Focused Practice

I. Answer the following questions:

1. What did M. Minsky propose?

2. How was the sample stage scanned?

3. When was a confocal microscope developed by Mojmir Petran?

4. What was it based on?

5. What is image quality directly related to?

6. How will the optical signal that reaches the detector be determined?

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

III. Speak on: Detectors in confocal microscopes.

Unit 56 Grammar: The Perfect Tenses. The Subjunctive Mood Word List:

1. sidereal clock	звездные часы
2. Quartz Crystal Oscillator	кварцевый генератор
3. celestial navigation	астронавтика
4. radio beacon	радиомаяк
5. line- of-sight radio navigation signal	радионавигационный сигнал по линии прямой видимости
6. timing accuracy	точность отсчета
7. Doppler shift	эффект Допплера

A Historical Review of Atomic Frequency Standards Used in Space Systems

Since the dawn of civilization man has used a wide range of periodic phenomenon for time keeping from the apparent periodic motion of heavenly bodies (sidereal clocks) to the periodic internal motion of electrons in atoms (atomic clocks). In this long development, the advent of the Quartz Crystal Oscillators is a very important milestone. The “Information Revolution” would simply not have occurred without the ubiquitous Crystal Oscillators. Historically, man’s need for accurate time was driven by the demands of navigation in vast open seas which are devoid of any landmarks. Celestial navigation required accurate portable clocks. With the development of radios, another class of navigation aids -radio beacons e.g. Loran, was born. Yet another technology-artificial satellites - made possible the more precise, line-of-sight radio navigation signals. The accuracy of radionavigation is critically dependent on the synchronization and timing accuracy of the transmitter and the receiver of the radio signals. The first satellite navigation was realized in the 1960’s, with the US Navy’s Navigation Satellite System known as TRANSIT. The TRANSIT satellites carried on-board Quartz Crystal Oscillators (XOs) for stable and precise frequency generation. This system was based on the Doppler Shift of the received signal. TRANSIT is still operational though no new satellites will be added to the system.

The remarkable navigational accuracy currently enjoyed by the users of satellite navigation systems is in major part due to the excellent performance of the onboard atomic frequency standards (AFSs).

Space-borne AFSs must maintain a high level of performance and stability throughout the mission duration in the harsh environment of space. The mechanical design must be such that the AFSs withstand the enormous shocks and vibrations of the launch. The extremes of temperature that may be encountered in space require that the thermal and mechanical designs be such that the AFSs maintain excellent frequency stability over a broad range of temperatures.