|
Applications The GPS (Global Positioning System) is a "constellation" of at least 24 well-spaced satellites that orbit the Earth and make it possible for people with ground receivers to pinpoint their geographic location. The location accuracy is anywhere from 1 to 100 meters depending on the type of equipment used. The GPS is owned and operated by the U.S. Department of Defense, but is available for general use around the world. GPS works like this: Military GPS allows accurate targeting of various military weapons including cruise missiles and precision-guided munitions, as well as improved command and control of forces through improved locational awareness. The satellites also carry nuclear detonation detectors, which form a major portion of the United States Nuclear Detonation Detection System. Civilian GPS receivers are required to have limits on the velocities and altitudes at which they will report coordinates; this is to prevent them from being used to create improvised missiles. Navigation
Mobile Satellite Communications Satellite communications systems permit "remotes" to communicate with "hubs" via satellites. A typical system uses satellites in geosynchronous orbit: this requires a directional antenna (usually a "dish") that is pointed at the satellite. When the "remote" is portable, as on a ship or a train, the antenna must be pointed based on its current location. Essentially all modern antenna controllers incorporate a GPS receiver to provide this location information. The remote uses its location for two distinct purposes: first, to point the antenna at the satellite, and second, to compute the distance to the satellite. The distance to the satellite is crucial when deciding when to transmit a TDMA burst. In this application, there are two distinct types of satellites and two distinct antennas: the GPS satellites are MEO and the GPS antenna is typically a 2cm sq. "patch antenna." The communications satellites are GEO and the communications antenna is typically 1m or larger. To a first approximation, the GPS system is less than 1% of the total cost of the remote system. Location-based services GPS functionality can be used by emergency services and location-based services to locate mobile phones. Assisted GPS is a GPS technology often used by the mobile phone because it reduces the power requirements of the mobile phone and increases the accuracy of the location obtained. GPS provides a location solution which is less dependent on the telecommunications network topology, but more dependent on the mobile phone than methods using radiolocation. The ability to locate a mobile phone to reasonable accuracy is mandated in the United States by E911 emergency services legislation. However, as of September 2006 such a system is not in place in all parts of the country. The mobile phone location may also be used to provide location specific information to the mobile phone, such as location specific advertising, or providing service information specific to the phone user's geographic location. Location-based games The availability of hand-held GPS receivers for a cost of about $90 and up (as of March 2005) has led to recreational applications including location-based games like the popular game Geocaching. Geocaching involves using a hand-held GPS unit to travel to a specific longitude and latitude to search for objects hidden by other geocachers. This popular activity often includes walking or hiking to natural locations. Other location-based games are played controversially by two or more teams on the streets of a city, but most of these are rather still in the stage of research prototypes than a commercial success. Aircraft passengers Most airlines allow passenger use of GPS units on their flights, except during landing and take-off when other electronic devices are also restricted. Even though inexpensive consumer GPS units have a minimal risk of interference, there is still a potential for interference. Because of this possibility, a few airlines disallow use of hand-held receivers for safety reasons. However, other airlines integrate aircraft tracking into the seat-back television entertainment system, available to all passengers even during takeoff and landing. Surveying More costly and precise receivers are used by land surveyors to locate boundaries, structures, and survey markers, and for road construction. There is also a growing demand for Machine Guidance such as Automatic Grade Control systems that use GPS positions and 3D site plans to automatically control the blades and buckets of construction equipment. Agriculture GPS Machine Guidance is used for tractors and other large agricultural machines via auto steer or a visual aid displayed on a screen, which is extremely useful for controlled traffic and row crop operations and when spraying. As well as guidance, GPS used in harvesters with yield monitors can provide a yield map of the paddock being harvested. Geophysics and geology High precision measurements of crustal strain can be made with GPS by finding the relative displacement between GPS sites, one of which is assumed to be stationary. Multiple stations situated around an actively deforming area (such as a volcano or fault zone) can be used to find strain and site velocities relative to a stable reference site. These measurements can then be inverted using the relationships between stress and strain to interpret the source and cause of the deformation. For example, measurements of ground deformation around a volcano can be used to interpret the source and cause—a dike, sill, or other body beneath the surface. Precise time reference Many systems that must be accurately synchronized use GPS as a source of accurate time. For instance, the GPS can be used as a reference clock for time code generators or NTP clocks. Also, when deploying sensors (for seismology or other monitoring application), GPS may be used to provide each recording apparatus with a precise time source, so that the time of events may be recorded accurately. Communications networks often rely on this precise timing to synchronize RF generating equipment, network equipment, and multiplexers. The atomic clocks on the satellites are set to "GPS time". GPS time is counted in days, hours, minutes, and seconds, in the manner that is conventional for most time standards. However, GPS time is not corrected to the rotation of the Earth, ignoring leap seconds and other corrections. GPS time was set to read the same as Coordinated Universal Time (UTC) in 1980, but has since diverged as leap seconds were added to UTC. The GPS day is identified in the GPS signals using a week number along with a day-of-week number. GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6 1980. The week number is transmitted in a ten-bit field, and so it wraps round every 1,024 weeks (7,168 days). The transmitted week number returned to zero at 00:00:19 TAI on August 22, 1999 (23:59:47 UTC on August 21 1999). GPS receivers thus need to know the time to within 3,584 days in order to correctly interpret the GPS time signal. A new field is being added to the GPS navigation message that supplies the calendar year number in a sixteen-bit field, thus performing this disambiguation for any receivers that know about the new field. The GPS navigation message also includes the difference between GPS time and UTC, which is 14 seconds as of 2006. Receivers subtract this offset from GPS time in order to display UTC time. They may further adjust the difference against the UTC time for a local time zone. New GPS units will initially show the incorrect UTC time, or not attempt to show UTC time at all, after achieving a GPS lock for the first time. However, this is usually corrected within 15 minutes, once the UTC offset message is received for the first time. The GPS-UTC offset field is only eight bits, and so it wraps round every 256 leap seconds (At the current rate of change of the earth's rotation, the first wraparound of this field is projected to occur in the year 2330. It is plausible that all current receivers will be obsolete long before this happens). There is also a leap second warning bit, to help GPS receivers tick UTC correctly through a leap second, but its use is troublesome because of misunderstandings about its semantics. History & Timeline The design of GPS is based partly on the similar ground-based radio navigation systems, such as LORAN developed in the early 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 U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, because of 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 on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion. The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. Using a constellation of five satellites, it could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the Timation satellite which proved the ability to place accurate clocks in space, a technology the GPS system relies upon. In the 1970s, the ground-based Omega Navigation System, based on signal phase comparison, became the first world-wide radio navigation system. The first experimental Block-I GPS satellite was launched in February 1978. The GPS satellites were initially manufactured by Rockwell International and are now manufactured by Lockheed Martin. In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 in restricted Soviet airspace, killing all 269 people on board, Ronald Reagan announced that the GPS system would be made available for civilian uses once it was completed. By 1985, ten more experimental Block-I satellites had been launched to validate the concept. The first modern Block-II satellite was launched on February 14, 1989. In 1992, the 2d Space Operations Squadron, which originally managed the system, was de-activated and replaced by the 50th Space Wing. The system achieved initial operational capability by December 1993 A complete constellation of 24 satellites was in orbit by January 17, 1994. In 1996, recognizing the importance of GPS to civilian users as well as military users, President Bill Clinton issued a policy directive declaring GPS to be a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset. In 1998, Vice President Al Gore announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and reliability, particularly with respect to aviation safety. On 2 May 2000, "Selective Availability" was switched off permanently, allowing users outside the US military to receive a full quality signal. In 2004, President George W. Bush updated the national policy, replacing the board with the National Space-Based Positioning, Navigation, and Timing Executive Committee. The most recent launch was September 25, 2006. The oldest GPS satellite still in operation was launched in February 1989. Navigation signals
Calculating positions
Best case The position calculated by a GPS receiver relies on three accurate measurements: the current time, the position of the satellite, and the time delay for the signal. Errors in the clock signal can be reduced using the method above, meaning that the overall accuracy of the system is generally based on the accuracy of the position and delay. The measurement of the delay requires the receiver to "lock onto" the same sequence of bits being sent from the satellite. This can be made relatively accurate by timing comparing the rising or trailing edges of the bits. Modern electronics can lock the two signals to about 1% of a bit time, or in this case about 1% of a microsecond. Since light travels at 299,792,458 m/s, this represents an error of about 3 meters (10 ft), the minimum error possible given the timing of the C/A signal. This can be improved by using the higher-speed P(Y) signal, assuming the same 1% accuracy in locking the retrieved P-code to the internally generated version. In this case the same calculation results in an accuracy of about 30 centimeters (1 ft). Since the P-code repeats at 10.23 MHz, it has a "repeat range" of about 30 kilometers (20 mi). This explains the terminology; when using the P-code, it was first necessary to calculate a coarse position with the C/A code in order to determine how to line up the P-code with the internally generated copy. However, several "real world" effects intrude and degrade the accuracy of the system. These are outlined in the table below, with descriptions following. When all of these effects are added up, GPS is typically accurate to about 15 meters (50 ft). These effects also overwhelm the P(Y) code's added accuracy. Atmospheric effects One of the biggest problems for GPS accuracy is that changing atmospheric conditions change the speed of the GPS signals unpredictably as they pass through the ionosphere. The effect is minimized when the satellite is directly overhead and becomes greater toward the horizon, since the satellite signals must travel through the greater "thickness" of the ionosphere as the angle increases. Once the receiver's rough location is known, an internal mathematical model can be used to estimate and correct for the error. Because ionospheric delay affects the speed of radio waves differently based on their frequencies, the second frequency band (L2) can be used to help eliminate this type of error. Some military and expensive survey-grade civilian receivers can compare the difference between the P(Y) signal carried in the L1 and L2 frequencies to measure atmospheric delay and apply precise corrections. This correction can be applied even without decrypting the P(Y) signal, as long as the encryption key is the same on both channels. In order to make this easier, the U.S. Government has added a new civilian signal on L2, called L2C, starting with the Block IIR-M satellites. The first Block IIR-M was launched in 2005. It allows a direct comparison of the L1 and L2 signals for ionospheric correction. The effects of the ionosphere are generally slow-moving and can easily be tracked. The effects for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location. This correction, say, "10 meters to the east" is also valid for other receivers in the same general location. Several systems send this information over radio or other links to the receivers, allowing them to make better corrections than a comparison of L1 and L2 alone could. The amount of humidity in the air also has a delaying effect on the signal, resulting in errors similar to those generated in the ionosphere but located much closer to the ground in the troposphere. The areas affected by these problems tend to be smaller in area and faster moving than the billows in the ionosphere, making accurate correction for these effects more difficult. Multipath effects GPS signals can also be affected by multipath issues, where the radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground, etc. This delay in reaching the receiver causes inaccuracy. A variety of receiver techniques, most notably narrow correlator spacing, have been developed to mitigate multipath errors. For long delay multipath, the receiver itself can recognize the wayward signal and discard it. To address shorter delay multipath from the signal reflecting off the ground, specialized antennas may be used. This form of multipath is harder to filter out since it is only slightly delayed as compared to the direct signal, causing effects almost indistinguishable from routine fluctuations in atmospheric delay. Multipath effects are much less severe in dynamic applications such as cars and planes. When the GPS antenna is moving, the false solutions using reflected signals quickly fail to converge and only the direct signals result in stable solutions. Ephemeris and clock errors The navigation message from a satellite is sent out only every 12.5 minutes. In reality, the data contained in these messages tends to be "out of date" by an even larger amount. Consider the case when a GPS satellite is boosted back into a proper orbit; for some time following the maneuver, the receiver’s calculation of the satellite's position will be incorrect until it receives another ephemeris update. Additionally, the amount of accuracy sent in the ephemeris is limited by the bandwidth; using the data from the satellites alone limits its accuracy. Further, while it is true that the onboard clocks are extremely accurate, they do suffer from clock drift. This problem tends to be very small, but may add up to 2 meters (6 ft) of inaccuracy. These sorts of errors are even more "stable" than ionospheric problems and tend to change on the order of days or weeks, as opposed to minutes. This makes correcting for these errors fairly simple by sending out a more accurate almanac on a separate channel. Techniques to improve accuracy The accuracy of GPS can be improved several ways: Selective availability When it was first deployed, GPS included a feature called Selective Availability (SA) that introduced intentional errors of up to a hundred meters (300 ft) into the publicly available navigation signals, making it difficult to use for guiding long range missiles to precise targets. Additional accuracy was available in the signal, but in an encrypted form that was only available to the United States military, its allies and a few others, mostly government users. SA typically added signal errors of up to about 10 meters (30 ft) horizontally and 30 meters (100 ft) vertically. The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly, for instance the entire eastern U.S. area might read 30 m off, but 30 m off everywhere and in the same direction. In order to improve 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 civilian ones among 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 it to the enemy. But since SA was also denying the same accuracy to thousands of friendly troops, turning it off presented a clear benefit. In the 1990s, the FAA started pressuring the military to turn off SA permanently. This would save the FAA millions of dollars every year in maintenance of their own radio navigation systems. The military resisted for most of the 1990s, but SA was eventually turned off in 2000 following an announcement by U.S. President Bill Clinton, allowing users access to an undegraded L1 signal. The US military has developed the ability to locally deny GPS (and other navigation services) to hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems. Such Navigation Warfare uses techniques such as local jamming to replace the blunt, world-wide degradation of civilian GPS service that SA represented. Military (and selected civilian) users still enjoy some technical advantages which can give quicker satellite lock and increased accuracy. The increased accuracy comes mostly from being able to use both the L1 and L2 frequencies and thus better compensate for the varying signal delay in the ionosphere. Satellites As of August 2006 the GPS system used a satellite constellation of 29 active Block II/IIA/IIR/IIR-M satellites (for the global coverage 24 is enough) in intermediate circular orbits. The constellation includes three spare satellites in orbit, in case of any failure. Each satellite circles the Earth twice each day at an altitude of about 20,200 kilometers (12,600 miles). The orbits are aligned so at least four satellites are always within line of sight from almost any place on Earth. There are four active satellites in each of six orbital planes. Each orbit is inclined 55 degrees from the equatorial plane, and the right ascension of the ascending nodes is separated by sixty degrees. The flight paths of the satellites are measured by five monitor stations around the world (Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs). The master control station, at Schriever Air Force Base, processes their combined observations and sends updates to the satellites through the stations at Ascension Island, Diego Garcia, and Kwajalein. The updates synchronize the atomic clocks on board each satellite to within one microsecond, and also adjust the ephemeris of the satellites' internal orbital model to match the observations of the satellites from the ground. Frequencies used Several frequencies make up the GPS electromagnetic spectrum: Carries a publicly usable coarse-acquisition (C/A) code as well as an encrypted precision P(Y) code. Usually carries only the P(Y) code, but will also carry a second C/A code on the Block III-R satellites. Carries the signal for the GPS constellation's alternative role of detecting missile/rocket launches (supplementing Defense Support Program satellites), nuclear detonations, and other high-energy infrared events. Two new signals are also being studied: Being studied for additional ionospheric correction. Proposed for use as a civilian safety-of-life (SoL) signal. This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2008. Receivers GPS receivers vary widely in accuracy because of the expense of adding more radio receivers needed to tune in more satellites. For instance, early consumer-grade receivers typically included six to eight receivers for the L1 C/A signal. As the computer industry has improved the state of the art in chipmaking, the cost of implementing these receivers has fallen dramatically, and even low-cost hand held receivers typically have twelve receivers today. More expensive units, known as "dual-frequency receivers", also tune in the L2 signals in order to correct for ionospheric delays. Another major factor in the accuracy of a GPS fix is the amount of processing applied to the received signals. This is a function of the performance of the electronics and the required battery life. These factors have also been dramatically affected by improved chip making, allowing even low cost modern receivers to outperform much more expensive earlier models. GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of a RS-232 port at 4,800 bps speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM. Receivers with internal DGPS receivers can outperform those using external RTCM data. The cost of implementing these receivers is also falling dramatically, and even low-cost units are commonly including WAAS receivers today. Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. NMEA 2000 is a newer and less widely adopted protocol. Both are proprietary and are controlled on a for-profit basis by the US-based National Marine Electronics Association. References to the NMEA protocols have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF protocol. Receivers can interface with external devices via a number of means, such as a serial connection, a USB connection or even a Bluetooth wireless connection. Relativity According to Einstein's Theory of relativity, because of their constant movement and height relative to the Earth Centered Inertial reference frame the clocks on the satellites are affected by their speed (special relativity) as well as their gravitational potential (general relativity). Consequently it was expected that when observed from the Earth's reference frame, satellite clocks would be perceived as running at a slightly faster rate than clocks on the Earth's surface. This amounts to a discrepancy of around 38 microseconds per day, when observed from the Earth. To account for this, the frequency standard on-board the satellites is set to run extremely slightly slower than its desired frequency on Earth, at 10.22999999543 MHz instead of 10.23 MHz—a difference of 0.00457 Hz. The satellite clocks are claimed to be well tuned when in orbit, making it a practical demonstration of the theory of relativity in a real-world system. Neil Ashby presented in Physics Today (May 2002) an account how these relativistic corrections are applied, and their orders of magnitude. The error introduced by relativistic effects can be as much as 15 meters. The GPS system also makes adjustments for the relativistic drift of the atomic clocks in the satellites. Parts of this correction are carried out in the satellites and parts in the receiver. Awards Two GPS developers have received the National Academy of Engineering Charles Stark Draper prize year 2003: One GPS developer, Roger L. Easton, received the National Medal of Technology on February 13 2006 at the White House. On February 10, 1993, the National Aeronautic Association selected the Global Positioning System Team as winners of the 1992 Robert J. Collier Trophy, the most prestigious aviation award in the United States. This team consists of researchers from the Naval Research Laboratory, the U.S. Air Force, the 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." GPS tracking A GPS tracking system uses GPS to determine the location of a vehicle, person, or pet and to record the position at regular intervals in order to create a track file or log of activities. The recorded data can be stored within the tracking unit, or it may be transmitted to a central location, or Internet-connected computer, using a cellular modem, 2-way radio, or satellite. This allows the data to be reported in real-time, using either web browser based tools or customized software. GPS jamming Jamming of any radio navigation system, including satellite based navigation, is possible. The U.S. Air Force conducted GPS jamming exercises in 2003 and they also have GPS anti-spoofing capabilities. In 2002, a detailed description of how to build a short range GPS L1 C/A jammer was published in Phrack issue 60 by an anonymous author. There has also been at least one well-documented case of unintentional jamming, tracing back to a malfunctioning TV antenna preamplifier. If stronger signals were generated intentionally, they could potentially interfere with aviation GPS receivers within line of sight. According to John Ruley, of AVweb, "IFR pilots should have a fallback plan in case of a GPS malfunction". Receiver Autonomous Integrity Monitoring(RAIM), a feature of some aviation and marine receivers, is designed to provide a warning to the user if jamming or another problem is detected. GPS signals can also be interfered with by natural geomagnetic storms, predominantly at high latitudes. GPS jammers the size of a cigarette box are allegedly available from Russia; their effectiveness is in question following their use in the Iraq War. The U.S. government believes that such jammers were also used occasionally during the 2001 war in Afghanistan. Some officials believe that jammers could be used to attract the precision-guided munitions towards non-combatant infrastructure; other officials believe that the jammers are completely ineffective. In either case, the jammers may be attractive targets for anti-radiation missiles. Low power jammers would have limited military usefulness and high power jammers would be easy to locate and destroy. During the Iraq War, the U.S. military claimed to destroy a GPS jammer with a GPS-guided bomb. Other systems Russia operates an independent system called GLONASS (GLObal NAvigation Satellite System), although with only twelve active satellites as of 2004, the system is of limited usefulness. Availability in Russia, Northern Europe and Canada is above 90%. At least four GLONASS satellites are visible 90% of time, which is notable considering GLONASS operates only 12 of 24 required satellites. Dual-band GPS and GLONASS receivers can use satellites from either system, increasing availability in regions where satellite visibility is a problem. There are plans to restore GLONASS to full operation by 2008 with assistance from India. The European Union is developing Galileo as an alternative to the United States-owned-and-operated GPS system. The People's Republic of China, Israel, India, Morocco, Saudi Arabia and South Korea joined the EU in this project. It is planned to be operational by 2010. See also | |||||||||||||||
|
| ||||||||||||||||
 |
|
| |