About GPS
Global Positioning System
The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite Systems (GNSS). Using 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.
Developed by the Department of Defense, is the official name of NAVSTAR GPS (Contrary to popular belief, NAVSTAR is not an acronym, but simply a name given by Mr John Walsh, who makes the key decisions at the time of budget for the GPS program [1]). The satellite constellation is managed by the U.S. Air Force 50th Space Wing. The cost to keep the U.S. system is approximately $ 750 million a year, [2], including the replacement of aging satellites, and research and development. Despite these costs, GPS is free for civilian use as a public good.
GPS has become a widely used aid to navigation worldwide, and a tool useful for mapping, surveying, commerce and scientific applications. GPS also provides a precise time reference used in many applications, including the scientific study of earthquakes, and synchronization of telecommunications networks.
Simplified method of operation
A GPS receiver calculates its position by measuring the distance between itself and three or more GPS satellites. Measuring the time between transmission and reception of each signal GPS microwave gives the distance to each satellite, since the signal travels at a known speed – the speed of light. These signals also have information on location of satellites and the health system in general (known as almanac and ephemeris data). In determining the position and distance of at least three satellites, the receiver can calculate its position trilateration [3]. Receivers are usually not very accurate clocks and therefore track one or more additional satellites, using their atomic clocks to correct the receiver's own clock error.
[edit] Technical description
GPS satellite on display Unlaunched in the San Diego Aerospace Museum
Unlaunched GPS satellite on display at the San Diego Aerospace Museum
[edit] segmentation system
The current GPS consists of three main segments. These are the space segment (SS), a control segment (CS), and a user segment (U.S.) [4].
[edit] Space segment
The space segment (SS) consists of GPS satellites in orbit, and space vehicles (SV) in GPS language. The design GPS 24 SVS calls are distributed equally among six circular orbital planes. [5] The orbital planes are centered on Earth, which rotates with respect to distance stars. [6] The six planes have approximately 55 ° inclination (tilt regards Ecuador Earth) and are separated by 60 ° right ascension of the node upward (angle along the Ecuador from a reference point to the intersection of the orbit). [2]
In orbit at an altitude of approximately 20,200 km (12.600 nautical miles or 10,900 miles; orbital radius of 26,600 kilometers (16,500 miles or 14,400 nm)), each SV makes two complete orbits each sidereal day, so passes over the same location on Earth once each day. The orbits are arranged so that at least six satellites are always within line of sight almost the entire surface of the Earth [7].
In September 2007, there are 31 actively broadcasting satellites in the GPS constellation. Additional Satellites to improve the accuracy of GPS receiver calculations by redundant measurements. With the increasing number of satellites in the constellation was changed to a system not uniform. This agreement has been shown to improve reliability and system availability, in relation to a uniform system, when multiple satellites no [8].
[edit] Control Segment
The flight paths of the satellites are tracked by the U.S. monitoring stations of the Air Force in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, along with monitor stations operated by the National Geospatial-Intelligence Agency (NGA). [9] The tracking information is sent Control Station Space Command Air Force master Schriever Air Force Base in Colorado Springs, which is operated by the 2nd Squadron Operations Space (2 NTP) of the United States Air Force (USAF). 2 contacts each GPS satellite SOPs regularly with a navigational update (using the ground antennas on Ascension Island, Diego Garcia, Kwajalein and Colorado Springs). These updates synchronize the atomic clocks aboard the satellites to within one microsecond and adjust the ephemeris of each satellite orbital model interior. The updates are created by a Kalman filter which uses inputs from the ground control stations, the space weather, and several other inputs. [10]
GPS receivers come in a variety of formats, from devices integrated into cars, phones and watches, to dedicated devices such as those shown here from manufacturers Trimble, Garmin and Leica (left to right).
Receptors GPS comes in a variety of formats, from devices integrated into cars, phones and watches, to dedicated devices like the ones shown here for manufacturers Trimble, Garmin and Leica (left to right).
[edit] User segment
The user's GPS receiver is the user segment (U.S.) of GPS system. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a very stable clock (often a crystal oscillator). They may also include a display to provide the location and speed information to the user. A receiver is often described by its number channels: this signifies how many satellites can be controlled simultaneously. Originally limited to four or five years, which has been increasing in the recent years so that, from 2006, receivers typically have between twelve and twenty channels.
A typical module OEM GPS receiver based on SiRF chipset Star III, which measures 15 × 17 mm, and is used in many products.
A typical OEM GPS receiver module based on SiRF Star III chipset, which is 15 × 17 mm, and is used in many products.
GPS receivers may include an entry for the differential corrections, using the RTCM SC-104 format. This is typically in the form of an RS-232 to 4800 bit / s speed. The data is actually sent at a rate much lower, thus limiting the accuracy of the signal sent through RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. From 2006, including affordable units typically include Wide Area Augmentation System (WAAS) receptor.
Many GPS receivers can use position data to a PC or other device using the NMEA 0183 protocol. NMEA 2000 [11] is a new protocol and less widely adopted. Both are owned and controlled by the U.S. National Marine Electronics Association. References to the NMEA protocols have been compiled from records public, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols also exist, as the SiRF and MTK protocols. Receptors can interact with other devices using methods including a serial connection, USB or Bluetooth.
[edit] Navigation signals
Main article: GPS signals
Dissemination of the GPS signal
Broadcast signal GPS
Each GPS satellite continuously broadcasts a Navigation Message at 50 bit / s giving the time of day, GPS week number and health information satellite (all transmitted in the first part of the message), an ephemeris (transmitted in the second part of the message) and an almanac (the last part of the message). The ephemeris data provides precise orbit of the satellite itself and the output is over 18 seconds, repeating every 30 seconds. The ephemeris is updated every 2 hours and is generally valid for 4 hours, with provisions for 6 hours of standby time. The time needed to acquire the ephemeris is becoming a significant element the delay in fixing the position first, because, as hardware becomes more capable, the time to lock to the satellite signals shrinks, but the data ephemeris requires 30 seconds (worst case) before it is received, due to low data transmission rate. The almanac consists of the orbit and the information side state for each satellite in the constellation and takes 12 seconds for each satellite present, with a new satellite data that are transmitted every 30 seconds (15.5 minutes in 31 satellites). The purpose of the data is to assist in the acquisition of satellites in the ignition, allowing the receiver to generate a list of satellites visible on the basis of the storage location and time, while an ephemeris from each satellite is needed to calculate the position fix using that satellite. On older hardware, lack of an almanac in a new receiver might cause delay before providing a valid position, because the search for each satellite was a slow process. Advances in hardware have made the acquisition process much faster, so do not have a calendar is no longer a problem. An important thing to note on the navigation data is that each satellite transmits only its own ephemeris, but transmits an almanac for all satellites.
Each satellite transmits its navigation message with at least two different spread spectrum codes: the coarse / acquisition (C / A) code, which is freely available for the public, and the precise (P) code, which is usually encrypted and reserved for military applications. The C / A code is a pseudo-random chip 1023 (PRN) Code 1,023 million in chips / s so that is repeated every millisecond. Each satellite has its own C / A code so that it can be uniquely identified and received separately of the other satellites transmitting on the same frequency. The P-code is a 10.23 Megachip / sec PRN code that repeats only every week. When the "anti-" mode is spoofing, as it is in normal operation, the P code is encrypted Y-code to produce the P (Y) code, which can only be decrypted by units with a valid decryption key. Both the C / A and P (Y) codes to spread the exact time of day for the user. Frequencies used by GPS are
* L1 (1575.42 MHz): Mix of Navigation Message, coarse-(acquisition C / A) code and encrypted precision P (Y) code, plus the new L1C on future Block III satellites.
* L2 (1227.60 MHz): P (Y) code, plus the new L2C code on the Block IIR-M and new satellites.
* L3 (1.381,05 MHz): Used by the nuclear detonation (NUDET) Detection System Payload (NDS) to signal detection of nuclear detonations and other high-energy infrared events. Used to enforce nuclear test ban treaties.
* L4 (1379.913 MHz): is being studied for additional ionospheric correction.
* L5 (1176.45 MHz): Proposal for use as a civilian security of Life (SoL) signal (see GPS modernization). This frequency falls in a range of international protection for aeronautical navigation, promising little or no interference in all circumstances. The first Block IIF satellite that provide this signal is set to be released in 2008.
[edit] Calculation of positions
[edit] Using the C / A code
For starters, the receiver captures C / A codes to listen for the PRN number, based on information from almanac who has previously purchased. As it detects each satellite signal is identified by its distinct C / A code pattern, then measures the time delay for each satellite. For this, the receiver produces an identical C / A sequence using the number of seeds as the satellite. By lining up the two sequences, the receiver can measure the delay and calculate the distance to the satellite, called the pseudorange [12].
Pseudoranges overlapping, represented as curves, are modified to obtain the probable position
Pseudoranges overlapping, represented as curves, are modified to obtain the likely position
Next, data from the orbital position, or ephemeris, from the navigation message is downloaded to calculate the exact position of the satellite. A potentially more sensitive receiver will acquire ephemeris data faster than a less sensitive receiver, especially in a noisy environment. [13] Knowing the position and distance of a satellite indicates that the receiver is somewhere on the surface of an imaginary sphere centered on that satellite and whose radius is the distance to it. Recipients can replace altitude a satellite, the GPS receiver translates to a pseudorange measured from the center of the earth.
Points are not calculated in three dimensional space, but in four spacetime dimensions, which means a measurement of the precise time of day is very important. The measure pseudoranges from four satellites have already been determined by the clock internal receiver, and thus have an unknown amount of clock error. (The error of real time clock or no matter the initial pseudorange calculation, since it is based on how much time has elapsed between the receipt of each of the signals. [clarify] [citation needed]) The four-dimensional point that is equidistant from the pseudoranges are calculated as a guess as to the location of the receiver, and the factor used to adjust the pseudoranges to be cut into four-dimensional gives a guess as the receiver clock offset. With each guess, a geometric dilution of precision (GDoP) vector is calculated based on the sky of the positions used on satellites. As more satellites are collected, more combinations of pseudoranges from four satellites can be processed to add more responses to the location and clock offset. The receiver determines which combinations to use and how to calculate the estimated position by determining the weighted average of these positions and clock offsets. After the final location and time are calculated, the location is expressed in a given coordinate system, for example, the latitude and longitude using the WGS 84 datum or a local geodetic system specific country.
[edit] Using the P (Y) Code
Calculating a position with the P (Y) signal is generally similar in concept, assuming one can decipher it. Encryption is essentially a safety mechanism: if a signal can be successfully decrypted, it is reasonable to assume that is a real signal being sent by a GPS satellite. [edit] By comparison, civil receivers are highly vulnerable to spoofing since correctly formatted C / A signals can be generated using available signal generators. RAIM features do not protect counterfeiting, since RAIM only checks the signals from a navigation perspective.
[edit] Accuracy and error sources
The position calculated by a GPS receiver requires the current time, satellite position and measures the delay of the received signal. The position accuracy depends mainly the satellite position and signal delay.
To measure the delay, the receiver compares the bit sequence received from the satellite with a version generated internally. By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset within approximately 1% a bit time, or approximately 10 nanoseconds for the C / A code Since GPS signals propagate nearly at the speed of light, this represents an error of about 3 meters. This is the minimum error possible using only the GPS C / A signal.
Position accuracy can be improved by using the highest chiprate-P (Y) signal. Assuming the same 1% accuracy short time, the high frequency P (Y) signal results in an accuracy of about 30 centimeters.
Electronics errors are one of accuracy of various degrading effects outlined in the table below. When taken together, autonomous civilian GPS horizontal position fixes are typically an accuracy of about 15 meters (50 feet). These effects also reduce the more precise P (Y) code's accuracy.
User ranking sources equivalent errors (UERE) Effect Source
Ionospheric effects ± 5 meters
Ephemeris errors ± 2.5 meters
Clock Errors satellite ± 2 meters
Multipath distortion ± 1 meters
Effects of the troposphere ± 0.5 meters
Numerical errors ± 1 meters
[edit] Atmospheric effects
Inconsistencies of atmospheric conditions affect the speed of GPS signals passing through the atmosphere Earth and the ionosphere. Correcting these errors is a major challenge to improving GPS position accuracy. These effects are smaller when the satellite is directly overhead and become more satellites nearer the horizon since the signal is affected for a long time. Once the location Approximate receptor is known, a mathematical model can be used to calculate and compensate for these errors.
Because ionospheric delay affects the speed of microwave signals differently depending on the frequency-a characteristic known as dispersion, both frequency bands can be used to help reduce this error. Some military and expensive survey-grade civilian receivers compare the different delays in the L1 and L2 frequencies to measure atmospheric dispersion, and apply a correction more precisely. This can be done in civilian receivers without decrypting the P (Y) signal carried on L2, by tracking the carrier wave instead of the code modulated. To facilitate this on lower cost receivers, a new civilian code signal L2, called L2C, was added to the Block IIR-M satellite, launched in 2005. This allows a direct comparison of L1 and L2 signals in the coded signal instead of the carrier wave.
The effects of the ionosphere generally change slowly, and can be calculated over time. The effects of any particular geographical area can be readily calculated by comparing the GPS-measured position to a known the location surveyed. This correction is also valid for other receivers in the same general location. Several systems send this information through the radio or other links to allow L1 only receivers to make ionospheric corrections. Data is transmitted through the ionosphere from satellite systems based augmentation satellites such as WAAS, which transmits in the frequency of the GPS with a special pseudo-random number (PRN), so that only one antenna and receiver are required.
Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the troposphere. This effect is much more localized and changes more quickly than ionospheric effects and is not frequency dependent. These traits make precise measurement and compensation of humidity errors more difficult than ionospheric effects.
Changes in altitude also change the amount of delay because the signal passes through less of the atmosphere at higher elevations. Since the GPS receiver calculates its approximate altitude, this error is relatively easy to correct.
[edit] effects of
GPS signals can also be affected by multipath issues, where radio signals are reflected in the surrounding terrain, buildings, canyon walls, hard ground, etc. These delayed signals can cause inaccuracy. A variety of techniques, most notably narrow correlator space, have been developed to mitigate multipath errors. For multiple delays, the receiver can recognize the wayward signal and discard it. To make shorter versus multipath delay of the reflected signal on the ground, specialized antennas can be used to reduce the power of the signal received by the antenna. Reflections short term are more difficult to filter, because they interfere with the real signal, causing effects almost indistinguishable from routine fluctuations in atmospheric delay.
The effects are much less severe in moving vehicles. When the GPS antenna is moving, the false solutions using reflected signals quickly to converge and not just the direct result of signals in stable solutions.
[edit] Ephemeris and clock errors
The navigation message from a satellite is sent only every 30 seconds. In fact, the data contained in these messages are usually "out of date" by an even greater amount. Consider the case when a GPS satellite is boosted back into a proper orbit; for some time after the maneuver, the calculation of receiver satellite position will be incorrect until it receives another ephemeris update. The onboard clocks are extremely accurate, but suffer from a certain deviation of the clock. This problem tends to be very small but can add up to 2 meters (6 feet) of inaccuracy.
This kind of error is more "stable" that the problems of the ionosphere and tends to change over days or weeks rather than minutes. This makes correction fairly simple by sending a more accurate almanac on a separate channel.
[edit] Selective Availability
The GPS includes a feature called Selective Availability (SA) that introduces intentional, changing little bit random errors of up to one hundred meters (328 feet) navigation signal available to the public to confuse, eg, missile guidance throughout reach the objectives. The extra precision is available in the signal, but in an encrypted format that was available only for the U.S. military, their allies and some others, mostly government users.
SA typically added signal errors of up to 10 meters (32 feet) horizontally and 30 meters (98 feet) vertically. The inaccuracy of the civilian signal was deliberately encoded so as not to change very rapidly, eg, the entire eastern U.S. could read 30 m off, but 30 m from the front everywhere and in the same direction. To enhance the usefulness of GPS for civilian navigation, Differential GPS was used by many civilian GPS receivers to greatly improve accuracy.
During the Gulf War, the shortage of military GPS units and the wide availability of civilians between personnel resulted in a decision to disable Selective Availability. This was ironic, as SA had been introduced specifically for these situations, allowing friendly troops to use the signal for accurate navigation, while at the same time denying the enemy. But since SA was also denying the same accuracy to thousands of troops friends, turning off or setting it to an error of zero meters (same thing), presented a clear benefit.
In the 1990s, the FAA began pressing the military to turn SA permanently. This would save the FAA millions of dollars each year to maintain their own radio navigation systems. The resistance army for most of the 1990s, and ultimately took an executive order that SA have withdrawn from the GPS signal. The amount of error was added "to zero" [14] at midnight on 1 May 2000, following the announcement by U.S. President Bill Clinton, allowing users access to the error free L1 signal. On the directive, the induced error of SA was changed to add no error to public signals (C / A code). Selective Availability is still a system capability of GPS, and error could, in theory, be reintroduced at any time. In practice, given the risks and costs that this leads to the U.S. and other sending countries, are unlikely to be reintroduced, and several government agencies including the FAA, [15] have stated that it is not intended to be reintroduced.
The U.S. Army has developed the ability to deny GPS locally (and other navigation services) to hostile forces in an area specific crisis without affecting the rest of the world or their own military system [14].
An interesting side effect of the selective availability of hardware is the ability to correct the frequency of the GPS cesium and rubidium atomic clocks to an accuracy of about 2 × 10-13 (one in five trillion). This represented a significant improvement in the raw precision of watches. [Edit]
On 19 September 2007, the Department of Defense announced that no purchase more satellites capable of implementing SA. [16]
[edit] Relativity
According to the theory of relativity, due to his constant movement and height relative to the Earth-centered inertial reference system, the clocks of the satellites are affected by their speed (special relativity), and its gravity potential (general relativity). For GPS satellites, general relativity predicts that atomic clocks at GPS orbital altitudes tick more rapidly, at about of 45,900 nanoseconds (ns) per day, because they are in a weaker gravitational field than atomic clocks on Earth's surface. Special relativity predicts that atomic clocks moving at GPS orbital speeds tick more slowly than stationary ground clocks by about 7,200 ns per day. When combined, the difference is 38 microseconds per day, a difference of 4465 pieces in 1010. [17]. To account for this, the frequency standard onboard each satellite is given a rate of compensation, before launch, making it run slightly slower than the desired frequency on Earth, specifically, at 10.22999999543 MHz instead of 10.23 MHz [18].
GPS observation processing must also compensate another relativistic effect, the Sagnac effect. The GPS time scale is defined in an inertial system but observations are processed in an Earth centered, Earth-fixed (co-rotation), a system in which simultaneity is not uniquely defined. The Lorentz transformation between the two systems amending the runtime of the signal, a correction having opposite algebraic signs for satellites in the Eastern and Western Europe celestial hemispheres. Making ignore this effect fails east-west in the order of hundreds of nanoseconds, or tens of meters in position [19].
The atomic clocks on board GPS satellites are precisely tuned, making the system practical application in the application of scientific theory of relativity in an atmosphere the real world.
[edit] GPS interference and jamming
Since GPS signals at terrestrial receivers tend to be relatively weak, it is easy other sources of electromagnetic radiation to desensitize the receiver, which makes acquiring and tracking satellite signals difficult or impossible.
Solar flares are one such natural emissions, with the potential to degrade GPS reception, and their impact can affect reception over half Earth facing the sun. GPS signals can also be interfered with by naturally occurring geomagnetic storms, mainly near the poles of the magnetic field Earth. [20] Another source of trouble is the metal embedded in the windshields of several vehicles to avoid ice formation, degrading reception just inside the car.
Man-made interference can also disrupt, or jam, GPS signals. In a well documented case, an entire harbor was unable to receive GPS signals due to unintentional interference caused by a malfunctioning TV antenna preamplifier. [21] intentional interference is also possible. In general, stronger signals can interfere with GPS receivers when they are within radio range or line of sight. In 2002, a detailed description of how build a short range GPS L1 C / A disturbing was published in the online magazine Phrack [22].
The U.S. government considers that such interference were used occasionally during the 2001 war in Afghanistan and the U.S. armed forces said that to destroy a GPS jammer with a GPS-guided bomb during the Iraq war. [23] This clamp is relatively easy to detect and locate, making it an attractive target for anti-radiation missiles. The British Defense Ministry tested a jamming system in the west the country in the UK on 7 and 8 June 2007. [24]
Some countries allow the use of GPS repeaters to allow reception of GPS signals interior and hidden places, however, under the laws of EU and UK, the use of these is prohibited signals may cause interference to other receivers GPS can receive data from both GPS satellites and the repeater.
Due to the possibility that both natural and man-noise, numerous techniques continue to be developed to deal with interference. The first is to not rely on GPS as a single source. According to John Ruley, IFR, pilots must have a alternate plan in case of a GPS malfunction "[25]. RAIM (CAIR) is a feature now included in some receivers, which is designed to provide to alert the user if it detects interference or other problem. The U.S. military has also deployed their Selective Availability / Anti-Spoofing Module (SAASM) at the Defense Receiver Advanced GPS (DAGR). In demonstration videos, the DAGR is able to detect jamming and maintain its blockade of the encrypted GPS signals during interference which causes receptor civilians who lost lock [26].
[edit] Techniques to improve accuracy
[edit] Increased
Main article: Augmentation GNSS
Methods to improve the accuracy increase is based on external information being integrated into the calculation process. There are many such systems in place and are generally named or described on the basis of how the GPS sensor receives the information. Some systems transfer information about the error sources (such as clock drift, ephemeris, or ionospheric delay), others provide direct measurements of the amount of the signal was in the past, while a third group provide additional navigational or vehicle information to be integrated into the calculation process.
Examples of systems increase include the Wide Area Augmentation System, Differential GPS, Inertial Navigation Systems and Assisted GPS.
[edit] precise control
The accuracy a calculation can also be improved through precise monitoring and measurement of the existing GPS signals in a complementary or alternative.
After SA, who is disabled, the largest error in GPS is usually the unpredictable delay through the ionosphere. The parameters of the spacecraft broadcast ionospheric model, but errors persist. This is one reason the GPS spacecraft transmit on at least two frequencies, L1 and L2. Ionospheric delay is a well defined function of frequency and total electron content (TEC) along the path, so measuring the arrival time difference between the frequencies of the EC Treaty and therefore determines the delay accurate in the ionosphere each frequency.
Receivers with decryption keys can decode the P (Y)-code transmitted on both L1 and L2. However, these keys are reserved for the military and "authorized" agencies and are not available to the public. No keys, it is still possible to use a codeless technique to compare the P (Y) codes on L1 and L2 to gain much of the same error information. However, this technique is slow, so is currently limited to specialized surveying equipment. In the future, other civil codes are expected to be broadcast on the L2 and L5 frequencies (see GPS modernization, below). So all users will be able to perform dual-frequency measurements and directly compute ionospheric delay errors.
A second way to handle with accuracy improvement is called Carrier-Phase (CPGPS). The error, which corrects this, is because the pulse transition of the PRN is not instantaneous, and therefore the correlation (satellite receiver sequence of play) operation is imperfect. CPGPS's approach uses the L1 carrier wave, which has a period 1000 times smaller than that of the C / A bit period, to act as an additional clock signal and resolve the uncertainty. The phase difference error in the normal GPS amounts to between 2 and 3 meters (6 to 10 ft) of ambiguity. CPGPS working within 1% of the perfect transition reduces this error to 3 centimeters (1 inch) of ambiguity. Al eliminate this source of error CPGPS with DGPS normally realizes between 20 and 30 centimeters (8 to 12 inches) of absolute accuracy.
Relative Kinematics Positioning (RKP) is another approach to a GPS-based precision positioning system. In this approach, determining the signal range can be resolved with an accuracy of less than 10 centimeters (4 in). This is done to resolve the number of cycles where the signal is transmitted and received by the receiver. This can be achieved through a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and techniques for ambiguity resolution through statistical tests, possibly with processing in real time (real time kinematic positioning, RTK).
[edit] GPS time and date of
While most clocks are synchronized to Coordinated Universal Time (UTC), the atomic clocks of the satellites are set to GPS time. The difference is that GPS time is not corrected to match the rotation of the Earth, so it contains no leap seconds or other corrections which are periodically to UTC. GPS time was set to coincide with the Coordinated Universal Time (UTC) in 1980, but since then they parted. The lack of corrections means that GPS time remains in a constant offset (19 seconds) with International Atomic Time (TAI). Periodic corrections are made to correct the clocks on board the relativistic effects and keep them synchronized with ground clocks.
The GPS navigation message includes the difference between GPS time and UTC, which from 2006 is 14 seconds. Receivers subtracting the calibration of GPS time to calculate the UTC and specific timezone values. New GPS units not show the correct UTC time until after receiving UTC offset message. The GPS-UTC field can accommodate 255 seconds jump (eight bits) which, at the current rate of change of Earth's rotation, is sufficient to last until 2330.
Unlike the year, month, date and format of the Julian calendar, the GPS date is expressed as a number of the week and a day-by-number a week. The week number is transmitted as a ten-bit field in the C / A and P (Y) navigation messages, and thus becomes zero again every 1024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on 6 January 1980 and the week number became zero again for the first time 23:59:47 UTC on 21 August 1999 (00:00:19 TAI August 22, 1999). To determine the current Gregorian date, a GPS receiver must have an approximate date (within of 3584 days) to correctly translate the GPS date signal. To address this concern the messages to modernize GPS navigation using a 13-bit field, that only repeats every 8192 weeks (157 years), and will not return to zero until about 2137.
[edit] The GPS modernization
Item Home: GPS modernization
Having reached the program's requirements for Full Operational Capability (FOC) on July 17, 1995 [27], the GPS completed its original design goals. However, additional advances in technology and new demands on the existing system led to efforts to modernize the GPS system. Classifieds Vice President and the White House in 1998 initiated these changes, and in 2000 the U.S. Congress authorized the effort, referring to it as GPS III.
The objective Project to improve the accuracy and availability of all users and includes new ground stations, new satellites and four additional navigation signals. New civilian signals are called L2C, L5 and L1C; the new military code is called M-Code. Initial Operational Capability (IOC) of the L2C code is expected in 2008. [28] A the goals of 2013 is set for the whole program, with incentives offered to contractors if it can be completed in 2011.
[edit] Applications
The System Global Positioning, while originally a military project, is considered a dual-use technology, which means it has important applications for both military and civilian industry.
[edit] Military
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The military uses GPS for the following purposes:
[edit] Navigation
GPS allows soldiers to find objectives in the dark or in unfamiliar territory, and to coordinate the movement of troops and supplies.
[edit] target tracking
Various military weapons systems use GPS to track potential ground and air targets before they are flagged as hostile. These weapon systems pass GPS coordinates of targets to precision-guided missiles to participate in the targets precisely.
Military aircraft, in particular those used in air-ground roles use GPS to find targets (for example, gun camera video of AH-1 Cobras in Iraq show GPS coordinates that can be searched in Google Earth).
[edit] missiles and missile guidance
GPS allows accurate targeting of various military weapons, including intercontinental ballistic missiles, cruise missiles and precision guided missiles.
Artillery projectiles with embedded GPS receivers able withstand the forces of 12,000 G have been developed for use in shells of 155 mm [29].
[edit] Search and Rescue
Fallen pilots can be found faster if you have a GPS receiver.
[edit] Map of recognition and creation
The military use GPS extensively to aid maps and recognition.
[edit]
The GPS satellites also have nuclear detonation detectors, which form an important part of the U.S. nuclear detonation detection system [30].
[edit] Civil
See also: GPS Applications
This antenna is mounted on the roof of a hut containing a scientific experiment that require precise timing.
This antenna is mounted on the roof of a hut containing a scientific experiment that require precise timing.
Many civilian applications benefit from GPS signals, using one or more of the three basic components of GPS, absolute location, relative movement, the transfer time.
The ability to determine the absolute location receptor allows GPS receivers to function as a surveying tool or as an aid to navigation. The ability to determine relative movement enables a receiver to calculate local velocity and orientation, useful in vessels or observations of the Earth. Being able to synchronize the clocks of the highest standards allows the transfer of time, which is critical in large communication and observation systems. An example is CDMA digital cellular. Each base station has a receiver GPS to synchronize the timing of its spreading codes with other base stations to facilitate the hand out of the cell and GPS support CDMA hybrid positioning mobiles for emergency calls and other applications.
Finally, GPS enables researchers to explore the Earth environment including the atmosphere, ionosphere and gravity field. GPS survey equipment has revolutionized tectonics by directly measuring the movement of faults in earthquakes.
To help prevent Civilian GPS guidance from being used in a military enemy or improvised weapons, the U.S. Government controls the export of civilian receivers. A manufacturer Headquartered in general can not export a GPS receiver unless the receiver contains limits restricting it from functioning when it is both (1), at a height of more than 18 kilometers (60,000 feet) and (2) traveling more than 515 m / s (1,000 knots) [31].
[edit] History
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The design of GPS is based partly on similar ground-based radio navigation systems such as LORAN and the Decca Navigator developed in the 1940s, and used during World War II. Additional inspiration for the GPS system came when the Soviet Union launched the first Sputnik in 1957. A team of scientists U.S. directed by Dr. Richard B. Kershner were monitoring the radio transmissions of Sputnik. They discovered that, due to the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location in the world, could determine where the satellite was in orbit by measuring the Doppler distortion.
The first satellite navigation system transit, used by the U.S. Navy, was successfully tested in 1960. Using a constellation of five satellites, could provide a solution Navigation approximately once per hour. In 1967, the U.S. Navy Timation satellite developed that demonstrated the ability to place accurate clocks in space technology is based on GPS system. In the 1970s, the ground-based Omega Navigation System, based on comparison of signal phase, became the first world wide radio navigation system.
The first experimental block and GPS satellite was launched in February 1978 [28]. The GPS satellites were initially manufactured by Rockwell International and are manufactured by Lockheed Martin.
[edit] Timeline
* In 1972, Air Force U.S. Central Inertial Guidance Test Facility (Holloman AFB) conducted tests to combat development of two prototype GPS receivers at White Sands Missile Range, ground-based pseudo-satellites.
* In 1978, the first experimental block was put in place the GPS satellite.
* In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 in restricted procedures Soviet airspace, killing 269 people on board, U.S. President Ronald Reagan announced that the GPS system will be made available for civilian uses once it was completed.
* In 1985, ten more blocks experimental satellites had been launched to validate the concept.
* On 14 February 1989 the first modern Block II satellite was launched.
* In 1992, the Space Wing 2, originally administer the system was deactivated and replaced by the 50th Space Wing.
* In December 1993, the GPS system achieved initial operational capability [32]
* January 17, 1994, a full constellation of 24 satellites in orbit was.
* The full operational capability was declared by NAVSTAR in April 1995.
* In 1996, recognizing the importance of GPS to civilian users and military users, U.S. President Bill Clinton issued a policy of declaring GPS Directive [33] to be a dual-use system and creating a Board Interagency GPS Executive management as a national asset.
* In 1998, USA Vice President Al Gore announced plans to upgrade GPS with two new civilian signals for the accuracy and reliability enhanced user in particularly with regard to aviation security.
* On May 2, 2000 selective availability was discontinued as a result of Executive Order 1996, allowing users to receive a non-degraded signal globally.
* In 2004, the United Kingdom States Government signed an historic agreement with European Community establishing cooperation related to GPS and Europe's Galileo system planned.
* In 2004, USA President George W. Bush updated national policy, replacing the executive board for the national space-based positioning, navigation and timing of the Executive Committee.
* November 2004, QUALCOMM announced successful tests of Assisted-GPS for mobile phones [3].
* In 2005, the first modernized GPS satellite was put into march began transmitting a second civilian signal (L2C) for enhanced user performance.
* The most recent release was on 17 November 2006. The oldest GPS satellite still in operation was launched in August 1991.
* On 14 September 2007, the aging mainframe-based Segment ground control system was the transition to the new architecture of the development of the Plan. [4]
[edit] Satellite numbers
Name Release Period Number of satellites launched, inc. launch failures currently in service
Block I 1978-1985 11 0
Block II 1985-1990 9 0
Block II 1990-1997 19 15 11
1997-2004 Block IIR 12 12
Block IIR-M 2005-3 3
Total of 54 (plus one non-initiated) 30 +1
1one test satellite
[edit] Awards
Two GPS developers have received the National Academy of Engineering Charles Stark years Draper Prize 2003:
* Ivan Getting, president emeritus of the Aerospace Corporation and engineer at the Massachusetts Institute of Technology, established the basis for GPS, improving on the ground World War II-based radio system called LORAN (Long-range radio navigation aid).
* Bradford Parkinson Professor of Aeronautics and Astronautics Stanford University, conceived the present satellite-based system in the 1960s and developed in conjunction with the U.S. Air Force.
One GPS developer, Roger L. Easton received the National Medal of Technology on 13 February 2006 in the White House [34].
On February 10, 1993, the National Aeronautic Association selected the Global Positioning System equipment, as winners of 1992, Robert J. Collier Trophy, the most prestigious award in aviation in the United States. This team consists of researchers from the Naval Research Laboratory, U.S. Air Force, Aerospace Corporation, Rockwell International Corporation, and IBM Federal Systems Company. The citation accompanying the presentation of the trophy honors the GPS Team "for the most significant development for safe and efficient navigation and surveillance of air and spacecraft since the introduction of radio navigation 50 years ago. "
[edit] Other systems
Main article: Global Navigation Satellite System
Other satellite navigation systems or the use of various states of development include:
* Beidou – China's regional system that China has proposed to extend into a global system named COMPASS.
* Galileo – a proposed global system is being developed by the European Union, along with China, Israel, India, Morocco, Saudi Arabia and South Korea, Ukraine plans to be operational in 2011-12.
* GLONASS – Russia's global system, which is being restored to full availability in association with India.
* Indian Regional Navigation Satellite System (IRNSS) – India's regional system proposals.
* QZSS – Japan proposed regional system, adding better coverage of the Japanese islands.
[edit] See also
Portal navigation satellite systems
Nautical Portal
* RAIM
* SIGI
* Radionavigation
* High sensitivity GPS
* Confluence Project Title Use GPS to visit integral degrees of latitude and longitude.
* Exif, GPS data transfer.
* Geotagging
* Geocaching
* NaviTraveler.com, – a community that shares the GPS.
* GPS Drawing Digital mapping and drawing with GPS tracks.
* GPS tracking
* GPS / INS
* Assisted GPS
* GPX (XML schema for the exchange of points of interest)
* ID Sniper Rifle
* OpenStreetMap, maps and pictures of the street free content (GFDL)
* Telematics: Many telematics devices use GPS to determine the location of mobile equipment.
* The American Practical Navigator Chapter 11 "satellite navigation"
* Point of Interest
* Car Navigation System
* NextGen
[edit] Notes
1. ^ Parkinson, BW (1996), Positioning Global System: Theory and Applications, chap. 1: Introduction and Heritage of NAVSTAR, Global Positioning System. pp. 3-28, American Institute of Aeronautics and Astronautics, Washington, DC
2. ^ Ab general NAVSTAR GPS Joint Program Office. 2006 Accessed 15 December.
3. ^ HowStuffWorks. How Receivers GPS working. Accessed May 14, 2006.
4. Globalsecurity.org ^ [1].
5. ^ Dana, Peter H. GPS Orbital Planes. August 8, 1996.
6. ^ What Indicates Global Positioning System We relativity. 2007 Retrieved on January 2.
7. ^ USCG Navco: GPS FAQ. Accessed January 3, 2007.
8. ^ Massatt, Paul and Brady, Wayne. "Optimizing performance through constellation management", Crosslink, Summer 2002, pages 17-21.
9. ^ General of the U.S. Coast Guard GPS News 9-9-05
10. ^ USNO. NAVSTAR Global Positioning System. Accessed May 14, 2006.
11. ^ NMEA NMEA 2000
12. ^ Http: / / gge.unb.ca / Resources / HowDoesGPSWork.html
13. ^ AN02 Assistance Network (HTML). Retrieved on 2007-09-10.
14. ^ Ab Office of Science and Technology Policy. Statement by the President to stop degrading GPS. May 1, 2000.
15. ^ FAA, selective availability. Retrieved January 6, 2007.
16. ^ Http: / / www.defenselink.mil/releases/release.aspx?releaseid=11335
17. ^ Rizos, Chris. University of New South Wales South. GPS satellite signals. 1999.
18. ^ The Global Positioning System by Robert A. Satellite Nelson, November 1999
19. ^ Ashby, Neil Relativity and GPS. Physics Today, May 2002.
20. ^ Space Environment Center. SEC Navigation Systems GPS Page. August 26, 1996.
21. ^ The search for an unintentional GPS disturbing. GPS World. January 1, 2003.
22. ^ Low-cost, portable GPS jammer. Phrack Issue 0x3c (60), Art 13]. Posted 28 December 2002.
23. ^ American Forces Press Service. CENTCOM Charts progress. March 25, 2003.
24. ^ [2]
25. ^ Ruley, John. AVweb. GPS interference. February 12, 2003.
26. ^ Commercial GPS Receivers: Facts for the Warfighter. Hosted on the website of the Joint Staff, together by the USAF GPS Wing DAGR program website. Retrieved on April 10, 2007
27. ^ U.S. Coast press release from the Guard. Global Positioning System completely operating
28. ^ Ab Hydrographic Society Journal. Advances in global navigation satellite. Issue # 104, April 2002. Accessed April 5, 2007.
29. ^ XM982 Excalibur Precision Guided Extended Range Artillery Projectile. GlobalSecurity.org (2007-05-29). Retrieved on 2007-09-26.
30. ^ Non-Proliferation of Sandia National Laboratory in technology programs and arms control.
31. ^ Arms Control Association. Control Regime Technology Missile. Accessed 2006 May 17.
32. ^ U.S. Department of Defense. Notice of the initial operating capability. December 8, 1993.
33. ^ Archives National and Records Administration. U.S. Global Positioning System POLITICS. March 29, 1996.
34. ^ United States Naval Research Laboratory. National Medal Technology for the GPS. November 21, 2005
[edit] References
Wikimedia Commons has media related to:
System Global Positioning
Government Links
* GPS.gov General public education website created by the U.S. Government
* National Space-Based PNT Executive Committee-established in 2004 to supervise the management of GPS and GPS at the national level increases.
* USCG Navigation Center-State of the GPS constellation, government policy, and links to other references. Also includes satellite almanac data.
* The Joint Program Office GPS (GPS JPO), responsible for designing and acquiring the system on behalf of the U.S. Government.
* U.S. Naval Observatory State of the constellation GPS
* U.S. Army Corps of Engineers manual: NAVSTAR HTML and PDF (22.6 MB, 328 pages)
* PNT Selective Availability Announcements
* GPS SPS Signal Specification, 2nd Edition "Official Standard Positioning Signal specification.
* Federal Aviation GPS Management FAQs
Introduction / tutorial links
* How does GPS work? Explains TomTom GPS navigation and maps digital
* Garmin GPS Academy of interactive video web site explaing what exactly GPS is and what it can do for you
Simplified explanation * HowStuffWorks "GPS and video about how GPS works.
Online * Trimble GPS Tutorial designed to introduce the principles behind GPS
* Simulation of GPS and GLONASS (Java) applet simulation and graphical representation of space vehicle motion including the calculation of dilution of precision (DOP)
Technical, historical themes and accessories links
* Dana, Peter H. "Global Positioning System Overview "
* Satellite Navigation: GPS and Galileo (PDF)-16-page document on the history and working of GPS, Galileo play in the next
* History of GPS, including configuration information for each satellite and launch.
* Chadha, Kanwar. "The GPS Global: Challenges in the GPS offers consumers Mainstream "Technical Article (1998)
* GPS Technical Guidance arms
* RAND history of GPS (PDF)
* GPS Anti-Jam Protection Techniques
* Crosslink Summer 2002 issue by the Aerospace Corporation satellite navigation.
* Improved weather predictions from Cosmic occultation data from GPS satellite signal.
* David L. Wilson 's GPS Accuracy Web Page A thorough analysis of the accuracy of GPS.
* Innovation: Spacecraft Navigator, Autonomous GPS positioning in the Highlands Orbits Example of GPS receiver designed for high altitude spaceflight.
* The recipient of the Navigator GPS Receiver GSFC Navigator spaceflight.
* Neil Ashby's Relativity in the Global Positioning System
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The satellite navigation systems
Historical Flag of United States Transit
Operating Flag of the Soviet Union Flag Russian Glonass GPS · Flag of the United States
Development Flag of the People's Republic of China, Beidou / COMPASS · Flag of Europe Galileo · Flag of India Flag of Japan IRNSS · QZSS
Related topics · EGNOS GAGAN GPS · · × F × C LAAS MSAS WAAS ·
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Time signal stations
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BPM shortwave · · CHU RWM WWV · · WWVH · YVTO
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Missing OMA weather stations · VNG
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