European Train Control System
ETCS is a signalling system that relies on central interlocking issuing movement authorities (MAs) to drivers of trains via either trackside balises (in Level 1 which usually also has signals) or by radio (in Levels 2 or 3). The MA tells the driver how far he can travel (in the same way that a yellow aspect tells a driver he can only proceed as far as the next signal).
On-train equipment calculates how fast the train can go according to how long its MA is. Calculating the speed depends on factors such as gradient, train weight and braking performance.
In Level 2, trackside detection equipment such as track circuits or axle counters tells the central interlocking where the train is.
In Level 3 this is removed and the train calculates its own position. This can be by using odometers to measure distance travelled and balises placed on sleepers to provide fixed reference points. Attention is turning to use systems such as GPS to determine position.
Radio links come courtesy of GSM-R, but more advanced systems are being investigated to cope with the limited capacity available with GSM-R.
According to European definitions, when ETCS is linked with GSM-R it becomes ERTMS (European Rail Traffic Management System).