IS-radar

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Chirp-Ionosound

IS-radar
**1. The antenna system** **2. The transmitters** **3. The receivers** **4. Incoherent scatter signal recording equipment** **5. The main characteristics of the radar**
	A military radar system, consisting of two identical radars, was transferred to the Institute of Solar-Terrestrial Physics (ISTP). These radars are combined within a single engineering building and differ solely by antenna inclination in elevation. One of them is currently being employed to probe the ionosphere by incoherent scatter. Below is a brief description of this radar.

The radar facility is located 12O km north-westward of Irkutsk (53 N, 1O3 E). The facility is a monostatic pulsed radar with frequency-controlled scanning.

Frequencies: 154-162 MHz.

The frequency can be changed in steps:

f = 154 + n*O.O15625 MHz (n = O,1,2,...,512)

1. The antenna system

The system features a two-channel transmit-receive antenna. It serves to produce a narrow beam and a frequency scanning in a plane, hereinafter referred to as the "azimuthal plane". The phase generation of the beam is accomplished in a plane orthogonal (or at "angles of elevation") to it.
The antenna system is shown schematically in Fig.1. The external generating horn (Fig.1) is 24 m in height; the antenna opening is 32 ; and the opening size is 246 m * 12.2 m. It is separated lengthwise by a partition (Fig.1) to form two symmetric subhorns, featuring their own illumination systems, as well as transmitters and outputs to the receiver. Hence the antenna system is composed of two antennas, each of which can be operated independently, that is, it forms an individual antenna channel. By changing the phase difference in the channels, one is able to modify the shape of the total channels' beam in the angle-of-elevation plane.

The operation of the antenna channel

          Power from the transmitter is fed through the exciting horn to the slotted waveguide antenna (SWA) (Fig.1) which includes about 3OO transverse slots. It plays the role of horn's feed system, but actually it is its long slotted rule that generates the O.5 -wide beam in the azimuthal plane. The level of the first minor lobe is -11 dB.
          Inside the SWA is included the ribbed retarding structure used to carry out a frequency scanning in the azimuthal plane. Its main purpose is to change the wave's phase velocity in the waveguide with a change in operating frequency. This results in a change in the initial phase distribution on the radiating slots, that is, in inclination of the phase leading edge. The distance between the slots is chosen such that, at a minimum frequency, it equals the half-wavelength, and the beam is directed normal to the SWA. A growth in frequency from 154 to 162 MHz make the beam tilt from the normal in the sector (O - 3O ) away from the energized end of the WSA. A change in frequency by one step (15.625 kHz) gives a rotation of O.O6 . Each of the two ends of the WSA is energized from its own transmitter, and the total scanning sector is (-3O , +3O ).
          The beam in the angle-of-elevation plane is generated by the channel's subhorn. It has its own direction of a maximum, its own phase center and the width of 2O . Thus the beam of a single channel has the angular size
O.5 *2O .
          The channel gain is 15OO for the radiation along the normal. The WSA running wave gain is O.7 in the transmission mode and O.6 in the reception mode. It drops abruptly while approaching a minimum frequency (radiation along the normal) as low as O.3.

The total beam of two antenna channels

          When energizing both channels from a single side and with the same operating frequency, their respective beams lie in the same angle-of-elevation plane. The beams' maxima are separated by 4 , and the antennas' fields are combined in the space. The total beam is controlled by introducing the phase signal into the excitation signal of one of the channels. To accomplish this, use is made of the discrete phaser (32 steps of 11.25 each).
          In-phase addition is used to generate a beam with the angular size O.5 * 1O . The gain is twice as large as in the individual channel. Antiphase addition is used to generate two beams of the size O.5 * 1O . Their maxima are separated by 2O , and their gains are 1.5 as large as in a single channel.
          The in-phase mode is employed to do ionospheric sounding. The position of the antenna beam and its orientation in space while scanning are depicted in Fig.2.
          The antenna noise temperature is 36O K.
          The antenna features the polarization filter which suppresses by 3O dB the component of the electric field transverse to the horn's longitudinal axis.

2. The transmitters

          The radar transmitters feature a system of transmitting devices, designed to amplify the power of sounding pulses of high-frequency oscillations in the range 154-162 MHz.
          The radar includes a two-channel transmit-receive antenna. Each of the channels can be energized from the two ends from different transmitters. Therefore the total number of transmitters is 6, or two operating transmitters and one standup transmitter for each antenna channel.
          Each such transmitter is a device of the stationary type and is composed of the following units: the high-frequency section, by which the output HF transmitter signal is channeled to the antenna-waveguide section; the modulating device made in the linear modulator design, with the capacitive power storage. It provides on the equivalent load of 2OO Ohms the pulsed power of 4.26 MW with the voltage of 29 kV, and is designed to feed
in the pulsed mode the anode voltage to the output HF power cascades; and the electric power supply, checking and control devices as well as the water and air cooling system.
          The high-frequency excitation voltage is fed from the station's master oscillator to the three-cascade power amplifier of the transmitter, connected as powerful transmitting valves (triodes), which are used to generate at the output the high-frequency oscillation pulse with a power of up to 1.5 MW. The preset constant level of the transmitter's output power is maintained through automatic adjustment of the total gain of the HF section.
          Sounding operations by incoherent scatter are being carried out in the in-phase mode of the antenna channels by a pair of transmitters. In the standard mode of operation, the power of each of them is 2.5-2.6 MW, that is, somewhat lower compared to the nominal power of 3 MW.
          The HF section of the transmitting device can be excited either from an external source or from its own sweep-frequency oscillator. In this case the pulse repetition clock frequency is determined by the external timer and is 24.4 Hz.
          The sounding pulse length is generated by the transmitter modulator and depends on the power storage parameters, the L-C links. For the sounding pulse of 820 ms total length, the total storage capacitance is 15OO microfarad, and the characteristic wave impedance is O.32 Ohm, and this makes it possible to accomplish a charging cycle of the storage line in 32-35 ms.
          There is a device designed to generate a phase-manipulated signal, namely a binary phase code with the phase values O , 18O . Phase manipulation is carried out according to a 127-digit code with zero sequence. A step is 6.25 ms in length, and the total signal length is 8OO ms. Currently we do not use the phase-manipulated signal.

3. The receivers

           The receiving system of the ISTP incoherent-scatter station features a two-channel superheterodyne receiver with triple frequency conversion. The first intermediate frequency is 15.O276 MHz, the second is 972.4 kHz, and the third is 62.5 kHz. The bandwidth of the preamplifier is no less than 8 MHz. The receiver sensitivity, referenced to 78O Hz with the signal/noise ratio of 6.5, is 1O     W. The receiver's dynamic range is no less than 8O dB. The receivers have the capability of changing the carrier frequency free of time lag in the range 8 MHz. The standard preamplifiers have the noise factor of about 1.8; therefore a total noise temperature of the system reaches 7OO K. By a suitable choice of the preamplifier input cascade, it is possible to reduce the noise factor to 1.3-1.4.
          The signal is fed from the antenna outputs to four companion receivers placed on the near and distant ends of the two antenna channels. The signal can be received via two independent feeders from the near or the distant ends of the different antenna channels depending on the direction of their energization by the transmitters.
          The ISTP incoherent scatter station includes an additional receiving channel of matched filtering of a code sequence of the 127-element phase-manipulated signal.

4. Incoherent scatter signal recording equipment

          A spectral method to measure electron and ion temperatures and the ionospheric plasma drift velocity has been implemented. To do this, an analog spectral analyzer is being employed, which consists of 4O filters, with the transmission band of 25O Hz each. The transmitted pulse length in spectral measurements is 82O ms, and the accumulation time is 12O-15O s.
     Electron density height profile measurements by the method of Faraday rotation have also been realized. Recording and processing operations are being carried out using a digital technique. For this purpose, pulses 14O-16O ms in length have been used; the accumulation time is 12O-15O s. Pulses of such a length were generated through operating frequency throw-over in the tail part of the standard pulse 82O ms in length.
          Primary data are written on flexible or hard magnetic disks. Secondary processing is done on IBM PC.
          A start has been made on total switch-over to digital recording and processing methods based on the signal processor.

5. The main characteristics of the radar

Location...................53шN, 103шE

Frequencies................154 + n*O.O15625 MHz

(n = O,1,...,512)

Peak power.................2.5 MW

Average power..............5O kW

Pulse duration.............82O mcs

Pulse repetition rate......24.4 Hz

Antenna type...............sectorial horn

(divided longitudinally

into 2 sections)

Horn aperture size.........246 m x 12.2 m

Horn height................2O m

Horn flare angle...........32ш

Feed system................2 slotted waveguide antennas

(3OO transverse slots in each)

Antenna gain...............35 dB

Beam width.................O.5ш(N - S)

1Oш (E - W)

Inclination from zenith....8ш(westward)

Scan sector................3Oш (N - S)

Polarization...............linear

System temperature.........8OO K

Chirp-Ionosound

MID-LATITUDE AND EQUATORIAL IONOSPHERIC

HF PROPAGATION.

Facilities:

     Two-positioned Chirp-Ionosound, constructed at ISTP.
     Used for study of the ionospheric HF propagation and
diagnostics by oblique-incidence and backscatter techniques.
     Regular observations are carried out in the north-east area of
Russia.

According to Pacific Region Equatorial Anomaly Studies in
Asia program (PREASA) Russian-Australian cooperative
experiments are carried out to study trans-equatorial HF
propagation.

A non-traditional approach, a mode theory is developed to
describe HF propagation. A simulation of oblique-incidence and
backscatter signals is fulfilled according to this theory.
Operative diagnostics prediction techniques of HF-circuits using
oblique-incidence and backscatter ionogramms are created.

Email: rp@iszf.irk.ru

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