A colleague pointed this out to me - and I thought I'd clear it up. The 'circle' is a Pk (Probability of Kill) test site.

An air launched weapon is launched at the centre of the circle, and either air or surface detonated. Impact damage can be assessed at each 5-10 meters on different types, and orientations of target. Thus you will have aircraft, vertical walls of differing materials, dummys of soliders in different protective gear (e.g. some in fatigues, some in NBC suits etc.), along with a number of wooden or steel posts of identical heights that are planted along one or more diameters of the circle.

In this case you have a 300 m diameter site which would indicate a relatively small conventional weapon, likely surface, or near ground detonated (the test site is too small for something like a fuel/air weapon - definately not nuclear ).

It's not a radar cross section range - you would have a single aircraft on a rotating pillar in the centre of a circular area. This area would be surrounded by a number (more than 3) radars of varying strengths.To cap that, nobody would be signature testing MiG 21's as shown in the picture - that data has been publicly available for decades).

Hope that helps.