the present invention relates to a system and a method for detecting and monitoring the distance between a patient and a reference point on an inflatable air mattress and for controlling the air supply to that mattress in relation to such distance . the patient distance sensing system includes a rigid support frame such as a bed frame , an inflatable air mattress positioned atop the bed frame with the upper surface of the inflatable air mattress forming a patient support surface , an air supply operable to provide controlled variations in air supply , and a series of patient distance sensing devices . such devices including a heterodyning proximation detector , a force responsive distance sensing device , and a light responsive sensing device . each device is operable to act separately or cooperatively in maintaining a therapeutically beneficial patient support surface through variable control of the delivery of inflation pressure to the air mattress . the heterodyning proximation detector is further operable to initiate inflation by way of a through space sensing of an approaching patient . the present invention further provides a preferred method for regulating the inflation of a therapeutic air mattress through operation of the described patient distance sensing system .