Why isn't radar used underwater instead of sonar if radio waves are faster than sound waves?

Asked by: Andrew Campbell


Radar is short for ra(dio) d(etecting a(nd) r(anging). Radar works by emitting pulses of electromagnetic waves toward a target and detecting a small portion of those waves that are reflected back to the receiving antenna. The transmission and reception is usually done by the same dish-shaped antenna. The time between transmission and reception is used to calculate the distance to the target by multiplying one-half the round-trip time by the speed of light to get an answer in terms of length.

Sonar or so(und) na(vigation) r(anging) works in a manner similar to radar, except sonar uses pulses of sound waves underwater to find the distance to a sound-reflecting target. Since the speed of sound is about 196,000 times slower than light(in sea water), the response time for sonar is proportionately longer.

Why not use radar underwater? The catch is that radar uses radio waves in the microwave frequency range, or approximately one centimeter in wavelength. This wavelength range is used because it is easier to direct the waves with small antennas in narrow beams. Unfortunately, Microwaves are strongly absorbed by sea water within feet of their transmission. This renders radar unusable underwater.
Answered by: Scott Wilber, None, President, ComScire - Quantum World Corporation

The reason is mainly because radar has a harder time penetrating large volumes of water. Contacts made by submarines are often dozens of miles away, and radar would have to be EXTREMELY powerful to reach that far in water, while sound (a mechanical wave) can make it that far.

Also, radar is only an active system allowing for your detection by passive sensors. Whereas sonar can be both passive and active. You can listen to sounds made by other subs' propulsion without giving away your position. That is very important in Anti-Submarine Warfare, and is why quieter submarines have an edge on survivability.

Then there's the prospects of electromagnetic jamming. Sonar jamming wouldn't be practical, because it would reveal your position and wouldn't mask your propulsion signature very well. The only way to mask your engine using audio countermeasures would be to generate a negative waveform of your signature and produce it in real time, which is very hard and also impractical.

So far, this has been because of military applications, but what about civilian use?

Sonar is much cheaper and works just as well for their purpose of mapping the ocean floor or whatever they do with it.

Also, there is not really much advantage to having a faster wave. Sound travels very fast in water compared to air, so you can hear things at distance in near real-time. It is also easier to measure the doppler effect with sound than radio.

Radar is just too impractical to use underwater.
Answered by: Justin Clifford, High School Student, Alpine, Utah