Learn / Guide 8
Aircraft on the radio: ADS-B and air band
What an aircraft transmits on 1,090 MHz, how far it can be received, how an RTL-SDR decodes it, and how to listen to the voice in AM.
intermediate 6 min read
The aircraft tells where it is
ADS-B stands for Automatic Dependent Surveillance-Broadcast. Each word says something:
- Automatic: no one needs to ask. The plane transmits on its own.
- Dependent: depends on the airplane's own navigation system, usually GPS.
- Broadcasting: the message goes out in all directions, in the clear, to any receiver.
Traditional radar discovers where the aircraft is by measuring the echo. In ADS-B, the aircraft calculates its own position and announces it. Air traffic control receives it, other aircraft receive it, and a cheap receiver by a window does too.
What goes into the message
Transmission is on 1,090 MHz. These are very short bursts: an 8-microsecond preamble and 112 bits of data, in pulses, at one million bits per second. Each message lasts a little over a tenth of a millisecond.
Every message carries the aircraft ICAO address, a 24-bit number written in hexadecimal, fixed for each aircraft. Brazil received from ICAO the block from E40000 to E7FFFF, so Brazilian registered aircraft have addresses starting with E4, E5, E6 or E7. The rest varies according to the type:
| Message type | Content | Frequency |
|---|---|---|
| Identification | flight callsign or registration, aircraft category | every 5 seconds, more or less |
| In-flight position | latitude, longitude, and barometric altitude | twice per second |
| Speed | ground speed, heading and rate of climb or descent | twice per second |
| Ground position | position and movement during taxi | less frequent |
| State | emergency, transponder code | from time to time |
The position arrives compressed in a format called CPR. The decoder needs two consecutive messages, one even and one odd, to resolve the position without ambiguity. That is why an aircraft first appears on the list with only address and altitude, and a few seconds later earns a spot on the map.
Not every plane appears. Older or small aircraft may have transponders without ADS-B: they reply to radar, but do not announce their position. Collaborative networks manage to locate some of them by comparing the signal arrival time at multiple receivers, a technique called multilateration.
As far as it can be received
At 1,090 MHz the signal travels in a straight line. It does not bend over the horizon and does not cross hills. Theoretical range depends on both heights:
distance in km ≈ 4.12 × (√antenna height + √airplane height), with heights in meters.
| Airplane altitude | Range with antenna 10 m off the ground |
|---|---|
| 1,000 m (approx.) | about 140 km |
| 5,000 m | about 300 km |
| 11,000 m (cruising) | about 445 km |
This is the rooftop, with a clear horizon on all sides. In the city, what matters is what is around you. Buildings, hills, and even your own house walls cut out entire slices of the sky.
With an RTL-SDR and a simple antenna, the range is usually partial: one side of the sky arrives well and the other barely appears. The coverage pattern on the map ends up being a portrait of the terrain and buildings around the antenna.
How an RTL-SDR receives
The RTL-SDR is a cheap USB receiver that delivers raw signal samples to the computer. All the heavy lifting is done by software.
- The receiver is tuned to 1,090 MHz and samples 2 million times per second, or slightly more.
- The decoder (the classic is dump1090 and its derivatives) looks for the preamble pattern amidst the noise.
- Once the preamble is found, it reads the 112 bits by the position of the pulses.
- The last 24 bits are an error check. Messages that fail the math are corrected if the error is a single bit, or discarded.
- Valid messages feed a table and a map, on a page served by the program itself.
The antenna is small because the wave is short: about 27.5 cm. A vertical dipole for 1,090 MHz has each leg measuring about 6.5 cm. What weighs most in the result:
- Height and clear view. An extra meter on the roof is worth more than any accessory.
- Short cable. Thin coaxial cable loses a lot at 1,090 MHz. Every meter counts.
- Vertical polarization. Aircraft antennas are vertical.
- Filter and amplifier right by the antenna. They help where cell phones and strong FM are nearby, and compensate for the cable.
The guide to first receiver brings the rest of the assembly.
Voice: aviation band in AM
The data shows where the plane is. Voice tells what is happening. Communication between pilots and control is between 118 and 137 MHz, in AM.
AM survived in aviation for a technical reason. In FM, when two stations transmit together, the stronger one wipes out the weaker one, and nobody notices there was a second one. In AM the two mix into a whistle, and the controller knows someone was stepped on.
What can be found:
- ATIS: continuous recording with time, runway in use and notices from an airport. It is the best first target because it is always on the air.
- Tower, ground and traffic: airport traffic.
- Zoom and center control: airplanes climbing, descending and en route.
- 121.500 MHz: international emergency frequency. Almost always silent.
The frequencies for each airport are public and are on aeronautical charts. In Brazil, the DECEA aeronautical information service publishes all of them.
One characteristic catches the eye of beginners: you hear the airplane and do not hear the tower. The airplane is high up, in direct line of sight. The tower antenna is on the ground, tens of kilometers away, behind the horizon.
Listening to the airband is one thing. Recording and publishing it is another, and the difference matters. The guide on what can be listened to and published deals with this.