The BSTS was intended to provide early, accurate, and reliable tactical warning and attack assessment (TW/AA) of ballistic missile launches, while also providing monitoring of peacetime launches. The BSTS concept used a wide field-of-view (WFOV) infrared mosaic sensor to cover the entire earth, below and above the horizon, while the large number of detectors would have provided high sensitivity and temporal resolution.
The mosaic sensor would have used Z-architecture focal plane technology, allowing a low sample rate with analog processing on the focal plane. The Mercury Cadmium Telluride (MCT) detectors were to provide high sensitivity and multicolor operation at a relatively high temperature. Communications systems would have used extrahigh-frequency (EHF) RF crosslinks and downlinks.
BSTS, however, proved too large and costly to field. Therefore, the Air Force selected the SBIRS-High/SBIRS-Low systems because their smaller size and lower cost meant that they could be deployed in larger numbers, creating a constellation of orbital platforms what would provide near constant real-time coverage of the entire surface of the earth.