Radio waves are not the only waves used for detecting things. By the start of the Second World War, SONAR (SOund Navigation and Ranging) systems had been around for some time - the British ASDIC system had been installed in ships since the early 1920s. In essence, sonar works in exactly the same way as radar: a stream of pulses are sent out and any returning echoes are analysed.
One important difference between sonar and radar is in determining bearing to the target. Chain Home used two receiving aerials for each station, set at ninety degrees to each other. By comparing the signal strengths, using a device called a goniometer, it was possible to tell the direction from which the echo was being returned. Very soon afterwards the familiar modern system of rotating antennae was introduced (at first, in Chain Home Low, the antennae were rotated by someone pedalling an adapted bicycle!). As with the Chain Home system this exploits the possibilities of highly directional antennae: the pulses are sent out in a narrow beam and so the position of the antenna determines the direction of the echo.
Using multiple hydrophones to increase directional sensitivity
It is not possible to build hydrophones (underwater microphones) with anything approaching that degree of directional sensitivity, so instead bearings to targets in sonar systems are generally found using multiple hydrophones. Sound travels at around 1550m/s (1 mile per second) in seawater, so the slight time difference between the arrival of an echo at different hydrophones can be used to get a bearing to the target. This is impractical for radar because light travels around 200,000 times faster and the time differences are therefore minute.
Post-war development of sonar
As with almost all other technologies useful for killing people or for avoiding being killed, sonar developed rapidly during the war, and afterwards, just like radar, found its way into civilian use. The first echo sounders (simple sonar sets measuring the depth of the sea and therefore unconcerned with direction) for yachts appeared in the 1950s, and almost every small craft now carries one. Amateur and commercial fishermen use fishfinding sonar, which can identify reflections from bodies of fish.
Although extremely useful, these simpler systems use a single transducer which acts as sound source and hydrophone, and therefore only measures in one direction - straight down. At a price it is now possible to buy side-scan sonar systems which show the seabed in 2D detail. They send out a fan-shaped pulse of sound, then use an array of hydrophones and a considerable amount of computing power to work out the shape of the bottom and anything on it.
Ultrasound medical imaging
Perhaps one of the most familiar uses of sonar scanning is ultrasound medical imaging. This works on exactly the same principle as side-scanning sonar: a beam of high frequency sound is sent out, reflects off an object within the body (bones, the boundaries between organs and so on) and is received by an array of microphones to be processed digitally and displayed on a screen. It's a non-invasive and extremely safe process and so is widely used to examine fetuses during pregnancy.
Echolocation is widely used in nature, most famously in bats. As they fly they emit a series of very high pitched chirps and by comparing the echoes they receive in their right and left ears can navigate with astonishing accuracy - and even locate insects in flight to eat. Humans can use the same technique. Some blind people, perhaps most notably Daniel Kish, have learned how to detect their surrounds in great detail by making a stream of palatal clicks with their tongues and listening to the echoes. Read more about Daniel and his technique.
This may seem like a long way from the team working desperately against time at Orfordness in the 1930s. However, the principles used are just the same and it's clear that we haven't yet got anywhere near fully exploiting the possibilities of echolocation.
This article is part of the Radar: What happened next? collection, which looks at different aspects of radar technology since the introduction of Chain Home, the first functioning radar defence system developed during the Second World War. This collection was inspired by the OU/BBC drama Castles in the Sky, featuring Eddie Izzard as Robert Watson-Watt, the father of radar.