Chapter 2: Header Keyword Definitions . ... gyro (roll), . ... This keyword can be blank for the following reasons: (1) For short observations, GSSEPRMS is not meaningful in this context and the keyword is blank, (2) If the commanded guiding is set to a mode with a single guide star only (FINE LOCK/GYRO, COARSE TRACK/GYRO, GYRO), this field will be blank, (3) If a two-FGS guide star acquisition failed, this field will also be blank and an anomaly keyword will appear at the end of the header record. ...