As of 31 March 2008 mode S transponders are mandatory for VFR flights (IFR earlier: as of 31 March 2007) in European airspace. It pays to know how they work. This text describes the interrogation methods, 24 bit addresses and more technical stuff.
In addition to the downlinking of Aircraft Identification, which is a prerequisite for Mode S Elementary Surveillance (ELS), other specified downlink parameters (DAP) may be acquired by the ground system to meet the requirements.
Document below describes EHS parameters and downlink parameters.
Excerpt from the document:
The first step in the introduction of SSR Mode S in Europe is known as Mode S Elementary Surveillance (ELS). Mode S EHS, which builds upon the concept of ELS and consists of the extraction of Downlink Aircraft Parameters (DAP), facilitates an increase in the safety and efficiency of the ATM operations and will enable the controllers to increase their efficiency in tactically separating aircraft.
The controller’s information is improved by providing actual aircraft derived data such as magnetic heading, air speed, selected altitude and vertical rate enabling to reduce the radio telephony (RT) workload and better assess the separation situations. Mode S EHS enables monitoring tools and safety nets which work on actual data, to be implemented or improved (MSAW, STCA) which in turn will allow safety levels to be maintained or improved despite the increase in traffic levels.
To continue reading see the pdf below: