Transponder beacon for railway doppler radar - transmits messages by returning signals with amplitude modulation by subcarrier

摘要

A Doppler radar, operating at appr. 35GHz, is used on a rail vehicle, to measure distance travelled by integrating the Doppler signal. It can also receive messages from transponder beacons near the track. In the beacon, a subcarrier at appr. 100MHz is modulated by the message to be sent. 35GHz is received, via a directional antenna, from the radar and fed to a mixer where it is amplitude modulated by the subcarrier. The mixer output is re-radiated back to the radar as 35GHz with two sidebands at + and - 100MHz. The radar contains a band-stop filter which suppresses one sideband. The output of the radar receiver thus consists of the Doppler signal plus the 100MHz modulated subcarrier. These are separated by filters and processed independently.