Positive control is used in areas which have radar and so is commonly referred to as radar control. The controller "sees" the airplanes in the control area and uses VHF voice to provide instructions to the flight crews to ensure separation. Because the position of the aircraft is updated frequently and VHF voice contact timely, separation standards (the distance by which one aircraft must be separated from another) are less. This is because the air traffic controller can recognize problems and issue corrective directions to multiple airplanes in a timely fashion. Separation standards are what determine the number of airplanes which can occupy a certain volume of airspace.