as a chopper. The chopper causes the image of the star to repeatedly appear and disappear on the photocell, producing a signal that was then smoothed to produce an alternating current output. The phase of that signal was compared to the one on the tape to produce a guidance signal. Star trackers were often combined with an INS. INS systems measure accelerations and integrate those over time to determine a velocity and, optionally, double-integrate to produce a location relative to its launch location. Even tiny measurement errors, when integrated, adds up to an appreciable error known as "drift". For instance, the N-1 navigation system developed for the SM-64 Navaho cruise missile drifted at a rate of 1 nautical mile per hour, meaning that after a two-hour flight the INS would be indicating a position away from its actual location. This was outside the desired accuracy of about half a mile. In the case of an INS, the magnetic tape can be removed and those signals instead provided by the INS. The rest of the system works as before; the signal from the INS roughly positions the star tracker, which then measures the actual location of the star and produces an error signal. This signal is then used to correct the position being generated from the INS, reducing the accumulated drift back to the limit of the accuracy of the tracker. These "stellar inertial" systems were especially common from the 1950s through the 1980s, although some systems use it to this day. Current technology Many models are currently available. There also exist open projects designed to be used for the global CubeSat researchers and developers community. Star trackers, which require high sensitivity, may become confused by sunlight reflected from the spacecraft, or by exhaust gas plumes from the spacecraft thrusters (either sunlight reflection or contamination of the star tracker window). Star trackers are also susceptible to a variety of errors (low spatial frequency, high spatial frequency, temporal, ...) in addition to a variety of optical sources of error (spherical aberration, chromatic aberration, etc.). There are also many potential sources of confusion for the star identification algorithm (planets, comets, supernovae, the bimodal character of the point spread function for adjacent stars, other nearby satellites, point-source light pollution from large cities on Earth, ...). There are roughly 57 bright navigational stars in common use. However, for more complex missions, entire star field databases