Optical calibration phase locked loop for the Shuttle Radar Topography Mission

Abstract

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 8 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 stays within 1 relative to the phase of the calibration tone at the inboard antenna.

The Shuttle Radar Topography Mission

“…The receivers were not identical mechanically or thermally, and the signal path length from receiving antenna to electronics was vastly different because of the 60‐m baseline. Rather than attempt to force the receiver phases to be identical, a calibration tone signal with common reference was distributed to the antennas over optical fiber cable to the deployed antenna [ McWatters et al , 2001]. The signals were injected into the receive paths at the antennas and detected in the data processing.…”

A comparison of internal calibration schemes for spaceborne single-pass InSAR applications

“…The block diagram of the stabilized microwave fiber optic link can be showed in [2], and it is the first single mode optical fiber technology used in spaceborne application. More details can be found in [1] [2] [3]. The reference signal is transferred via the 60m long mast, and then serves as an internal calibration signal for the outboard receiving channel.…”

Phase Difference Measurement of Under-Sampled Sinusoidal Signals for InSAR System Phase Error Calibration

“…Phase difference measurement of sinusoidal signals [1,2,3,4,5,6,7,8,9] is one of the most important research topics in applications such as phase error calibration of the spaceborne single-pass interferometric synthetic aperture radar (InSAR) system [10,11,12,13], power system monitoring [14], radio frequency communication [15], and laser ranging [16]. For the spaceborne single-pass InSAR system, a possible interferometric phase error can arise from relative phase differences between the two receiver channels, because the two signal receivers are not identical mechanically or thermally, and the signal path length from receiving antenna to electronics is vastly different because of the 60 m baseline [12].…”

“…For the spaceborne single-pass InSAR system, a possible interferometric phase error can arise from relative phase differences between the two receiver channels, because the two signal receivers are not identical mechanically or thermally, and the signal path length from receiving antenna to electronics is vastly different because of the 60 m baseline [12]. Therefore, an internal calibration signal with common reference is distributed to the antennas over an optical fiber cable to the deployed antenna [10,11,12,13], and the phase difference of the internal calibration signals (usually sinusoidal signals) received separately from the primary and secondary antennas needs to be measured. More than that, the frequency of the calibration signal is generally high.…”