

A pro-Russian separatist holds a stuffed toy found at the crash site of Malaysia Airlines flight MH17, near the settlement of Grabovo in the Donetsk region, July 18, 2014. (REUTERS/Maxim Zmeyev)

One of the remaining mysteries about the Malaysian airliner crash is why the pro-Russian Ukrainian separatists would've gone after a civilian airliner when until now they'd been targeting primarily military aircraft — low-flying helicopters and the like. That's a departure from the separatists' previous pattern. What changed?

Without speculating too much on the rebels' motives, there are a few things we might be able to glean by looking at the technology that was used in the incident. To date, the rebels had reportedly been using man-portable surface-to-air missiles. But that equipment isn't capable of striking high-flying aircraft, hence the use of SA-11/17 mobile missile platforms, which can engage targets as high up as 82,000 feet.

That's some sophisticated equipment. Speaking to reporters Friday, President Obama said U.S. intelligence officials had gathered strong evidence suggesting the separatists had been trained by Russian forces.

Here's the thing: A well-trained radar operator should have been able to distinguish between a civilian airliner and military aircraft.

All aircraft carry transponders — radio equipment that sends out signals about a plane and what it's doing. Military and civilian transponders use different patterns; the most commonly used modes for commercial flights are known as Mode C and Mode S, the latter of which is also used in a positioning system called ADS-B, a term you might be familiar with from following our coverage of that other Malaysia Airlines flight, MH370.

Meanwhile, military planes use a different identification system whose different modes are numbered rather than lettered. In the military, this system is known as Identification Friend-or-Foe (IFF); for civilian purposes, it's called Secondary Surveillance Radar (SSR).

What distinguishes one mode from another is generally how far apart two radio pulses are, along with any information that's included in the transmission. Here's one illustration that tries to depict the differences.

If you want an idea of how complicated this can get, read this.

A missile platform like the SA-11 Gadfly relies on its operator to know which radar contacts are safe to shoot and which are not; this means the transponder codes are crucial information. Once fired, the SA-11's missiles hunt down the target with a 95 percent interception rate, no matter who's on the receiving end. The more advanced SA-17 Grizzly can apparently determine when it's latched onto a civilian transponder code, but that won't stop the operator from taking the shot.

So we're left with two possibilities: Either the Ukrainian rebels were deliberately trying to shoot down an airliner or they were trained just well enough to operate the controls but lacked the sophistication to distinguish between different transponder codes.

The evidence seems to point in favor of the latter, for a few reasons. First, shooting down a civilian airliner seems like it would work against the rebels' strategic interest. Second, the SA-11 appears to have been only recently introduced to the theater. Third, phone calls released by the Ukrainian government purporting to be a conversation between rebel leaders suggest that the separatists only found out they'd downed a commercial airliner after the fact.

Sen. Mark Kirk (R-Ill.), hinting strongly at the involvement of Russian trainers, said Friday he doubted the rebels' capabilities.

"The Buk missile system is such a complicated radar-guided system," he told the Post. "I would think a bunch of Ukrainian hillbillies would not have an ability to operate it efficiently."

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