EDSU … Russian F.C.R What is it !!!!

Hi, Just joined this forum and found it to be quite informative.
For the past 12 years Ive had an interest in combat aircraft
particularly Russian. This came about as a result of reading a
technical article about the performance of the Mig 21 in Vietnam.
Since then I have not looked back.
However, my question is this… what is EDSU !!!. I first
saw reference to this in a technical book called The SU-27 Russian
Super Fighter by Milbrook books. This was refering to the aircrafts
Fire Control Systems>>>>>>> ANY HELP APRECIATED <<<<<<<<<<<<<<
THANKS