How do you apply Doppler processing to distinguish moving targets from stationary ones?

1 The Doppler effect

The Doppler effect is the phenomenon that occurs when a wave source and an observer are in relative motion. The frequency of the wave perceived by the observer changes depending on whether the source is approaching or receding. For example, the sound of a siren becomes higher as it approaches you and lower as it moves away. The same principle applies to electromagnetic waves, such as radar signals.

2 The Doppler frequency

The Doppler frequency is the difference between the transmitted frequency and the received frequency of a radar signal. It depends on the speed and direction of the target relative to the radar. If the target is moving toward the radar, the received frequency is higher than the transmitted frequency, and the Doppler frequency is positive. If the target is moving away from the radar, the received frequency is lower than the transmitted frequency, and the Doppler frequency is negative. If the target is stationary or moving perpendicular to the radar, the received frequency is equal to the transmitted frequency, and the Doppler frequency is zero.

3 The Doppler filter

The Doppler filter is a device that separates the radar signals based on their Doppler frequencies. It can be implemented as a bank of filters, each tuned to a specific Doppler frequency range. The output of each filter corresponds to a different velocity bin, or a group of targets with similar velocities. For example, a Doppler filter bank can have four filters: one for zero Doppler frequency, one for positive low Doppler frequency, one for positive high Doppler frequency, and one for negative Doppler frequency.

4 The Doppler spectrum

The Doppler spectrum is a graphical representation of the power distribution of the radar signals across the Doppler frequency range. It shows how many targets and how much clutter are present in each velocity bin. The shape and width of the Doppler spectrum depend on the characteristics of the targets, such as their size, shape, orientation, and number. The Doppler spectrum can be used to analyze the target motion and identify the dominant velocity components.

5 The Doppler processing

Doppler processing is the process of using a Doppler filter and Doppler spectrum to distinguish moving targets from stationary ones. It involves transmitting a pulse or continuous wave of a known frequency and recording the received signal. The Doppler frequency of the received signal is then calculated by comparing it to the transmitted frequency. This signal is then passed through a Doppler filter bank and its output is plotted as a function of the Doppler frequency to obtain the Doppler spectrum. Peaks and valleys in this spectrum are associated with target velocities and clutter levels, allowing for identification.

6 The benefits and challenges

Doppler processing offers several benefits, such as enhancing target detection and measurement, providing information about target motion and direction, and improving target resolution and discrimination. However, it is important to note that Doppler processing requires a high signal-to-noise ratio and a stable frequency source for accurate Doppler frequency estimation. Additionally, Doppler ambiguity and aliasing can occur if the Doppler frequency exceeds the Nyquist frequency or the pulse repetition frequency of the radar. Also, the target aspect angle and the radar beamwidth can cause variations in the Doppler frequency and spectrum.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?