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Aircraft signal guide

The receiver is tuned to 1090.000 MHz with a 2.4 MS/s sample rate. It observes Mode S replies and extended squitter transmissions. Classification describes the decoded frame; it does not imply that every family is always present.

Families

Family Source Typical content
ADS-B Aircraft, usually DF17 Identity, barometric/GNSS position, velocity, status, operational capability
Mode S Aircraft replies outside ADS-B Surveillance altitude, identity, Comm-B, all-call, air-to-air data
TIS-B Ground surveillance rebroadcast Traffic reports for targets seen by ground radar or another link
ADS-R Ground ADS-B rebroadcast ADS-B traffic translated from another data link
Mode A/C Legacy short replies Squawk identity or pressure altitude, without a Mode S CRC
Other Reserved or unclassified decoded forms Frames that do not map cleanly to the supported families 
flowchart LR
    Spectrum[1090 MHz spectrum] --> Pulses[Pulse preambles]
    Pulses --> Short[Mode A/C short replies]
    Pulses --> ModeS[Mode S frames]
    ModeS --> Replies[DF 0/4/5/11/16/20/21/24]
    ModeS --> DF17[DF17 aircraft broadcast]
    ModeS --> DF18[DF18 rebroadcast]
    DF17 --> ADSB[ADS-B]
    DF18 --> TISB[TIS-B]
    DF18 --> ADSR[ADS-R]
DF Meaning
0 Short air-to-air surveillance
4 Surveillance altitude reply
5 Surveillance identity reply
11 All-call reply
16 Long air-to-air surveillance
17 ADS-B extended squitter
18 Extended squitter or ground rebroadcast
19 Military extended squitter
20 Comm-B altitude reply
21 Comm-B identity reply
24 Comm-D extended-length message

ADS-B type codes

Inside an extended squitter, the five-bit type code gives the payload family:

  • TC 1–4: aircraft identification and category
  • TC 5–8: surface position
  • TC 9–18: airborne barometric position
  • TC 19: airborne velocity
  • TC 20–22: airborne GNSS position
  • TC 28: aircraft status
  • TC 29: target state and status
  • TC 31: aircraft operational status

Power measurements

readsb reports decoded signal and noise in dBFS, decibels relative to the receiver’s digital full scale. Values closer to 0 dBFS are stronger. These numbers are useful for comparing this receiver with itself over time.

They are not calibrated dBm at the antenna connector. The hardware does not expose antenna standing-wave ratio, impedance, receiver temperature, coax loss, absolute antenna gain, or a wideband waterfall while dedicated to decoding.

quadrantChart
    title Receiver adjustment clues
    x-axis Low decoded rate --> High decoded rate
    y-axis Weak relative power --> Strong relative power
    quadrant-1 Productive reception
    quadrant-2 Possible overload
    quadrant-3 Poor placement or feedline
    quadrant-4 Busy but weak environment

What is unavailable simultaneously

The same RTL-SDR tuner cannot remain centered at 1090 MHz and also receive 978 MHz UAT, VHF airband voice, NOAA weather radio, broadcast FM, or other distant bands. Add a second receiver for simultaneous monitoring. Stopping readsb and retuning this receiver also pauses the airplanes.live feed.