Any special equipment needed for the altitude component for badges today? Like an altitude encoding transponder? Or does a simple GPS data logger suffice?

From the current Sporting Code https://www.fai.org/igc-documents :
"FLIGHT RECORDER 1.1.5 An IGC-approved device to record pressure altitude and GPS position and altitude. A given FLIGHT RECORDER may be approved for all flights, all badges, or Silver through Diamond badge claims only.
POSITION RECORDER 1.1.6 A NAC-approved device to record GPS data for Silver or Gold badge claims only."

FR's have special barometric sensors. PR's can too, but most use GPS altitude, which has a error band of 100m applied.

A list of PR's approved by NACs is at http://www.ukiws.demon.co.uk/GFAC/po..._recorders.htm .