SAW wireless, passive sensor spread spectrum platforms

SAW technology has been identified as a possible solution for NASA´s long term needs for ground, space-flight, and space-exploration sensor requirements. SAW has many unique advantages over possible competing technologies, which include the following properties: passive, radiation hard, operable over wide temperature ranges, small, rugged, inexpensive, and identifiable. The purpose of this paper is a focus on the platform and system constraints; not on any particular sensor. For remote sensing, the device and system platform is the essential element which enables the sensor information to be obtained. This paper´s emphasis will be on orthogonal frequency coded (OFC), and single frequency CDMA sensor tags. The parameters of interest are device insertion loss, system range, code diversity, center frequency and bandwidth. It will be shown that multi-frequency and OFC tags offer the possibility of much greater ranges and better code collision properties than the more conventional commercial SAW CDMA RFID tags. Reflectors can have less than 1 dB insertion loss, and when using unidirectional transducers, overall SAW device loss can be less than a few dB. Antenna size and gain are key parameters for small foot print and achievable range. By using spread spectrum techniques, processing gain can be encoded in the reflectors, the transducer, or both. The paper will present the fundamental engineering equations which define the system range, receiver dynamic range, minimum detectable signal and processing gain. Theoretical system performance of both the multi-frequency and single frequency SAW RFID sensor will be discussed.