The Star Tracker (STR) on the Swarm satellites is comprised of three Camera Head Units (CHUs) mounted on the innermost end of the optical bench. It retrieves information on the precise satellite orientation and attitude in space. Nominally, the attitudes of all three heads are provided simultaneously at 2 Hz rate, however one head is regularly blinded by the Sun leaving the attitudes of just two heads. The attitudes of the 2-3 CHU are combined into one attitude, the attitude of the STR Common Reference Frame (CRF).