2. EHS (Mode-S Enhanced Surveillance)

Let’s hack into the EHS messaged too! More information on aircraft air speeds.

The Mode-S Enhanced Surveillance (EHS) provides to the air traffic controller more information than what is included in the ADS-B (aka Mode-S Elementary Surveillance). It responds to the ATC Secondary Surveillance Radar, and broadcasts specific parameters non-independently. Hence it is only available in the area where ATC presents.

There are quite a few very interesting data contained within various types of the EHS messages. Such as: airspeeds (IAS, TAS, Mach), roll angles, track angles, track angle rates, selected altitude, magnetic heading, vertical rate, etc.

There are a few challenges to decode this information:
  • Which aircraft does one message came from?
  • What is the type of one message (aka which BDS code) most likely to be?
  • How reliable is the information that has been decoded?

This part of the book covers only a selected common types of EHS messages. For a complete Python implementation: https://github.com/junzis/pyModeS/blob/master/pyModeS/ehs.py

Chapter structure: