The Doppler-spectrum indicates how the echo power is distributed according to the Doppler-shift frequency. The Doppler-shift frequency is directly related to the radial velocity of the scatterer. A pulsed Doppler radar uses phase change of the echo between pulses to calculate the Doppler shift frequency and consequently only velocities that correspond to radial displacements smaller than a fourth of wavelength in pulse repetition period are unaliased. In the picture the unaliased velocity intervall is ±5 m/s.
The ranges of the 10 spectra displayed are shown in kilometers on the right
hand side of the picture. The velocity range of the spectra goes from 5 m/s
towards the radar at the left end and 5 m/s away from the radar at the right
end of the points
The relatively thin peaks in the spectra are caused by relatively large
insects.
According to Kauri Mikkola (Finnish Museum of Natural History)
the moth species involved was probably Agriopis aurantiaria.
The measurement lasted 0.34 seconds and the differences in
the speeds are mostly explained by the individual differences in the flight
directions and speeds of the insects. However, the calculated mean motion
using many pulse volumes is near the wind speed and direction.