On Monday, March 30, 2015 at 12:56:07 PM UTC-7, Martin Gregorie wrote:
On Mon, 30 Mar 2015 17:05:44 +0000, Benedict Smith wrote:

At 16:47 30 March 2015, Martin Gregorie wrote:
I've not seen anything to show this is anything but some reporter's
imagination.

Have a look at the flightradar 24 forum,
http://forum.flightradar24.com/threa...lysed-the-raw-
data-from-the-transponder-of-4U9525-and-found-some-more-data They detail
how this was found and have also released the raw data so anyone can
check it.
Ben.

Thanks for that.

I didn't know that Mode S would carry that sort of data: presumably its
there as a way to spot a fat-fingered an altitude change setting.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

/*acronym alert */

Mode-S in a sufficiently configured aircraft will reply to UF20 (uplink format 20) request for different BDS (Binary Data Store) register contents, like in this case a BDS register containing vertical intent (aka A/P altitude select) data. You will only see those replies from the aircraft when the ground based Mode-S interrogator asks for it, typically as part of a Mode-S EHS (Enhanced Surveillance) facility. EHS like this is is used in many locations in Europe, Unfortunately not used in the USA.

Mode S can potentially transmit a lot of detail about a suitably configured aircraft and it's operation. Another example is that Mode-S is used as part of TCAS-II to coordinate TCAS-II resolution advisories between two TCAS-II equipped aircraft (so for example both don't get told to climb into each other). Those Mode-S broadcasts can be monitored from the ground and controllers (at least in principle) alerted to an aircraft's TCAS-II system issuing a RAs.

Even Mode-S transponders used in gliders, like the Trig TT-21 and TT-22 are Level 2 transponders and so will can reply to UF20 requests, but the Vertical Intent register won't contain data...

The data that ADS-B actually transmits also comes from (different) BDS registers (containing just basic aircraft specs, pressure altitude and GPS data), but is relatively limited and certainly does not include the Vertical Intent data.

A good article is at http://defenseelectronicsmag.com/sit.../512RFDSF3.pdf