On Fri, 10 Aug 2012 17:24:47 -0700, Korben Dallas
wrote:

the old obstacle for doing this was lower iss orbit. the iss orbital
altitude was intentionally reduced to make it reachable by space
shuttle. now, with shuttle out of the picture, the iss was transferred
to a notably higher orbit, making it much easier to shoot progress ships
straight to the iss.

I think the altitude difference has actually been pretty marginal
since the Shuttle's return to flight in 2005/2006. They did let the
Station get a little lower than normal for a couple of the real
heavyweight flights like Kibo and S6, but that wasn't constant.

There were several reports at the time of the launch that it was the
new TsVM-101 digital flight computer (introduced on Soyuz TMA-01M in
Oct 2010) that made this possible. Now that the TsVM-101 computer has
a few flights under its belt, Russia was ready to try rapid
rendezvous.

Brian