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Atomic clocks in gps satellites

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We live in a world governed by precise time. In a world without greater accuracy many of today’s technological inventions would be impossible. We rely on high-precision clocks to navigate. GPS uses atomic clocks as its time base and all the satellites that are orbiting the world now have atomic clocks built into them. What sets atomic clock apart is that the inner rhythm of atom remains unchanged. Clocks that rely on their oscillation are the most accurate in the world. Specifically, a second was defined as the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state undisturbed by external fields.
This article details how GPS devices get their time more accurately using cesium atomic clock.

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Atomic clocks in gps satellites

  1. 1. G.NARAYANAMMA INSTITUTE OF TECHNOLOGY & SCIENCE (FOR WOMEN) TOPIC: ATOMIC CLOCKS IN GPS SATELLITES. (under SATELLITE COMMUNICATIONS) A.V. Sridevi I year in Bachelor’s of Engineering Electronics & Telematics G.Narayanamma Institute of Technology & Science Email- akondisridevi999@gmail.com
  2. 2. Abstract: We live in a world governed by precise time. In a world without greater accuracy many of today’s technological inventions would be impossible. We rely on high-precision clocks to navigate. GPS uses atomic clocks as its time base and all the satellites that are orbiting the world now have atomic clocks built into them. What sets atomic clock apart is that the inner rhythm of atom remains unchanged. Clocks that rely on their oscillation are the most accurate in the world. Specifically, a second was defined as the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state undisturbed by external fields. This article details how GPS devices get their time more accurately using cesium atomic clock. Introduction: A)THE GLOBAL POSITIONING SYSTEM (GPS): The GPS is developed, by United States Department of Defense (DoD), to replace the TRANSIT1 system. It was designed to solve the need of continuous position, time and velocity, for the end-user, anywhere around the globe and at any time. Satellite constellation was designed to provide a continuous global positioning capability with at least four visible GPS satellites at all times. It was found that a set of 24 evenly spaced satellites, placed in nearly circular orbits with a radius of approximately 26,560 km, and inclined 55◦ to the equatorial plane would provide a global coverage. There are 6 orbital planes each consisting of 4 satellites. A GPS receiver's job is to locate four or more of these satellites, figure out the distance to each, and use this information to deduce its own location .This operation is based on a simple mathematical principle called trilateration. The GPS satellite launches started in 1978 with the first GPS satellite generation called Block I. Second-generation of GPS satellites were launched, beginning in 1989 and they were called Block II, when only in 1995 the system became fully operational. At present, the GPS constellation consists of 31 satellites (13 Block IIA, 12 Block IIR, 6 Block IIR-M). Today, the fifth generation of satellite, Block IIR-M, is being launched and future generations of satellites are already in development. (Pellerin,2006, ,USNO) B) ATOMIC CLOCKS An atomic clock is a clock that uses the resonance frequencies of atoms as its resonator. The resonator is "regulated by the frequency of the microwave electromagnetic radiation emitted or absorbed by the quantum transition (energy change) of an atom or
  3. 3. molecule." The advantage of this approach is that atoms resonate at extremely consistent frequencies. If you take any atom of cesium and ask it to resonate, it will resonate at exactly the same frequency as any other atom of cesium. Cesium-133 oscillates at 9,192,631,770 cycles per second. This sort of accuracy is completely different from the accuracy of a quartz clock . In a quartz clock, the quartz crystal is manufactured so that its oscillating frequency is close to some standard frequency; but manufacturing tolerances cause every crystal to be slightly different, and things like temperature will change the frequency. A cesium atom always resonates at the same known frequency, that is what makes atomic clocks so precise. The world’s smallest atomic clock. C) TIME SCALES: Unit of time, the second, is one of the fundamental units in International System of Units (SI) units system. The concept of measuring time, as part of generation of precisely synchronized signals aboard spacecraft and measurement of their transition time, is the heart of GPS. . A synchronization error of 1µs in a satellite clock will introduce an error of 300m in the pseudorange with a corresponding error in the position estimate. 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. WORKING PINCIPLE OF CESIUM ATOMIC CLOCK: In a cesium clock like these, liquid cesium is heated to a gaseous state in an oven. A hole in the oven allows the atoms
  4. 4. to escape at high speed. These particles pass between two electromagnets whose field causes the atoms to separate into two beams, depending on which spin energy state they are in. Those in the lower energy state pass through the ends of a U-shaped cavity in which they are irradiated by microwaves of 3.26-cm wavelength. The absorption of these microwaves excite transitions of many of the atoms from the lower to the higher energy state. The beam continues through another pair of electromagnets, whose field again divides up the beam. Those atoms in the higher energy state strike a hot wire, which ionizes them. Thereafter, a mass spectrometer selects only the cesium atoms from any impurities and directs them onto an electron multiplier. The frequency of the microwaves is adjusted until the electron multiplier output current is maximized, constituting the measurement of the atoms' resonance frequency. This frequency is electronically divided down and used in a feedback control circuit ("servo-loop") to keep a quartz crystal oscillator locked to a frequency of 5 megahertz (MHz), which is the actual output of the clock, along with a one-pulse-per-second signal. The entire apparatus is shielded from external magnetic fields. ATOMIC CLOCK & ITS IMPACT ON GPS SATELLITES: In addition to longitude, latitude, and altitude, the Global Positioning System (GPS) provides a critical fourth dimension – time. Each GPS satellite contains multiple atomic clocks that contribute very precise time data to the GPS signals. GPS receivers decode these signals, effectively synchronizing each receiver to the atomic clocks. This enables users to determine the time to within 100 billionths of a second, without the cost of owning and operating atomic clocks. The Global Positioning System (GPS) area is under continous development and its effects, on our daily life, increases rapidly. Due to technology improvement and the inquiring of more applications for precise GPS services (e.g. positioning, velocity, time synchronization or etc.), an investigation of the GPS source errors need to be done. One of the dominants error effect is GPS time synchronization, which is the reason each satellite carries in it an atomic clock. Although atomic clocks are very precise, they are not an ideal clocks, they also have clock errors. In Telecommunications network, every network element requires proper synchronization to minimize transport errors. Hence atomic clocks are used. But the accuracy of an atomic clock depends on two factors. The first factor is temperature of the sample atoms—colder atoms move much more slowly, allowing longer probe times. The second factor is the frequency and intrinsic width of the electronic transition. Higher frequencies and narrow lines increase the precision. ADVANTAGES OF ATOMIC CLOCKS IN GNSS: 1) Precise synchronization of communications systems, power grids, financial network and other critical
  5. 5. infrastructure. 2) More efficient use of limited radio spectrum by wireless networks. 3) Improved network management and optimization, making traceable time tags possible for financial transactions and billing. Commu PAADVANTAGES OF ATOMIC CLOCKS IN GPS SATELLITE: 1) Major investment banks use GPS to synchronize their network computers located around the world. Large and small businesses are turning to automated systems that can track, update, and manage multiple transactions made by a global network of customers, and these require accurate timing information available through GPS. 2) Power companies and utilities have fundamental requirements for time and frequency to enable efficient power transmission and distribution. Repeated power blackouts have demonstrated to power companies the need for improved time synchronization throughout the power grid. Analyses of these blackouts have led many companies to place GPS-based time synchronization devices in power plants and substations. By analyzing the precise timing of an electrical anomaly as it propagates through a grid, engineers can trace back the exact location of a power line break. 3) Instrumentation is another application that requires precise timing. Distributed networks of instruments that must work together to precisely measure common events require timing sources that can guarantee accuracy at several points. GPS-based timing works exceptionally well for any application in which precise timing is required by devices that are dispersed over wide geographic areas. EXAMPLES OF PRECISION TIMING USING GPS 1) The U.S. Federal Aviation Administration (FAA) uses GPS to synchronize reporting of hazardous weather from its 45 Terminal Doppler Weather Radars located throughout the United States. 2) New applications of GPS timing technology appear every day. Hollywood studios are incorporating GPS in their movie slates, allowing for unparalleled control of audio and video data, as well as multi-camera sequencing. FUTURE SCOPE Recent improvements in cesium clock include replacement of the state-selection magnets with laser beams, which can select and detect the required transition with greater efficiency and hence less noise from, the radiating atoms. Heralding a new age of terrific timekeeping, a research group led by a National Institute of Standards and Technology (NIST) physicist has unveiled an experimental strontium atomic clock that has set new world
  6. 6. records for both precision and stability. The world’s most accurate atomic clock based on neutral atoms has been demonstrated by physicists at JILA,a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder. The JILA strontium clock would neither gain nor lose a second in more than 200 million years. Bathed in red laser light at exactly the right frequency, strontium atomic clocks tick ‘430’ million times per second. As GPS becomes modernized, further benefits await users. REFERENCES:GPS among national l 1) http://www.gps.gov/applications/timing/ 2)http://en.wikipedia.org/wiki/Global_Positi oningSSystem. 3)http://www.kowoma.de/en/gps/satellites.ht m 4)http://www.livescience.com/37452-physici sts-tes- optical-atomic-clock-method.html

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