Cross-Correlator Implementations Enabling Aperture Synthesis for Geostationary-Based Remote Sensing
Doctoral thesis, 2018
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.
Erik J Ryman
Chalmers, Computer Science and Engineering (Chalmers)
A 3-GHz Reconfigurable 2/3-Level 96/48-Channel Cross-Correlator for Synthetic Aperture Radiometry
Proceedings of European Solid-State Circuits Conference (ESSCIRC),; (2017)
Paper in proceedings
1.6 GHz Low-Power Cross-Correlator System Enabling Geostationary Earth Orbit Aperture Synthesis
IEEE Journal of Solid-State Circuits,; Vol. 49(2014)p. 2720-2729
A SiGe 8-Channel Comparator for Application in a Synthetic Aperture Radiometer
Proceedings - IEEE International Symposium on Circuits and Systems,; (2013)p. 845-848
Paper in proceedings
Radio observations are traditionally being performed using a parabolic dish antenna, however, both the dish diameter (the aperture), and observed frequency is proportional to the achievable resolution. A larger antenna or a higher frequency means better resolution. Achieving a high resolution in the radio spectrum will, thus, require a very large dish. This problem is increased by the fact that the very high orbit for geostationary operation means the earth is significantly further away than in lower orbits.
Aperture synthesis is a method, originally developed for radio astronomy, where an array of antennas is used for emulating a single large antenna. In radio astronomy, aperture synthesis has made imaging with a resolution, that would otherwise require antennas with diameters of several kilometers or even as large as the Earth, possible. The emulated antenna size is equivalent to the longest distance between any two receivers of the array.
In the field of remote sensing of our atmosphere from orbit, aperture synthesis is a relatively new idea and has this far only been implemented by one satellite in low earth orbit. The advantage of this method compared to using a single parabolic dish is that an array of small receivers can more easily be folded during launch, fitting within the relatively restricted payload volume of current launchers. The array could then be extended to several meters. Another advantage is that while a parabolic dish can only sample one pixel at a time, requiring a scanning technique, the array can sample an entire image simultaneously.
Aperture synthesis require large amounts of signal processing. Performing this processing could be very costly in terms of power dissipation. For satellites, the available power and the ability to get rid of the generated heat, is very restricted. This thesis presents the work of creating electronics that handle the most critical part of this signal processing, known as cross-correlation. Cross-correlation is a mathematical operation performed, pair-wise, on all possible combinations of signals received by the array. The cross-correlation has to be performed on the satellite before sending the data to earth for further processing, this means that the electronic circuits performing these calculations have to be very power-efficient while handling the large amounts of calculations required. It also has to be able to operate in space where radiation levels are much higher than at ground-level, increasing the risk of calculation errors. The thesis presents circuit designs, the ideas behind these, and testing of the circuits. It also discusses larger cross-correlator systems for future satellites.
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4432
Chalmers University of Technology
Room EA, EDIT building, Rännvägen 6B
Opponent: Roland Weigand, European Space Agency (ESA), The Netherlands