Optical calibration phase locked loop for the Shuttle RadarTopography Mission

The Shuttle Radar Topography Mission (SRTM) is an interferometric synthetic aperture radar system that flew on the space shuttle in February 2000, SRTM has an inboard antenna in the shuttle cargo bay and an outboard antenna at the end of a 60-m mast, extending from the cargo bay. In order to meet the elevation mapping accuracy requirement, the relative phase delay between the radar signals received via the outboard channel, compared with the inboard channel has to be known to within 80 at 5.3 GHz. This paper describes the design solutions and constraints, the devices, the analysis, and validation used to implement an optical calibration loop for SRTM. The calibration method involves injecting a tone into one panel of the inboard antenna, and sending an optical copy of the tone via a fiber-optic cable to be injected into the outboard antenna. A portion of the optical signal is reflected off an outboard partial mirror and travels back via the fiber to the inboard calibration system. There, it is converted back into a radio frequency tone and its phase is compared with the phase of the original tone. As the temperature of the mast fiber changes, a phase error is detected in the phase comparator. This error is used to control a custom designed optical phase shifter connected in series with the mast fiber. This phase-locked-loop guarantees that the phase of the calibration tone at the outboard stages within 1° relative to the phase of the calibration tone at the inboard antenna