Времена группы Perfect действительного залога(Active Voice)

Совершенные времена используются для выражения действия, совершившегося к настоящему моменту (Present Perfect), к определенному моменту в прошлом (Past Perfect) или действие, которое совершится к определенному моменту в будущем (Future Perfect). Используя времена группы Perfect, говорящий обращает внимание на результат, вытекающий из совершенного действия, а не на время его совершения.

Present Perfect образуется при помощи вспомогательного глагола to have в форме настоящего времени и III формы смыслового глагола (Participle II).

I have written my test. He has not (hasn’t) written the test yet. Have you written the test?

Past Perfect образуется при помощи вспомогательного глагола to have в форме прошедшего времени и III формы смыслового глагола (Participle II).

We had finished our work when he phoned. He had not (hadn’t) phoned before we finished. Had you come when he phoned?

Future Perfect образуется при помощи вспомогательного глагола to have в форме будущего времени III формы смыслового глагола (Participle II).

She will have read this book by the end of the week. I shall not haveread my book by this time. Will you have read your book?

Exercise. Запишите предложения в отрицательной и вопросительной форме.

1. Leo will have come by 10.

2. I had cooked supper when mother came from work.

3. He has been to New York this month.

4. They have just started the meeting.

5. We shall have tested the equipment by the end of the year.

Radar Components

Найдите в словаре и запишите в тетрадь транскрипцию данных слов. Прочитайте их вслух несколько раз.

Amplify, adequate, reliable, efficient, design, key, pseudo, cause, equate, false, cycle.

Words to be learnt

store- накапливать, сохранять

to amplify- усилить

carrier (wave) - несущая

bandwidth- полоса пропускания, полоса частот

reliable- надежный

to maintain- обслуживать, содержать в исправности

direct current (dc)- постоянный ток

either…or…- или…или…, либо…либо…

to cause- вызывать, являться причиной

in order to…- для того, чтобы…

to depend on (upon) - зависеть от…

average value- среднее значение

to consume- потреблять

to determine- определятьto

to exceed- превышать

false alarm- ложный аварийный сигнал

to ajust- регулировать, выверять, корректировать

to reduce- снижать, уменьшать

close to…- близко к…, вплотную

duration- длительность, продолжительность

Text

A practical radar system requires seven basic components.

1. Transmitter. The transmitter creates the radio wave to be sent and modulates it to form the pulse train. The transmitter must also amplify the signal to a high power level to provide adequate range. The source of the carrier wave could be a Klystron, Traveling Wave Tube (TWT) or Magnetron. Each has its own characteristics and limitations. The radar transmitter produces the short duration high-power RF pulses of energy that are radiated into space by the antenna. The radar transmitter is required to have the following technical and operating characteristics:

-the transmitter must have the ability to generate the required mean RF power and the required peak power

-the transmitter must have a suitable RF bandwidth.

-the transmitter must have a high RF stability to meet signal processing requirements

-the transmitter must be easily modulated to meet waveform design requirements.

-the transmitter must be efficient, reliable and easy to maintain and the life expectancy and cost of the output device must be acceptable.

One main type of transmitters is the keyed-oscillator type. In this transmitter one stage or tube, usually a magnetron produces the RF pulse. The oscillator tube is keyed by a high-power dc pulse of energy generated by a separate unit called demodulator. This transmitting system is called POT (Power Oscillator Transmitter). Radar units fitted with a POT are either non-coherent or pseudo-coherent. Power-Amplifier-Transmitters (PAT) is used in many recently developed radar sets. In this system the transmitting pulse is caused with a small performance

in a waveform generator. It is taken to the necessary power with an amplifier following (Amplitron, Klystron or Solid-State-Amplifier). Radar units fitted with a PAT are fully coherent in the majority of cases.

2. Receiver. The receiver is sensitive to the range of frequencies being transmitted and provides amplification of the returned signal. In order to provide the greatest range, the receiver must be very sensitive without introducing excessive noise. The ability to discern a received signal from background noise depends on the signal-to-noise ratio (S/N). The background noise is specified by an average value, called the noise-equivalent-power (NEP). This directly equates the noise to a detected power level so that it may be compared to the return.

In the receiver, S/N sets a threshold for detection which determines what will be displayed and what will not. In theory, if S/N = 1, then only returns with power equal to or greater than the background noise will be displayed. However, the noise is a statistical process and varies randomly. The NEP is just the average value of the noise. There will be times when the noise exceeds the threshold that is set by the receiver. Since this will be displayed and appear to be a legitimate target, it is called a false alarm. If the SNR is set too high, then there will be few false alarms, but some actual targets may not be displayed known as a miss). If SNR is set too low, then there will be many false alarms, or a high false alarm rate (FAR). Some receivers monitor the background and constantly adjust the SNR to maintain a constant false alarm rate, and therefore are called CFAR receivers.

Some common receiver features are:

a.) Pulse Integration. The receiver takes an average return strength over many pulses. Random events like noise will not occur in every pulse and therefore, when averaged, will have a reduced effect as compared to actual targets that will be in every pulse.

b.) Sensitivity Time Control (STC). This feature reduces the impact of returns from sea state. It reduces the minimum SNR of the receiver for a short duration immediately after each pulse is transmitted. The effect of adjusting the STC is to reduce the clutter on the display in the region directly around the transmitter. The greater the value of STC, the greater the range from the transmitter in which clutter will be removed. However, an excessive STC will blank out potential returns close to the transmitter.

c.) Fast Time Constant (FTC). This feature is designed to reduce the effect of long duration returns that come from rain. This processing requires that strength of the return signal must change quickly over it duration. Since rain occurs over and extended area, it will produce a long, steady return. The FTC processing will filter these returns out of the display. Only pulses that rise and fall quickly will be displayed. In technical terms, FTC is a differentiator, meaning it determines the rate of change in the signal, which it then uses to discriminate pulses which are not changing rapidly.

3. Power Supply. The power supply provides the electrical power for all the components. The largest consumer of power is the transmitter which may require several kW of average power. The actually power transmitted in the pulse may be much greater than 1 kW. The power supply only needs to be able to provide the average amount of power consumed, not the high power level during the actual pulse transmission. Energy can be stored, in a capacitor bank for instance, during the rest time. The stored energy then can be put into the pulse when transmitted, increasing the peak power. The peak power and the average power are related by the quantity called duty cycle, DC. Duty cycle is the fraction of each transmission cycle that the radar is actually transmitting.

4. Synchronizer. The synchronizer coordinates the timing for range determination. It regulates that rate at which pulses are sent (i.e. sets PRF) and resets the timing clock for range determination for each pulse. Signals from the synchronizer are sent simultaneously to the transmitter, which sends a new pulse, and to the display, which resets the return sweep.

5.Duplexer. This is a switch which alternately connects the transmitter or receiver to the antenna. Its purpose is to protect the receiver from the high power output of the transmitter. During the transmission of an outgoing pulse, the duplexer will be aligned to the transmitter for the duration of the pulse. After the pulse has been sent, the duplexer will align the antenna to the receiver. When the next pulse is sent, the duplexer will shift back to the transmitter. A duplexer is not required if the transmitted power is low.

6. Antenna. The antenna takes the radar pulse from the transmitter and puts it into the air. Furthermore, the antenna must focus the energy into a well-defined beam which increases the power and permits a determination of the direction of the target. The antenna must keep track of its own orientation which can be

accomplished by a synchro-transmitter. There are also antenna systems which do not physically move but are steered electronically.

7. Display. The display unit may take a variety of forms but in general is designed to present the received information to an operator. The most basic display type is called an A-scan (amplitude vs. Time delay). The vertical axis is the strength of the return and the horizontal axis is the time delay, or range. The A-scan provides no information about the direction of the target.

Задание I. Опишите функцию каждого из компонентов радара, правильно соединив части предложений

1. Radar transmitter…	…focuses the energy into a well- defined beam which determines the direction of the target.
2. Radar receiver…	…regulates the rate of the pulses and resets the timing clock for each pulse.
3. Power supply…	…amplifies the returned signal.
4. Syncronizer…	…connects the transmitter or the receiver to the antenna and protects the receiver from the high- power output of the transmitter.
5. Duplexer…	…provides all radar components with electrical power.
6. Antenna…	…presents the received information to the operator.
7. Display…	…sends the radio wave in the form of pulse.