The Global Positioning System ( GPS ), originally Navstar GPS , is a satellite-based radio navigation system owned by the United States government and operated by the United States Air Force Union. This is a global navigation satellite system that provides geolocation and time information to GPS receivers anywhere on or near Earth where there is an unobstructed line of sight to four or more GPS satellites. Barriers such as mounts and buildings obstruct relatively weak GPS signals.
GPS does not require users to transmit any data, and it operates independently from phone or internet reception, although this technology can improve the usefulness of GPS positioning information. GPS provides critical positioning capabilities for military, civil, and commercial users worldwide. The United States government creates systems, maintains them, and makes them freely accessible to anyone with a GPS receiver.
The GPS project was launched by the US Department of Defense in 1973 for use by the US military and began operating in full in 1995. It was permitted for civilian use in the 1980s. Technological advances and new demands on existing systems have driven efforts to modernize GPS and implement the next generation of Block IIIA GPS satellites and the Next Generation Operational Control System (OCX). Announcement from Vice-President Al Gore and the White House in 1998 initiated this change. In 2000, the US Congress passed a modernization effort, GPS III. During the 1990s, GPS quality was degraded by the United States government in a program called "Selective Availability", but this was no longer the case, and was stopped in May 2000 by law signed by President Bill Clinton. New GPS receiver devices that use L5 frequency to start launching in 2018 are expected to have much higher accuracy and show devices in 30 centimeters or just under one leg.
The GPS system is provided by the United States government, which can selectively deny access to the system, as it did with the Indian military in 1999 during the Kargil War, or lower the service at any time. As a result, a number of countries have developed or are in the process of setting up other global or regional navigation systems. Russia's Global Navigation Satellite System (GLONASS) was developed simultaneously with GPS, but suffered from an incomplete world coverage until the mid-2000s. GLONASS can be added to GPS devices, making more satellites available and allowing positions to be repaired more quickly and accurately, up to two meters. China BeiDou Navigation Satellite System is due to reach global reach by 2020. There is also EU Galileo positioning system, and NAVIC India. The Japanese Quasi-Zenith Satellite System (scheduled to start in November 2018) will be a GPS-based augmentation satellite system to improve GPS accuracy.
Video Global Positioning System
Histori
The GPS project was launched in the United States in 1973 to overcome the limitations of previous navigation systems, integrating ideas from some of its predecessors, including a number of secret engineering design studies from the 1960s. The US Department of Defense developed the system, which originally used 24 satellites. Originally developed for use by the US military and fully operational in 1995. Use of civilians was allowed from the 1980s. Roger L. Easton from the Naval Research Laboratory, Ivan A. Getting from Aerospace Corporation, and Bradford Parkinson from the Applied Physics Laboratory is credited with creating it.
The GPS design is partly based on similar ground-based radio-based navigation systems, such as LORAN and the Decca Navigator, developed in the early 1940s.
predecessor
When the Soviet Union launched its first artificial satellite (Sputnik 1) in 1957, two American physicists William Guier and George Weiffenbach, at the Johns Hopkins University Applied Physics Laboratory (APL) decided to monitor their radio transmissions. Within a few hours they realized that, because of the Doppler effect, they could determine where the satellite was along its orbit. The APL Director grants them access to UNIVAC to perform the required weight calculations.
The following spring, Frank McClure, deputy director of APL, asked Guier and Weiffenbach to investigate the inverse issue - determining the user's location, remembering the satellites. (At that time, the Navy was developing Polaris missiles launched by submarines, which required them to know the location of the submarine.) It took them and APL to develop the TRANSIT system. In 1959, ARPA (renamed DARPA in 1972) also played a role in TRANSIT.
TRANSIT was first successfully tested in 1960. It uses a constellation of five satellites and can provide navigation improvements about once per hour.
In 1967, the US Navy developed the Timasi satellite, which proved the feasibility of placing accurate clocks in space, the technology required for GPS.
In the 1970s, the ground-based OMEGA navigation system, based on phase comparison of signal transmission from station tide, became the world's first radio navigation system. The limitations of this system push the need for a more universal navigation solution with greater accuracy.
While there is a wide need for accurate navigation in the military and civilian sectors, almost nothing is seen as a justification for the billions of dollars required in research, development, deployment, and operation for constellation of navigation satellites. During the Cold War arms race, the nuclear threat to the existence of the United States is a need that justifies these costs in the view of the United States Congress. This deterrent effect is why GPS is funded. That was also the reason for ultra secrecy at the time. The nuclear triad consists of naval ballistic missiles launched by the United States Navy (SLBM) along with the United States strategic bombers (USAF) and intercontinental ballistic missiles (ICBM). Considered important for nuclear deterency posture, accurate determination of the SLBM launch position is a multiplier force.
Proper navigation will enable US ballistic missile submarines to get an accurate position repair before they launch their SLBMs. The USAF, with two-thirds of the nuclear triad, also has requirements for a more accurate and reliable navigation system. The Navy and Air Force developed their own technology in parallel to solve essentially the same problem.
To improve the survival of ICBM, there are proposals for using mobile launch platforms (comparable to SS-24 and SS-25 Russia) so the need to improve the launch position is similar to the SLBM situation.
In 1960, the Air Force proposed a radio navigation system called MOSAIC (MObile System for Accurate ICBM Control) which is essentially a 3-D LORAN. A follow-up study, Project 57, worked in 1963 and "in this study that the concept of GPS has been born." That same year, the concept was pursued as Project 621B, which has "many attributes you now see in GPS" and promises increased accuracy for Air Force bombers as well as ICBMs.
Updates from the Navy TRANSIT system are too slow for high speed Air Force operations. The Naval Research Laboratory continued progress with their Timation satellite (Time Navigation), first launched in 1967, and with the third in 1974 carrying the first atomic clock into orbit.
Other important GPS predecessors come from different US military branches. In 1964, the United States Army rounded the first sequential collision satellite (SECOR) used for geodetic surveys. The SECOR system includes three ground-based transmitters from known locations that will send signals to satellite transponders in orbit. A fourth ground-based station, at an undetermined position, can then use the signal to correct its location appropriately. The last SECOR satellite was launched in 1969.
Development
With parallel developments in the 1960s, it was realized that superior systems could be developed by synthesizing the best technology of 621B, Transit, Timation, and SECOR in multi-service programs. However, satellite position orbit errors, caused by variations in the field of gravity and radar refraction, among others, must be resolved. A team led by Harold L Jury of the Pan Am Aerospace Division in Florida from 1970-1973, using the assimilation of real-time data and a recursive estimate to do so; modeling systematic and remaining errors to manageable levels to enable accurate navigation.
During the Labor Day weekend in 1973, a meeting of about twelve military officers at the Pentagon discussed the creation of the Defense Satellite Navigation System (DNSS) . At this meeting the real synthesis of being a GPS was created. Later that year, the DNSS program was named Navstar , or the Using Time and Start Navigation System. With individual satellites associated with Navstar names (such as the Transit and Timation predecessors), a more complete covering name is used to identify the constellation of Navstar satellites, Navstar-GPS . Ten "I Block" satellite prototypes were launched between 1978 and 1985 (additional units were destroyed in launch failures).
The ionospheric effects on radio transmissions through the ionosphere are investigated in the geophysical laboratory of the Air Force Cambridge Research Laboratory. Located at Hanscom Air Force Base, outside Boston, the lab was renamed the Air Force Geophysical Research Laboratory (AFGRL) in 1974. AFGRL developed the Klobuchar Model to calculate ionospheric correction to GPS locations. Of note is the work done by the Australian Space Experts Elizabeth Essex-Cohen at AFGRL in 1974. He was worried about the curvature of the radio waves passing through the ionosphere of the NavSTAR satellites.
After Korean Air Lines Flight 007, a Boeing 747 carrying 269 people, was shot down in 1983 after straying into the USSR banned air space, around the Sakhalin and Moneron Islands, President Ronald Reagan issued a directive to make GPS available for free for civilian use, after it is developed enough, as a common good. The first Block II satellite was launched on February 14, 1989, and satellite 24 was launched in 1994. The cost of the current GPS program, excluding user equipment costs, but including satellite launch costs, has been estimated at about USD 5 billion (dollars last year).
Initially, the highest quality signal is provided for military use, and the signal available for civilian use is deliberately degraded (Selective Availability). This changed with President Bill Clinton signing policy directives to shut down Selective Selection May 1, 2000 to provide the same accuracy to civilians given to the military. The directive was proposed by US Defense Secretary William Perry, due to the growing growth of differential GPS services to improve civilian accuracy and eliminate US military gains. In addition, the US military is actively developing technology to deny GPS services to potential enemies on a regional basis.
Since its deployment, the US has implemented some improvements to GPS services including new signals for civilian use and increased accuracy and integrity for all users, all while maintaining compatibility with existing GPS equipment. The modernization of satellite systems has been an ongoing initiative by the US Department of Defense through a series of satellite acquisitions to meet the growing needs of the military, civil, and commercial markets.
In early 2015, high quality, FAA class, Standard Positioning Service (SPS) GPS receivers provide horizontal accuracy better than 3.5 meters, although many factors such as receiver quality and atmospheric problems can affect this accuracy.
GPS is owned and operated by the United States government as a national resource. The Department of Defense is a GPS servant. The Interagency GPS Executive Board (IGEB) oversaw GPS policy issues from 1996 to 2004. Afterwards, the National Executive-Based Positioning, Navigation and Timing Executive Committee was established by the presidential directive in 2004 to provide advice and coordinate federal departments and agencies on matters relating to GPS and related systems. The executive committee is co-chaired by the Deputy Secretary of Defense and Transport. Its membership includes equivalent level officials from the Department of State, Commerce and Homeland Security, Joint Chiefs of Staff and NASA. The components of the president's executive office participate as observers for the executive committee, and the FCC chairman participates as a liaison.
On February 10, 1993, the National Aeronautic Association chose the GPS Team as the 1992 Robert J. Collier Cup winner, the nation's most prestigious aviation award. The team combines researchers from Naval Research Laboratory, USAF, Aerospace Company, Rockwell International Company, and IBM Federal System Company. The quote honors them "for the most significant development for safe and efficient navigation and airborne surveillance and spacecraft since the introduction of radio navigation 50 years ago."
Two GPS developers received the Charles Stark Draper National Academy of Engineering Award for 2003: Ivan Getting, emeritus president of The Aerospace Corporation and engineer at the Massachusetts Institute of Technology, established the basis for GPS, improved the World War II ground-based land system called LORAN ( Loi/ng> range R adio A id to N avigation).
GPS developer Roger L. Easton received the National Technology Medal on February 13, 2006.
Francis X. Kane (USAF Col., Ret.) Inaugurated into the US Air Force Room and Missile Pioneer Hall of Fame at Lackland AFB, San Antonio, Texas, March 2, 2010 for his role in the development of aerospace technology and GPS concept engineering design performed as part of Project 621B.
In 1998, GPS technology was inducted into the Space Hall of Space Science Foundation of Fame.
On October 4, 2011, the International Astronautical Federation (IAF) awarded the 60th Anniversary Award Global Positioning System (GPS), nominated by IAF members, the American Institute for Aeronautics and Astronautics (AIAA). The IAF Honors and Awards Committee recognizes the uniqueness of the GPS program and the exemplary role it has played in building international collaborations for the benefit of humanity.
Maps Global Positioning System
The basic concept of GPS
Fundamentals
The GPS concept is based on the time and position of a known GPS satellite. Satellites carry very stable atomic clocks that are synchronized with each other and with ground clocks. Each drift of the correct time is maintained in the corrected soil every day. In the same way, satellite locations are known to be very accurate. GPS receivers have hours too, but they are less stable and less precise.
GPS satellites continuously transmit data about their current time and position. A GPS receiver monitors multiple satellites and solves the equations to determine the exact position of the receiver and its deviation from the actual time. At a minimum, four satellites must be in receiver's view for it to count four unknown quantities (three coordinates of position and hour deviation from satellite time).
More details
Each GPS satellite continues to broadcast the signal (carrier wave by modulation) which includes:
- The pseudorandom code (sequences one and zero) known to the receiver. By setting the timing of the generated receiver version and the version of the code as measured by the recipient, the arrival time (TOA) from the point specified in the code sequence, called the times, can be found in the clock time of the receiver
- Messages that include transmission time (TOT) from the code period (in GPS time scale) and the current satellite position
Conceptually, the receiver measures the TOAs (according to the watch itself) from four satellite signals. From TOA and TOT, the receiver forms four flight time values ​​(TOF), which (given the speed of light) is approximately equal to the range of the receiving satellite. The receiver then calculates the three-dimensional position and the clock drift of all four TOFs.
In practice the receiver position (in the Cartesian three-dimensional coordinates with origin in the center of the Earth) and the offset of the receiver clock relative to the time GPS is calculated simultaneously, using the navigation equation to process the TOFs.
The location of the solution centered on the receiving Earth is usually converted into latitude, longitude and high relative to the ellipsoidal Earth model. The altitude can then be converted further to the height relative to the geoid (eg, EGM96) (basically, sea level on average). These coordinates can be displayed, for example, on a moving map view, and/or recorded and/or used by some other system (eg, vehicle guide system).
User-satellite geometry
Although usually not explicitly formed in receiver processing, the conceptual time-out difference (TDOA) defines the measurement geometry. Each TDOA corresponds to a hyperboloid revolution (see Multilateration). The line connecting the two involved satellites (and their extensions) forms the hyperboloid axis. The receiver is located at the point where three hyperboloids intersect.
It is sometimes mistakenly said that the user's location is at a three-ball junction. Although it is easier to visualize, this is only the case if the receiver has a clock synchronized with the satellite clock (that is, the receiver measures the correct range to the satellites rather than the range difference). There are significant performance benefits for users who carry clocks synced with satellites. Most importantly, only three satellites are needed to calculate the positioning solution. If this is an important part of the GPS concept so that all users need to carry a synchronized clock, then a small number of satellites can be used. However, the cost and complexity of user equipment will increase significantly.
Receiver in continuous operation
The above description is representative of the receiver's initial situation. Most recipients have a track algorithm, sometimes called a tracker , which combines a collection of satellite measurements collected at different times - essentially, utilizing the fact that consecutive recipient positions are usually adjacent to each other. After a series of measurements are processed, the tracker predicts the recipient location corresponding to the next set of satellite measurements. When new measurements are collected, the recipient uses a weighting scheme to combine new measurements with tracking predictions. In general, the tracker may (a) improve the receiver's position and timeliness, (b) reject the poor measurement, and (c) estimate the speed and direction of the receiver.
Tracker loss is that changes in speed or direction can only be calculated by delay, and the derivative direction becomes inaccurate when the distance traveled between two position measurements falls below or near random error of position measurement. The GPS unit can use the Doppler shift measurement of the received signal to calculate the speed accurately. More sophisticated navigation systems use additional sensors such as compasses or inertial navigation systems to complement GPS.
In a typical GPS operation as a navigator, four or more satellites must be visible to obtain accurate results. The solution of the navigation equation gives the receiver position along with the difference between the time stored by the receiver's clock-on-board and the actual days, thus eliminating the need for receiver-based clocks more precisely and possibly impractical. Applications for GPS such as time transfer, signal timing, and synchronization of mobile base stations make use of this inexpensive and highly accurate time. Some GPS applications use this time to display, or, in addition to basic position calculations, do not use them at all.
Although four satellites are required for normal operation, less is applicable in special cases. If one variable is already known, the recipient can determine its position using only three satellites. For example, a ship or an aircraft may have a known height. Some GPS receivers may use additional instructions or assumptions such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle's computer, to provide positions (possibly degraded) when less than four satellites are visible.
Structure
GPS currently consists of three main segments. This is a segment of space, control segments, and user segments. U.S. Air Force develop, maintain, and operate space and control segments. GPS satellites broadcast signals from outer space, and each GPS receiver uses this signal to calculate its three dimensional location (latitude, longitude, and altitude) and current time.
Space segment
The space segment (SS) consists of 24 to 32 satellites in medium Earth orbit and also includes a charge adapter to the amplifier required to launch them into orbit.
The space segment (SS) consists of an orbiting GPS satellite, or a Space Vehicle (SV) in the GPS language. The GPS design was originally called for 24 SV, eight each in three circular orbits, but these were modified into six orbiting planes with four satellites each. The six orbital planes have about 55 Â ° inclination (the slope relative to the Earth's equator) and separated by a 60 Â ° elevation rise from the ascending node (angle along the equator from the reference point to the orbital junction). The orbital period is one and a half day sidereal, ie, 11 hours and 58 minutes so that the satellite passes the same location or almost the same location every day. Orbits are arranged so that at least six satellites always face each other from almost any place on the surface of the Earth. The result of this goal is that the four satellites are not evenly spaced (90 Â °) in each orbit. In general, the angular difference between satellites in each orbit is 30 Â °, 105 Â °, 120 Â °, and 105 Â ° apart, which is 360 Â °.
Orbiting at an altitude of about 20,200 km (12,600 mi); an orbital radius of about 26,600 km (16,500 mi), each SV makes two complete orbits each day sidereal, repeating the same dirt tracks daily. This is very helpful during development because even with only four satellites, true alignment means the four are visible from one place for several hours each day. For military operations, repetition of overland routes can be used to ensure good coverage in the battle zone.
As of February 2016, there were 32 satellites in the GPS constellation, 31 of which were in use. Additional satellites improve the accuracy of GPS receiver calculations by providing redundant measurements. As the number of satellites increases, the constellation is changed to a non-uniform arrangement. Such arrangements prove to increase the reliability and availability of the system, relative to a uniform system, when some satellites fail. Approximately nine satellites are visible from any point on the ground at one time (see animation on the right), ensuring considerable redundancy over the minimum of four satellites required for a position.
Control segment
The control segment (CS) consists of:
- the main control station (MCS),
- alternative master control station,
- four dedicated ground antennas, and
- six special monitor stations.
MCS can also access ground antennas of the US Air Force Satellite Control Unit (AFSCN) (for additional command and control capability) and NGA (National Geospatial-Intelligence Agency) monitoring stations. The satellite flight path is tracked by a dedicated US Air Force monitoring station in Hawaii, Kwajalein Atoll, Ascension Island, Diego Garcia, Colorado Springs, Colorado and Cape Canaveral, along with NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington DC. Tracking information was sent to the MCS Air Force Command at Schriever Air Base 25 km (16 mi) ESE Colorado Springs, operated by the US Air Force Operating Space Squadron 2 (2 SOPS). Then 2 SOPS contact each GPS satellite regularly with navigation updates using a special landline or shared antenna (AFSCN) (GPS-specific landline located in Kwajalein, Ascension Island, Diego Garcia, and Cape Canaveral). This update synchronizes atomic clocks on satellite boards into several nanoseconds to each other, and adjusts the ephemeris of each model of the satellite's internal orbit. Updates are made by Kalman filters that use input from ground monitoring stations, space weather information, and other inputs.
Satellite maneuvers are not exact by GPS standards - so to change the orbit of satellites, satellites should be marked unhealthy , so the recipients do not use them. After satellite maneuvers, engineers tracked new orbits from the ground, uploaded new ephemeris, and marked the satellite back healthy.
The operation control segment (OCS) currently serves as a record control segment. It provides operational capabilities that support GPS users and makes GPS operational and performs in specifications.
OCS successfully replaced the 1970s legacy mainframe computer at Schriever Air Force Base in September 2007. After installation, the system helped enable the upgrades and provided a foundation for a new security architecture that supported the US armed forces.
OCS will continue to be a ground record control system to a new segment, Next Generation GPS Operation Control System (OCX), fully developed and functional. The new capabilities provided by OCX will be the foundation for revolutionizing the GPS mission capabilities, enabling Air Force Space Command to greatly enhance the GPS operational service to US combat troops, civil partners, and numerous domestic and international users. The OCX GPS program will also reduce costs, schedules and technical risks. It is designed to provide savings of 50% savings through efficient software architecture and Performance-Based Logistics. In addition, GPS OCX is estimated to cost millions less than the cost to improve OCS while providing four times the ability.
The OCX GPS program represents an important part of GPS modernization and provides significant information upgrades over the current GPS OCS program.
- OCX will have the ability to control and manage the legacy GPS satellites as well as next generation GPS satellite satellites, while allowing a full array of military signals.
- Built on flexible architecture that can quickly adapt to the changing needs of current and future GPS users enabling direct access to GPS data and constellation status through safe, accurate and reliable information.
- Provide warfighter with more secure information, actionable and predictable to increase situational awareness.
- Enables newly modernized signals (L1C, L2C, and L5) and has M-code capabilities, which can not be done by legacy systems.
- Provides a significant increase in the information assurance of the current program including detecting and preventing cyber attacks, while isolating, containing, and operating during the attack.
- Supports higher volume near real-time commands and capabilities and control capabilities.
On September 14, 2011, the US Air Force announced the completion of the Initial Design Review of GPS OCX and confirmed that the OCX program is ready for the next development stage.
The OCX GPS program has lost a major milestone and is driving the launch of GPS IIIA after April 2016.
User segment
The US (US) user segment comprises hundreds of thousands of US military users and allies of secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service (see GPS navigation device). Generally, a GPS receiver consists of an antenna, tuned to a frequency transmitted by satellites, receiver processors, and a very stable clock (often a crystal oscillator). They can also include a view to provide location and speed information to users. The recipient is often described by the number of channels: this indicates how many satellites can be monitored simultaneously. Initially limited to four or five, this has been increasing over the years so that, in 2007, receivers typically had between 12 and 20 channels. Although there are many receiver manufacturers, they almost all use one of the chipsets produced for this purpose.
The GPS receiver can input input for differential correction, using RTCM SC-104 format. This is usually an RS-232 port with a speed of 4800 bits/sec. The actual data is sent at a much lower rate, which limits the accuracy of the transmitted signal using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. In 2006, even low-cost units generally included Wide Area Augmentation System (WAAS) receivers.
Many GPS receivers can transmit position data to a PC or other device using NMEA 0183 protocol. Although this protocol is officially designated by the National Marine Electronics Association (NMEA), references to this protocol have been compiled from public records, allowing open source tools such as gpsd to read protocol without violating intellectual property law. Other proprietary protocols exist, such as the SiRF and MTK protocols. The receiver can interact with other devices using methods including serial connection, USB, or Bluetooth.
Apps
Although initially a military project, GPS is considered a dual-use technology, meaning having significant military and civilian applications.
GPS has become a widely used and useful tool for trading, scientific usability, tracking, and surveillance. Accurate GPS times facilitate daily activities such as banking, mobile phone operations and even control of the power grid by enabling synchronized hand transfer.
Civil
The US government controls the export of some civilian recipients. All GPS receivers capable of functioning over 18 km (60,000 feet) in height and 515 m/s (1,000 knots), or designed or modified for use with unmanned aerial vehicles such as, eg ballistic or cruise missile systems, are classified as ammunition (weapons)) - which means they need an Export Department license.
This rule applies even to pure civilian units that receive only L1 and C/A (Coarse/Acquisition) frequencies.
Disabling operations above these limits frees recipients from classification as munitions. Interpretation of different vendors. Rules refer to operations at altitude and target speed, but some receivers stop operating even when stationary. This has caused problems with several amateur radio balloon launches that regularly reach 30 km (100,000 ft).
This limit applies only to units or components exported from the United States. The growing trade in the various components exists, including GPS units from other countries. It is expressly sold as ITAR-free.
Military
In 2009, military GPS applications include:
- Navigation: Soldiers use GPS to find destinations, even in darkness or in unfamiliar territory, and to coordinate troops and provide movement. In the US armed forces, the commander uses the Digital Assistant Commander and ranks below using Digital Assistant Soldiers .
- Target tracking: Military weapon systems use GPS to track potential ground and air targets before marking them as enemies. These weapon systems pass target coordinates to precision-guided ammunition to enable them to accurately target them. Military aircraft, especially in air-to-ground roles, use GPS to find targets.
- Missile and projectile guides: GPS allows accurate targeting of various military weapons including ICBM, cruise missiles, precision-guided ammunition and artillery shells. The embedded GPS receiver is able to withstand the acceleration of 12,000 g or approximately 118 km/sec 2 has been developed for use on a 155-millimeter (6.1-inch) howitzer shell.
- Search and save.
- Reconnaissance: Patrolling can be managed more closely.
- GPS satellites carry a set of nuclear detonation detectors composed of optical sensors called bhangmeter, X-ray sensor, dosimeter, and electromagnetic sensor (EMP) sensor (W-sensor), which forms the main part of the American Nuclear Detection Detection System Union. General William Shelton has stated that future satellites can lower this feature to save money.
GPS type navigation was first used in the war in the 1991 Persian Gulf War, before GPS was fully developed in 1995, to help Coalition forces navigate and maneuver in war. The war also shows the GPS vulnerability to a standstill, when Iraqi forces install jamming devices on possible targets that emit radio sounds, disrupting the reception of weak GPS signals.
Communications
The navigation signals transmitted by GPS satellites encode various information including satellite positions, internal clock state, and network health. These signals are transmitted on two separate carrier frequencies that are common to all satellites in the network. Two different encodings are used: public encryption that allows navigation with lower resolution, and encrypted encryption used by the U.S. military.
Message format
Each GPS satellite continually broadcasts a navigation message at frequencies L1 (C/A and P/Y) and L2 (P/Y) at a rate of 50 bits per second (see bitrate). Each complete message takes 750 seconds (12 1/2 minutes) to complete. The message structure has the basic format of a 1500-bit frame consisting of five subframes, each subframe measuring 300 bits (6 seconds). Subframes 4 and 5 are sub-divided 25 times each, so complete data messages require full 25 frame transmissions. Each subframe consists of ten words, each 30 bits in length. Thus, with 300 bits in the subframe times 5 subframes in frames multiplied by 25 frames in a message, each message has a length of 37,500 bits. At the 50-bit/s transmission rate, this gives 750 seconds to transmit all the almanac (GPS) messages. Each 30-second frame starts exactly in minutes or half minutes as indicated by the atomic clock on each satellite.
The first subframe of each frame encodes weekly numbers and times of the week, as well as data on satellite health. The second and third subframes contain ephemeris - exact orbits for satellites. The fourth and fifth subframes contain almanac , which contains rough orbit and status information of up to 32 satellites in the constellation as well as data related to error correction. Thus, in order to obtain an accurate satellite location of this transmitted message, the receiver must demodulate messages from each of the satellites included in the solution for 18 to 30 seconds. To collect all transmitted almanacs, the receiver must demodulate the message for 732 to 750 seconds or 12 1/2 minutes.
All satellites broadcast on the same frequency, encoding signals using a unique double passcode (CDMA) division so that recipients can distinguish individual satellites from each other. The system uses two different types of CDMA coding: coarse/acquisition codes (C/A), accessible to the general public, and exact codes (P (Y)), which are encrypted so that only the US military and other NATO countries who have been granted access to the encryption code can access it.
Ephemeris is updated every 2 hours and is generally valid for 4 hours, subject to renewal every 6 hours or more in non-nominal conditions. Almanacs are updated usually every 24 hours. In addition, data for a few weeks after uploading in case of a transmission update delaying data upload.
Satellite frequency
All satellites are broadcasted on the same two frequencies, 1.57542 GHz (signal L1) and 1.2276 GHz (signal L2). Satellite networks use CDMA spread-spectrum techniques where low bitrate message data is encoded in different high-rate pseudo-random (PRN) sequences for each satellite. The recipient must be aware of the PRN code for each satellite to reconstruct the actual message data. The C/A code, for civilian use, transmits data on 1,023 million chips per second, while P code, for US military use, transmits at 10.23 million chips per second. The actual internal reference of the satellite is 10.22999999543 MHz to compensate for the relativistic effect that makes observers on Earth feel different time references with respect to the transmitters in orbit. The carrier L1 is modulated by the codes C/A and P, whereas the L2 carrier is only modulated by the code P. The P code can be encrypted as P (Y) code only available for military equipment with proper decryption key. Both C/A and P (Y) code give users the right time.
L3 signal at 1.38105 GHz frequency is used to transmit data from satellite to earth station. This data is used by the US Nuclear Detection System (NUDET) Detection (USNDS) to detect, locate, and report nuclear detonation (NUDET) in the Earth's atmosphere and near space. One of its uses is the enforcement of a nuclear test ban agreement.
The L4 band at 1.379913 GHz is being studied for additional ionosphere correction.
The L5 frequency band at 1.17645 GHz is added in the process of modernizing the GPS. This frequency falls into an internationally protected range for flight navigation, promising little or no interruption in all circumstances. The first Block IIF satellite providing this signal was launched in May 2010. On 5 February 2016, the 12th and final Block IIF satellite was launched. L5 consists of two carrier components that are in quadrature phase with each other. Each carrier component is a two-phase shift key (BPSK) modulated by a separate bit train. "L5, a third Civilian GPS signal, will ultimately support life-of-life applications for aviation and provide better availability and accuracy."
In 2011, a conditional abandonment was given to LightSquared to operate a terrestrial broadband service near the L1 band. Although LightSquared had applied for a license to operate in band 1525-1559 in early 2003 and was issued for public comment, the FCC asked LightSquared to establish a study group with the GPS community to test the GPS receiver and identify problems that may arise due to greater signal strength of the network terrestrial LightSquared. The GPS community does not object to the LightSquared application (previously MSV and SkyTerra) until November 2010, when LightSquared submits applications for modifications to the Ancillary Terrestrial Component (ATC) authorization. This archiving (SAT-MOD-20101118-00239) amounted to a request to run several orders of greater force in the same frequency band for terrestrial base stations, essentially changing what should be a "quiet environment" for signals from outer space as equivalent to mobile networks. Testing in the first half of 2011 has shown that the impact of the lower 10à , · MHz spectrum is minimal for GPS devices (less than 1% of total GPS devices are affected). The top 10 MHz intended for use by LightSquared may have some impact on the GPS device. There are some concerns that this can seriously lower the GPS signal for many consumer uses. Aviation Week magazine reports that the latest testing (June 2011) confirmed the "significant disturbance" of GPS by the LightSquared system.
Demodulation and decoding
Since all satellite signals are modulated to the same L1 carrier frequency, the signal must be separated after demodulation. This is done by assigning each satellite a unique binary sequence known as the Gold code. The signal is decoded after demodulation using the addition of a Gold code corresponding to the satellite monitored by the receiver.
If the almanac information has been previously obtained, the receiver takes the satellites to be heard by their PRN, unique numbers in the range 1 to 32. If the almanac information is not in memory, the recipient enters the search mode until a key is obtained on one of the satellites. To get the key, there needs to be an unobstructed line of sight from the receiver to the satellite. The receiver can then obtain an almanac and determine which satellites to listen to. Because it detects the signal of each satellite, it identifies with different C/A code patterns. There may be a delay of up to 30 seconds before the first position estimate due to the need to read ephemeris data.
The navigation message processing allows the timing of transmission and the current satellite position. For more information see Demodulation and Decoding, Advanced.
Description of problem
The receiver uses messages received from satellites to determine the satellite position and time sent. Components x, y, and z of satellite positions and delivery times are defined as x i , y i , z i , s i ] where the subscript i shows the satellite and has a value of 1, 2,... , n , where n Ã,> = Ã, 4. When the receipt time of the message indicated by the on-board receiver clock is t? i , the actual reception time is t i = t? i - b , where b is the receiver clock bias of the much more accurate GPS clock used by satellite. The receiver clock bias is the same for all received satellite signals (assuming the satellite clock is all perfectly synchronized). Message transit time is t? i - b - s i , where s < sub> i is satellite time. Assuming the message is passed at the speed of light, c , the distance traveled is ( t i - b - s i ) c .
Untuk n satelit, persamaan yang harus dipenuhi adalah:
di mana d i adalah jarak atau rentang geometrik antara penerima dan satelit:
Mendefinisikan pseudoranges sebagai , kami melihat mereka merupakan versi bias dari rentang yang benar:
- .
Karen
Source of the article : Wikipedia
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