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

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

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

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

Cross-Correlator Implementations Enabling Aperture Synthesis for Geostationary-Based Remote Sensing

An ever-increasing demand for weather prediction and high climate modelling accuracy drives the need for better atmospheric data collection. These demands include better spatial and temporal coverage of mainly humidity and temperature distributions in the atmosphere. A new type of remote sensing satellite technology is emerging, originating in the field of radio astronomy where telescope aperture upscaling could not keep up with the increasing demand for higher resolution. Aperture synthesis imaging takes an array of receivers and emulates apertures extending way beyond what is possible with any single antenna. In the field of Earth remote sensing, the same idea could be used to construct satellites observing in the microwave region at a high resolution with foldable antenna arrays. If placed in a geostationary orbit, these could produce images with high temporal resolution, however, such altitudes make the resolution requirement and, hence, signal processing very demanding. The relentless development in miniaturization of integrated circuits has in recent years made the concept of high resolution aperture synthesis imaging aboard a satellite platform viable.The work presented in this thesis addresses the challenge of performing the vital signal processing required aboard an aperture synthesis imager; namely the cross-correlation. A number of factors make the application challenging; the very restrictive power budgets of satellites, the immense amount of signal processing required for larger arrays, and the environmental aspects of in-space operation. The design, fabrication and evaluation of two cross-correlator application-specific integrated circuits (ASICs), one analog-to-digital converter (ADC) ASIC and one complete cross-correlator back-end is presented. Design concepts such as clocking schemes, data routing and reconfigurable accuracy for the cross-correlators and offset compensation and interfacing of the ADCs are explained. The underlying reasons for design choices as well as ASIC design and testing methodologies are described. The ASICs are put into their proper context as part of an interferometer system, and some different cross-correlator back-end architectures are explored.The result from this work is a very power-efficient, high-performance way of constructing cross-correlators which clearly demonstrates the viability of space-borne microwave imaging interferometer back-ends

1.6 GHz Low-Power Cross-Correlator System Enabling Geostationary Earth Orbit Aperture Synthesis

We present a 64-channel cross-correlator system for space-borne synthetic aperture imaging. Two different types of ASICs were developed to fit into this system: An 8-channel comparator ASIC implemented in a 130 nm SiGe BiCMOS process technology performs A/D conversion, while a single 64-channel digital cross-correlator ASIC implemented in a 65 nm CMOS process performs the signal processing. The digital ASIC handles 2016 cross-correlations at up to 3.6 GS/s and has a power dissipation of only 0.13 mW/correlation/GHz at a supply voltage of 1 V. The comparator ASIC can handle sample rates of at least 4.5 GS/s with a power dissipation of 47 mW/channel or 1 GS/s with a power dissipation of 17 mW/channel. The assembled system consists of a single board measuring a mere 136 x 136 mm(2) and weighing only 135 g. The assembled system demonstrates crosstalk of 0.04% between neighboring channels and stability of 800 s. We provide ASIC and system-board measurement results that demonstrate that aperture synthesis can be a viable approach for Earth observation from a geostationary Earth orbit

A SiGe 8-Channel Comparator for Application in a Synthetic Aperture Radiometer

We present a high-speed low-power 8-channel comparator tailored for the application of sampling antenna signals in a cross-correlator system for space-borne synthetic aperture radiometer instruments. Features like clock return path, per-channel offset calibration and bias current tuning make the comparator adaptable and gives the possibility to adjust the comparator for low power consumption, while keeping performance within the requirements of the cross-correlator system. The comparator has been implemented and fabricated in a 130-nm SiGe BiCMOS process. Measurements show that the comparator can perform sampling at a rate of 4.5 GS/s with a power consumption of 48 mW/channel or 1 GS/s with a power consumption of 17 mW/channel

Correlators for Interferometric Radiometry in Remote Sensing Applications, A Scaling Perspective

Correlators are extensively used in the field of radio interferometry. Two different types are considered for two applications; autocorrelators for spectrometry and cross-correlators for aperture synthesis. We concentrate on satellite-based applications where power budgets are very restrictive. Several satellites are already employing correlators for interferometric measurements, and future projects are targeting even larger systems in terms of spectral channels in the case of spectrometry and baseline counts in the case of aperture synthesis. Thus, it is important to develop correlators with increasing channel count, either using ASIC technology scaling or by constructing larger systems from several ASICs. Building on earlier ASIC designs, we examine how larger correlator systems can be constructed and the implications this has, in terms of power dissipation, system complexity, and ASIC count. Our findings indicate that, for large systems, having a very high channel count per ASIC is indeed of interest for keeping system complexity and power dissipation down by reducing both ASIC and I/O count, especially for cross-correlators

Custom versus Cell-Based ASIC Design for Many-Channel Correlators

While ASICs are efficient in terms of area utilization, performance, and power dissipation, ASIC design requires significant development resources. We compare two approaches to implementing ASIC correlators for interferometric imagers and spectrometers: The first approach, custom design, gives very high performance and area utilization, but is complex and time consuming. The second approach, cell-based design, reduces design time, but leads to lower performance and area utilization. In our evaluation, we consider two different correlator architectures: Autocorrelators for spectrometry, and cross-correlators for synthetic aperture imaging. Based on both 65-nm CMOS and 28-nm FD-SOI process technologies, our results show that for implementations for a limited number of channels, the cell-based approach may prove useful since it offers relatively short development time while still providing acceptable area utilization and performance. For larger designs, however, the area overhead of cell-based design becomes a major concern, especially for autocorrelator architectures