Signal-to-Noise power ratio available from photomultipliers used as star detectors in star tracking systems. A method of assessment

In space navigation, or attitude control of satellites, where guidance is effected by reference to a set of axes defined by a stabilized platform, continuous monitoring of the reference axes will be required in order to compensate for the inevitablelong-term drift of the stabilizing gyros. Changes in the directions of distant¿fixed¿ stars provide a convenient monitoring reference; in order to usesuch a monitoring reference, two or more star tracking systems must be employed. Theconcept of a star tracking system as (a) a direction measuring device demanding, forsatisfactory operation, a minimum signal/noise ratio, and (b) a star detecting device providing a signal/noise ratio which is dependent upon certain system parameters, is briefly stated. A method of deriving an asymptotic approximation to the availablesignal/noise ratio as a function of stellar visual magnitude is described; in deriving this asymptotic function, the relationships between a number of system parametersare clarified and given graphical significance