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How the GPS works?

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How the GPS works?

Mensagem  joseflor em Sab 25 Jul 2009 - 10:52

How the Global Positioning System works

How the Global Positioning System works
Like many of today’s technology breakthroughs, GPS was originally a military system. Initially four NAVSTAR satellites, the first launched in 1978, formed the backbone of system. As satellites go, they aren’t very big: about 1.5m wide and 5m long. In orbit (17,450Km out), they weigh only 850Kg.
Each satellite contains four extremely accurate atomic clocks (one second in three million years!). This time information and satellite identification is transmitted on two L-band carriers around 1.575GHz.
Today there are 32 of these satellites, which provide coverage to every point on the planet. At least three satellites would normally be “visible” from anywhere; more important areas have up to twelve satellites available from which to obtain data.
Because the exact position of each satellite is known at any instant in time, a GPS receiver on the ground (or in the air, or at sea) can work out precisely how far away that satellite is by comparing the time-stamped transmitted signal to the time it actually received that signal.
Doing the same thing with the signal from a second satellite enables the GPS receiver to determine its position between the two. Adding a third signal enables a location to be established; i.e. a three-dimensional “fix”.
And adding a fourth signal (or more) enables errors to be virtually eliminated, giving even more accuracy.
Design accuracy is within 30m of true position. Until 2000, accuracy for civilian and non-US-aly users was only 100m because of “selective availability” or SA errors, deliberately introduced into the system to make it more difficult for non-friendly armed forces to use.
But, former US President Clinton ordered SA be removed on the 1st May 2000, to allow all users access to the military-precision signal.
Achieved accuracy is usually better then 30m and often significantly better – most vehicle GPS systems can show exactly where a vehicle is on the road to within a few metres, an accuracy of at least 5m or even better. That’s not too bad from 17,450Km away!
The GPS system is fairly unaffected by weather; rain and cloud generally have little impact but wet foliage and even dense tree cover can cause problems.
During the “Desert Storm” war in the Middle East GPS was used extensively to obtain positions in completely featureless desert and often in blinding sandstorms. In fact, GPS has been credited with having a decisive role in the UN forces’ success.
Most of today’s GPS receivers required an initial “fix” from no more then three satellites to establish their position. Once the signal is received and position determined. Therefore it is ideal in very poor signals areas.
It can take almost a minute to receive and analyze enough signals to determine position from a “cold start”. Once the receiver knows where it is, a “hot start” gives a position in about eight seconds. While operating, the information is updated about every 100ms.
While the majority of GPS units are fully self-contained, somehave the ability to output data for recording, further analysis, etc. the output from the module is data in the form of NMEA-0183 sentences. NMEA stands for the National Marine Electronics Association for all GPS data output. An NMEA sentence contains an address field, a data field and a checksum.
Within the data field can be such information as latitude and longitude, north or south of the equator, east or west of 0 degree meridian, speed over ground in knots, course over ground in degrees true, the data and time, and whether the data is valid or not.
By the way, the reason that the exact position of the GPS satellite is always known is that they themselves use signals from the others satellites to exactly determine their own position.
And positioning is not the only use for GPS: its highly accurate time signals are used in a huge variety of applications worldwide.

Photo comment:
Basic Positioning (simplified to one plane only): if the GPS receiver (at point A) knows it is a certain time away from the red satellite, it must be somewhere on the red circle. Similarly, if it also knows it is a certain time way from the blue satellite, it can only be where the red and blue circles intersect (point A & C). If a third (green) satellite is added, it can only be at point A. Once it knows it is at point A, even if the GPS receiver temporarily loses data from one or two satellites it knows it cannot be at point B, C or D so it takes its data from one satellite and works with that data until another comes into view. In the real GPS world, all of the circles are actually spheres, so the system operates in all three dimensions and can therefore give height.

Source: SC Jan. 2009

Última edição por joseflor em Seg 27 Jul 2009 - 12:57, editado 2 vez(es)
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How Atomic Clocks works

Mensagem  joseflor em Seg 27 Jul 2009 - 12:48

How Atomic Clocks works?
When the power goes out and is later restored, how do you know what time to set your clocks to? Have you ever wondered how time is regulated?
In the United States, the standard of time is regulated by the US Naval Observatory's Master Clock (USNO), the official source of time for the Department of Defense. The effects of these mechanisms are felt by all of us in the f­orm of alarm clocks, computers, answering machines and meeting schedules.

Are Atomic Clocks Radioactive?
Atomic clocks keep time better than any other clock. They even keep time better than the rotation of the Earth and the movement of the stars. Without atomic clocks, GPS navigation would be impossible, the Internet would not synchronize, and the position of the planets would not be known with enough accuracy for space probes and landers to be launched and monitored.
Atomic clocks are not radioactive. They do not rely on atomic decay. Rather, they have an oscillating mass and a spring, just like ordinary clocks.
The big difference between a standard clock in your home and an atomic clock is that the oscillation in an atomic clock is between the nucleus of an atom and the surrounding electrons. This oscillation is not exactly a parallel to the balance wheel and hairspring of a clockwork watch, but the fact is that both use oscillations to keep track of passing time. The oscillation frequencies within the atom are determined by the mass of the nucleus and the gravity and electrostatic "spring" between the positive charge on the nucleus and the electron cloud surrounding it.

What Are The Types of Atomic Clocks?
Today, though there are different types of atomic clocks, the principle behind all of them remains the same. The major difference is associated with the element used and the means of detecting when the energy level changes. The various types of atomic clocks include:
Cesium atomic clocks employ a beam of cesium atoms. The clock separates cesium atoms of different energy levels by magnetic field.
Hydrogen atomic clocks maintain hydrogen atoms at the required energy level in a container with walls of a special material so that the atoms don't lose their higher energy state too quickly.
Rubidium atomic clocks, the simplest and most compact of all, use a glass cell of rubidium gas that changes its absorption of light at the optical rubidium frequency when the surrounding microwave frequency is just right.
The most accurate atomic clocks available today use the cesium atom and the normal magnetic fields and detectors. In addition, the cesium atoms are stopped from zipping back and forth by laser beams, reducing small changes in frequency due to the Doppler effect.

How Does a Practical Cesium Atomic Clock Work?
Atoms have characteristic oscillation frequencies. Perhaps the most familiar frequency is the orange glow from the sodium in table salt if it is sprinkled on a flame. An atom will have many frequencies, some at radio wavelength, some in the visible spectrum, and some in between the two. Cesium 133 is the element most commonly chosen for atomic clocks.
To turn the cesium atomic resonance into an atomic clock, it is necessary to measure one of its transition or resonant frequencies accurately. This is normally done by locking a crystal oscillator to the principal microwave resonance of the cesium atom. This signal is in the microwave range of the radio spectrum, and just happens to be at the same sort of frequency as direct broadcast satellite signals. Engineers understand how to build equipment in this area of the spectrum in great detail.

To create a clock, cesium is first heated so that atoms boil off and pass down a tube maintained at a high vacuum. First they pass through a magnetic field that selects atoms of the right energy state; then they pass through an intense microwave field. The frequency of the microwave energy sweeps backward and forward within a narrow range of frequencies, so that at some point in each cycle it crosses the frequency of exactly 9,192,631,770 Hertz (Hz, or cycles per second). The range of the microwave generator is already close to this exact frequency, as it comes from an accurate crystal oscillator. When a cesium atom receives microwave energy at exactly the right frequency, it changes its energy state.
At the far end of the tube, another magnetic field separates out the atoms that have changed their energy state if the microwave field was at exactly the correct frequency. A detector at the end of the tube gives an output proportional to the number of cesium atoms striking it, and therefore peaks in output when the microwave frequency is exactly correct. This peak is then used to make the slight correction necessary to bring the crystal oscillator and hence the microwave field exactly on frequency. This locked frequency is then divided by 9,192,631,770 to give the familiar one pulse per second required by the real world.

When Was The Atomic Clock Invented?
In 1945, Columbia University physics professor Isidor Rabi suggested that a clock could be made from a technique he developed in the 1930s called atomic beam magnetic resonance. By 1949, the National Bureau of Standards (NBS, now the National Institute of Standards and Technology, NIST) announced the world’s first atomic clock using the ammonia molecule as the source of vibrations, and by 1952 it announced the first atomic clock using cesium atoms as the vibration source, NBS-1.
In 1955, the National Physical Laboratory in England built the first cesium-beam clock used as a calibration source. Over the next decade, more advanced forms of the clocks were created. In 1967, the 13th General Conference on Weights and Measures defined the SI second on the basis of vibrations of the cesium atom; the world’s time keeping system no longer had an astronomical basis at that point! NBS-4, the world’s most stable cesium clock, was completed in 1968, and was used into the 1990s as part of the NIST time system.
In 1999, NIST-F1 began operation with an uncertainty of 1.7 parts in 10 to the 15th power, or accuracy to about one second in 20 million years, making it the most accurate clock ever made (a distinction shared with a similar standard in Paris).

How Is Atomic Time Measured?
The correct frequency for the particular cesium resonance is now defined by international agreement as 9,192,631,770 Hz so that when divided by this number the output is exactly 1 Hz, or 1 cycle per second.
The long-term accuracy achievable by modern cesium atomic clocks (the most common type) is better than one second per one million years. Hydrogen atomic clocks show a better short-term (one week) accuracy, approximately 10 times the accuracy of cesium atomic clocks. Therefore, the atomic clocks have increased the accuracy of time measurement about one million times in comparison with the measurements carried out by means of astronomical techniques.
The National Company in Massachusetts produced the first commercial atomic clocks using cesium. Today, they are produced by various manufacturers, including Hewlett Packard, Frequency Electronics, and FTS. New technology continues to improve performance. The most accurate laboratory cesium atomic clocks are thousands of times better than commercially produced units.

Some Definitions

Atomic Clock - A precision clock that depends for its operation on an electrical oscillator regulated by the natural vibration frequencies of an atomic system (as a beam of cesium atoms)

Atom - The smallest particle of an element that can exist either alone or in combination; the atom is considered to be a source of vast potential energy

Cesium 133 - An isotope of cesium used especially in atomic clocks and one of whose atomic transitions is used as a scientific time standard

SI Second (atomic second) - The interval of time taken to complete 9,192,631,770 oscillations of the cesium 133 atom exposed to a suitable excitation

Chip-Scale Atomic Clock Unveiled by NIST
Chip-Scale Atomic Clock Unveiled by NIST

Source: How stuf works, Merriam-Webster Online, nist.gov
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