Abnormal data from dump1090 Raw Output

FlightAware ADS-B Flight Tracking
1

Has anyone seen abnormal data from dump1090-fa Raw Output (port 30002) packets? Notice how the altitude data fluctuates within a couple minutes:

*8DA5AD7459 862 675A446EEA35BD0;
*8DA5AD7459 862 2D7059DB72018D8;
*8DA5AD7459 860 6757E46E1F5103D;
*8DA5AD7459 860 6753046C73C567C;
*8DA5AD7459 860 6752046C21EDE53;
*8DA5AD7459 260 2D5C99D4CE0395D;
*8DA5AD7459 2E0 6736846239B6AE8;
*8DA5AD7459 2E0 67352461A88FD56;
*8DA5AD7459 0E0 2D4AB9CDD1B610B;
*8DA5AD7459 0E0 2D4899CCEBABD6A;
*8DA5AD7459 0E0 2D4659CBEC46CB8;

Are the raw packets error checked before being sent out?

Appreciate any inputs.

Herman

2

Yes, dump1090-fa only forwards messages with correct CRC (where there is proper CRC coverage).

Those messages have correct CRCs and the altitudes look fine; what is the abnormal data you’re referring to?

*8da5ad7459862675a446eea35bd0;
CRC: 000000
DF:17 AA:A5AD74 CA:5 ME:59862675A446EE
  Baro altitude: 3300 ft

*8da5ad74598622d7059db72018d8;
CRC: 000000
DF:17 AA:A5AD74 CA:5 ME:598622D7059DB7
  Baro altitude: 3300 ft

*8da5ad74598606757e46e1f5103d;
CRC: 000000
DF:17 AA:A5AD74 CA:5 ME:598606757E46E1
  Baro altitude: 3200 ft

*8da5ad74598606753046c73c567c;
CRC: 000000
DF:17 AA:A5AD74 CA:5 ME:598606753046C7
  Baro altitude: 3200 ft

*8da5ad74598606752046c21ede53;
CRC: 000000
DF:17 AA:A5AD74 CA:5 ME:598606752046C2
  Baro altitude: 3200 ft

*8da5ad74592602d5c99d4ce0395d;
CRC: 000000
DF:17 AA:A5AD74 CA:5 ME:592602D5C99D4C
  Baro altitude: 3000 ft

*8da5ad74592e06736846239b6ae8;
CRC: 000000
DF:17 AA:A5AD74 CA:5 ME:592E0673684623
  Baro altitude: 2900 ft

*8da5ad74592e067352461a88fd56;
CRC: 000000
DF:17 AA:A5AD74 CA:5 ME:592E067352461A
  Baro altitude: 2900 ft

*8da5ad74590e02d4ab9cdd1b610b;
CRC: 000000
DF:17 AA:A5AD74 CA:5 ME:590E02D4AB9CDD
  Baro altitude: 2800 ft

*8da5ad74590e02d4899ccebabd6a;
CRC: 000000
DF:17 AA:A5AD74 CA:5 ME:590E02D4899CCE
  Baro altitude: 2800 ft

*8da5ad74590e02d4659cbec46cb8;
CRC: 000000
DF:17 AA:A5AD74 CA:5 ME:590E02D4659CBE
  Baro altitude: 2800 ft
3

I thought the altitude data are the 3 hex characters behind 0x59. In these packets, the Q-bit is 0, which means x 100. I only found the formula to decode x25 but don’t have the formula to decode x100. How should I decode in this case?

Thanks,
Herman

4

See DO-260B / ED-102A §2.2.3.2.3.4.3 “Altitude Encoding” in ADS-B Airborne Position Messages
which in turn refers to the Mode C encoding specified in ICAO Annex 10 Vol IV chapter 3. Or see the dump1090 implementation.

(If you are building your own decoder, you really need a copy of the specifications in some form)

5

This is amazing. Decoding the altitude, now I am buying 3 huge documents, DO-260B, DO-181E and ICAO Annex 10 (close to 3000 pages total). But thank you, obj, very much for pointing me to the right direction.

Herman

6

Yeah, getting the specifications is a hassle. There are draft versions floating around online if you don’t need the 100% official version.

7

I have been looking extensively online and there’s where I found some partial spec with x 25 feet formula. We have RTCA membership so I downloaded the DO-260B and DO-181E today. Just have to put in an order for the ICAO ANNEX 10. Your reply saved the day. Now at least I know there’s nothing wrong with the AVR packets.