APPLICATION OF LASERS IN RADAR SYSTEMS

ABSTRACT

The article describes the use of lasers in radar. Long range with relatively low power consumption, high accuracy of measuring range and angular coordinates, low noise in receivers, difficulty in creating interference, small size and weight, all this makes the use of laser radar systems promising. Especially promising is laser location in space for tracking satellites, for radar of planets.

АННОТАЦИЯ

В статье описано использование лазеров в радиолокации. Большая дальность действия при относительно низком энергопотреблении, высокая точность измерения дальности и угловых координат, низкий уровень шумов в приемниках, сложность создания помех, малые габариты и масса - все это делает перспективным использование лазерных радиолокационных систем. Особенно перспективна лазерная локация в космосе для слежения за спутниками, для радиолокации планет.

Keywords: radar system, laser, noise, location, communication, navigation.

Ключевые слова: радиолокационная система, лазер, помехи, локация, связь, навигация.

One of the important parameters of the optical location system is the signal-to-noise ratio. At optical frequencies, the radiative noise of the external environment is of great importance. The amount of noise varies depending on the time of day and the weather. Communication is greatly influenced by the radiation of the sun and stars. This effect is especially noticeable in location and navigation systems using the signals of optical quantum generators.

The main advantages of laser radar systems are as follows: long range with relatively low power consumption, high accuracy in measuring range and angular coordinates, low noise in receivers, difficulty in creating interference, small size and weight. All this ensures the prospects for the use of optical radar systems. Especially promising is optical location in space when tracking satellites, for radar of planets, etc. The radar system for determining the distance to the target contains a laser transmitter, a trigger mechanism, an optical receiver with a filter of monochromatic light reflected from the target; a reader associated with an optical receiver and a trigger device.

Figure 1. Scheme of the optical locator

Figure 1 shows a diagram of an optical radar system. Laser 1 is a rod 2 made of an active substance, for example, ruby. The rod is surrounded by a gas-discharge lamp 3, which receives pulses from the pump energy source 4. Synchronizer 5 activates the source 4, which ignites the lamp 3, as a result of which the laser emits a beam 6 of coherent light towards the target. The synchronizer also provides a horizontal sweep of the beams of two oscilloscopes 7 and 8 - the system's readers. The output beam of the laser is fixed by the detector 9, which is connected to the oscilloscope 7. A pulse 10 appears on the oscilloscope, corresponding to the moment of transmission of the output laser pulse. The beam 6 of the laser is reflected from the target 11 and after some time is received by the optical receiver 12. The beam 13 reflected from the target hits the parabolic reflector 14 and is focused in the photocell 15. The photocell is connected to the oscilloscope 8, which registers the light pulse received from the target. The time difference between pulses 10 and 16 on both oscilloscopes is a measure of the distance from the system to target 11. An improved radar system is proposed. It allows you to detect moving objects, accurately measure the distance to them, the angular coordinates and the speed of their movement.

The optical locator (Figure 2, a) consists of a transmitting part, which includes a laser 1 and a deflection system 2, which produces a mechanical or electrical discontinuous sweep of the laser beam.

Figure 2. Improved optical range radar system

The deflected beam passes through the optical system 5 and surveys space in azimuth and elevation. The transmission of a light signal is not continuous, and the beginning of the emission of each pulse occurs at a strictly defined moment in time. To this end, during transmission, the modulator interrupts the light for the time that the deflecting device needs to change the position of the beam in space. This allows you to accurately measure the moment of return of the reflected beam and, therefore, the distance to the target. Electronic deflection of the beam can be carried out, for example, using an ultrasonic cell or in another way. The return beam reflected by different points of the view area is received by the optical system 4 and then mixed in the mixer 5 with the optical radiation of the laser 6. The mixer creates a light beam whose center frequency is equal to the transmission frequency and the envelope frequency is equal to the difference between the transmitted and received frequencies by the receiver. The beat signal only appears if the beam comes from a target that has a certain radial velocity with respect to the locator. The frequency of this signal is proportional to the Doppler frequency of the object and hence to the radial velocity. Device 7 deflects the beam from the output of the mixer simultaneously with the sweep so that the receiving device receives only one beam reflected from the target. Such a device eliminates interference created by the sun when illuminating the viewing area. Device 7, which ensures the selection of useful signals that carry information during reception, is at the input of the photomultiplier. The interference suppression system (Figure 2, b) consists of a photocathode 1 and a photomultiplier 2, which amplifies the electron beam and creates a signal at the output. The signal amplitude is proportional to the energy of the received light beam. The system also contains a device 3 that causes the deflection of the electron beam, and a screen 4 impervious to electrons with a hole 5. The deflection of the electron beam is regulated simultaneously with the sweep carried out during reception so that at the moment corresponding to a strictly defined direction, sighting, only a part of the electron beam, obtained from the reflected signals, was deflected to the hole and transferred to the photomultiplier. The device that causes the deflection is electrically controlled, for example by changing the voltage on the electrodes of the deflecting system. Photomultiplier 8 (Figure 2, a) at the output creates an electrical signal, the frequency of which is equal to the beat frequency at the output of mixer 5 (Figure 2, b) and, therefore, is proportional to the target speed. This signal is then sent to three special devices of the system 6, 9, 10. Device 10, which performs coarse filtering of the signal frequency, transmits it to oscilloscope 1 through various output channels, in accordance with the frequency range in which it is located. Device 10 consists of three filters whose passbands are contiguous and cover a common range of frequencies occurring according to the target's velocity range. The signal coming from the target, the speed of which is outside this range, is practically suppressed by the filter system. The outputs of the three filters are connected to the inputs corresponding to different colors of the multicolor oscilloscope 11 beam, for example, three-color. An image of the observed zone is obtained on the oscilloscope, while the screen is scanned in such a way that the points depicting the observed targets give the relative angular coordinates of these targets. Dots of different colors correspond to different target speeds. Targets with too slow or too fast speeds do not appear on the oscilloscope screen. At the same time, the electrical signal from the photomultiplier is fed to systems 6 and 9, which measure the range and angular coordinates of the target, located on the oscilloscope screen, as well as the speed. Range measurement is performed in the manner described above. The speed is measured by a device that consists of filters, the common input of which is supplied with an electrical signal from a photomultiplier. The filters have very narrow passbands and adjacent edges, with the set of passbands covering the same frequency range as the set of three filters. These filters separate the input signal according to its frequency, which allows you to determine the speed of the target. The accuracy obtained from this speed measurement is determined by the bandwidth of each filter. The Doppler frequencies obtained using the considered optical radar are quite high even at relatively low target speeds. For example, at a wavelength of 1 micron, the Doppler frequencies of a target whose radial velocity is in the range of 3.5–110 km/h fluctuate from 2 to 60 MHz. In a radar operating at a wavelength of 0.1 m, the Doppler frequencies obtained at the same target speeds fluctuate in the range of 20-600 Hz. Increasing target Doppler frequencies improves radar performance. This is one of the main advantages of such an optical radar compared to conventional radars.

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