Are radar and sonar equivalent technologies

Basics of radar technology

The differences between radar, sonar and lidar

The similarities between radar, sonar and lidar should be clear: all three technologies use a time-of-flight measurement of transmitted signals for their reflections and calculate a distance and often even a pictorial representation of an environment.

Fig. 1: Transverse waves (above) versus longitudinal waves

Figure 1: Transverse waves (above) versus longitudinal waves

radar

In radar, electromagnetic waves are used to measure the transit time. Their speed of propagation 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 types of propagation and propagation conditions for the electromagnetic waves, which influence the performance of the radar in a desired direction.

Electromagnetic waves are transverse waves, they oscillate on a plane perpendicular to the direction of propagation. Exactly in which direction is determined by the polarization. Different polarization is used in order to utilize different reflection properties of the objects to be located in a targeted manner for better detection.

sonar

With a sonar, sound waves are used to measure the transit time. Usually it is ultrasound: the frequency is therefore not in an audible range. Sound waves only propagate as longitudinal waves: they only vibrate on the line of the direction of propagation. A polarization similar to electromagnetic waves is not possible. Sound waves need a medium for propagation. Every medium has a specific speed of propagation. An image calculation is only possible if an approximately homogeneous medium of propagation exists. This is the case underwater, for example, which is why sonar is mostly used in maritime applications.

Lidar

Lidar uses light pulses to measure the transit time and can therefore best be compared with radar. Most lasers are used as transmitters. Their very high frequency is in the petahertz range and is usually given as its equivalent in a 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 identical terms in optics and can therefore only be compared with caution.

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

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

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

With lidar, measurements of Doppler frequencies are subject to different technological conditions. In the case of light, the term coherence is only used within a certain coherence length and is usually in the micro range. After that, there are indefinite phase jumps that prevent further coherent signal processing. In contrast, in the case of radar with fully coherent generation of the transmission signal, this coherence length is infinite. Due to the restriction to one coherence length, CW and FMCW applications with lidar are practically not feasible.

For this reason, with a radar the depolarization is only measured as a pure rotation of the polarization plane, since the different sub-components of a reflection coherently superimpose to form a wave with a new polarization direction. The different coherence length thus has a direct effect on the reflection at volume targets, where reflections from radar coherently overlap and can even be completely canceled (see circular polarization). That is why the use of lidar is particularly widespread in weather observation. There are also military applications as laser rangefinders, but these can only be used with good optical visibility (as a reminder: the independence of visibility and light conditions with radar).