47 CFR § 78.113 - Frequency monitors and measurements.
(a) The licensee of each CARS station shall employ a suitable procedure to determine that the carrier frequency of each transmitter is maintained within the tolerance prescribed in § 78.111 at all times. The determination shall be made, and the results thereof entered in the station records: when a transmitter is initially installed; when any change is made in a transmitter which may affect the carrier frequency or the stability thereof; or in any case at intervals not exceeding one year.
(b) The choice of apparatus to measure the operating frequency is left to the discretion of the licensee. However, failure of the apparatus to detect departures of the operating frequency in excess of the prescribed tolerance will not be deemed an acceptable excuse for the violation.