Radar (Radio Detection and Ranging); Sonar (Sound Navigation and Ranging)

Two similar but different technological advances that figured prominently in the Allied victory in World War II were radar and sonar, both techniques for detecting the location and speed of enemy aircraft or submarines.

During World War II, radar played a critical role in the British victory in the Battle of Britain, an aerial battle fought largely between August 1940 and the end of that year.  In August of 1940 Britain stood alone in the war in Europe, the German army having defeated Britain’s Polish allies in September of 1939, and the French in June of 1940.  By August, Hitler had given up attempts to persuade the British to sign a peace treaty favorable to Germany.  He started planning an invasion of Great Britain, but first he had to deal with the British air forces.  Hitler had reason to be optimistic.  The British had only 800 aircraft to try to hold back the onslaught of over 3,000 German planes.  The British victory in this battle was largely due to a series of radar stations that had been built along the southern and eastern coasts of Britain in 1939.  These radar stations enabled the British to determine the direction, altitude, and speed on oncoming German aircraft while they were still 50 to 60 miles away, and thus concentrate their limited fighter forces against them.

The use of sonar was largely directed against German submarines.  Following their failure in the Battle of Britain, the Germans began to use submarine warfare in an attempt to cut off British shipping, upon which Britain depended for vital war materials and food.  The use of sonar allowed the powerful British surface fleet to detect the direction and depth of these submarines and destroy many of them.  

Radar and Sonar Compared

What makes these two technologies similar is that they both locate enemy ships or planes at a distance by sending out invisible waves that strike the ship or plane, and bounce off and return to a wave detector.  The returning waves provide information as to the direction of the enemy craft, as well as their velocity. How are these waves similar?  Radar waves and sonar waves exhibit the properties of all waves, including wavelength, frequency, velocity, reflection, and changes in wavelength resulting from the motion of the wave source relative to a stationary detector (the Doppler Effect).

Sonar makes use of sound waves to detect distant objects.  (Radar uses radio waves which are more like light than sound.)  The use of sonar was “invented” by bats and dolphins long before it was employed by humans. Bats locate flying insects at night by producing sound waves with such a high frequency that they cannot be heard by the human ear.  These waves strike the insect and are reflected to the bat’s ear, allowing the bat to detect the location of the insect.  Biologists call this echolocation.  Sound waves, which are called compression waves, are produced by vibrating objects.  When a vibrating object moves toward us, the air molecules are pushed closer together.  When the object vibrates away from us, the reverse happens.  You can simulate this in one dimension with a long spring, like a slinky toy.  Stretch the spring, and push a few coils of the spring closer together.  When the compressed coils are released, a wave of compression will move away from its original location.  When this compression wave strikes the fixed other end of the spring, a reflected wave can be seen.  Sound waves travel longer and faster in water that they do in air.  The velocity of sound in water is about 1.6 kilometers/second. As early as 1822 Daniel Colloden used an underwater bell to calculate the speed of sound underwater in Lake Geneva, Switzerland this early research led to the invention of dedicated sonar devices by other inventors. In 1906, American naval architect Lewis Nixon invented the first sonar-like listening device to detect icebergs. During World War I (1914-18), a need to detect submarines increased interest in sonar. French physicist Paul Langévin constructed the first sonar set to detect submarines in 1915.