www.radartutorial.eu www.radartutorial.eu Radar Basics

The differences between radar, sonar and lidar

The similarities between radar, sonar and lidar should be clear: all three technologies use a run-time measurement of transmitted signals to their reflections and use this to calculate a range and often even an image of its environment.

Direction of oscillation
Propagation direction

Figure 1: Transverse waves (top) versus longitudinal waves (bottom)

Schwingungsrichtung
Ausbreitungsrichtung

Figure 1: Transverse waves (top) versus longitudinal waves (bottom)

Schwingungsrichtung
Ausbreitungsrichtung

Figure 1: Transverse waves (top) versus longitudinal waves (bottom)
(see Java applet on transverse and longitudinal waves)

Radar

In radar, electromagnetic waves are used for run-time measurement. Their propagation speed in the atmosphere is close to the speed of light. The frequency of these waves is chosen between about 30 kilohertz and (currently) 230 gigahertz. Within this frequency spectrum, there are different propagation modes and propagation conditions for the electromagnetic waves which influence the performance of the radar in a desired direction.

Electromagnetic waves are transversal waves, they oscillate on a plane transverse to the direction of propagation. In which direction exactly, that is determined by the polarization. Different polarization is used to exploit different reflection properties of the objects to be located for better detection.

Sonar

Sonar uses sound waves to measure the run time. Mostly it is ultrasound: the frequency is therefore not in an audible range. Sound waves propagate only as longitudinal waves: they oscillate only on the line of the propagation direction. Polarization similar to electromagnetic waves is not possible. Sound waves require a medium for propagation. Each medium has a specific propagation speed. An image calculation is only possible if an approximately homogeneous propagation medium exists. This is the case under water, for example, which is why sonar has mostly maritime applications.

Lidar

Lidar uses light pulses for travel time measurement and is, therefore, most comparable to radar. Lasers are used as transmitters mostly. Their very high frequency is in the petahertz range and is usually specified as their wavelength in a vacuum. It can also be in the infrared range. Different polarization is possible, but terms from radar technology can have very simplified meanings compared to equivalent terms in optics and therefore can only be compared with caution.

Figure 2: Coherence lengths in light: relatively large coherence length at the top, small coherence lengths at the bottom, the arrows refer to the chaotically arranged phase jumps, which can also be a change of the polarization direction at the same time.

Figure 2: Coherence lengths in light: relatively large coherence length at the top, small coherence lengths at the bottom, the arrows refer to the chaotically arranged phase jumps, which can also be a change of the polarization direction at the same time.

Figure 2: Coherence lengths in light: relatively large coherence length at the top, small coherence lengths at the bottom, the arrows refer to the chaotically arranged phase jumps, which can also be a change of the polarization direction at the same time.

Measurements of Doppler frequencies are subject to other technological conditions in lidar. With light, the term coherence is used only within a certain coherence length and is usually in the micro range. After that, indefinite phase jumps occur, which prevent further coherent signal processing. In radar with a fully coherent generation of the transmitted signal, however, this coherence length is infinite. Due to the limitation to one coherence length, CW and FMCW applications are practically not feasible with lidar.

For this reason, in radar, the depolarization is also measured only as a pure rotation of the polarization plane since the different partial components of a reflection are coherently superimposed to a wave with a new polarization direction. The different coherence length thus has direct effects on the reflections at volume targets. These reflections can coherently overlap cancel each other (see circular polarization). Therefore, the use of lidar is especially widespread in weather observation. As laser rangefinders, there are also military applications but they can only be used with good optical visibility (remember: the independence of visibility and light conditions with radar).