Radar Echo Signature From Birds

Free–flying birds flight paths are traced with a high power tracking radar. The radar locks to the target, and the amplitude of the radar echo varies as the area of the target changes. This happens e.g. every wing beat. Hence, it is (at least theoretically) possible to estimate the wing–beat frequency of a tracked bird from the variation of the radar echo amplitude.

The Pico Technology ADC-200 is a high speed PC–based measuring instrument. Using the versatile and easy to use driver libraries supplied with the unit, we have developed a software that samples the radar echo signal.

Another external signal, derived from the tracking circuitry of the radar, is fed to the trigger input of the ADC200. The trigger frequency is locked to 84 Hz. Triggering occurs slightly before the maximum amplitude of the echo signal.

Upon each trigger input, the ADC200 takes 15 samples with 20 ns interval and transfers data to the PC. Later, the software extracts peak values of every sampling sequence. The peak values are then displayed as in figure 1, and frequency content is calculated with FFT analysis.

Figure 1. Raw data (wave) of about 4 seconds sampling — randomly chosen:

Figure 2. The fft chart of the data above. The wing beat frequency of this particular Swift (Apus apus) is on average 7.82 Hz: