Microwave interferometry provides a means of synthesizing large scanning antennas that are not otherwise physically practical for spaceborne Earth observational systems. By cross-correlating multiple receivers of an array, high-resolution images are synthesized from a sparse — or thinned — array of small antennas rather than relying on extremely large mechanically scanned antennas. For Earth observations from space, high-datarate cross correlators are required that operate with low power and have low mass and complexity. ASIC (application-specific integrated circuit) cross-correlators are an enabling technology for space-based interferometry. An ASIC CMOS (complementary metal-oxide semiconductor) cross-correlator was developed to correlate outputs from a large number of receivers.
The ASIC can digitize up to 128 analog signals at 1 Gsample/s with 2-bit accuracy, then cross-correlate 64 of these signals with the remaining 64 signals synchronously, and then serialize the resulting data through an 8-bit bus. This ASIC is targeted for application in NASA’s GeoSTAR system for high-spatial-resolution sounding of atmospheric temperature and water vapor at 50 to 183 GHz from geostationary orbit.
Highlights of this mixed-signal ASIC include the integration of 128 high-speed analog to digital converters (ADCs) on the same chip with the digital totalizer and cross-correlator matrix. Each of the 128 analog inputs to the ASIC contains independent blocks with a 3-stage VGA (variable gain amplifier), ADC, and automatic gain correction loops. Each digitized signal is accumulated in a totalizer circuit that is later used to measure signal amplitude. In parallel, each of the digitized signals is paired with 64 signals from the opposing inputs for cross-correlation. Each of these 4,096 cross-correlators consists of a look-up table to generate the 3-bit product, followed by an accumulator that records a weighted sum of the signal cross-products. The accumulation period, or integration time, is typically 9 ms. This corresponds to 9 million multiplyaccumulate cycles at the 1-GHz sample rate. The totalized and cross-correlated data is used to generate high-resolution radiometric images using a Fourier-transform. As viewed from a geostationary platform, these radiometric images of the Earth are used to retrieve contiguous water vapor and temperature profiles with very high temporal and spatial resolution.
The power consumption of the ASIC’s digital part is approximately 2 W. Due to a 4-bit full-adder and 22-bit ripple counter, the ASIC accumulates 26 bits of integration data over the integration period.