Student Information System 
Safety Instrumented Systems 
Single Instance Store 
Signal Intelligence Service 
Specialized Information Services 
Statutory Instruments 
Single Instance Storage 
Software Interface Specification 
Side Impact Sensor 
superconductor-insulator-superconducto… 
Secret Intelligence Service 
Simms 
safety injection system 
SAIL Interface System 
Scanning Imaging Spectrometer 
Schengen Information System 
Science Instrument System 
Science Integration Staff 
Secretarial Information System 
Shuttle Information System 
Shuttle Interface Simulator 
Signals Intelligence Service 
Simulator Interface System 
Single Integrated Simulation 
Software Implementation Specification 
Software Integrated Schedule 
Solar Imaging Suite 
Solar Isotope Spectrometer 
Sound-in-Sync 
Special Information Systems