If you’ve ever seen the dramatic final scene of “Strategic Air Command” with Jimmy Stewart, you’ve seen a Precision Approach Radar, or PAR, approach. A PAR begins with vector- to-course, much like an ILS, but also adding trend info to help the pilot build situational awareness.
There’s no navaid. All you need is a voice radio. ATC uses special ground radar to give precise headings and tell the pilot when to “begin descent.” The controller also gives trends such as “left of course, and holding” or “going slightly above glideslope.” Trends are for guidance and situational awareness.
Laterally, we do our best to match the controller’s headings, even if they’re wrong. Like a pilot, the controller has to find the heading that makes the blip fly the black line. If we arbitrarily tweeked it, ATC would never know what heading we were on and couldn’t reliably get us on course. Glidepath, on the other hand, is strictly pilot stuff, with ATC advising trends. Starting with “begin descent” I note our groundspeed and watch the other pilot. If his rate exceeds what’s required, I think, “The controller’s going to call us going below glideslope” and two potatoes later he does. When I take new pilots out to fly the PAR, I give them essentially the same heading and rate schpiel as in this article. Then I sit and patiently watch. Those who fly the airplane precisely hear “on course, on glidepath” and those who don’t, well, don’t.
It’s not magic; it’s science. The PAR is actually an excellent illustration of how a well-flown ILS is accomplished. Although PARs are getting rare even at military fields, most bases that have them allow civilian practice so long as the aircraft don’t touch down. If you have the opportunity, give it a go.