Figure 1. Hatchet Fish

ABSTRACT

This article examines radar and electronic warfare in nature. It briefly defines electronic warfare and investigates how some animals use electronic combat techniques for hunting and evasion. In particular, it delves into the electronic weapons of bats and moths. It describes how the bat uses different rader and electronic warfare techniques to locate its prey and for manoeuvring - pulse frequency repetition agility, frequency modulation, Doppler, multimode and monopulse operation, tracking and even pulse integration and processing. It also explains how the moth combats this through its radar-warning receiver or through hiding in clutter to deceive or evade the bat. To counteract this, the bat further develops its electronic contermeasures. Finally it puts forward case on how such knowledge can help to address some of the military problems in radar and electronic warfare.

Dr Aaron Chia Eng Seng


INTRODUCTION

"A flying object homes in on its target. Flying in darkness, it finds its target by sending out streams of short pulses. But the target does not give in so easily. It detects the homing pulses and dodges evasively to avoid attack. Sometimes, it counter attacks with blasts of pulses of its own to confuse and disorientate the attacker."

Is this a picture of modern aerial and electronic warfare? Yes, but it is warfare in nature between bats and their prey, the moths. Bats use a form of sonar known as echolocation to locate their prey. To accomplish this, bats emit very high frequency sounds that bounce off moths and other insects, giving them an estimate of the prey's relative location. Bats have developed a technique to filter out the most powerful sounds so that they can concentrate on the faint return signals. Moths have in turn evolved a defence in the form of a soft covering on their bodies and wings, which absorbs the bat chirps. In response, bats have evolved new chirp frequencies that can be used to identify the moths' fuzzy coating. To counter this, the moths have enhanced their stealth technology with a jamming technique that involves emitting their own sounds to jam the bats' return signals. This is often coordinated with elaborate evasive manoeuvres. Bats respond by adopting an elaborate flight pattern that can overwhelm a moth's senses, and also by periodically turning off their echolocation, making the moth's jamming technique less effective. Nature's electronic warfare arms race continues...

In this article, the author examines radar and electronic warfare in nature. The first part defines radar and electronic warfare and looks at how some animals use electronic combat techniques for hunting and evasion. The second part delves into the electronic weapons of the bat and moth, and shows how they can help to address some of the military issues in radar and electronic warfare. A glossary of biological, radar and electronic warfare terms can be found at the end of the article.

WHAT IS RADAR AND ELECTRONIC WARFARE?

Radar is an electromagnetic system for the detection and location of objects. It operates by transmitting a particular type of waveform and detecting the nature of the echo signal. It is designed to `see' in conditions unfriendly to normal human vision. These include darkness, haze, fog, rain or snow. Moreover, it can measure the distance or range to the object. Echolocation is a system of orientation or radar involving the use of echoes produced by the animal for gathering information about the environment.

Electronic warfare is a military action where the objective is to control the electromagnetic spectrum (Schleher, 1999). To accomplish this, both offensive electronic attack (electronic countermeasure) and defensive electronic protection (electronic counter-countermeasure) are required. In addition, electronic warfare support (electronic support measure) actions are necessary to supply the intelligence and threat recognition that allow implementation of both electronic attack and protection.

NATURE'S ELECTRONIC WEAPONS

Although there is no known animal species that uses electromagnetic waves at radio frequencies or microwaves, examples of animals using sound, light and electric field for electronic combat abound. The ultrasonic sensor of a bat is the "mother of all radars" used for navigation, surveillance and hunting food. Apart from bats, there are other creatures that use acoustic echolocation. Dolphins have highly sophisticated sonars that are not yet fully understood by humans. There are echolocating birds too: the oilbird and the White-rumped Swiftlet or the Blacknest Swiftlet (Bossel, 2001a). To combat the bat's echolocation, moths have developed ears that can detect bats' signals and trigger a series of clicking noises that jam the bat's detector (self-protection jammers). Some electric fishes create an electric field around themselves, which is used to detect and communicate with other fish. They have even adopted frequency hopping where they hop from one frequency to another to avoid jamming.

In the depths of the ocean are the habitats of some real electronic warfare experts (Bossel, 2001b). The Hatchet Fish takes advantage of the low illumination by looking upwards and waiting for the shadow of some other animal swimming by. Through evolution, prey animals have found stealth to be a countermeasure. Some fish and cephalopods exhibit bioluminescence where their skin is able to emit just about the right amount of light to compensate for their own shadow and thus render themselves undetectable. One notable example is the squid. Other animals have developed chromatophores. They can adapt the colours and texture of their skin such that it matches the structure of their hiding place. An example of such animals is the octopus. Some species, like the cuttlefish, produce sticky and luminescent mucus when put under stress by an attacker. This slime adheres to the predator and makes it a distinguishable target for predators further up the food chain (target illumination). The anglerfish features a luminescent appendix to its lower chin. This appendix is used as bait (decoy) for other predators while the anglerfish is lying in ambush. Glass worms produce a sort of liquid that contains hundreds and thousands of luminescent particles when pursued by a predator. In similar fashion, the jellyfish is able to jettison some of its twinkling tentacles and escape, mostly unharmed. These particles serve to confuse the predator's senses, just like chaff that is jettisoned from modern aircraft to confuse a hostile radar or missile seeker head.

BATS' ELECTRONIC WEAPONS

Contrary to popular belief, bats can see fairly well. However, their eyes are useful sensors only under daylight conditions and they only hunt in the dark. To support their night activities, bats have developed an active acoustic sensor in the ultrasonic frequency range. Apart from differences in frequencies and the wave medium between acoustical waves and radio waves, a bat's sensor is very similar to radar: chirped signal, target tracking by Doppler estimation, terrain avoidance function, and fine angle measurement based on the monopulse principle.

Bats' echolocation sensors are used for navigation, obstacle avoidance and hunting. Occasionally, they emit a short cry that serves to 'illuminate' the scene ahead. Echoes are picked up by their ears and analysed in the brain. The time between sending out a cry and receiving a response enables the bat to determine the distance between the bat and whatever objects that happen to be around. Stationary objects yield an echo that is a replica of the pulse sent. Moving objects are revealed by an echo at a slightly lower or higher frequency because of Doppler estimation (Moving Target Indicator (MTI). Furthermore, echoes from mosquitoes, moths and butterflies exhibit fluctuations that are caused by the flutter of their wings, a concept similar to unintentional modulation on pulse. This enables bats to tell the difference between a moth and a leaf swaying in the wind. Some bats feed on fish rather than on insects. These bats can detect the perturbations on the water surface when a fish is close to it. This enables them to find their submerged prey.

Bats begin their pursuit of prey while in the search mode, emitting pulses with long duration and inter-pulse intervals. Once they have detected a potential prey, they decrease their pulse duration, shorten the time interval between pulses and change the intensities of the pulses as they proceed from the search phase to the capture of the target through the approach, track and terminal phases of the echolocation attack sequence.