It would be possible to set the Wifi SSID of the PDA to a usefull value and enable 1/2/5 (call that X) second updates.

This would mean anyone in the area will receive updates on the location of the PDA. For Example a Wifi Scan will show

   G-BMAA LAT LON Speed Heading ALT 

In some form of packed HEX for example (to save space) as there is 32bytes for use.   On the Ground or other PDA flight stations, it can listen for Wifi SSID's and plot their locations on the map.

Richard P