PURPOSE: To detect an antenna tracking error by an inexpensive system using one receiver and with simplified constitution.

CONSTITUTION: An electromagnetic wave generated from a wave generating source is received by four feed horns 1-4 arranged on orthogonally intersecting two axes, and the output of them are switched by a switch 7 and a hybrid 8, and they are amplified and frequency-converted by one receiver 9. Then, they are detected by envelope detectors 10-13 by making synchronize with the switching timing of the switch 7. After that, error voltages are generated by subtractors 14, 15 and integrators 16, 17.