On March 25, 2019 the U.S. Missile Defense Agency (MDA) conducted FTG-11, a salvo test of the US Ground-based Midcourse Defense (GMD) system in which two interceptors were fired, just under a minute apart, at a single ICBN-range missile.  

In a press release later that day, MDA described the test as follows:

“This test was the first salvo engagement of a threat-representative ICBM target by two Ground Based Interceptors (GBI), which were designated GBI-Lead, and GBI-Trail for the test. The GBI-Lead destroyed the reentry vehicle, as it was designed to do. The GBI-Trail then looked at the resulting debris and remaining objects, and, not finding any other reentry vehicles, selected the next ‘most lethal object’ it could identify, and struck that, precisely as it was designed to do.”

The MDA press release stated that “Initial indications show the test met requirements.”  It also quoted MDA Director Air Force Lt. General Samuel A. Greaves:

“The system worked exactly as it was designed to do, and the results of this test provide evidence of the practicable use of the salvo doctrine within missile defense. The Ground-based Midcourse Defense system is vitally important to the defense of our homeland, and this test demonstrates that we have a capable, credible deterrent against a very real threat.”

At yesterday’s (April 3, 2019) hearing on missile defense before the Strategic Forces Subcommittee of the Senate Armed Services Committee, General Greaves reaffirmed the success of the test.  Asked by Senator Angus King whether the test was a success, General Greaves stated that although there were nine months of data to review, the “initial look says it was a complete success.”

Senator King then asked him to “Define complete success, did the bullet hit the bullet?”

After giving some background information on testing, General Greaves stated:

“But this test was different because we launched within a very short period of time two Ground-Based Interceptors operationally released by the combatant commander using their operational processes –which is very important — and the lead interceptor intercepted the ICBM-representative threat.  But what’s most important is that it created a debris field — and this test has been 10 years or more in the making — and the importance of that was the trailing — the second — interceptor was able to discern the debris from the next most lethal object — I can talk about it in a classified forum — and also intercepted that object. What that means is [an] enemy concept of operations which seeks to confuse our missile defense system by launching junk or debris would not be successful, that’s why it was a success.” [My transcription from the video of the hearing.]

 Both the press release and the General Greaves quote immediately above make it clear that after the first intercept, there remained both debris and at least one other object.  All of these other objects must have had an appearance sufficiently different from the warhead so that the first interceptor could select the warhead.  And at least one of these other objects must have had an appearance sufficiently different from the debris so that the second interceptor could identify it.

So although when Senator Dan Sullivan asked General Greaves if the second interceptor had hit the largest debris fragment, General Greaves replied “Yes, sir,” he immediately clarified this saying that it hit “the next most lethal object” which he defined as ‘the next object that most closely resembles a threat vehicle.”

But what did FTG-15 actually prove?  Did it provide “evidence of the practicable use of the salvo doctrine” as the press release claimed?  Did it demonstrate that such the salvo doctrine would defeat enemy attempts to confuse the system as General Greaves quote suggests? I think it is clear the answer to both questions is “No.”

Strictly speaking, the test did demonstrate that the GMD system could intercept a target resembling a warhead against a field of debris, none of which resembles a warhead.  But why would any enemy try to defeat the system with such an obviously ineffective tactic (unless the warhead was disguised to look like debris)?  Moreover, the debris field that an attacker might deploy along with its warhead likely would look quite different than the debris field produced by an interceptor hitting a warhead or other object.  Finally, there was no need to launch two interceptors to conduct this demonstration.  Debris could have been launched along with the first warhead, and there would have been no need to waste a second $70 million GBI interceptor.

More importantly, the fundamental purpose of the salvo firing doctrine is to compensate for low missile defense system reliability, the interceptor reliability in particular.  For example, if the GBIs were believed to be 60% reliable (although this is slightly higher than its success rate in intercept tests – see table at end of this post — there is not enough test data to know their what actual reliability is), then salvo firing N interceptors gives a probability of killing the target of P_KN = 1 – (1-0.6)^N.   So for one interceptor P_K1 = 0.6, for a two-interceptor salvo P_K2 = 0.86, and for a three interceptor salvo P_K3 = 0.936.  This is why many observers of the GMD system have speculated that the GMD firing doctrine is at least a three or four interceptor salvo against a single attacking missile.  This assumes that the failure of any one interceptor is statistically independent of the others.

A two interceptor salvo firing doctrine benefits the defense against an attacking missile when the first interceptor fails.  There is no benefit from the second interceptor if the first interceptor destroys the target, as was the case in FTG-11. 

More importantly, when an attacker takes deliberate and effective steps to defeat the defense (countermeasures), the salvo doctrine will be much less effective.  This is because if a countermeasure (and not an interceptor reliability problem) causes the first interceptor to fail, then there is a substantial probability that the countermeasure will also cause subsequent interceptors to fail.  For example, consider a case in which the warhead is disguised to look like a decoy and is accompanied by five credible light-weight decoys.  If the interceptor is unable to distinguish between the disguised warhead and the decoys, it will have to choose one of six potential targets.  Assuming  the interceptor has a same reliability as assumed above, 0.6, the chance of a successful intercept of the warhead with a single interceptor  is only P_K1 = 0.6 * (1/6)  = 0.10. In a salvo of two interceptors, if the first interceptor fails to destroy the warhead, the odds for the second one are slightly better, because there is an 60% chance it will see only five targets, so the kill probability for this interceptor is P­_K1­ = 0.6*0.6*(1/5) + 0.4*0.6*(1/6) = 0.112, and the overall success rate for the two intercept salvo is only P_K2 = 0.201.  The salvo firing doctrine does not solve the countermeasure problem, which has long been recognized as the most important and difficult problem facing an above-the-atmosphere defense such as the GMD system

It is worth noting that FTG-11 was not MDA’s first attempt to intercept a target against a debris field.  On November 1 2015, MDA conducted test FTO-02 event 2.  In this test, a short range ballistic missile target was successfully intercepted by a THAAD interceptor, creating a debris field.  An Aegis SM-3 Block IB TU interceptor then attempted to intercept a medium-range ballistic missile against the background of this debris field.  However, the SM-3 missile failed before it could make the intercept attempt.  The target was then destroyed by a second THAAD interceptor, although it is unclear (to me) if the target was still in the vicinity of the debris field.  However, even prior to FTO-02, MDA had conducted “tests to verify interceptor performance in debris clouds” although these most likely were of systems such as the Aegis SM-3 or THAAD and involved only a single interceptor.

I think FTG-11 is better regarded as two separate intercept tests, albeit separated by less than a minute.  I have accordingly updated my Tables of Intercept Tests post of November 30, 2018, counting both tests as successes.  The GMD table from that post is below: