ADSBee m1421: Inside the World’s Smallest Dual-Band ADS-B Receiver

If you’ve spent any time in the RTL-SDR world, you already know the drill: plug a dongle into a Pi, run dump1090, watch planes crawl across a map on port 8080. It works beautifully for a home ADS-B feeder sitting on a windowsill. It works a lot less beautifully if what you actually want to bolt ADS-B reception onto is a drone, a weather balloon payload, or anything else where every milliamp and every gram counts.

That’s the gap the ADSBee m1421 was built to fill — and it’s a genuinely interesting piece of open-source RF engineering, not just another “small SDR” story.

The backstory: from three chips to two

ADSBee isn’t new. The project, from a small outfit called Pants for Birds LLC, started with the original ADSBee 1090 — a full-featured ground-station-grade receiver built around an RP2040 doing PIO-based Mode S/ADS-B decoding, a CC1312 handling the 978 MHz UAT band, and an ESP32-S3 tacked on for WiFi/Ethernet feeding. Three processors, a discrete RF frontend with a couple of power-hungry LNA stages, USB/UART/WiFi/Ethernet outputs, GNSS input for MLAT — basically a Pi-class feeder shrunk onto one board, drawing around a watt.

That’s great for a rooftop feeder. It’s overkill — in size, weight, and power — for anything you’d want to fly.

So the team spent roughly eight months redesigning the receive chain around a different pair of chips: a TI CC1314R10 low-power sub-GHz RF MCU and a Semtech LR2021 radio. The result, announced on Hacker News as “the world’s smallest dual-band aircraft tracker,” is the ADSBee m1421 — a module that does everything the original does on the RF side, minus the WiFi/Ethernet/GNSS extras, in a package that weighs under 2 grams and sips less than 60 mW.

What’s actually inside the m1421

The split of labor is elegant. The LR2021 provides best-in-class sensitivity for demodulating the high-baud-rate OOK waveform used by 1090 MHz Mode S. The CC1314’s sub-GHz radio core handles 978 MHz UAT reception directly, while its Cortex-M33 does everything else — FEC, full Mode S and UAT decoding, and maintaining an onboard aircraft dictionary for up to 400 simultaneous targets — running as a bare-metal, no-RTOS super-loop that polls the LR2021 over SPI.

There’s no SDR dongle, no FPGA, and no host computer anywhere in this picture. The whole receive-and-decode pipeline lives on ~19 mA at 1.8–3.3 V.

Headline specs:

Output-side, the m1421 speaks the protocols you’d expect from an aviation-grade tracker: raw packets, GDL90, MAVLINK1/2, Mode S Beast, Aircraft JSON, and Pants for Birds’ own information-rich CSBee format — all over a plain UART console with a full AT command set for baud rate, gain, logging, and output selection, with settings persisted to flash.

There’s also a SYNC line that puts the whole device into a SRAM-retained STANDBY sleep and, cleverly, hands the LR2021’s SPI bus over to an external MCU on wake — useful if you want to repurpose the LR2021’s 2.4 GHz radio for LoRa or mesh duty when ADS-B reception isn’t needed.

The tradeoff nobody hides

What I liked most, digging through the project’s Hacker News “Show HN” thread, is how upfront the maker (posting as CoolNamesAllTkn) was about what you give up for that power budget. Because the LR2021 can only receive Mode S in its maximum-dynamic-range configuration, the m1421 decodes DF17 extended squitter only — no DF4/5/11 surveillance/altitude replies, and no per-packet RSSI or timestamps. That second bit rules out multilateration (MLAT) applications out of the box, something the team says they’re still noodling on for a future firmware update.

In other words: if you’re building an airport-grade ground station that needs every squitter type and precise per-packet timing, you still want the three-chip ADSBee 1090. If you’re building a drone detect-and-avoid system, a solar- or coin-cell-powered flight tracker, or general-aviation cockpit avionics where DF17 traffic and UAT weather/traffic uplink is what actually matters, the m1421’s tradeoffs are the right ones — and, per the same thread, that’s exactly where their early customers are using it.

Fully open, right down to the KiCad files

The whole family — firmware for both the 1090 and 1421 lines, shared decoders and aircraft-dictionary code, KiCad hardware design files, LTspice/Excel RF calculations, and even the datasheet sources — lives in one GitHub repo under GPL-3.0. Firmware for the 1421 builds through a Docker toolchain and flashes either over JTAG with a J-Link or over the UART ROM bootloader using an included programmer jig or a browser-based Web Serial console — no debugger required for the common case. That’s a refreshingly complete openness for a commercial product; the hardware isn’t just “inspired by” open source, the actual board files and RF simulations are in the repo.

Pricing keeps with that hobbyist-friendly spirit: the standard m1421 module runs about $48 in single quantities (an NDAA-compliant variant is currently $75, with the team working toward cost parity by the end of the year), and there’s a compact 1421 Developer Kit with a power-combined dual-band antenna, USB programmer, and wiring harness for anyone who wants to get a receiver talking over UART on a bench without designing a carrier board first.

Why this matters if you’re coming from RTL-SDR

For the RTL-SDR crowd, ADSBee m1421 is a nice reminder that “software-defined” isn’t the only path to good ADS-B reception anymore — sometimes a purpose-built RF MCU pair beats a general-purpose SDR on power and size, at the cost of some flexibility. It won’t replace your rtl_adsb/dump1090 feeder station; it’s not trying to. But if you’ve ever wanted ADS-B traffic awareness on something that flies, runs off a battery, or needs to disappear inside a 2-gram footprint, it’s very much worth a look — and since it’s fully open source, it’s also a genuinely good reference design to study if you’re curious how UAT decoding actually works under the hood.

• Datasheet, firmware, and hardware files: github.com/PantsForBirds/adsbee
• Product page / ordering: pantsforbirds.com/adsbee
• Original Show HN discussion: news.ycombinator.com/item?id=49455557

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