"Multiple-input multiple-output (MIMO) radar techniques make use of multiple transmitters and multiple receivers to improve the spatial characterization of targets. In the case where the Bragg scattering k → -vector can be assumed to be the same for all transmit-receive paths, MIMO methods can be seen as a way of increasing the number of effective receivers. In the last decades, there has been scientific interest in determining the spatial characteristics of ionospheric and atmospheric irregularities on the subtransmit beam scale, allowing the study of processes in their intrinsic scales, otherwise inaccessible using simple beamforming techniques. Interferometric methods, including aperture synthesis imaging, were used in the past with a single transmitter and multiple receivers [single-input multiple-output (SIMO)]. In this paper, we present the first implementation of MIMO techniques to improve the spatial resolution of aperture synthesis radar imaging of daytime equatorial electrojet irregularities observed using the Jicamarca Radio Observatory (JRO). Our implementation uses two spatially separated transmitters and four spatially separated receivers. In order to separate the contributions of each transmitter, time, code, and polarization diversity experiments have been tested. We find that all three diversity approaches can be used for ionospheric irregularities, but time and polarization diversity are not applicable in all situations due to the range and Doppler width of the echoes, and due to magnetoionic radio propagation effects. The results are evaluated by comparing new MIMO imaging results against the currently used SIMO imaging technique. We present and discuss the theoretical and practical aspects of the MIMO approach, so they can be applied to study other targets not only at JRO but also at other modular coherent scatter radars."
