V.I. Kulakova – Ph.D.(Eng.), Head of Department - Main Product Engineer, LLC «Special Technology Center» (Saint Petersburg)
S.B. Makarov – Dr.Sc.(Eng.), Professor, Director of Institute of Physics, Nanotechnology and Telecommunications of Peter The Great St. Petersburg Polytechnic University
The paper presents the results of an experimental study of a synthetic aperture direction finder [1−3]. The problem of radio direction finding is solved by measuring a radio source signal on two small unmanned aerial vehicles (UAVs). Individual narrow-band stationary sources emitting in frequency range from 30 MHz to 3 GHz are considered. The aim of the paper is experimental study of the angular resolution values and systematic errors in the spatial angle achieved in real conditions in the entire range of operating frequencies. Section 1 of the paper presents general description of the direction finder and its accuracy. fig. 2 shows the block diagram of the di-rection finder. Its achievable accuracy depends on the duration of the aperture synthesis, the emitter wavelength, the values of the UAV travel speeds, as well as the trajectories of the UAV relative to the emitter. tabl. 1 shows the values of the required aperture synthesis time for a set of carrier frequencies Fc in order to achieve angular resolutions δθ = 0,5° and 0,1°, provided that only one UAV moves at speed of 30 m/s, and the viewing angle of the emitter is 90°. Section 2 lays out the major factors that affect achievable accuracy in real conditions. These factors include the accuracy of antenna phase center positioning, the influence of phase instability introduced into the signal in the receivers, as well as the accuracy of frequency synchronization of two receivers. The first problem is solved by integrated SINS/GNSS navigation system developed for high-precision antenna phase center positioning . The accuracy of the developed navigation system is sufficient to ensure conditions of coherent radio signal accumulation at the time intervals presented in tabl. 1 for δθ = 0,1° in the entire range of operating frequencies. For second problem the impact of phase noise of the receivers is investigated in stationary conditions. fig. 3 presents examples of the phase instability present in the trajectory signal when highly stable quartz oscillators are used. The results confirm that it is possible to synthesize apertures for desirable time intervals. The latter problem is related to the receiver tuning errors for a given frequency, which leads to errors in measuring the Doppler frequency in the emitter signal and, consequently, to systematic spatial errors of the direction finder. Frequency synchronization is performed using pulse-per-second signal from GNSS receiver. RMS of systematic error in the spatial angle due to the residual error in the frequency synchronization is 0,1° for the entire range of operating frequencies. The final section contains the results of flight experiments. During the first experiment two UAVs was used to find a radio source emitting a monochromatic signal at carrier frequency of 2200 MHz (fig. 4, tabl. 2). The duration of the synthesized aperture was 2.5 s. Emitter location was determined with an error of 16 m, which is 0.4% of the range to the nearest UAV. In the second experiment, a frequency-modulated signal generator at a frequency of 520 MHz was used (fig. 5, tabl. 3). The experiment involved three UAVs that located emitter in 80 s with an error of 12 m (the synthesis time was 12 s). During the third experiment, two UAVs were used to determine the location of a radio station that periodically emits a signal with single-band modulation at a carrier frequency of 35 MHz (fig. 6). The emitter location was determined with an error of 10 m (the synthesis time ranged from 2 to 34 s). The experimental results confirmed the high accuracy of the synthetic aperture direction finder.
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