For those that follow Squwak codes here is an interesting article on the use of Squwak code 1000.
http://www.airtrafficmanagement.net/2012/04/mode-s-surveillance-moves-up-a-gear/From 1700 today a number of airliners in central Europe will now be identified solely by ADS-B ELS. This means that they will no longer be allocated a discrete Squwak code by ATC, but will instead set their transponders to the common transponder code A1000
