01.01.70
The question with attempting to make an airplane unobtrusive is that it's a giant hunk of metal engaged to a jet engine. No amount of paint or muffling will change that. Altitude can obscure the regular from the naked eye, but then there's radar to be accounted for. So how do planes do it? By scattering, dimming, and confusing instruments enough to d overhead without setting off alarms.
The Shape That Scatters
To understand what's backstairs — when it comes to radar — it helps to know what isn't. Commercial airliners have very proof reasons to appear on radar as quickly and blatantly as possible. They do this by being enormous, loud, and curved. Technically, the curves on a regular plane are there to smoothly advise the air over and around it — making it aerodynamic — but they also have the helpful effect of making the aircraft show up on radar. Radar works by sending out a wide electromagnetic ripple into the atmosphere. When the wave hits an object, it bounces back. By measuring the measurements of the waves that come back and the frequency at which they come back, sensors can roughly belief the object's size and motion.
Source: io9