Calibrating the amplitude and phase imbalances in AgileDARN HF radar

The Agile Dual Auroral Radar Network (AgileDARN) radar has been developed as an all-digital high-frequency (HF) coherent scatter radar. Based on phased array antenna and digital signal processing techniques, digital beam forming (DBF) is employed for both transmission and reception, which steers toward the desired direction based on the relative phases within the array elements. However, the amplitude and phase imbalances among channels will lead to substantial degradation of DBF performance, including the beam direction deviation, broadening of the main lobe, deteriorated side lobes, antenna gain degradation, and ultimately affects the radar detection accuracy. Here, we present a complete method for calibrating these imbalances in transceivers and linear array elements in AgileDARN radar: internal calibration and external calibration. During the internal calibration, the calibration signals with the same amplitude and phase as the multi-pulse sequence are sent to all transceivers internally. The variance of the outputs illustrates the transceivers' imbalances and can be processed to implement calibration. Further, the meteor echoes and linear least squares fitting method are employed for external calibration, which calibrates the amplitude and phase imbalances among the cables and phased array elements. The technique has been successfully implemented, and we present the results of measured data observed by the AgileDARN HF radar before and after calibration.