Atmospheric measurements have, in recent years, become very important for increasing our knowledge of weather and climate phenomena. With this increasing interest comes a demand for better Earth coverage and shorter imaging intervals. Performing observations from satellites in geostationary orbit, where image update time becomes independent on orbit time, means that continuous observation of the visible hemisphere can be achieved. Observations in the radio spectrum (frequencies lower than infra-red and visible light) has the potential to measure atmospheric temperature and humidity. It also makes it possible to see through clouds, not possible with infrared or visible light observations.
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.