Railway track circuits

1. A railway track circuit apparatus comprising a track circuit transmitter and a track circuit receiver,

wherein the transmitter generates a QPSK modulated signal, which is transmitted into the track circuit, said signal carrying a digital message, and a unique track circuit identifier code, and

wherein the QPSK modulated signal is detected by the receiver, the receiver only indicating that the track circuit is clear having received a QPSK modulated signal of amplitude greater than a threshold and carrying the correct track circuit identifier code.

2. The apparatus according to claim 1 , wherein the QPSK signal is constrained to a narrow frequency band to produce a QPSK signal with a high form factor.

3. The apparatus according to claim 1 , wherein the QPSK modulated signal is a differential form of a QPSK modulated signal, i.e. a QDPSK modulated signal.

4. The apparatus according to claim 1 , wherein the receiver only indicates that the track circuit is clear having decoded the QPSK signal and checked that the sum of all phase coherent symbol amplitudes in the message is greater than a predefined threshold.

5. The apparatus according to claim 1 , wherein the data transmitted in the QPSK signal also carries internal transmitter information to the receiver.

6. The apparatus according to claim 5 , wherein the internal transmitter data indicates the current transmitter output amplitude, which is used by the receiver to determine signal attenuation along the track circuit.

7. The apparatus according to claim 1 , wherein data transmitted in the QPSK signal can be supplied to the transmitter from an external system, transmitted along the track circuit and received by the track circuit receiver, which outputs the data to an external system.

8. The apparatus according to claim 1 , wherein for track to train communication, the QPSK signal is also receivable by a train-borne receiver.