Title of Presentation: Self-Calibrating High-Efficiency L-Band T/R Modules for Phase Stable Array Antennas
Primary (Corresponding) Author: Wendy N. Edelstein,
Organization of Primary Author: NASA Jet Propulsion Laboratory
Co-Authors: Constantine Andricos, Gregory Sadowy
Abstract: L-band repeat-pass Interferometric Synthetic Aperture Radar (InSAR) techniques have been shown to provide very accurate and systematic measurements of surface deformation and surface strain accumulation due to seismic and volcanic activity as well as assessing natural or man-made deformation caused from subsidence, flooding or landslides. Numerous studies and reports have concluded that an L-band phased-array InSAR instrument would be best suited to meeting the Solid Earth science needs. While existing repeat-pass InSAR instruments (e.g., SIR-C, ERS-1/2, Radarsat-1) have been able to provide sufficient accuracy over small scales or in select areas, future systematic measurements will be required over very large areas (300 km) for a variety of surface conditions and over long time intervals. Given that many areas around the globe will exhibit low temporal correlation over time due to vegetation, it is important that future InSAR systems employ temporally stable systems (ie., phase stability over time) operating at longer wavelengths (ie., L-band).
For repeat-pass interferometric SAR systems, variations in the overall insertion phase of the antenna over a data acquisition interval will manifest themselves as along-track undulations in the interferometric phase data. These undulations, which can not be removed through spatial averaging, would be indistinguishable from large-scale surface displacements and would therefore corrupt the InSAR observations. To achieve the required temporal stability using a phased-array radar system, highly stable and efficient transmit/receive (T/R) modules are required. In order to achieve 1 cm displacement accuracy, artifacts from variations in T/R-module insertion phase should be kept well below this level. If variability of the T/R-module insertion phase is allocated 1 mm in the surface-displacement error budget, an absolute phase stability of 1-deg is required. To reduce the frequency of on-orbit calibration, it is desirable that this stability requirement be maintained over long intervals (days to weeks to months).
While a self-calibrating T/R module capability has been sought for years, a practical and cost effective solution has never been demonstrated. An Advanced Component Technology (ACT) project is addressing this requirement for amplitude and phase stable T/R modules. The work described in this paper adds new capability to a previously developed 60% efficient L-band T/R module by adding an internal closed-loop self-calibration circuit. The integral adaptive calibrator will result in only a modest increase in module size and power, while providing a new capability not currently available in existing T/R modules. The novelty of the T/R module described herein is the integrated adaptive calibrator which compensates for all environmental and time related changes and provides very stable phase and amplitude characteristics. This is achieved using a gain and phase detector, recently available as a low cost single-chip Bi-CMOS integrated circuit. The self-correcting feature eliminates costly T/R module calibration and allows for increased tolerance of T/R module components. This approach provides an inexpensive way to implement improved phase stability where no extensive individual phase matching is required. Therefore the T/R module and phased-array antenna manufacturing and test costs will be reduced.