UFO Detection
Alien craft regulary enter and leave the Earth's atmosphere. Spotting them and shooting them down is an important goal.
Detection Facilities
The above can be built in your base for the purpose of detecting UFOs nearby.
(cue notes from someone who knows more about how this is done then Bomb Bloke does)
Your craft can also detect UFOs via onboard scanners. If an alien vessel moves out of range of your land-based detection systems, you may be able to resume tracking by sending one of your ships to where you believe the UFO to be (based on its speed and direction).
Detections are rolled every 30 minutes, on the half hour, so if you are advancing the game an hour or day at a time you may find it jumps forward to exactly a half hour interval as it spots a new UFO.
Finally, the UFO activity graphs act as a sort of MUFON, allowing you to see activity that your radars cannot. The graphs are updated every 30 minutes, on the half hour. You could even check the graphs every so often and build zero radars if you wanted, and if you see a blip, send aircraft. This worldwide coverage can be easily used this way for the first few months. After that, UFO activity becomes too intense and the graphs lose their resolution, but by that time you should aim to have decoders already built, or on the way.