The best way to understand the radio spectrum is to pull something real out of it. This is a field guide to three receive-only captures anyone can reproduce with a cheap SDR: aircraft broadcasting their own position, weather-satellite imagery, and the quiet chatter of the devices around your house on 433 MHz. It is the hands-on companion to how an SDR works - and the interactive below decodes a real aircraft message field by field.
//Capture 1: Aircraft (ADS-B, 1090 MHz)
Every airliner overhead broadcasts its identity, position and altitude roughly twice a second, in the clear, on 1090 MHz. This is ADS-B - the system air traffic control and every flight-tracking website runs on - and it is the single most satisfying first capture, because within minutes you have planes appearing on a map you built.
The method: an RTL-SDR dongle, a 1090 MHz antenna (a quarter-wave whip is about 69 mm), and dump1090 (the maintained forks are dump1090-fa and readsb). Point the software at 1090 MHz and it decodes the frames and plots them.
What you are receiving is a 112-bit Mode S extended squitter. The demo above decodes the standard example frame: a downlink format marking it as ADS-B, a 24-bit ICAO address that uniquely identifies the airframe, a type code that says what the message carries, a barometric altitude, and a CRC. The one genuinely fiddly part is position: to save bits, each frame carries only an ambiguous fraction of latitude and longitude (Compact Position Reporting), so you need an even frame and an odd frame received close together to resolve an exact location. Altitude and identity come from a single frame; position takes the pair.
//Capture 2: Weather Satellites (137 MHz)
For years the classic second capture was a NOAA weather satellite: as one passed overhead you could receive its 137 MHz APT downlink - a 2400 Hz subcarrier, 120 lines a minute - and decode a live greyscale image of the clouds over your own location, from a satellite, with a hand-made antenna. It is a wonderful thing to do and it taught a generation of hobbyists FM and AM demodulation in one afternoon.
The technique, for the record: an FM demodulation of the 137 MHz carrier recovers the 2400 Hz subcarrier, which is AM-demodulated to recover image brightness line by line, two video channels side by side (typically a visible and an infrared) plus telemetry calibration wedges. A right-hand circularly polarised antenna and a pass that comes high over your horizon make or break it.
//Capture 3: The 433 MHz Neighbourhood Survey
The most quietly revealing capture needs no satellite and no aircraft - just tune to 433.92 MHz and listen to your own street. This ISM band is where an enormous population of cheap unlicensed devices chatters: tyre-pressure sensors, wireless weather stations, doorbells, remote controls, home sensors. Most use simple on-off keying in short bursts, and the tool rtl_433 decodes over two hundred device protocols straight to readable JSON.
Run it for an hour and you get an unsettling little census: the temperature from a neighbour's weather station, a car's tyre sensors announcing themselves as it parks, a doorbell press three houses down. None of it is encrypted because none of it was ever thought worth encrypting - which is exactly the lesson. (US-market devices often sit at 315 MHz instead; 433.92 MHz is the Region-1 case for Australia and Europe.) Receiving this open device telemetry is fine; transmitting on the band or acting on anything private is not.
//Where the Original Data Goes
This is a reproducible method, not a results post - the demo frame is the standard teaching example from mode-s.org, not something logged off a specific rooftop. The point of writing it as a guide first is honesty: the original-data value of this site comes from real captures with real provenance, not plausible-looking numbers. The planned follow-ups are the actual runs - the aircraft logged over one location across a week, the 433 MHz census of one street, decoded and mapped - and those become the pieces only a sensor on the ground can produce. If you run these captures yourself, that is the move: publish what you received, dated and sourced, not what the internet already has.
dump1090/readsb for aircraft, an FM+APT decode chain or SatDump for satellites, rtl_433 for the ISM survey. Antennas: a 1090 MHz whip for ADS-B, a V-dipole or QFH for 137 MHz, a 433 MHz whip for the survey. The HackRF works too and goes far wider, but for these three the cheap dongle is the right tool.//Sourcing and Framing
The ADS-B frame format and the worked example are from Junzi Sun's mode-s.org decoding guide; the decoded identity, type code and altitude were verified against it. The NOAA POES decommissioning dates are from NOAA's own OSPO status page (June-August 2025); do not state any APT bird as currently live without checking a current source. The 433 MHz device landscape and rtl_433 are from that project's documentation.
Framing, as across the signals cluster: receive-only, open or own signals, nothing operational, and a hard line against intercepting private communications. The interpretation is mine - that the 433 MHz survey is the capture that teaches the most, because it shows how much of ordinary life is broadcast unencrypted by things nobody thinks of as radios.