Clouds, rain and radar – Addendum

My piece on May 12 calling out Prime Minister Narendra Modi’s baffling conviction in his knowledgeability about surveillance radar systems and atmospheric attenuation has prompted some criticism as to its scientific accuracy. Apart from the compulsively defensive bhakts, some scientists also wrote to me saying that Modi was technically in the right to have claimed clouds and rain could have worked in the Indian Air Force’s (IAF’s) favour on the day of the Balakot airstrikes in February.

Much of this criticism hinged on the premise that military radar-surveillance systems operated on the X band (approx. 7-11 GHz), popular during World War II. However, many – if not most – of the surveillance units that the Defence R&D Organisation has built for the IAF operate over the L and S bands (1-2 GHz and 2-5 GHz, resp.). Consider the following official descriptions of four of them:

Central Acquisition Radar – “a ground based mechanically scanning S-band pulse Doppler radar for air space surveillance to detect and track air targets with reliability, even under hostile EW operational environment for the Indian Air Force.”

Tactical Control Radar – “a Tatra VVL mounted, mobile stand-alone medium range, all weather 3D surveillance radar for detection and identification of aerial targets. Pertinent data can be collected … 20 km away from the radar. The radar operates in S-band and [can track] airborne targets up to 90 km for fighter aircrafts and 65 km for UAVs, subject to radar horizon. The antenna is mechanically rotated in azimuth to provide 360º and 50º elevation coverage upto 10 km height.”

BHARANI – “L-band 2D radar [is] a light weight, battery powered and compact sensor which provides 2D surveillance solution to alert Army Air Defence Weapon Systems mainly in mountainous terrain against hostile aerial targets like UAVs, RPVs, helicopters and fixed wing aircraft flying at low and medium altitudes.”

ASLESHA – “a multifaceted ground based S-band 3D low level lightweight surveillance radar for deployment in diverse terrains like plains, deserts, mountain tops and high altitude regions. Aslesha detects and tracks heterogeneous air targets, including helicopters, fighters and UAVs at low and medium altitudes.”

According to a USAF paper published in 1975, the amount of signal attenuation through clouds is exponentially higher at higher frequencies. According to the log-log plot (below) presented in the paper, it is nonexistent to minimal in the L band and is at best 0.1 dB/km for S band radiation passing through dense clouds. The X band that spans 7-11 GHz is susceptible to greater attenuation, up to nearly 10x when passing through dense clouds.

However, rainfall attenuates S band radiation more than cloud-cover does. According to the same paper, radio waves with frequencies in the range 3-5 GHz are attenuated by up to 5-70-times as much as L band radiation when the rainfall rate is between 50 mm/hr and 150 mm/hr, resp. (apparently possible between December and March over Pakistan’s Khyber Pakhtunkhwa province).

Does this mean Modi was partly right? No, for the following reasons, which should clarify that the point of my piece was also different: The way Modi phrased his comment suggested his opinion came from a place of relative ignorance. The fact that the BJP Twitter handle tweeted and then deleted it shows they had no idea what they were talking about. So to claim Modi could have been right is disingenuous when he himself attributed it to “raw wisdom”, and made no effort to be clear or coherent despite the fact that he was on national television talking about national defence.

Instead, I believe he was simply fortunate that his comment in this case was sufficiently vague to allow him to be right. And given his utterances in the last five years, I have no reason to believe he was in the know; if he was, that is not what he would have said. Finally, it was also silly to suggest the IAF hadn’t already thought about sources of attenuation. It is the foundation of this conviction that I felt compelled to criticise, for reasons I discussed in my piece:

It has become clear that education has little to do with spewing pseudoscience … and a lot to do with brooking challenge and dissent. If the patriarch of a middle-class household is going to claim that the cosmic arrangement of a few stars thousands of light-years away is why he was fired that day, the absence of a counter-voice is only going to legitimise his spurious beliefs. And when Prime Minister Modi is this man, and it would seem he is, the simple dissemination of facts is not going to cure our society of this problem.

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