A Cross-Correlator for the Remote Sensing of Earth by Synthetic Aperture

Licentiate thesis, 2011

In light of our changing climate and the unpredictability of severe weather; a better understanding of our climate and increased weather forecast accuracy are in high demand. Humidity and temperature distribution profiles with high temporal resolution can significantly increase our knowledge of highly dynamic weather phenomena and improve weather forecasts. Microwave sounding from low earth orbit is extensively used for humidity and temperature measurements in the atmosphere because of its much better cloud penetrating properties compared to visible and infrared light. Performing these observations from geostationary earth orbit (GEO) would give the additional advantage of large coverage and no revisiting times. Microwave sounding from GEO is however demanding, this because of the large aperture required to reach acceptable spatial resolution. Synthetic aperture interferometry, widely used in ground based radio astronomy, has been proposed as a solution to overcome this obstacle. Cross-correlation is a signal processing algorithm that is a central and highly calculation-intensive part of aperture synthesis. CMOS process technology scaling, and the decreasing power per performance figures that have followed, has finally reached a point where these kinds of instruments are viable for space deployment. This thesis presents a cross-correlator chip that has been designed, fabricated and extensively evaluated, paving the way for larger correlator systems based on similar design concepts. Routing and synchronization schemes were developed for the purpose of handling the massively parallel calculations and the signal distribution and timing issues specific to synthetic aperture cross-correlators. The chip presented shows significant improvements over previous correlators in power per performance evaluations.