sounds like u need to find a geosynchronus satellite with a fairly constant transmit power, a dish antenna (and possibly an LNB) pointed at it, and a receiver to compute the received power transmitted from the satellite. If you use an LNB, you could use a fairly cheap low frequency spectrum analyzer as the receiver...just want to be sure it has a computer interface on it. Then an automatic rain guage to mount on the roof...also computer connected.
If u want to do a few frequency points, u need multiple antenna dishes/lnbs.....and maybe a computer controlled IF switch.
The location of the satellite with respect to your receiving antenna location will be important. A satellite on the horizon will show much more rain fade than one relatively overhead.