Multiple instruments rely on static air pressure, i.e. pressure unaffected by airspeed.
To my understanding, no such air should exist around an aircraft. Air in motion exhibits a lower pressure measured laterally, and a higher pressure measured head-on (ram air).
Switching to an in-cabin alternate static source usually decreases measured pressure, as the cabin altitude is above outside altitude, due to its equalizing with accelerated outside air.
The only solution I could think of was to measure pressure at an angle, where both effects (Bernoulli and ram, so to speak) cancel out. Now such an angle would be airspeed-dependent. How can a reliable pressure be obtained, without computational aid, over a range of airspeeds?