Vaisala Oy

Eero Salasmaa

Henry Andersson

A method for measuring the velocity (v) of a moving target (A). Radio signals are transmitted by means of radio transmitter-receiver arrangements of both the measuring station (B) and the target station (A) and received at the opposite stations (A,B), the frequencies of which signal include Doppler shifts (fd) to be observed both at the measuring station (B) and in the measuring target (A). On the basis of the Doppler shifts (fd), the escape and/or approach velocity (v) of the target to be measured is determined relative to the measuring station (B). Radio signals are transmitted from the measuring station (B) and the target station (A), the frequencies of which signals deviate from each other by a relatively small frequency difference (fk) or by a Doppler frequency shift (fd), the absolute value of which small frequency difference remains at least as high as the absolute value of the largest expectable Doppler frequency shift (fd.sub.max). A difference frequency or difference frequencies (fb, fa) of the frequency including the Doppler shift (fd) and received at the measuring station (B) and the target station (A) and of the reference frequency of essentially the signal frequency range developed locally are formed in such a way that a low-frequency or possibly a zero-frequency signal at the target station is obtained as the difference frequency or frequencies (fb, fa). On the basis of the difference frequencies (fb, fa), the Doppler frequency shift (fd) is determined, by means of which the velocity (v) of the target (A) is obtained.

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Eero Salasmaa