MICROWAVE PHOTONIC METHOD FOR MEASURING DOPPLER SHIFT OF RADAR ECHO SIGNAL WITH TANDEM AMPLITUDE-PHASE MODULATION
Introduction. The Doppler shift (DS) of the microwave echo signal reflected from a moving object is fundamental parameter for determining the radial velocity and has an important role in radar and wireless communications. Besides DS, the direction of movement is also very important, according to which one can judge where the target is moving - towards the observer or vice versa. Microwave signals demanded in radar cover the frequency range from MHz to hundreds of GHz. However, the measurement of DS, implemented with the use of an electronic component base, can't cover such a large frequency range and, moreover, is extremely sensitive to electromagnetic interferences. Thanks to radio-photonic methods, the range and the accuracy of DS measurements are significantly increased. The aim of the work is to present and experimentally prove the new radiophotonic DS measuring tool. Method and experimental technique. There is presented a novel method for measuring the Doppler shift of a radar microwave echo signal reflected from a moving object, based on radio photonics technologies. The DS measuring tool has the same structure as the sequential radio-photon link with filtration and consists of a laser, a block of electro-optical modulators and a filter that is a fiber Bragg grating (FBG), and a photodetector (PD). The block of electro-optical modulators, in contrast to the known solutions based on a two-port Mach-Zehnder amplitude modulator, is built on two sub-units, consisting of tandem one-port amplitude and phase modulators connected in series. Thus, the measuring tool has a very simple structure and low cost, has a wide operating frequency range up to 40 GHz, determined by the bandwidth of the modulators and the FBG filter, wideband or narrowband PD depending on the type of the signal processing at the output, and high operational reliability due to simple procedures of calibrating and managing the temperature regimes of elements, especially if it is made on the basis of integrated photonic circuits. Research findings showed that the measurement of DS in the range ± 100 kHz at the sounding frequency of roughly 6.5 GHz can be implemented with an error of a fraction of a Hz and the error in measuring the velocity of the moving object of fractions of mm/s.