M50 to J2000 reference frame conversion

hi folks,
how/where can i learn about M50 to J2000 reference frame conversion?
i've recently picked up an interest in tracking satellites. read a
couple of books on the topic, took a grad level class (on sat
communication rather than the tracking of it) and after using packaged
software to track satellites i am now working my way thru writing a
program solely for learning a thing or two about science related to
space. implemented the SDP4/SGP4 algos from "spacetrack report #3" in
java and am now porting it to c++. also found and implemented algorithms
for julian to georgian conversion. now i'm stuck with the reference
frame conversion. i'm decent with trig and such so i figured it wouldn't
a big deal but since i don't know what the aries mean of 1950 is
actually based on it's kinda hard to do the conversion. where can i
learn more? i've tried googling but nothing concrete turns up. public
libraries have a slim pickin' as far as science and math books are
concerned (i tried).

any help/suggestion would be greatly appriciated. and if the software
turns out to be any good, i intend to GPL it so that anyone interested
can have fun with it too.

thanks,
anoop