Airspeed
You should always be aware that Pro Controller displays only ground speed for aircraft. It is important to keep that in mind before reminding a pilot of speed restrictions you have issued. Obviously, in a no-wind situation, the pilot's indicated airspeed will be lower than what you see on your display. Especially at higher altitudes, it is up to you to compensate for the difference, and not assign an unreasonable airspeed restriction.

Indicated airspeed
The aircraft's airspeed indicator depends on the effect of air being forced into a small tube mounted on the outside of the aircraft (pitot tube). This airspeed indication is affected by the density of the air, which changes with altitude and ambient air temperature. As the aircraft climbs, the air becomes less dense, and the airspeed indicator shows a lower speed than the aircraft is actually moving through the air.

Most pilots rely on indicated airspeed to control the aircraft, because it is a true representation of how the aircraft will behave in respect to stall speeds and other known aircraft characteristics.

True airspeed
True airspeed is the speed the aircraft is moving through the air. It has no relation to winds. In some aircraft, a true airspeed indicator is provided along with the normal "indicated airspeed" gauge. Most pilots estimate their true airspeed before a flight, and calculate the actual true airspeed during the flight. This data is needed for flight planning purposes. It is used along with wind data to arrive at - - - you guessed it, ground speed.

Ground speed
While true airspeed is the speed an aircraft moves through the air with no regard to the wind, ground speed is the speed the aircraft is moving over the ground. That said, just remember that the speed you see on your radar is ground speed, and the speed the pilot normally sees is indicated airspeed. Your speed will usually show a higher value than the pilot's, depending on his altitude, unless he has a strong headwind. Unfortunately, some pilots display true airspeed instead of ground speed because it is one of the FS options. Be aware of that possibility when an aircraft appears to slow considerably below an assigned airspeed restriction.

SPEED RESTRICTIONS
In Turkey an airspeed restriction applies of a maximum of 250 knots KIAS below FL100. Further, you may not operate an aircraft at or below 2500 feet above the surface within 4 nautical miles of the primary airport of a Class C or Class D airspace area at a speed greater then 200 knots KIAS, HOWEVER this restriction may be lifted as soon practical by the Controller.

For example if its quiet and there are no traffic conflicts, it may be desirable to lift the restriction for a departing aircraft, once its established on its SID. By the same token Controllers are advised to keep the restriction in force to ensure separation of traffic at busier times or locations.

Holding Speeds
See this section regarding holdings.