Friday, 11 September 2009
Hunting in the dark
These two hunters use echo location to find their quarry. But the bat is more sophisticated in extracting information from the returned echoes than the equipment in the aeroplane. This implies no disrespect to the engineers who designed the radar in the Junkers. In an intense, darwinian struggle of barely 5 years, through move and countermove, German, British and American engineers devised most of the techniques of what is now called Electronic Warfare, EW.
The equipment in the aircraft could determine the distance to a target with some accuracy, but the direction could only be discriminated with an accuracy of about 5 degrees. Bats, on the other hand, (and here I’ll speak of the characteristics of various species, not just the Horseshoe bat in the picture) can also extract Doppler information. Doppler is used to determine the speed of a target, by measuring how much its motion has changed the frequency of the returned echo. In 1940 the Doppler effect had been known of for a 100 years but it took the Hamburg campaign, Operation Gomorrah, before radar engineers exploited it. The raids, in July 1943, a week of intense air attacks, night and day, was the most effective strategic bombing effort that the RAF were ever to carry out.
The effectiveness of the attacks was largely due to the deployment of a new EW measure, chaff. Both Germany and Britain had known that by dropping bundles of metal foil, of the correct size, they’d be able to create havoc on radar. Both sides resisted using it, in case their opponents hadn’t already thought of it. But in 1943, Harris, commander of the RAF bomber force, was given permission to use chaff in the Hamburg attacks. Huge amounts were dropped making the actual aircraft indistinguishable to the radars of the day.
For once the bombers were able to deliver a crushing blow. Largely unhindered by radar controlled flak guns and night fighters the bomber force was able to bomb with uncommon accuracy. Very large fires were started and losses on the ground were huge. Hamburg was considered by both the allies and Germany to be a major event. But the bombers were never able to be as effective again, just 4 months after Hamburg Germany had come up with a countermeasure.
The countermeasure was doppler discrimination. Doppler is used by bats to distinguish between moving prey and stationary trees. They detect the small changes in frequency of the received echoes. The bigger the frequency change, the greater the speed of the target. The same phenomena effects radio frequency echoes. Dropped chaff falls straight down, unlike the bomber stream which had a cruising speed of around 200 mph. In response to Hamburg, German radars were modified to distinguish moving targets by detecting the doppler shift. They were able to sort out the targets, from the chaff.
But bats can do even better, the Doppler effect of the particular motion of the wings of a prey can characterise the returned echo. Some bats use this to distinguish between different species of insects. By the end of the war electronics had caught up with bats and was able to do this too, and use the modulation effect of spinning propellers to help identify types of potential target.
Biologists talk of co-evolution, where the development of a new trait in one species influences the evolution of another. Some species of moths have evolved a sensitivity to the calls of a particular bat’s echolocation. They use it to take evasive action. This too has it’s airborne counterpart with aircraft using directional receivers that ‘listen out’ on the known frequencies of enemy radars. Physics restricts what frequencies can be used and the trick is for the hunter to keep changing frequencies. It turns out that bats too, in their need to keep one step ahead of the moths have learned to frequency hop, in order to pre-empt the moths sensitivity to the usual sound.
Bats exploit frequency sweeping, changing the note of the burst of sound they emit in order to more accurately determine the range to the prey, accuracies of under a millimetre are possible. Electronic implementations of these techniques, using modern systems are very software intensive. They require a wealth of intensive, maths heavy, code. Yet the bat has evolved a nervous system that is quite capable of cracking the same problem.
Is there anything humans have come up with that hasn’t already been achieved by bats? Bats seem to be lone hunters while radar equipped nightfighters receive guidance from the ground. Nowadays we have airborne networks with fighters, dedicated radar warning aircraft and ground stations all exchanging data continuously and automatically. I’d be surprised if it turns out that bats ever figured out how to hunt in cooperation, analysing echoes when the position of the transmitter is unknown could be just too difficult for nature.
But evolution has come up with some pretty amazing things. Watch out, if it turns out that a study on bats, or any of the other creatures that use echolocation suddenly disappears from view. Maybe the men in black have confiscated all the copies! It could just be that nature came up with something that man as only just go around to understanding, and now engineers somewhere are keen to copy it and work it into a new weapon system, before the other guys do.
WW2 German airborne radar
Dyson, Disturbing the Universe