I am not a controller, but if an aircraft has been assigned a Mach/IAS and they are catching up the one in front - as noted on your radar display - then assign the trailing aircraft another 10kts IAS slower. You don't need to know their groundspeed. (Sometimes pilots don't set the requested IAS/Mach - sometimes ATC can see what speed they have actually set !!)
But the originally ‘assigned IAS’ should have been enough to resolve the potential conflict. Why wasn’t it? (The answer has a ‘G’ and an ‘S’ in it.) And what does the controller do if ‘assigning’ the trailing aircraft another ‘10kts IAS slower’ doesn’t solve the problem? If pilots don't set the requested IAS/Mach, the pilots are 'naughty'. But it's ultimately the ground speed and track that are gonna cause collisions that will kill people.
Re your second point; Aircraft are not designed to fly by selected groundspeed.
Of course not. But it’s a parameter that’s right there on those GPS and radar screens. Plus track.
There is no selection window or data box in the FMGS to select a groundspeed, and there are no groundspeeds dictated on SID and STAR plates. There is nowhere to set a groundspeed bug. If you think about it, it could be incredibly dangerous to do so - it could cause an aircraft to stall or overspeed if the winds were particularly strong tail winds or head winds.
I have thought about it. I understand that GS does not equal IAS, and I understand it’s air speed that keeps an aircraft in the air. But that’s precisely why an aircraft can have e.g. have a groundspeed twice its stall speed, yet still stall and, conversely, a groundspeed half its stall speed, yet still not stall and continue flying to eventually collide with another aircraft.
The ultimate point of this thread is that prosecuting a pilot for flying an aircraft so as to exceed VNE (an airspeed) on the basis of the aircraft’s recorded groundspeed is … what is the appropriate adjective: nonsensical? incompetent? malicious? Choose your own.
We can select or program a Mach number or an IAS. (And, no, we are not going to get out our CRP-5's to work out what IAS we need to fly a groundspeed, then work it all out again when we change our heading !)
I’ve obviously confused some people by not making clear that the point of this thread was not to suggest that pilots should ‘program’ or otherwise try to manage an aircraft’s ‘safe passage through the air’ by managing groundspeed. My apologies for the confusion. Managing groundspeed is about knowing and managing ETA and collision risk.
To know your TAS or groundspeed, you need accurate OAT probes and air data computers and some form of very accurate inertial or GNSS position determining computer. Not all aircraft will have this - certainly not all of the same accuracy. But all aircraft - even a basic Cessna 152 - have an IAS readout.
No Cessna 152 has ever been the subject of air traffic control instructions as to the IAS to be flown. At least not on the planet on which I live.
But every Cessna 152 pilot – or at least those who progress to cross-country navigation - should be able to work out TAS from IAS and temperature and pressure altitude. ‘Accurate’ instruments, including OAT measurements on Cessnas, have fat margins of error that don’t result in too many people going too fast or too slow, or getting lost, that often.
And very accurate GNSS position determining computers? I think there have been hundreds of thousands of aircraft flying around with them, for quite some time.