Locking system for detecting and tracking the center frequency of absorption lines in gases

A method to detect a gas absorption line that includes alternately transmitting and sweeping (402) two radio frequency (RF) signals through an absorption cell (102), wherein the two RF signals are transmitted at different frequencies separated by a range and are swept across a span of frequencies in a microwave and millimeter wave regions of a frequency spectrum. Receiving (404) the RF signals by a receiver and analyzing (406) the received signals by a closed loop system for relative absorption by a gas due to an absorption line of the gas in the span of the swept frequencies. Detecting (408) the absorption line of the gas when the two RF signals straddle the gas absorption line and the relative absorption by the two RF signals is equal.