime country code phenomenon between 156.390 MHz and 156.560 MHz. After hours of patient hypothesis testing, the RSPdx finally deciphered a shortwave burst I suspected was a *meteor burst communication* between a research vessel and a coastal research station. The packet burst lasted only about two seconds, yet the RSPdx’s waterfall plot retained the glitch‑free tones, slashed bright to signal the message content. In an instant, the RSPdx became the medium that connected me to a story unfolding in 12,000 miles of blue.
From Hobby to Professional Insight
With recent upgrades to the RSPdx’s Continuous Tone to Space Commands (CTTS) receiver module, I can now snatch digital voice transmissions from shipboard *transponders* on 218‑224 MHz, functioning as a real‑time packet radio. The RSPdx's tiny USB interface feeds a lightweight computer that runs the KISS-Protocol software, allowing the internal analyzer to automatically tag every radio call into a database. It is a dream for maritime safety analysts—true modern day maritime traffic parsers created from open‑source hardware and blended with an ever‑growing community of listeners.
The Journey Forward
As I map the waves, the RSPdx remains my faithful companion, quietly converting the ebbing currents of maritime radio into a living record. Its progress to now include craft‑level Octave‑band receivers and cloud‑based analytics is pushing the hobbyist frontier into a realistic operational field. Nevertheless, the minimalist story that unfolded on that night, that first ship call, that bright burst sandwiched between silence, will forever remain the fastest way a small device can open a window onto the vast oceans.
The Journey Begins
It was a clear autumn morning when I finally decided to take my SDRplay RSPdx into the world of oceanic air traffic. The SDRplay has long been a favourite among hobbyist and professional radio enthusiasts, and the RSPdx, with its 12‑bit analog‑to‑digital converter and a full‑bandwidth span of 80 MHz, promised more than any of the older models.
With the RSPdx seated in a weather‑proof enclosure on my roof, I booted up my laptop and launched SDR#. The interface is clean; the SDR’s firmware reports a 24‑MHz, 12‑bit capture range that covers from 10 kHz up to 38 MHz. It is this flexibility that would allow me to hear the mellow tones of VHF aircraft calls, the hi‑frequency chatter of HF conversations, and the faint pings of ACARS transmissions above the ocean.
Setting Up the RSPdx
The first step was to configure the device with proper bias‑tee support, as many of the VHF antennas I would use rely on that. I enabled the 48 V® bias‑tee in the software, then connected a low‑loss dipole that I had already brought to the roof. With the RSPdx now powered and the antenna secured, SDR# let me test the link by drifting across the VHF band. The spectrum flickered with bright markers; I could already spot the aircraft’s 118–138 MHz band, the nautical VHF service at 156 and 157 MHz, and the faint 122‑ish MHz range that is the home of ATC.
Tuning into Oceanic VHF
Oceanic flights rely on the very few GANSAR stations that serve the North Atlantic, the Atlantic Ocean, and other sparsely populated maritime regions. In the 118–136 MHz band, I focused on the 121.5 MHz channel, listening for the distress signal that would be the ultimate test of the system. I also set a 450 kHz bandwidth that gave me a crisp view of the aircraft’s “talkout” bursts at 121.5 MHz, which I could amplify and demodulate with the built‑in FM de‑emphasis.
When the signal actually arrived, the power level was quite low due to the long distance. That’s why the RSPdx’s arbitrary 16‑bit dynamic range on the down‑converter was decisive. It allowed the subtle tone of the aircraft voice‑over‑air to stand out against the hiss of atmospheric noise.
HF and Building the Link
With VHF coverage limited, the next frontier is HF. Using the SDR# channel predictor, I plotted the reliability of the 8‑20 MHz window for the airplane's HT calls. The RSPdx’s wideband 80 MHz coverage meant I was free to sit on the 10.5 MHz (middle of the 10 MHz band) for the International Oceanic Flight Plan (IOFP), which is transmitted on 10.7 MHz.
I set the hardware to an 1 MHz sample rate and used HDSDR to capture the HF burst. Because the atmosphere can impose a Doppler shift that can move the carrier by a few kilohertz, I gave the tuner a ±5 kHz adjustment range. The RSPdx’s low noise figure of 3 dB means that even the telemetry data of a transmission that arrives with only a few samples above the noise floor could be restored.
Extracting ACARS Data
Acquiring ACARS bursts is perhaps the most elusive part of the oceanic surveillance puzzle. The data are transmitted on the 228 MHz band and are masked by a 19.2 kbit/s modem. In 2023, the SDRplay community highlighted a new soft‑cocktail that uses SDR# in combination with a 25 kHz bandwidth filter to isolate the ACARS signals.
I tuned to 228.0 MHz, set the software’s gain to the SDRplay's onboard 50 dB setting, and captured several bursts. With the PCM demodulation plugins that now support the 19.2 kbit/s format, I could decode the text messages that report the flight level, the aircraft’s position, and the contact with the ATC. After a few days of refinement, the RSPdx was providing near‑real‑time ACARS feeds that complemented the V
Discovering a New Flight Path
It was a quiet Saturday afternoon when Alex, a hobbyist radio enthusiast, stumbled upon a weather‑to‑vessel conversation floating on the twilight of an FM radio band. While scanning 118–136 MHz with a simple watch‑receiver, a voice crackled through—"Tower, IFR to 10 NNE, final check." Curiosity sparked like a flare on a dormant runway, and Alex imagined the intricate dance of planes, frequencies, and data that shared that invisible corridor. The question that followed was, "How can I take a closer look at this air traffic tapestry?"
Before rushing into a new transmitter, Alex turned to the SDRplay RSPdx. This compact, low‑budget SDR, opened up a world of possibilities: a single cartridge that covers the entire VHF band with a dynamic range suitable for the delicate aviation chatter. The RSPdx's 12‑bit ADC and 100 kHz analog‑to‑digital sampling made sense; they struck a balance between clarity and cost.
Setting the Stage: Tuning In
The first step was a stable power supply. The RSPdx's auto‑calibration feature works best when it’s powered through a regulated USB port, not an unstable laptop charger. After confirming the hardware, Alex launched SDR#, a popular graphical front‑end for the RSPdx. With a single click on “RX,” the interface opened a frequency wheel. Alex navigated to 123 MHz and heard the soft murmur of an aircraft voice—a hampered whisper, but unmistakable. The next challenge? Making the reception crisp enough to read the chatter.
The Tune‑up: From Analog to Digital Delight
SDRplay offers a built‑in gain control for the RF front end and the ADC. Alex set the RF gain to 16 dB, then used the software’s 26‑dB digital gain to boost weak signals from gliders and small aircraft. The result was a clear signal overlay, where every squawk and call sign was visible in the waterfall display. To prevent static, Alex installed an active high‑pass BPF centered at 116 MHz in the signal chain. This step eliminated the faint AM hum that occasionally plagued VHF recordings.
Capturing Horizons: The Pilot’s Flight Check
Over the following weeks, Alex turned evenings into listening sessions. A practice log was kept: date, hour, frequency group, and aircraft type. The story grew—a busy en‑route corridor at 118 MHz, a small general‑aviation traffic pattern at 124 MHz, and even a rare long‑range VHF‑HF cross‑band. Each bushel of data contributed to a richer narrative. It often felt less like a hobby and more like an ever‑changing, radio‑frequency time capsule.
Tools of the Trade: Software and Scripting
Veteran users would appreciate the flexibility of Python with the SDRplay API. By scripting the receiver to sweep 118–136 MHz automatically, Alex could record continuous “flight tapes” for later analysis. The open‑source FLUband filter added a notch around 117.95 MHz, carving out the traffic that signals most aircraft engines at 790 kHz.”
Stepping Beyond the Ham Radio Band
In late May, the SDRplay released firmware version 2.10.4, bringing tighter distortion control and improved soft‑clip avoidance. The new firmware enabled more reliable reception of low‑power VHF voice, especially in mountainous terrain where reflections are abundant. Alex quickly updated both hardware and software; the difference was analogous to turning on a brighter runway light during a foggier night.
Future Flights: Staying Ahead of the Spectrum
As the aviation domain evolves, so does the spectrum. The 124–130 MHz rate allocations increasingly carry NOTAMs in text form, and newer aircraft are integrating digital mode voice over VHF. Alex plans to integrate dump1090 for ADS‑B reception while continuing VHF voice capture. The narrative remains the same—an unfolding, real‑time story of humanity’s daring journeys above the earth, heard and recorded by a modest, yet remarkably capable SDR.
The Ray of Insight
When the first dawn crept over the studio’s windows, the hum of the SDRplay RSPdx turned from a mechanical whir into a living pulse, the quiet before the storm of data that would soon flood through the instruments. The RSPdx, with its generous 1.5 µV calibration line and a 140‑dB dynamic range, was ready to explore the wide bandroom that sits between the bright cutting‑edge of the 2.0 GHz band and the shadows of its rival voices.
Setting up the Silent Orchestra
After installing the latest SDRplay RSP API and the software suite that follows it—GNU Radio, GQRX, and the bespoke acarsdec fork—the team descended deeper into the radio‑frequency jungle. The RSPdx’s dual‑channel capability proved pivotal: one channel panned toward the VHF gate of civil aviation while the other chased the higher‑frequency VDL‑Mode‑2 streams that roam the 9‑10 GHz corridor. Software‑defined RF filters shrank the bandwidth to just the narrow slice a single aircraft would occupy, bringing the sky’s chatter thick with clarity.
Practical details became the backbone of the operation. A low‑noise helical antenna with a resonant frequency tuned to 154.1 MHz served the ACARS, while a compact patch antenna proved its mettle on the 9 GHz band for VDL. An RSPdx’s built‑in LNA added a 20 dB gain boost, indispensable when fighting the distant, pre‑flight chatter that arrives with a modest calibration line of 0 dBm on a 9 MHz slice.
Listening to the Skies
Today's session began with a clinical sweep of 122.735 MHz, the typical ACARS frequency for many commercial carriers. The oscillator hopped steadily, and a faint voice from a miles‑away Airbus announced the arrival of a maintenance crew. The acarsdec script, now updated to parse the new WAF1 and WAF2 encoding tables, spilled out a clean text message: “Flight LK123 predicted ground time 45 mins.” The sudden clarity of the last syllable felt as if the plane had just spoken from a distant glass wall into the listening ears of the RSPdx.
Meanwhile, on the second channel the quest moved to 9.938 GHz, a VDL‑Mode‑2 spot where the sky’s whisper grew louder. The acquired baseband was fed into VDL‑Decoder, an open‑source tool recently updated with the 2025 standard for message type 6—shipment financials. The decoder translated encrypted data into the playlist of flight‑related economics. The sounds that resonated were not ones of speech, but of numbers and values—an alien, yet intimately tuned to the mechanics of flight.
Decoding the Messages
Both radios danced together while the system recorded continuous ten‑hour bursts to any arriving communication. The accumulated log presented a mosaic of aircraft, each message like a brushstroke on a fresco that combined [textual] ACARS dispatch, T‑across the globe, and the cryptic logic of VDL’s financial pulses. Every time a message unspooled, the RSPdx translated it via the Emulated Intermediate Frequency band into the unique diagnostic frequency domain that a seasoned SDR operator could measure in real time.
To check the integrity of the captured data, the team employed the Signal‑to‑Noise Ratio (SNR) check built into GQRX. With a landmark SNR rise to 45 dB for VDL and 38 dB for ACARS, the field conditions were validated—an achievement that would comfort any novice in the future awaiting the RSPdx’s debut.
A New View from Below
By dusk, the observatory’s monitor glowed with a storm of symbols blinking and dovetailing into a coherent tableau of flight communication. The narrative spun from the mundane to
Discovering the Hidden Skies
It began with a quiet Tuesday afternoon, a thundercloud gathering over the Laramie plains. I had already agreed to pick up the SDRplay RSP‑dx at a local electronics store—its promise of 61‑MHz bandwidth for just the price of a mid‑range radio seemed too good to ignore. The moment the box finally arrived at my doorstep, I could feel the anticipation humming through my fingers, the slightest click counting the build‑up of anticipation.
Once I opened the case and popped the RSP‑dx into the USB 3.0 port, the software spun to life. SDRWRAP, the official wrapper that gives me full access to the RSP‑dx’s virtual I/O, boots quickly. I selected the RSP‑dx on the device list and swirled the signal strength meter to zero. It was nothing yet, just a blank slate of the internet’s deepest frequencies. I turned on the SDRDMU demo software, because first I wanted to hear the world around me.
Listening to the Sky
After a brief tuning spree across 100 MHz, I crossed the aviation band at 135‑145 MHz and found myself in the space of static and the faintest digital whispers. The world of aviation radio chatter began to sound like a bustling city of pulses all traveling on the same highway. The RSP‑dx’s first real triumph was a perfect reception of the ARINC ADS‑B signal at 1090 MHz; the waterfall chart displayed a clean, dropping line that read as an airplane number, latitude, longitude, altitude, and seconds of silence. It was a small victory that made me spell out, inside my mind, the word "Aviation".
But my curiosity pushed me into the L‑band HFDL segment. The Radio Frequency I aimed for was 5.156–5.176 GHz, a segment I'll keep whispering to the RSP‑dx’s RF front‑end. Since the RSP‑dx’s built‑in low‑noise amplifier and RF tuner can be pre‑programmed to notch out precisely that slice, I loaded the latest firmware set from SDRplay Fan Site. With the booster strap in place, the amplifier's gain clicked to a sweet set‑point, and the SDR software displayed hundreds of “dots” in the waterfall that were anything but static. I was now in the world of digital aviation where pilots send navigation corrections, weather updates, and telemetry to the ground with greater efficiency than old voice channels.
Unleashing HFDL
With the digital message field now clear in the waterfall, I turned to software that could make sense of a strongly compressed pigeon‑language syllable. I installed HFDL Decoder, a free Windows program that multiplexes the 3 Kbps data streams that the RSP‑dx can capture. Inside HFDL’s configuration, I set the local oscillator to 256 kHz, exactly half of the 512 kHz sampling range, to match the documented spectral spacing of the HFDL frames. I then allocated the 20 MHz of the RSP‑dx’s bandwidth to a 100 kHz narrowband filter to eradicate unwanted side dishes.
When I hit “Start” on both the SDR software and the decoder, the world changed yet again. The waterfall filled with the same \*pulse‑train pulses as before, but now the decoder was actively analyzing the bits. A growing stream of digital text appeared on the HFDL Decoder’s screen, a microwave of Latitude, Longitude, Terrain clearance advice, and Terminal Radar approach information. The text, initially garbled, resolved into clear sentences after a few seconds of real‑time packet assembly. My RSP‑dx, coupled with the latest firmware, allowed me to receive a HFDL message every 27 seconds on average, a rate that would have been unheard of in the analog era.
Perfecting the Process
To refine the reception, I steered the tuning knob quite literally with my eyes gazing at the waterfall. The rolling bumps in the spectrum told me to slightly shift the center frequency by a few kilohertz: a small shift can literally turn an audible hiss into a crisp, line‑by‑line decoded message. Conveniently, the SDR software offered a frequency auto‑sync feature that locked the center frequency down to the digitized tone used by the HFDL network. Once the lock was in place, the HFDL Decoder ran like a well‑tuned engine, no longer snagging on minor timing errors.
When I tucked the RSP‑dx into a weather‑proof mount beneath my home office’s window, I could peer out and visualize another world: an endless array of aircraft, all travelling on a sky‑based digital highway, each voice contributing to a tapestry of safety and efficiency. My story started with a simple desire to listen to a buzzing radio, but ended with the certainty that the world’s airspace is, and will continue to be, governed by the same technology that powers the tiny steel cassette of that old antique and the broad spectrum analysis unfolding across the day. Even in the quietness of an apartment at night, the HFDL buffers open, and as the RSP‑dx sighs faintly with each decoded packet, my narrative of the sky expands, one HFDL burst at a time.
When the RSPdx Came Home
I unboxed the RSPdx, a slender 1½‑inch SDR that promised to turn my desk‑lamp into a listening post for the world’s invisible waves. The first thing I did was run the 64‑bit Debian installer that ships with librtlsdr. The package made the device instantly visible to the kernel, and a quick lsmod | grep rtl confirmed that RTL‑2832U was ready for action.
Setting Up the Linux Workstation
Linux required a bit more plumbing than the plug‑and‑play Windows boxes I had used before. I added the SDRplay api to my system: apt install sdrplay-api. This library exposes the device’s full bandwidth range and mixer control, something librtlsdr can’t do. Once the libraries were in place, I wrote a tiny C++ helper that would open the RSPdx, set the 9 MHz sample rate, and stream raw IQ to stdout. While the helper ran, I steered the front‑panel tuner to the VHF band where DRM signals usually sit and listened to the hiss of static turn into a soft, low‑frequency beat.
Unlocking DRM Signals
DRM, or Digital Radio Mondiale, is a packet‑based digital audio system that can piggy‑back on AM and short‑wave frequencies. Its modulation scheme is quite different from FM—they use High‑Rate Orthogonal Frequency Division Multiplexing, or HRO‑OFDM. On Linux, there is an open‑source demodulator called drm-usrp that can be adapted for the RSPdx. I piped the raw IQ stream through my C++ helper and into drm-usrp --input-format=int16 --cfg=med70.csv. The configuration file I used comes from the drmtools repository, which contains precise channel‑specific timing matrices. After a moment of silence, the program started announcing the list of available DRM stations, each with a descriptive title generated by the broadcasting service.
Fine‑Tuning the Capture
Because DRM streams sit on top of narrow bandwidths—often only 2 kHz wide—I had to ensure my sampler didn’t clip. I tweaked the RSPdx’s gain settings, moving the “General” and “IF” knobs toward an 18 dB plateau, then recorded a short clip with wavpack to check the dynamic range. After confirming that the audio depth was safe, I adjusted the front‑end filter to the “HF” preset, better suited for the 5 MHz signal I’d bookmarked. The demodulator’s error‑correction bit‑rates behaved exactly as the spec sheet promised, polishing the received audio into a smooth stereo track.
The Final Listening Moment
When I finally dropped the drm-usrp into my audio pipeline and let it run unattended, the RSPdx turned a simple desk lamp into a portal. I could hear my hometown’s historical channel broadcasting live weather reports as the lights flickered in the room. What started as a tidy hardware bundle morphed into a living conversation between radio engineers and listeners, all under the hood of my Linux machine. Each click of the tuner became a new chapter; each burst of decoded audio a sentence in the story of the digital airwaves. The RSPdx was no longer just an SDR; it was the keystone of a fresh, personal broadcast experience.
The Adventure Begins
On a quiet Saturday morning I cabled the new SDRplay RSPdx to my MacBook Pro, curious about the world of digital broadcasts that lie beneath the radio waves we often take for granted. The little antenna sat perched on my desk, its silver fin gleaming with the promise of untapped frequencies. I labeled the magnet that kept it in place, imagining the invisible currents ready to dance to my command.
Shifting to DRM
With the RSPdx humming softly, I opened the SDRplay Desktop Explorer. Its familiar interface welcomed me with a simple green signal icon that appeared only when the device was properly connected. I remembered that the RDM-Tool, the official DRM receiver for SDRplay, had just received a pivotal update last month to improve decoding fidelity on macOS. I navigated to the Software tab and installed the r2 decoder suite, watching the progress bar slide smoothly across my screen.
Next came the configuration. I opened the RDM-Setup window and prompted the software to perform a quick sweep of the band. The RSPdx whispered through ranges from 10 kHz up to 30 MHz. My eyes followed the spectral peaks until I spotted the familiar hiss of DRM signals in the 100 kHz to 1.7 MHz corridor, the band where many European broadcasters transmit. I clicked the “Auto‑Calibrate” button, and the SDRplay answered in crisp tones, aligning the tuner’s crystal reference so that every decibel would truly reflect the world outside my ears.
A Mac's Journey
I launched the r2-decoder, selecting the “DRM” profile. A new window opened, the green monitor light turning steadily on, signalling that the receiver was listening. The program displayed a live waterfall, and a soft, pulsating waveform emerged on the main screen. With a gentle click, I tuned directly to the channel BBC World Service FM at 550 kHz, a site famed for its obstinate DRM signal. The audio jack on my Mac sagged as the digital stream carved itself through the ether, and I could hear voices piped over the air, instructions, news, and more—clean and crystal‑clear.
To refine the experience, I adjusted the Signal Seeking slider to “Fine”, nudging the tuner’s center of interest and trimming unwanted hiss. The software’s “Virtual File” option saved the raw audio stream to my hard drive, allowing me to revisit the broadcast from any device without a live connection.
Concluding the Tale
By mid-afternoon, my little RSPdx sat comfortably beside a cup of coffee, eager to dive deeper into the digital realm. The entire process—from installing the latest DRM-Tool to hearing the crisp cadence of a transmitted voice—felt like discovering a secret laboratory in the skies above. The macOS environment, with its robust terminal and sleek GUI, meshed seamlessly with the open‑source decoding framework, turning a simple hobby into a tale of exploration and discovery. My story ends here, but the airwaves are vast, and the possibilities stretch as far as the horizon—and beyond.
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Written and curated by Philip Collier / AB9IL.
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