ESM IN ELECTRO-OPTICS

ESM IN ELECTRO-OPTICS

INTRODUCTION

1. Laser warning Receivers have been developed to provide warning against the illumination of a platform by a potentially hostile Laser Designator, Illuminator or Dazzle weapon. Presently LWR’s are predominantly found on Tanks and other Armoured Fighting Vehicles. In future years this balance will change when in response to the increasing threat from Lasers weapons, Helicopters and fixed Wing aircraft will demand their inclusion in integrated Defensive Aids Suites (DAS).
OBJECTIVES
2. The objective for this section is to explain the design, operation and limitation of LWR’s. The following areas will be covered:
a. The types of Laser to be detected.
b. LWR’s.
c. Laser propagation.
d. System design.

TYPES OF LASER TO BE DETECTED

1. LWR’s are designed to detect pulsed Lasers. CW lasers are not considered as worth detecting, as they are usually associated with very short range weapon fuses where there is insufficient time to make any difference. CW Lasers are difficult to manufacture for long range use. Therefore LWR’s look to detect coherent, pulsed, fast rise-time radiation. Rather like a RWR the following parameters can be determined:

a. Wavelength.
b. Pulse width.
c. Pulse repetition Frequency.
d. Bearing.
e. Location. (If possible)

LASER PROPAGATION

2. Lasers produce very narrow beam widths. Although there are some sidelobes, these are generally much smaller than for conventional radar beams and are thus very rarely detected. However, the Laser beam is spread out to varying degrees by atmospheric scattering and turbulence. The resulting beam is therefore no longer a narrow and can be detected over a much wider area.


3. As well as being able to detect Lasers that are directly illuminating the target the LWR has to be able to identify and alert against signals from beam scatter which may not necessarily be aimed against the platform mounting the LWR. However, beam divergence is variable and in many cases the beam from a target designator will still not cover the whole of the target. Although direct illumination of a LWR is the ideal case, they must be able to detect the low power levels from scattered signals. LWR’s perform better in poor weather due to increased scattering.

4. Another factor that causes scattering of the Laser beam is from refection of the beam as it hits other parts of the target. This is known as Target Slash and is a function of type of surface and angle of incidence of the Laser. Dirt, Smoke and Dust in the environment will also cause further scattering. It is also possible to be illuminated by a signal reflected of a nearby friendly unit that is being illuminated. There may therefore be many false alarms.

5. What makes a laser signal stand out from other sources of light is the property of coherence where the beam is in-phase and consists of a single frequency waveform. The spread of the wavefront is usually about 1 meter with the only variation from this belonging to a few carbon dioxide doppler Lasers.

6. Background noise that makes the low power laser signal difficult to detect can originate from other low power sources such as steady state Solar reflectance or thermal self emission. Other signals causing interference or false alarms can come from the following:

a. Sun glint off water.
b. Gun flashes.
c. Fires.
d. Lightning.
e. Electro-magnetic interference.
f. Cosmic rays.
g. Shot noise and thermal heating from the Sun.


LASER WARNING SYSTEM DESIGN

7. LWR’s are usually fairly simple focal plane arrays with low angular resolution, enough only to alert the crew and enable the deployment of countermeasures such as smoke. Low frequency filters are used to remove background noise. As Lasers transmit pulses of nanosecond or microsecond duration LWR’s are optimised to detect multiple pulses of this duration. False alarms and background noise can be eliminated, as they tend to produce predominantly single pulses. Only Lightning produces similar signals.

8. Another way to eliminate background clutter is to analyse signal rise time. In the case of a Laser beam this has a very fast rise time while background noise generally has a slower rise time. Unfortunately some clutter has almost a fast rise time as a Laser signal and it takes very good processing to enable discrimination, although it is possible.


SUMMARY

9. Laser beams, by definition, are very narrow and difficult to detect. A warning receiver must be able to pick up the diffuse scattered energy which may have initially been operating at a low power setting.

10. Because of the effectiveness of laser guided weapons, there is a proliferation of systems on the battlefield and laser warning receivers are being fitted to an increasing number of platforms.

11. Countermeasures against laser targeting include the use of manoeuvre to break lock, the use of smoke, or a counter-strike.