From Ainonline.com:
Safety Alert for Operators 15006 was published by the FAA to ensure that pilots realize the need to keep their aircraft transponders turned on to the altitude-reporting mode even when operational on the ground in airport movement areas. The FAA uses runway safety systems, such as airport surface detection equipment model X (ASDE-X) and advanced surface movement guidance and control system (A-SMGCS), at many airports in the U.S. to determine aircraft and vehicle locations when operating on an airport surface.
Both of these systems use data from transponders to obtain accurate aircraft and vehicle locations to increase airport surface safety and efficiency. Nationwide, the agency said that airports with ASDE-X report an average of 20 non-compliance transponder events per day, even with explicit airport diagrams or ATIS notification, or both, directing pilots to operate with transponders on. To address these problems, aircraft operating on all airport movement areas at all airports—not just those that are ASDE-X equipped—must taxi with their transponders on in the altitude-reporting mode.
