FMCW—Operating in the frequency modulated continuous wave (FMCW) mode, the radar measures the distance to stationary targets. By modulating the frequency, also referred to as an FMCW ramp or chirp, the radar measures the response of the reflected wave to derive range, velocity and the angle of the target. Figure 1 shows how the target’s range, velocity and angle are derived in FMCW mode.
The range resolution depends on the transmitter’s carrier sweep bandwidth; the higher the bandwidth, the higher the resolution of the radar sensor. The velocity resolution depends on dwell time and carrier frequency; the higher the carrier frequency or dwell time, the higher the resolution. Angular resolution depends on the carrier frequency; the higher the carrier frequency, the better the resolution.
Compared to laser detection, which measures a single spot, or camera detection, which only captures a 2D image within the camera’s field of view, FMCW radars provide a continuous, inherent average of the information from the target’s reflection. This provides a wide, 3D field of view by measuring the distance, speed and angle, from as close as a few centimeters to several hundred meters between the sensor and the targets.
Range-Doppler—In range-Doppler mode, the range and speed of the target are analyzed. Range-Doppler is one of the most powerful modes because it processes multiple transmit ramps or chirps simultaneously using a 2D Fourier transform. The processed range-Doppler data is displayed in a 2D map that enables targets with different velocities to be separated, even if they are the same distance from the sensor. This is important to distinguish multiple targets moving at high speed in different directions, e.g., resolving complicated air traffic scenarios with targets traveling in opposite directions or during overtaking maneuvers.
DBF—With digital beamforming (DBF), the distance and the angle to the target are displayed. The receive signals from the four receive channels are used to estimate the angle of the target, and the data contains the spatial distribution of the targets in the xy-plane. In the DBF mode, the system is configured the same as FMCW; however, the down-converted IF signals are processed differently. After calculating range, the angle information of the target is calculated by evaluating the phase differences between the four receive channels. The DBF mode requires a calibration of the radar front-end to eliminate deterministic phase variations between the receive channels.