Recently, there was a controversy about a statement made by Prime Minister Narendra Modi, ostensibly making a connection between cloud cover and the efficiency of RADAR. He was criticised for his statements which, according to many, lacked scientific validity. Scientists, the world over, tend to be critical of government policies and Modi is no exception.
Last week, Modi, in an interview, had said: “The weather was not good on the day of air strike. There was a thought that crept in the minds of the experts that the day of strike should be changed. However, I suggested that the clouds could actually help our planes escape the radars.”
Here are some facts from science:
In simplest terms, a radar comprises of a transmitter which sends radio waves along specific directions. The signals are reflected off the target which are used to construct an image of the target. If the target is moving at a specific velocity, there is a shift in the frequency of the signal which can be used to identify the target speed. As the received signal is just above the noise floor, a number of factors can influence the radar system and rainfall and clouds can certainly influence the measured signal.
Although radio waves are transparent to weather conditions like fog, clouds and rain, change in weather conditions can influence scattering and overall propagation. We have all seen telephone lines which are basically transmission lines are which are used to carry signals. Empty space can be represented as an array of field cell transmission lines which has a physical variable called impedance, which in some way obstructs the flow of signals. This quantity is directly governed by refractive index of the medium. For vacuum, the value of refractive index for radio waves is 1.
However, for radio waves propagating in water, its values increased approximately by factor of 3 to 10 depending on frequencies. It simply shows that the presence of moisture in air can influence propagation of signal in space.
Modi was talking in the context of Balakot strikes by the Indian Air Force. Very little information is available on the frequency ranges which Pakistan Air Force currently uses for its radar-based detection. However, Radar bands in general, operate over broad frequency ranges.
For example, the major bands along with their frequency ranges are L (1-2 GHz), S (2-4 GHz), C (4-8 GHz), X (8-12 GHz), Ku (12-18 GHz), K (18-27 GHz), Ka (27-40 GHz) ,V (40-75 GHz) and W (75-110 GHz) which are used for different applications. The X (8-12 GHz) band is mainly used for military applications like missile guidance. It is called X band as for long, it was a secret band widely used in the second world war. A typical airport surveillance radar, which detects the position of an aircraft in the terminal area operates at 2.7 to 2.9 GHz and 1.03 to 1.09 GHz). It can cover an area of 96 Km at an elevation of 25,000 feet.
Radars operating at such frequencies are not significantly affected by change in weather conditions. However, when the weather conditions are extreme, they can find it hard to detect a fighter aircraft zooming at very high speeds.
A number of researchers have authored papers on the subject of attenuation of radio waves by rain, fog and clouds. A detailed report by Rand Corporation for US Air Force was published way back in 1975. According to it, for a dense cloud, the attenuation of the signal could be 0.1 dB/km for X band radar. It implies signal attenuation by a factor of 10 if the target is 50 Km from the source. The attenuation could increase by a factor of 10 if there is rainfall at the rate of 25 cm/hr.
According to Meneghini et al. (1986), signal attenuation by cloud and precipitation is a serious problem associated with airborne or spaceborne millimetre wave operation. Lhermitte (1990) wrote in the Journal of Atmospheric And Oceanic Technology, that at 15 GHz the attenuation coefficient is 0.12 dB per mm per hour of rain intensity. It implies that if the rain intensity is 1 cm/hr, the attenuation of signal power can be in the range of 1.2 dB or approximately 31%. For a 30 GH z signal, the attenuation under heavy tropic rain could be in the range of 30 dB (a factor of 1,000). Besides rain, lightning-based scattering can also attenuate radar signals over short periods which can open new opportunities for fighter aircraft.
In fact, attenuation of radio waves is widely used in measuring rain intensity and moisture content. Below 1 GHz, the attenuation is not that significant, but heavy rains, clouds and lightning effects can still make some impact on the measurement process. Having said all that, it must be stated that as a pilot in an aircraft also communicates with the ground station using radio waves, the attenuation can also act as a bottleneck in maintaining a seamless communication link with the ground station. That is the reason why a lot of aircraft accidents happen during bad weather.
However, when the target is well defined, the risk can be averted. In a war, a lot of risky decisions need to be made.
To sum up, Modi’s statement does hold strong scientific basis which can be corroborated by existing research on the subject. The X band radar is significantly attenuated by rains, clouds and fog and related climatic conditions. For lower bands, the attenuation is less significant, but in high-speed warfare, slight change in conditions can offer huge leverage.
(The author is a Postdoctoral Associate at MIT. He earned his doctorate from Cambridge University for his work on radio signal sensing using microstructures. He has published papers in the field of electromagnetism and antennas in leading journals like Physical Reviewer Letters, Transactions of the Royal Society and Annalen der Physik.)
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