Phase calibration of an aperture synthesis imaging array by means of incoherent scattering

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

University of Tromsų, Tromsų, Norway

For a radar interferometer, phase calibration is an important, and often challenging technical step to be completed before further progress, e.g. aperture synthesis imaging, can be achieved. We have found a way to achieve phase calibration by means of regular incoherent scattering from the ionosphere.

As part of the EASI project (ESR aperture synthesis imaging), we have found that regular incoherent scattering over moderate integration periods (a few to a few tens of seconds) result in quite stable phase differences between the signals seen in the different antennas, even when the coherence (normalized cross-spectrum) is below detectability.

Since the transmit beam is the only plausible source of backscattering inhomogeneity which might produce such a cross-correlation, this offers a very simple phase calibration procedure for the interferometer -- the phases of the received signals are adjusted so that the phase differences due to incoherent scattering is zero in all baselines. In the EISCAT_3D project, interferometry is going to be an integral part of the system, and this technique should be of interest.

We present the cross-correlation and phase observations which underlie this calibration procedure, and the resulting enhancement of detected coherence for localized scatterers, including satellites and naturally enhanced ion-acoustic echoes. We also present simulations from synthetic incoherent scatter signals which demonstrate how such stable phase differences can arise, even for very low coherence levels.