The Message Filter parameter on the ADS-B Decoder determines which Mode S downlink format (DF) values are accepted for CRC checking and payload decoding. All other DF values are silently ignored.
Options
"Extended Squitter Only" (default)
Processes only DF 17, 18, and 19 — the three downlink formats that carry ADS-B Extended Squitter messages. This is the recommended setting for aircraft tracking applications.
- DF 17 — ADS-B message from a Mode S transponder
- DF 18 — ADS-B message from a non-Mode-S transponder (e.g., UAT re-broadcast, TIS-B)
- DF 19 — Military Extended Squitter
For all three, the 24-bit PI field is verified directly against the computed CRC-24. If the values match, the message is decoded.
"All Messages"
Processes all downlink formats that the decoder has implemented CRC handling for. This mode enables full Mode S surveillance reception in addition to ADS-B.
Supported DFs in this mode:
| DF | Name | Payload length | CRC method |
|---|
| 0 | Short Air-Air Surveillance (ACAS) | 56 bits | AP field XORed with CRC to recover ICAO address; passes if ICAO is known |
| 4 | Surveillance Altitude Reply | 56 bits | AP field XORed with CRC to recover ICAO address; passes if ICAO is known |
| 5 | Surveillance Identity Reply | 56 bits | AP field XORed with CRC to recover ICAO address; passes if ICAO is known |
| 11 | All-Call Reply | 56 bits | PI field must equal computed CRC |
| 16 | Long Air-Air Surveillance (ACAS) | 112 bits | AP field XORed with CRC to recover ICAO address; passes if ICAO is known |
| 17 | Extended Squitter | 112 bits | PI field must equal computed CRC |
| 18 | Extended Squitter / Non-Transponder | 112 bits | PI field must equal computed CRC |
| 19 | Military Extended Squitter | 112 bits | PI field must equal computed CRC |
| 20 | Comm-B Altitude Reply | 112 bits | AP field XORed with CRC to recover ICAO address; passes if ICAO is known |
| 21 | Comm-B Identity Reply | 112 bits | AP field XORed with CRC to recover ICAO address; passes if ICAO is known |
For DF 0, 4, 5, 16, 20, and 21 the Address/Parity (AP) field does not carry the CRC directly. Instead, the decoded ICAO address is accepted only if it already exists in the decoder’s internal aircraft dictionary (plane_dict). This means those message types can only be decoded for aircraft that have previously been observed via a DF 11, 17, 18, or 19 message.
Comparison
| Capability | Extended Squitter Only | All Messages |
|---|
| DF 17 ADS-B Extended Squitter | Yes | Yes |
| DF 18 ADS-B from non-Mode-S transponder | Yes | Yes |
| DF 19 Military Extended Squitter | Yes | Yes |
| DF 11 All-Call Reply (ICAO address harvest) | No | Yes |
| DF 0 / 16 Short/Long Air-Air Surveillance | No | Yes |
| DF 4 / 20 Surveillance Altitude Reply | No | Yes |
| DF 5 / 21 Surveillance Identity Reply | No | Yes |
| CPU load | Lower | Higher |
| Use case | Aircraft tracking | Mode S surveillance |
When to use each mode
Use "Extended Squitter Only" when:
- You only need aircraft position, velocity, and identification data.
- You want to minimise CPU load.
- You are feeding data to a map or flight-tracking application.
Use "All Messages" when:
- You need Mode S secondary surveillance radar (SSR) data.
- You want to populate the aircraft dictionary as quickly as possible by harvesting ICAO addresses from All-Call Replies (DF 11).
- You are performing RF monitoring or research that requires visibility into all transponder traffic.