Ground speed vs indicated airspeed
Two aircraft can both be flying at 250 knots and moving across the ground at completely different speeds. This isn't a glitch. It's wind.
Indicated airspeed (IAS)
IAS is what the pilot sees on their airspeed indicator. It measures how fast air flows over the wings. This is what matters for flying the airplane - stall speeds, flap limits, and structural limits are all based on IAS.
When ATC says "reduce speed to 210 knots," the pilot sets 210 on their airspeed indicator. Done.
At sea level on a calm day, IAS roughly equals the aircraft's actual speed through the air. At altitude, the air is thinner, so an aircraft indicating 250 knots might actually be moving through the air at 400+ knots (that's true airspeed, or TAS). At high altitude, pilots switch from IAS to Mach number for speed reference. But the pilot still flies by IAS because the aerodynamics care about airflow, not ground movement.
Ground speed (GS)
Ground speed is how fast the aircraft moves across the earth's surface. This is what shows up on the controller's radar scope. It's what determines how quickly an aircraft covers distance, how long until it reaches the next fix, and how much spacing you actually have between two targets on the screen.
Ground speed = true airspeed + wind effect.
Wind is the difference
A 737 cruising at 280 knots IAS (roughly 460 knots TAS at FL350) with a 100-knot headwind has a ground speed of 360 knots. That same aircraft with a 100-knot tailwind? Ground speed 560 knots. The pilot hasn't touched anything - same power, same IAS - but one is covering ground 55% faster than the other.
This matters for spacing. Two arrivals 10nm apart, both at 250 knots IAS, will maintain 10nm spacing in calm winds. Add a 40-knot headwind and they're both slower over the ground, but still maintaining the gap. Now give the lead aircraft a tailwind and the trailing one a headwind (which happens at altitude when wind shifts with position), and your 10nm gap starts closing fast.
What controllers see
Controllers see ground speed on the radar scope. The data block shows GS, and the target's movement across the screen reflects GS. You don't see IAS directly.
This creates a mental translation problem. You tell an aircraft to slow to 210 knots. They comply immediately - their IAS drops to 210. But what you see on the radar depends on the wind. In a strong headwind, 210 IAS might look like 170 knots ground speed. In a tailwind, that same 210 IAS could show 250 ground speed on your scope.
Why this matters for sequencing
Approach controllers live and die by this. When you're building a sequence of arrivals to the runway, you need consistent spacing. But you're issuing IAS commands while watching GS results.
A common trap: you've got two aircraft both assigned 210 knots, nicely spaced at 5nm. The lead one hits a headwind pocket and its ground speed drops to 180. The trailing one is still in calmer air at 200 ground speed. That 5nm gap is closing at 20 knots, which gives you roughly 7 minutes before they're on top of each other. Not a crisis, but you need to catch it.
Good controllers develop a feel for the wind. They watch which direction ground speeds are running and adjust their speed assignments accordingly.
In radarcontrol.io
Both IAS and ground speed are simulated. Wind layers at different altitudes affect ground speed realistically, so two aircraft at the same IAS will show different ground speeds depending on the wind at their altitude.
Speed commands (s210, s250) set indicated airspeed, not ground speed. This matches real-world operations. The ground speed you see on the radar data block reflects the wind effect.
The iasKts and gsKts properties are both available in the aircraft data if you're writing scripts. You can use ground speed differences to detect spacing problems early.
One thing to keep in mind: the sim's wind model uses layers that vary by altitude but are consistent across the airspace at any given altitude. Real wind has more local variation (terrain effects, jet stream boundaries), but the core relationship between IAS, wind, and GS works the same way.
Related: What is a flight level? | How arrivals are sequenced | What is a METAR?
Guides: Command reference | Arrivals guide
Play Chicago Center - watch how wind affects ground speed across the midwest.