IBSS (Infrared Background Signature Survey) was a test of infrared sensor technology for SDIO (Strategic Defense Initiative Organization). The IBSS sensors were mounted on a SPAS-II pallet and used to observe the Space Shuttle's engines firing from a distance away. They also observed a variety of common rocket propellants, including nitrogen tetroxide, MMH, and UDMH, released by three Chemical Release Observation subsatellites launched from Get-Away Special canisters on the Shuttle.