Airspy Mini and R2 with airspy_adsb: maximum ADS-B reception

Your feeder has been running stably for months, the antenna is on the roof, LNA and filter are installed – and still the message rate stagnates. By now one name comes up in feeder forums: Airspy. The Airspy Mini and Airspy R2 receivers, together with the airspy_adsb decoder, are considered one of the most capable combinations for ADS-B reception on 1090 MHz. This guide explains what an Airspy does differently from an RTL-SDR stick on a technical level, for which stations it is worth it, how to connect airspy_adsb with readsb or PiAware and which adjustments you should make – and which better not.

The short answer

An Airspy Mini or R2 with airspy_adsb is worth it if your antenna setup is already good and you want to get more out of it. The advantage lies in the 12-bit converter: it copes with strong and weak signals at the same time much better than the 8-bit converter of an RTL-SDR. In return, the Airspy costs many times as much as a stick and needs noticeably more processing power. For setup, the easiest route is the community project airspy-conf, which installs airspy_adsb as a service and reconfigures your existing decoder (readsb or dump1090-fa) accordingly. Start with 12 MSPS and work up the gain from below.

How the Airspy ranks among the other receivers is shown in the SDR stick comparison for ADS-B. This article is about practice with the Airspy itself.

What makes an Airspy different

A classic RTL-SDR stick is a repurposed DVB-T receiver. Its converter works at 8 bits, so only 256 possible values per sample. For ADS-B that is a real bottleneck: an aircraft on approach directly above your roof delivers a very strong signal, a plane 300 kilometers away an extremely weak one – and both have to fit into this small value range at the same time. If you turn the gain up, the stick overloads on nearby aircraft. If you turn it down, the distant ones disappear into the noise.

The Airspy receivers address exactly this. According to the manufacturer, Airspy Mini and Airspy R2 work with a 12-bit converter at 20 MSPS, reach 10.4 effective bits, 70 dB signal-to-noise ratio and 95 dB spurious-free dynamic range (SFDR). Airspy specifies the noise figure as 3.5 dB between 42 and 1002 MHz, the front end has tracking RF filters and tolerates up to +10 dBm at the input. For ADS-B that means: more room between “clipped” and “lost in the noise”.

Then there is the high sampling rate. A Mode S bit lasts one microsecond. An RTL-SDR typically samples ADS-B at 2.4 MSPS, so about two and a half times per bit. airspy_adsb works at 12 or 20 MSPS and thus has significantly more sample points per bit available to cleanly reconstruct weak or overlapping messages. What lies behind the messages is explained in the article transponder Mode A, C and S.

Airspy Mini or R2?

Both devices share the core. The differences lie in output rates, connectors and size.

	Airspy Mini	Airspy R2
Frequency range	24–1700 MHz	24–1700 MHz
Converter	12 bit, 20 MSPS	12 bit, 20 MSPS
Noise figure (42–1002 MHz)	3.5 dB	3.5 dB
Bias tee	4.5 V, software-switchable	4.5 V, software-switchable
Clock accuracy	0.5 ppm	0.5 ppm
External clock input	no	yes, 10–100 MHz (MCX)
Expansion	none	GPIOs, clock outputs, additional ADC inputs
20 MSPS with airspy_adsb	officially not supported	intended

For ADS-B the last row matters most – and in practice it is less important than it looks. According to airspy-conf, 12 MSPS delivers almost the same result as 20 MSPS. The compact Mini is therefore the obvious choice for most feeders. You need the R2 more if you want to experiment with an external reference clock or use the receiver for other projects as well. If you already own an R2, you lose nothing.

When switching is worth it

An Airspy is not a miracle cure. It improves decoding, not reception. What the antenna doesn’t pick up, even the best converter can’t decode. That is why the order applies: first antenna location, then cable, LNA and filter, and only then the receiver. How to approach the first steps is described in finding the best antenna location and increasing ADS-B range with LNA and filter.

Feeder forums also repeatedly stress that an Airspy only plays to its strengths with a filtered preamp. An Airspy does have a large dynamic range, but strong interfering signals outside 1090 MHz – cellular, broadcast, radar – still burden the converter. A bandpass filter in front of the receiver keeps them out. The Airspy brings the bias tee for a mast-mounted LNA itself.

How airspy_adsb and readsb work together

With an RTL-SDR feeder, one program does everything: readsb or dump1090-fa accesses the stick, decodes the signals and distributes the data. With an Airspy this work is split between two programs.

• airspy_adsb talks directly to the Airspy. It takes in the raw data, decodes the Mode S messages and outputs them with timestamps in Beast format.
• readsb or dump1090-fa now runs only in network mode. It receives the finished messages from airspy_adsb, builds the aircraft list from them, feeds the map and makes the data available to feeders.

In the default configuration of airspy-conf, airspy_adsb offers its data on port 47787 and additionally connects to port 30004 of the local decoder. With PiAware, the receiver type is instead set to “relay” and pointed to port 47787. All other feeders – for Flightradar24 or PlaneFinder, for example – access the Beast output of readsb or dump1090-fa as usual, typically port 30005. So for them nothing changes because of the Airspy. How to serve several networks in parallel is shown in feeding several networks at once.

Besides this two-program team there has also been an unofficial dump1090-fa variant with direct Airspy support since 2021, which works without a separate airspy_adsb. Its developer explicitly presented it in the FlightAware forum as an unofficial FlightAware version. The common route remains airspy_adsb with readsb or dump1090-fa.

Step by step: setup with airspy-conf

The airspy-conf project by wiedehopf on GitHub automates the setup. It downloads the right airspy_adsb program for your platform, sets up a systemd service and reconfigures your decoder. The author himself points out that the script doesn’t have to work as intended in every environment – so check the current project page beforehand.

If you want to get rid of airspy-conf later, the uninstall script restores the backed-up decoder configuration and shuts down the service.

Sample rate: 12 or 20 MSPS?

The airspy-conf configuration knows two values: 12 and 20 MSPS. The default is 12 – and for good reason.

The project’s recommendation is clear: 20 MSPS only if you don’t care about CPU load and the device runs in a moderate environment. If your MLAT client shows synchronization problems or an unstable clock after switching, go back to 12. How MLAT works and why clean timestamps matter so much is explained in MLAT/multilateration explained.

Setting the gain correctly

You set the Airspy’s gain in 22 steps from 0 to 21, each corresponding to about 3 dB. The default configuration uses an automatic setting. For a fixed setting, airspy-conf describes two approaches:

• RMS method (default): Start at low gain and increase it step by step until the strongest aircraft reach 0 dBFS – or until you no longer see aircraft in the range around −45 dBFS.
• SNR method: Increase the gain until the displayed signal-to-noise ratio no longer improves, then go back one step.

If 21 is too much, reduce by one step at a time. If you run a preamp at the antenna, the optimal value is usually lower than without one. Every change needs time: traffic fluctuates over the day and the week. So compare at least one full day with normal traffic before you turn it further.

The most important options at a glance

In the configuration file you will find a line with further options. You can leave most of them at their defaults. The overview is based on the comments in airspy-conf’s default file and the program help. Which options your version knows is shown by calling airspy_adsb -h.

Option	Function	Practical note
-C	CPU target value (5–95)	airspy_adsb adjusts the filtering work to the available power
-E	upper limit of the preamble filter	higher values cost more CPU; the default file names a higher value for maximum performance if CPU load allows it
-f	bits for error correction	1 is recommended
-b	switch on the bias tee	only with an LNA at the antenna that is powered via the cable
-p	bit packing	reduces the USB data rate but costs a bit more CPU
-x	DX mode	generates additional erroneous messages, don’t use if you feed networks
-v	verbose log	helpful for troubleshooting

You enter GAIN and SAMPLE_RATE on their own lines, not in the options line. After every change you restart the service.

Processing power and hardware

The biggest practical difference from the RTL-SDR is CPU load. An RTL-SDR decoder runs alongside on almost any Raspberry Pi. At 12 or 20 MSPS, airspy_adsb has to process many times the data. In forum reports, older models like the Raspberry Pi 3 reach their limits at high sampling rates. The -C option sets which CPU load airspy_adsb aims for.

A few basic rules follow from this:

• Plan enough reserve: If tar1090, graphs1090, several feeders or entirely different services also run on the same computer, the CPU shouldn’t already be maxed out by airspy_adsb.
• Take cooling seriously: Both the computer and the Airspy get warm in continuous operation. For 20 MSPS with the Mini, airspy-conf explicitly recommends heat sinks or active cooling. For outdoor enclosures that applies doubly, see running an ADS-B receiver outdoors.
• Stable power supply: The Airspy and possibly an LNA via the bias tee draw power from the USB port. A weak power supply leads to dropouts and lost samples.
• Mini PC as an alternative: airspy-conf also offers program versions for x86 computers. If you are thinking about a used thin client or mini PC, you will find ideas in ADS-B reception without a Raspberry Pi.

Measure success instead of guessing

Whether the Airspy is worth it you decide not by feel but from your statistics. By default, airspy_adsb writes its own metrics to a statistics file, and readsb provides message rate, positions and range. The tool for this is graphs1090, which we present in detail in measuring reception performance.

A fair comparison looks like this:

1. Note the values of your current station over at least a week: messages per second, aircraft with position, maximum range.
2. Swap only the receiver, leaving antenna, cable, LNA and filter unchanged.
3. Compare another full week, ideally under similar traffic conditions.
4. Afterward change gain or sample rate one at a time.

It is typical that an Airspy gains mainly on nearby and very distant aircraft at the same time – that is, where an 8-bit stick is torn between overload and noise. How large the gain is at your location can’t be predicted in general.

What to keep in mind

• The legal side stays the same: Whether Airspy or RTL-SDR – the same rules apply to receiving and passing on the data. Is receiving ADS-B data legal? gives the overview.
• Bias tee only with a matching LNA: The 4.5 volts on the antenna jack can damage devices that can’t handle remote power. Switch on -b only if your LNA is designed for it.
• Updates preserve your settings: With airspy-conf’s update script you update the program without losing your options.
• One receiver, many users: If you want to listen to aviation radio or ACARS in parallel, you need a second receiver. The Airspy is fully occupied in feeder operation. For starting points, see receiving aviation radio with an SDR stick and receiving VDL Mode 2.

By the way, you can use your station’s data not only to send to networks but also for spotting: how to complete sightings after the fact with your own ADS-B logs is shown in matching sightings with ADS-B.

Conclusion

The Airspy Mini and R2 are the tools for feeders who have already maxed out their station. The 12-bit converter with large dynamic range and airspy_adsb with a high sampling rate decode weak and strong signals at the same time better than an 8-bit stick. Setup via airspy-conf is manageable: prepare readsb or dump1090-fa, run the script, tune the gain. The price is higher purchase cost and significantly more CPU load. If you still work with a window antenna and a long cable, you are better off investing first in location, LNA and filter.