Fiber optic line phase noise and drift effects on bistatic imaging radar performance

Instrumentation grade, coherent, bistatic radar cross section (RCS) measurement systems require a reliable low-noise method to link the reference, local oscillator (LO) and intermediate frequency (IF) coherent signals between the transmit and receive subsystems. One approach to this is the use of a fiber optic link (FOL). Phase noise tests were performed on a distributed feedback (DFB) type laser transmitter-photodiode receiver link with a delay distance of 2.26 kilometers. The measurements were conducted using a standard Hewlett Packard HP 3048 A phase noise test set, and a variety of link configurations at a variety of frequencies from 100 MHz to 5 GHz. Further, system tests have been performed to incorporate a FOL into a coherent bistatic instrumentation radar system local oscillator path, and perform image processing on an emulated target. A Lintek elan system was configured to emulate bistatic operation by separating the transmit and receive local oscillator lines, allowing one of them to be excited through a fiber optic link. A target with one or two scatterers was emulated using a delay line between a sample of the transmit signal and a receiver injection port. This paper describes the theoretical effects of phase noise (close-in and higher frequency) on target scatterer imaging performance, describes the FOL tests and the architecture of the bistatic radar tests performed, and presents the results of the tests, including baseline system performance and performance using the FOL. Images representing closely spaced scatterers are presented, and the point spread function of a long dwell (40 seconds) data file, representing an ISAR process is presented. One of the limiting factors in the use of a FOL for long dwell times is that of the variable delay, caused by variation of the propagation velocity with laser frequency drift (dispersion), and the change in index of refraction with temperature. A method for mitigating the effects of this drift is presented.