Gosh, we’ve come quite a way from BAK in GA bugsmashers squawking altitude and groundspeed to transport category aircraft squawking every parameter including the galley sink level and the cockpit-selected IAS and FL/ALT.
I now get why pilot-reported IAS was useful before the advent of reliable EFIS/FMS and ADS systems, in airways. I’m still not quite sure why an instruction to ‘fly 10 knots IAS faster (if you can)’ or ‘fly 10 knots IAS slower (if you can)’ wouldn’t achieve the desired outcome, based on whatever information happens to be available to ATC these days. (Bit strange that you don’t get to see the selected IAS and FL/ALT data, le P?). If it’s about separation, that must be about predicting conflicts and I don’t understand how that can be done without an extrapolation from position, track and groundspeed (except in the case of aircraft flying e.g. flying the same airway and STAR at the same levels/altitudes - I get it that if each complies with an instruction to fly at or above or below a specified AIS you can predict whether they are all ‘catching up’ to, or ‘spreading apart’ from or maintaining ‘a’ distance from each other).
All interesting and important stuff - thanks.