Take a look at this link, this might help.
http://ourworld.compuserve.com/homepage ... sky_p3.htm
Also in the Radio Engineers Handbook
there is a plot of look angle vs frequency. I thought I had a soft copy of it but I misplaced it.
You are right in assuming that you can have a system noise temeprture that is quieter than what the
antenna sees expecially in the VHF band.