Interferometric imaging (WP5) for EISCAT_3D

Belyey, V., Grydeland, T. and La Hoz, C.

University of Tromsø, Tromsø, Norway

The magnetic field geometry at high latitudes means that there are no inherent symmetrical directions in the brightness distribution of potential ionospheric targets for radar imaging. Thus, it is recommended that full 2-dimensional imaging be incorporated into the EISCAT _3D design form the start. Measurements in range provides the 3rd dimension, ergo 3D. The adopted two-level architecture for antenna beam forming is an optimum global solution that also satisfies the requirements of interferometric imaging, as the module antennas of the second level constitute the antenna units upon which antenna baselines can be constructed with great flexibility. Thus it is not necessary to adopt a fixed module configuration since it can be synthesized in software on demand. The visibility function (the observable) and the brightness distribution (the image) are Fourier transforms of each other. The former is defined in baseline space and the latter in angle space subtended by the image. They are reciprocal of each other. Therefore, the size of the image is determined by the shortest baseline and the image resolution by the longest baseline. Thus, the shortest spacing (or length for dense packing) of the modules will determine the size of the image. In order to obtain a resolution of 10-20 m it will be necessary to have baselines of the order of a kilometer. Thus outlying receive-only modules separate from the core transmitting array will be necessary to achieve this resolution. Simulations of module antenna configurations with advantageous geometric patterns consistent with these conditions have been carried out. Real time monitoring of a selected subset of baselines to identify events worth of imaging is recommended. The detection of a pre-programed threshold will trigger the saving of the full set of baselines for on- or off-line imaging.

A useful condition on the accuracy of the visibility function phases— which determine the quality of the measured image — is that the timing system random variations (time jitter) should be a small fraction of the period of the radar wave. The choice of 1/40th of the period implies that the timing system has to be accurate to within 100 ps for a 250 Mhz radar. Since the module beam-forming algorithms include add/multiply operations on the signals over the tens (possibly more) of antenna elements of a module, the timing accuracy can be relaxed accordingly, since the signal's timing fluctuations are random and independent. An important and useful (global) calibration of the baseline phases of radar imaging has been discovered. The nature of incoherent scattering implies that the phases of the visibility function averages to zero over the image, a consequence of incoherent scattering signals having (locally) uniform brightness distribution and being spatially uncorrelated. The measured non-zero phases of the visibility function of pure incoherent scatter are the calibration values for each baseline.