Global positioning system

The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). Utilizing a constellation of at least 24 Medium Earth Orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its location, speed, direction, and time.

Other similar systems are the Russian GLONASS, the upcoming European Galileo positioning system, the proposed COMPASS navigation system of China, and IRNSS of India. Developed by the United States Department of Defense, GPS is officially named NAVSTAR GPS (Contrary to popular belief, NAVSTAR is not an acronym, but simply a name given by John Walsh, a key decision maker when it came to the budget for the GPS program).

The satellite constellation is managed by the United States Air Force 50th Space Wing. The cost of maintaining the system is approximately US$750 million per year, including the replacement of aging satellites, and research and development.

Following the shooting down of Korean Air Lines Flight 007 in 1983, President Ronald Reagan issued a directive making the system available for free for civilian use as a common good. Since then, GPS has become a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce, scientific uses, and hobbies such as geocaching.

GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.

Simplified method of operation. A typical GPS receiver calculates its position using the signals from four or more GPS satellites. Four satellites are needed since the process needs a very accurate local time, more accurate than any normal clock can provide, so the receiver internally solves for time as well as position. In other words, the receiver uses four measurements to solve for four variables: x, y, z, and t. These values are then turned into more user-friendly forms, such as latitude/longitude or location on a map, then displayed to the user.

Each GPS satellite has an atomic clock, and continually transmits messages containing the current time at the start of the message, parameters to calculate the location of the satellite (the ephemeris), and the general system health. The signals travel at the speed of light through outer space, and slightly slower through the atmosphere. The receiver uses the arrival time to compute the distance to each satellite, from which it determines the position of the receiver using geometry and trigonometry.

Although four satellites are required for normal operation, fewer may be needed in some special cases. If one variable is already known (for example, a sea- going ship knows its altitude is 0), a receiver can determine its position using only three satellites. Also, in practice, receivers use additional clues (doppler shift of satellite signals, last known position, dead reckoning, inertial navigation, and so on) to give degraded answers when fewer than four satellites are visible.

Notes

1. Global Positioning System (GPS) – система глобального позиционирования.

2. Global Navigation Satellite System (GNSS) – глобальная навигационная спутниковая система.

3. GLONASS (global navigation satellite system) – российский вариант глобальной (спутниковой) системы (радио)определения местоположения, система GLONASS.

4. Constellation – 1) созвездие; 2) совокупность, группа.

5. Geocaching – геокешинг, геокладоискательство (игра в "поиск кладов" с использованием GPS-навигаторов).

6. User-friendly – удобный для пользования.

7. Ephemeris – 1) эфемериды, таблицы положения небесных тел; 2) эфемеридная информация (напр. о координатах ИСЗ).

8. Doppler shift – доплеровский сдвиг частоты, доплеровская частота.

9. Dead reckoning – навигационное счисление (пути).

10. Inertial navigation – инерциальная навигация.

OPTICAL ENGINEERING

Optical engineering is the field of study that focuses on applications of optics. Optical engineers design components of optical instruments such as lenses, microscopes, telescopes, and other equipment that utilize the properties of light.

Other devices include optical sensors and measurement systems, lasers, fiber optic communication systems, optical disc systems (e.g. CD, DVD), etc.

Because optical engineers want to design and build devices that make light do something useful, they must understand and apply the science of optics in substantial detail, in order to know what is physically possible to achieve (physics and chemistry). However, they also must know what is practical in terms of available technology, materials, costs, design methods, etc.

As with other fields of engineering, computers are important to many (perhaps most) optical engineers. They are used with instruments, for simulation, in design, and for many other applications.

Engineers often use general computer tools such as spreadsheets and programming languages, and they make frequent use of specialized optical software designed specifically for their field. Optical engineering metrology uses optical methods to measure micro-vibrations with instruments like the laser speckle interferometer.

4,000 years ago there were some signs and indications that early optical engineers used optical applications. People who designed and built the Stonehenge3 and Pyramid of Cheops used basic optical engineering principles. These structures had a connection with the earth and sun.

These early engineers knew light travels in straight lines and understood the cycle of the seasons, which made these structures relative to the calendar and the compass. In 350 BC, Plato and Aristotle argued about the accurate nature of light. Plato thought vision was achieved by the discharge of optical beams from the eyes.

Aristotle believed vision is accomplished when particles from the object releases into the pupil of the eye.

In 300 BC, Euclid, who wrote and studied optics and geometry, wrote the book Optics, which heavily contributed to the study of the science of optics.

Optical engineering is the engineering discipline that focuses on the design of equipment and devices that function by using light. It is based on the science of optics, a field of physics that studies the properties and behaviors of visible light and its two nearest neighbors on the electromagnetic spectrum, infrared and ultraviolet.

The practice of optical engineering is ancient, and the use of mirrors, shaped and polished crystals, or containers of clear water for purposes such as magnification or focusing sunlight to start fires is more than 2,000 years old.

In modern times, this field is important to a very wide array of technologies, including optical instruments such as microscopes and binoculars, lasers, and many commonly used electronic and communication devices.

Some practical applications of optics can be done using a model of electromagnetic radiation based on classical physics. This is because the predictions of modern quantum mechanics diverge noticeably from classical mechanics only at the atomic or subatomic scale or under extremely unusual conditions such as nearabsolute zero temperatures.

Many modern optical technologies are based on how individual photons interact with atoms and particles, where the predictions of classical mechanics cease to be a useful approximation of reality, and so the science of quantum optics is necessary to understand and master these phenomena. Materials science is also important knowledge for optical engineering.

The design of many devices that use light to view or analyze objects involves optical engineering. Viewing instruments such as binoculars, telescopes, and microscopes use lenses and mirrors to magnify images, while corrective lenses for eyeglasses and contact lenses refract incoming light to compensate for defects in the wearer's vision.

Thus, their creation demands considerable scientific knowledge of how these optical components will affect incoming light. Successful optical lens design requires understanding of both how a lens composition, structure, and shape will affect the functioning of an optical device, and how a lens shape and materials will affect factors such as the device's mass, size, and distribution of weight, as well as its ability to operate in different conditions.

The design of devices called spectrometers cannot be done without optical engineering.

A spectrometer uses the properties of incoming photons to discover information about the chemical composition or other traits of the matter that the light has been emitted by or interacted with.

Spectrometers exist in a wide array of different types and are enormously important to modern science and industry, in applications ranging from identifying the composition of minerals to quality control in the metalworking industry to studying the motion of other galaxies.

Optical engineering is likewise essential to fiber-optic technology, which transmits information through cables using pulses of light instead of electricity.

Optical fibers are flexible materials that can be used as waveguides, materials that can guide the direction of light. They guide light as it travels by taking advantage of a phenomenon called total internal reflection, which keeps the light channeled down the core of the fiber.

The design of optical fibers requires an understanding of how light is refracted as it moves through different media, along with the refractive qualities of different materials.

Fiber-optics is essential to modern communication technologies, such as telephones, high-speed Internet, and cable television, due to their enormous capacity.

The design of lasers, which produce narrow beams of coherent light, also relies heavily on optical engineering. Lasers work by energetically exciting a material, called a gain medium, until it begins releasing energy in the form of photons.

Designing a working laser involves knowledge of both the quantum properties of light and of different materials that can be used as gain media in order to create photons with the qualities necessary for the laser's intended use and of how optical equipment such as lenses and mirrors can focus that light.

Laser technology is widely used in modern life. It is the basis for optical disk media formats such as CDs and DVDs, the detection technology LIDAR (light detection and ranging), and in many industrial applications.

Notes

1. Spreadsheet – электронная таблица.

2. Laser speckle interferometer – интерферометр с использованием лазерной спекл-структуры.

3. Stonehenge – Стоунхендж (один из самых больших и известных в мире кpомлехов; сооружён в 1900–1600 до н.э.; расположен близ Солсбери, графство Уилтшир).