Strange ADS-B transmission, always same signal strength probably ground station

FlightAware ADS-B Flight Tracking
1

Does anyone know what kind of data is contained in these messages ( I receive about 3 per second when there aren’t too many planes around)

*903c1fb7c284000045a33256efb9;
CRC: 000000
RSSI: -24.6 dBFS
Time: 188255603511.00us
DF:18 AA:3C1FB7 CF:0 ME:C284000045A332
 Extended Squitter (Non-Transponder) Reserved for surface system status (24/0)
  ICAO Address:  3C1FB7 (ADS-B, non-transponder)

*903c1fb7c194d9800a5f89dd9605;
CRC: 000000
RSSI: -24.6 dBFS
Time: 188256352593.58us
DF:18 AA:3C1FB7 CF:0 ME:C194D9800A5F89
 Extended Squitter (Non-Transponder) Reserved for surface system status (24/0)
  ICAO Address:  3C1FB7 (ADS-B, non-transponder)

*903c1fb7c29400004a93cd903997;
CRC: 000000
RSSI: -24.6 dBFS
Time: 188256354593.58us
DF:18 AA:3C1FB7 CF:0 ME:C29400004A93CD
 Extended Squitter (Non-Transponder) Reserved for surface system status (24/0)
  ICAO Address:  3C1FB7 (ADS-B, non-transponder)

*903c1fb7c1a4d9800a6278b6bafc;
CRC: 000000
RSSI: -24.6 dBFS
Time: 188257103380.17us
DF:18 AA:3C1FB7 CF:0 ME:C1A4D9800A6278
 Extended Squitter (Non-Transponder) Reserved for surface system status (24/0)
  ICAO Address:  3C1FB7 (ADS-B, non-transponder)

*903c1fb7c2a4000049a9f0c384d7;
CRC: 000000
RSSI: -24.6 dBFS
Time: 188257105380.17us
DF:18 AA:3C1FB7 CF:0 ME:C2A4000049A9F0
 Extended Squitter (Non-Transponder) Reserved for surface system status (24/0)
  ICAO Address:  3C1FB7 (ADS-B, non-transponder)

*903c1fb7c1b4d9800a655dcf569b;
CRC: 000000
RSSI: -24.6 dBFS
Time: 188257844086.67us
DF:18 AA:3C1FB7 CF:0 ME:C1B4D9800A655D
 Extended Squitter (Non-Transponder) Reserved for surface system status (24/0)
  ICAO Address:  3C1FB7 (ADS-B, non-transponder)

*903c1fb7c2b40000816a08f2415a;
CRC: 000000
RSSI: -24.6 dBFS
Time: 188257846086.75us
DF:18 AA:3C1FB7 CF:0 ME:C2B40000816A08
 Extended Squitter (Non-Transponder) Reserved for surface system status (24/0)
  ICAO Address:  3C1FB7 (ADS-B, non-transponder)

*903c1fb7c1c4d9800a6856c8376f;
CRC: 000000
RSSI: -25.2 dBFS
Time: 188258605529.25us
DF:18 AA:3C1FB7 CF:0 ME:C1C4D9800A6856
 Extended Squitter (Non-Transponder) Reserved for surface system status (24/0)
  ICAO Address:  3C1FB7 (ADS-B, non-transponder)

*903c1fb7c2c40000533a3277e230;
CRC: 000000
RSSI: -25.2 dBFS
Time: 188258607529.17us
DF:18 AA:3C1FB7 CF:0 ME:C2C40000533A32
 Extended Squitter (Non-Transponder) Reserved for surface system status (24/0)
  ICAO Address:  3C1FB7 (ADS-B, non-transponder)

*903c1fb7c1d4d9800a6b4588e84d;
CRC: 000000
RSSI: -24.6 dBFS
Time: 188259356554.75us
DF:18 AA:3C1FB7 CF:0 ME:C1D4D9800A6B45
 Extended Squitter (Non-Transponder) Reserved for surface system status (24/0)
  ICAO Address:  3C1FB7 (ADS-B, non-transponder)

*903c1fb7c2d40000570a3dcc4e1d;
CRC: 000000
RSSI: -24.6 dBFS
Time: 188259358554.75us
DF:18 AA:3C1FB7 CF:0 ME:C2D40000570A3D
 Extended Squitter (Non-Transponder) Reserved for surface system status (24/0)
  ICAO Address:  3C1FB7 (ADS-B, non-transponder)
2

Those look like ground vehicles.

Have you entered your station lat and long in the command line options for whatever version of dump1090 you are using?

For the generic dump1090 the options look like this:

–lat Reference/receiver latitude for surface posn (opt)
–lon Reference/receiver longitude for surface posn (opt)

I believe that ground vehicles transmit an abbreviated version of their position (since they are always near the airfield) so dump1090 needs to know your approximate position to decode the ground vehicles.

Good luck,

-Dan

3

Thank you for the tip. dump1090 knows my location and i have even reduced the default range to 230 nm as i’m limited by terrain. Which should make it even easier to decode partial position reports.

Quite sure that the signal is stationary as a car moving around would change signal strength i’m quite sure. It is literally a constant signal.

4

Yes. It’s definitely stationary.

PlanePlotter is my primary way of viewing my local data. I checked with a PlanePlotter user approx 290 degrees and 80 nautical miles from your location. That user is receiving a constant signal like yours with a hex=3C1FB5 (very similar to your 3C1FB7). The 3C1FB5 appears to be a ground station and is not broadcasting position or altitude.

Here in the United States DF18 messages are TIS-B which is traffic being rebroadcast by FAA ground stations. I have an FAA tower (data communication, not airport tower) approx 3 miles away and frequently receive DF18 data with aircraft traffic, but the tower itself does not constantly ping like you are seeing.

This page has some decoding info on DF18 messages:
https://mode-s.org/decode/adsb/introduction.html

I tinkered with the page above and a hex/binary/decimal calculator and came up with this.

Original hex
903c1fb7c1c4d9800a6856c8376f
type hex data parity
903 c1fb7c 1c4d9800a6856 c8376f

903= Type DF18 Capability=0
c1fb7c = icao hex

The first part of the data for each decodes as Type Code=24 which is listed as “Reserved” in the online doc. That’s probably the “Reserved for surface system status” you see above in your decodes.

Bottom line. It’s probably part of an ADS-B ground station or possibly some type of ADS-B “beacon” mounted on a tower or obstruction used to confirm their systems are operating correctly.

Either way it’s interesting. I always enjoy an excuse to learn something new…!

-Dan

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