PURPOSE: To enable a query signal pulse from an own airplane to be prevented from being synchronized with a query signal pulse from another airplane equipped with the same airborne navigation device by adding a response signal pulse to the query signal pulse from the device on the own airplane, and jittering timing for the occurrence of the query signal pulse.

CONSTITUTION: A clock pulse (a) having the predetermined period is divided at a divider circuit 1, according to a plurality of division ratios, and rising pulses (b1) to (d1) are respectively outputted from three rising pulse detectors 2 on the basis of divided output of (b) to (d). In an OR circuit 3, a response pulse (e) from a ground TACAN station or DME station and extracted from a received wave (h) via a response pulse detector 5, is added to the aforesaid pulses (b1) to (d1), thereby obtaining a theoretical sum of pulses. According to the aforesaid construction, a random pulse train (f) different from a random pulse train generated in another airplane is outputted. In addition, a variable divider 4 counts variably the aforesaid pulse train (f), and can output a jitter pulse (g) not synchronized with a jitter pulse from another airplane for determining timing for a query pulse signal transmitted from an airborne navigation device.