On the hot debate that has overflown across media on certain confusion created after a television interview of Prime Minister Narendra Modi where he spoke about cloudy weather and radar technology vis-a-vis the Balakot airstrike, Tapan Misra, former Director Space Application Centre, ISRO, has set the record straight when it comes to the possibility of very cloudy and rainy weather helping invading fighter aircrafts in escaping after completion of their tasks.
Misra is one of the finest brains on radar imaging and hyper spectral payloads. He has underscored that Radars use micro waves, which is reasonably transparent to clouds under nominal weather condition. But under dense cloud or heavy rain condition, they get absorbed quite heavily, to the tune of ~1 to ~10 dB per km. For lay men’s knowledge, dB is a logarithmic scale and non linear. 10 dB is 10 times, 20 dB is 100 times, 30 dB is 1000 times. On other side 1 dB is 1.1 times, 3 dB is 2 times, 6 dB is 4 times. Higher the frequency of the radar, more the attenuation as given in the diagram below.
Radars, Misra points out, are operated at different microwave frequencies and nomenclatured in a funny way: L band 1-2 GHz(Giga Hertz), S band 2-4 GHz, C-band 4-8 GHz, C-band 8-12 GHz, Ku – band 12-18 GHz and so on. As you see in the plot, as you go to higher and higher frequencies, more and more losses are encountered.
According to Misra, Space borne imaging radars (like RISAT SAR) see through clouds quite well as the look straight down and encounter small cross section of clouds and rains ranging from ~100 m to ~1-2 km radar pulses propagate to target and then reflected or scattered back. So they undergo double the loss (called two way loss) than common satellite communication signals, which suffer one way losses. One SAR image shows that even for small propagation distance, under heavy cloud condition SAR imaging system loses the signals.
All of us, Misra goes on to state, gave experienced some time or other, loss in Ku band DTH signal in our homes even under moderate cloud condition, even though signal reaches in vertical direction, encountering minimum cloud cross section. Hence, he goes on to emphasise, when you lose such DTH signal, loss will be more than 10 dB I.e, signal is attenuated more than 10 times.
He further says, consider tracking radar cases, radar pulse travels almost horizontally when the aircraft is far from the radar of the order of a few tens of km or more. Clouds are spread horizontally far and wide. The tracking radar signal travels through cloud and rain by almost tens to hundreds of km. Under heavy cloud or rain conditions, the loss may be tens of dB to hundreds of dB depending on the frequency.
The friendly fighter and normal passenger and cargo aircrafts are tracked by transponder mode. The received weak radar signal is amplified by the transponder in the aircraft and returned back. So losses in transmission is compensated by increasing the transponder gain. So normally transponder tracking is more resilient for atmospheric cloud or rain losses. Further such radars are operated at low frequencies in L and S band, minimising the losses.
But enemy fighter aircrafts switch off the transponders, Misra states, adding only way to track them is by skin mode tracking, where signals bouncing or scattered back from aircrafts are detected. Normally now a days, fighter aircrafts, by design, have low radar cross section, this means only a small fraction of incident microwaves , to the tune of one tenth to one hundredth is bounced back. Coupled with atmospheric losses, the effective tracking range of the radars are reduced under very adverse weather coconditions. Since, radar visibility of objects increases with square of frequency, for better tracking of enemy aircrafts under skin mode, tracking radars are operated in higher frequencies like S, C or X band. So they suffer more losses in bad weather condition and effective tracking ranges are reduced. So under bad weather, enemy aircraft is detected when they are so close to the target that hardly any reaction time by launching counter strike is available.
Misra explains that bad weather helps attacking aircrafts to ‘escape’ easily. However, needless to say, in operating military fighter missions under inclement weather, it calls for superior skill of the pilot and superior instrumentation and capability of the fighter aircrafts.