Title of Paper: Development of a Hybrid RF/Laser Radar
Principal Author: Dr. Christopher Allen
Abstract: With funding from NASA's Instrument Incubator Program (1998), researchers at The University of Kansas Radar Systems and Remote Sensing Laboratory have developed a hybrid RF/laser radar. This sensor combines modern radar signal processing with fiber optic components. It achieves an improved receiver sensitivity (about -90 dBm) by applying heterodyne optical downconversion, RF pulse compression, and digital signal processing. Compared to lidars like GLAS and MOLA, this sensor has a lower peak transmit power while providing orders of magnitude more measurements per second.
Two detection schemes have been evaluated: envelope detection and direct downconversion. Envelope detection provides the benefit of discarding the effects of optical phase variations on the detected signal consequently avoiding many temporal correlation issues, however it is less efficient in terms of the resulting signal-to-noise ratio (SNR). Direct downconversion to baseband is more SNR efficient, however the baseband signal contains the effects optical phase variations, which include laser phase noise, effects of atmospheric turbulence, and frequency shifting due to Doppler effects.
Test results support the feasibility of a satellite-based altimeter (600 km altitude), capable of making more than 4000 range measurements per second with 10 cm accuracy using less than 10 W peak transmit power. The present breadboard operates at 1319 nm, however the overall concept is wavelength independent. With future funding we plan to demonstrate coverage over a swath by using independent transmit/receive beams from a shared optical telescope by placing an array of optical fibers in the telescope's focal plane.
Benefits of this development may include increased system reliability, reduced power requirements, smaller sensor mass and volume, improved eye-safety, and lower probability of detection. In addition, the system architecture allows the sensor to be reconfigured and offers flexibility. While the original design focused on altimetry, other operating modes are possible, including three-dimensional terrain mapping.