SA - Selective Availability, from when GPS time and ephemerides data was dithered to degrade the navigation message to nonqualified receivers.
SBIR - Small Business Innovative Research which yielded developments like the composite ballistic screen
Scramjet - Supersonic-combustion ramjet
SEPIC - Single Ended Primary Inductance Converter
SDC - Service Delivery Card
SDK - software development kit (On2 DirectShow VP8 SDK)
SISTM - Sport Integration System (Sports music system)
SOC - system-on-a-chip - as for a digital media processor
SSR - Super Self Refresh memory technology that reduces self refresh current in Elpida's DDR SDRAM
STTR - Small Business Technology Transfer (Ferrium C61 C64 alloys)
SVN - software virtual network (SVN software virtual network battlefield communication simulation)