Any aircraft can be tracked by radar at anytime providing that there is radar coverage in that area and that the plane is at an altitude to be ‘seen’ by radar. This radar works like a speed trap police radar gun. It sends out a signal, it bounces off the target and returns to the source. The transmitter knows the speed of radio waves it can measure the change of frequency cause by the movement of the vehicle and record this in terms of speed. Or it could record the information in terms of vehicle position. Radar systems are refined for each of those purposes.
Some drivers have a device that can detect these signals from the a police radar trap and warn them that they are under surveillance. The plane can, and in commercial airspace, is required, to have a transmitter that gives a discrete signal that says ‘this is me … flight number 123 … I’m at a height of 39000’ .
So like the policeman and the speedster each can transmit and detect signals. The pilot doesn’t have to do anything. Before take off he sets the code he’s given so wherever he is the information will be transmitted to anyone who can detect it. This helps air traffic control to identify individual aircraft. This transmitter should be on throughout the flight. It has no influence or bearing on the operating safety of the plane itself … just information to air Traffic Control.
So if it’s on or off doesn’t cause the flight to be unsafe … it doesn’t affect how the plane flies.
The ground radar jus on all the time and can see the planes within its range … as long as they are high enough. Different radar systems have different uses so the one covering the approach and landing is technically different from the long range ones. They all ‘identify’ and plot positions of planes, and keep them apart.
Civilian radar cover the areas that civil planes fly in … along the airways.
Military radar looks everywhere for everything in case of attack.
Here is the dilemma, even if civilian radar lost radar contact with the plane isn’t it curious that military hasn’t ‘seen’ it?
The ACARS and telemetry services from a plane send different information and from which it’s possible to calculate the position of the plane (possibly).
Engine monitoring from the ground will only be as good as the range of the system. These are areas outside my knowledge, but are broadly correct.
So loss of radio (voice communication) is different from loss of radar contact (plotting position) and is different from radar identification (ATC information).
You can imagine how confused the press must be if they don’t understand these important differences.
Here is a link to a very interesting video, showing speeded up air traffic movements. “SafeAir 123 You are cleared for take off.”