Here is a video showing how an engagement looks like for operators of a Buk-M1 Launcher:
https://www.youtube.com/watch?v=LSXMhaFntrU
At 1.19 we see the operator's consoles, horizontal situation being displayed on the larger, circular orange display. I'm not sure what the right retangular orange display is for - vertical situation, precise target tracking, missile tracking ?
There is also a green oscilloscop-like display - not sure what it does, and a TV display for targeting using a visual channel.
From 1.24 to 1.29 we see a target engagement on the circular display - first the radar is scanning wide 120deg horizontal sector (7deg in elevation), we see a blip of a target detected. As the target is selected, the scanning is quickly reduced to 10deg zone, and just after that - target is locked on and the beam is tracking it.
All this looks quite "analog" - no syntetic situation display with computer generated target markers tagged with speed and altitude data - only analog "blips" on radar, the range is determined by the radar display grid, the speed - probably by the rate of movement on the display, and the altitude of the target - well, I have no idea if it's displayed anywhere ? Maybe after the target is locked on, some numerical or at least analog data about speed, course and altitude is calculated and displayed... somwhere....
The target detection, identification and selection is usually done by the battery's command center vehicle (which have data from the big search radar, with digital displays and syntetic computer-generated target symbols on them) - operators in command center would easily see difference between Su-25, An-26 or a B777 crusing at 33k. The radar on the TELAR Buk launcher vehicle is a simple one, used mainly for for locking on target, tracking and illlumination. It does have some target detection capability, but is very simple and gives little data about the targets - they are just "blips".
A calm and experienced operator could probably say the target is too high to be a Su-25 or An-26, but an inexperienced one in a rush ? I imagine that with such displays they could easily lock-on the MH17 thinking it's a lower flying ukrainian plane. Target speed, size or altitude just doesn't show on those displays, or this kind of data is not easily accesible and interpreted.
Didn't they know there is a commercial passenger traffic high overheads ? They could see the contrails with their naked eye.... Maybe the operators just arrived at the scene (with the vehicle) and just didn't happen to think about it.... That capabilities of their SAM system reaches to heights of civilian traffic and they can detect/engage a civilian plane mistakenly... Seems that in their minds a detected target just had to be military ukrainian one....
Well, in my language there is a proverb about giving a razor to a monkee...
Someone who gave a SAM system with such capabilities to a group of rebels, should be brought to justice....