Fighting for space

A-SAT test would require two years of preparation, making it hard to argue that it was an impulsive government decision

By Ajai Shukla

Business Standard, 2nd April 19

Last Wednesday’s anti-satellite (A-SAT) test, in which a Defence Research & Development Organisation (DRDO) interceptor missile destroyed a satellite 300 kilometres (km) above the earth, raises several questions. There are allegations that the test was politically motivated and conducted with an eye on the coming general elections rather than by any real need to validate and demonstrate India’s A-SAT capability. Others have argued this underscores the government’s commitment to military R&D. To grasp the technicalities of the debate, one must understand three interlinked technologies: ballistic missiles, ballistic missile defence (BMD) and A-SAT interception.

A ballistic missile works on the simplest of principles, akin to the country fair game of ringing a duck. Where a ballistic missile lands depends largely upon the direction and force with which it is thrown.

Typically, a long-range ballistic missile like the Agni-V is stored in, and fired from, a hermetically sealed canister. At launch, a gas generator inside the canister ejects the 50-tonne missile upwards at 20-25 metres per second. Once outside, its massive first-stage rocket motor ignites, propelling the missile upwards. Within 30 seconds, it goes supersonic and, a minute later, when the first stage finishes and the second stage ignites, the missile is travelling at five times the speed of sound. By the time the third stage burns out and the “boost phase” finishes, the missile is 1,200-1,400 kilometres above the earth, moving purely on momentum at six times the speed of sound.

That momentum carries the warhead 5,000 kilometres towards the target during its “mid-course phase”, with gravity progressively pulling it down towards the earth. While re-entering the atmosphere, intense friction heats the missile’s nose cone to 3000-4000 degrees centigrade. During the 45 seconds of re-entry, the missile’s carbon-resin nose cone sacrifices itself, burning off layer after layer and absorbing the heat without passing it on to the warhead inside.

During this “terminal stage”, to make the missile’s trajectory unpredictable and difficult to intercept, the warhead starts manoeuvring. At the same time, a guidance system homes the missile onto the strike point, compensating for variations in launch and during the mid-course phase. At the end of its 5,000-kilometre journey, the Agni-V lands within “a few hundred metres” of its intended strike point, says the DRDO.

A ballistic missile’s actions are all pre-programmed to make it fully autonomous after launch. Receiving commands during its flight would make the missile vulnerable to enemy disruption.

Anti-ballistic missile (ABM)

An ABM system is geared to shoot down ballistic missiles and is deployed around high-value targets like cities or national command posts. Detecting, identifying, profiling and then shooting down an incoming ballistic missile is highly complex, especially since the enemy might have fired several missiles to assure himself of successfully striking the target.

A missile can theoretically be shot down anytime in its trajectory: During its boost, mid-course or terminal phases. It is most vulnerable during its boost phase, when it is travelling relatively slowly and cannot manoeuvre or deploy countermeasures. But the boost phase lasts for barely 300 seconds (5 minutes), giving an interceptor insufficient time to reach a launch area that could be thousands of km away.

The mid-course phase, which could last 1200 seconds (20 minutes), provides a longer window for engaging the incoming missile. However, it would still be far away, travelling extremely fast and possibly deploying decoys. The ABM system must distinguish the decoys from the mother vehicle and also identify the missile, its target and its terminal velocity – parameters essential for engaging the missile.

The terminal phase, therefore, provides the most realistic opportunity for engagement. The computerised ABM command system would have calculated engagement parameters and the interceptor missile must now travel less distance. Further, atmospheric drag would have slowed down the incoming missile significantly.

Since any error might result in a city being reduced to smoking rubble, ABM systems engage incoming ballistic missiles at two levels. An exo-atmospheric interceptor engages the enemy missile 150 km above the earth. Meanwhile, an endo-atmospheric interceptor provides back-up, engaging the aggressor at 30-40 km altitude. For further assurance, two interceptors of each kind are launched, increasing the chances of a kill. 

With every second critical, the key to a successful ABM engagement is to detect the enemy missile early, giving oneself more time to identify, profile and intercept it. Ideally surveillance satellites should monitor enemy territory to pick up the “plume” of the missile when it is fired, with detection automatically activating the ABM system. While the DRDO is working on such satellites, the current ABM system relies on the so-called Long Range Tracking Radar (LRTR), derived from the Israeli Green Pine radar, which detects aggressor ballistic missiles 1,000 kilometres away.

The crucial endgame is the exquisitely precise business of interception. The incoming ballistic missile would have slowed down to about two km/second, while the interceptor, which has less than two minutes to reach 150 km altitude, would be accelerating to the same hypersonic velocity. Given their relative velocity of four km/second, ensuring that the ABM “kill vehicle” explodes within ten metres of the incoming missile requires an accuracy of about one-thousandth of a second.

This has been actually achieved. As the interceptor thunders up towards the incoming missile, an ABM uplink station beams it the target data – its position, velocity and manoeuvres. Powered by its first two solid-fuel stages, the interceptor guides itself towards the target. Once the “kill vehicle” radar seeker locks onto the incoming missile, its liquid fuel “divert thrusters” manoeuvre it towards it. At the precise millisecond it is closest to the aggressor missile, a proximity fuse detonates the kill vehicle, destroying or disrupting the ballistic missile. 

Anti-satellite (A-SAT)

Shooting down a satellite requires technologies almost identical to those that have driven the DRDO’s ABM tests since 2008. One key difference is that a satellite is engaged at a higher altitude – at 300 km, rather than 30-150 km in the ABM tests. An upgraded propulsion system is required to take the kill vehicle up to that altitude. 

Further, striking a satellite requires more stringent accuracy, since the target moves significantly faster (about 27,000 km per hour) than the incoming ballistic missiles simulated in ABM tests. On the other hand, engaging a satellite is easier because it moves along a well-known flight path and behaves in an entirely predictable manner without engaging in evasive manoeuvres like an incoming ballistic missile.

The timing of the A-SAT test raises other questions. On the one hand, a decision to demonstrate A-SAT capability would require at least two years of DRDO preparation to translate into a test, making it hard to argue that this was an impulsive government decision directed towards electoral benefits. However, it is also hard to argue, as many do, that India was driven to test for fear of being left out of a space security regime, being discussed in Geneva. As Christopher Clary of the University at Albany tweeted: “Isro (Indian Space Research Organisation) is a space superstar and India is a nuclear power. There was a zero per cent probability that a space security regime was going to be negotiated without India’s buy-in.”