Abstract: In a microwave landing system the individual words of a train of command words generated by the antenna beam steering unit are applied to a polynomial generator where each is, seriatim, summed with a polynomial check word generated in a previous operation. The result is shifted once toward the most significant bit to increase the degree of the resultant polynomial. Division by a predetermined generator polynomial is accomplished and the remainder is stored as the check word to be used in the next to follow operation. The check word polynomials are compared at preset intervals with stored check words to monitor the train of command words.

Abstract: An optical position code detector which senses information that is recorded on a record carrier. The record carrier preferably has scan marks for synchronizing operation of the detector and at least one track with positionally encoded data marks which include one or more imprinted alphanumeric characters. The detector includes photodetectors which sense light that is reflected from the record carrier. The intensity of the reflected light varies in dependence on whether or not data marks are present and also varies due to the presence of spurious marks on the record carrier. When a scan mark appears, a comparison circuit is activated which compares the voltage from each photodetector with a reference voltage which is cycled from a low magnitude to a relatively high magnitude. The comparison circuit distinguishes data marks from spurious marks with a high resolution. A logic circuit is connected to the comparison circuit to indicate whether or not a position code is present on the record carrier.