Bill Sweetman has a very good article on the Buk missile system, and how it was likely deployed, and how this contributed to the shoot-down of the Malaysian Airlines jet:

With mounting evidence that Malaysia Airlines Flight MH17 was shot down by Ukrainian separatist rebels who believed they were engaging a military aircraft, attention is focusing on the Russian-built Almaz-Antey Buk-M1 ground-based air defense system (GBADS) that destroyed the airliner.

The Buk-M1 (SA-11 Gadfly to NATO) can be used by minimally trained operators to deliver a lethal attack, without the safeguards built into other comparable GBADS, an Aviation Week analysis shows. It is also one of the two GBADS — both of Soviet origin — that are most widely distributed in conflict zones with the potential for large-scale, cross-border or civil violence.

The feature that makes the Buk-series weapons uniquely dangerous was introduced in the 1970s when Tikhomirov NIIP, now part of Almaz-Antey, designed the system to replace the 2K12 Kub low-altitude missile system, known to NATO as the SA-6 Gainful. (The similar names are coincidental: “Kub” means “cube” and “Buk” means “beech.”)

The Buk transporter-erector-launcher is designed to operate both as a part of an integrated air defense system (IADS) and independently, and this capability is what probably led to the incident:

The designers of the replacement Buk system had anticipated this problem. In addition to a new radar vehicle – the Phazotron 9S18M, Snow Drift to NATO – they fitted each launch vehicle with its own X-band multi-mode radar, under a radome on the front of the rotating launch platform. The vehicle is defined as a transporter/erector/launcher and radar (Telar). Similar to a fighter radar, the Telar radar (known to NATO as Fire Dome) has search, track and illuminator functions and can scan through a 120-deg. arc, independent of the movement of the platform.

This feature may have been a crucial factor in the destruction of MH17. The Fire Dome radar’s main job was to permit simultaneous engagement of more targets – one per Telar – under control of the battery’s 9S18M Snow Drift. But the Soviet military and the designers installed a set of backup modes that would permit the Telars to detect and attack targets autonomously, in the event the Snow Drift was shut down or destroyed by NATO’s rapidly improving anti-radar missiles.

The autonomous modes are intended for last-ditch use by the Telar operators, not the more highly trained crews in the battery command vehicle. According to an experienced analyst of Russian-developed radar, the automatic radar modes display targets within range. The operator can then command the system to lock up the target, illuminate and shoot.

Critically, these backup modes also bypass two safety features built into the 9S18M Snow Drift radar: a full-function identification friend-or-foe (IFF) system and non-cooperative target recognition (NCTR) modes. The IFF system uses a separate interrogator located above the main radar antenna and most likely will have been upgraded to current civilian standards.

The 9S18M introduced new NCTR processing technology, according to a 1998 interview with Buk designer Ardalion Rastov. NCTR techniques are closely held, but one of the most basic – jet engine modulation, or the analysis of beats and harmonics in the radar return that are caused by engine fan or compressor blades – should easily discriminate among a 777 with high-bypass turbofans, a turboprop transport or an Su-25 attack fighter.

There is no sign of an IFF interrogator on the Buk Telar’s Fire Dome radar or elsewhere on the vehicle. In normal operation, it would not be necessary since the target’s identity would be verified (according to the prevailing rules of engagement) before target data was passed to the Telar. Other GBADS also leave identification to the main search radar and the command-and-control center; however, the launch units cannot engage and fire without central guidance. The Buk’s combination of lethality and lack of IFF/NCTR is unique.

In a heavy SEAD (Suppression of Enemy Air Defenses) environment, the capability to operate autonomously, with the main battery turning on only briefly, and then passing the general targeting parameters to the TELs would be highly desirable.

However, in a situation like the Ukrainian civil war, where the rebels clearly are not operating an IADS, nor they are even operating as a complete Buk system.  They are operating as a lone TEL.

No central guidance, acquisition radar, no IFF, no trained command center, no kidding.

Why on earth airlines were routing aircraft through the Ukraine in such a situation, and the rebels possession of Buk TELs was well known, is completely beyond me.