Focused Practice

I. Answer the following questions:

1. What periodic phenomena has man used for time keeping?

2. What does the radionavigation accuracy depend on?

3. When was the first satellite navigation realized?

4. What was TRANSIT based on?

5. What is the remarkable navigational accuracy of satellite navigation systems due to?

6. When and where must space-borne AFSs maintain a high level of performance and stability?

II. Analyse the grammar structures underlined in the above text.

III. Speak on: The remarkable navigational accuracy of satellite navigation systems.

Unit 57 Grammar: The Adjective. Degrees of Comparison. The Infinitive Word List:

1. acronym	аббревиатура
2. unidirectional array	однонаправленная антенна
3. CRT-cathode-ray-lube	электроннолучевая трубка
4. horizontal line	развёртка
5. microwave frequency	сверхвысокая частота
6. to tilt	наклонять, поворачивать
7. returns	полезные сигналы, отраженные от цели
8. plan-position indicator (PPI)	контрольно-измерительный прибор (индикатор) кругового обзора
9. deflection coil	отклоняющая катушка
10. tracking algorithm	алгоритм слежения
11. phased array	фазированная антенная решетка
12. radio detection and ranging	нахождение цели и определение расстояния до неё с помощью радиоволн
13. reflection	помеховые сигналы, отраженные от местных предметов
14. pip	отметка, выброс на экране
15. PPI	программируемый периферийный интерфейс
16. Digital PPI Displays	цифровые PPI дисплеи

A Display System for Phased Array Radars

The principles of radar (an acronym formed from ‘radio detection and ranging’) were first elaborated at the beginning of the 20th century. Early radar antennas were fixed, unidirectional arrays that gave only range information. The antenna transmitted radio frequency (RF) pulses at regular intervals, amplified the reflections and displayed them on a CRT-based instrument similar to an oscilloscope. This was called an A-scan display, and produced a horizontal line with ‘pips’ corresponding to the positions of targets. This system had several problems, all due to the low frequency of the radio signals used at the time: it required a very large antenna, which could not be transported or redirected; it could detect only large targets; and the accuracy of range information was poor.

The Second World War provoked a concentrated effort on radar research in Britain, the USA and elsewhere. Improvements in RF electronics led to the use of microwave frequencies. These could be transmitted and received by a parabolic antenna, which was smaller and more transportable. The new antenna could be rotated about a vertical axis and used to scan the entire horizon, and the axis could be tilted to scan at different heights.

The beam always emerges along the normal axis of the antenna and reflections return along the same axis. The radar receiver converts the reflections, or returns, and displays them on a CRT to give a plan view of the area around the antenna. This type of display is known as a PPI, or plan-position indicator.

In the first PPI displays, which were purely analogue, a bright axial line drawn by an electron beam appeared to rotate around the face of a cathode ray tube. The rotation of the line was controlled by electromagnetic coils, similar to the deflection coils in a television or monitor CRT, but fed by analogue waveforms supplied by sensors on the antenna mounting.

Digital computers were introduced into radar for various reasons. They could be used to process the returns in a more complicated way to reduce noise and clutter (reflections from insignificant objects). They could be programmed with tracking algorithms that could display identification symbols over objects in the PPI display. They could draw maps over the PPI display, showing geographical information modified to suit the operator’s task.