The signal emitted by a radar system is a microwave signal, which can be described using a complex wave representation. This implies that the signal can be entirely represented by a complex number, which characterizes both its magnitude and its phase at a certain moment of time. In the SAR context, the complex number is usually represented by a real part, the in-phase component (I), and an imaginary part, the quadrature component (Q), from which the corresponding magnitude and phase can be retrieved. In single look complex SAR data, each of these components is pictured in a single image channel. The terminology comes from electrical engineering, whereby the quadrature component is 90° out of phase with respect to the reference frequency and the in-phase component. This is necessary in order to retrieve the phase information during A/D conversion. The I component can be expressed as the signal amplitude multiplied by the cosine of the phase. The Q component corresponds to the amplitude of the signal multiplied by the sine of its phase. Using both components as input, the magnitude and phase for each signal echoes and location can be retrieved.
The relationship between I/Q terms and the magnitude and phase of the signal can be best represented using a phasor. A phasor represents a complex number and its phase and amplitude equivalent. It can be best illustrated by a 2D vector in a Cartesian coordinate system, which projections on the horizontal and vertical axes represents the real and imaginary part, respectively. The length of the vector correspond to the signal’s amplitude and its direction (angle between the horizontal axis and the vector) characterizes the phase of the signal. Using simple mathematical considerations, the relationship between I/Q and amplitude and phase can be established.
Each signal echo and pixel of a complex SAR image can be represented with such a phasor and the necessary amplitude and phase information can be accordingly retrieved.