CC BY
137
4
ИНФРАСТРУКТУРА СТРАНЫ ИЗУЧАЕМОГО ЯЗЫКА
GLOBAL NAVIGATION SATELLITE SYSTEMS
Т.Р. Латыпов Научный руководитель: Т.В. Примакина
Тема доклада - «Навигационные спутниковые системы». Описываются принципы работы навигационных систем, источники их ошибок и неточностей, а также современные способы их преодоления.
Satellites already play a significant role in our daily lives, aiding communication, exploration and research. Navigation is perhaps one of satellite's most successful applications, and for consumers, receivers are becoming ever more affordable and reliable. The recent signing of cooperative agreements between the United States, the European Union and Russia will expand the system of navigation, laying the foundation for a compatible and interoperable Global Navigation Satellite System, the GNSS. With this relatively young technology improved accuracy, better reception and altogether new applications lie in wait for us in the near future.
Radio-based navigation systems were developed in the early twentieth century, and were used in World War II. As this technology advanced, both ships and airplanes used ground-based radio-navigation systems. The disadvantage of using a system that uses ground generated radio waves is that a choice has to be made between a high-frequency system that is accurate, but does not cover a wide area, and a low-frequency system that covers a wide area, but is not very accurate.
The history of satellite navigation goes back to the era of the 'space race'. With the launch of Sputnik I in 1957, the Russians had to keep the Doppler Effect in mind: To maintain radio contact with a moving object, you have to keep changing your frequency. The Monitor Station would search over a certain frequency range until it could acquire a lock on Sputnik's signal. By calculating the frequency shift, the velocity relative to the station could be determined. Consequently, the satellite's position in orbit could be calculated. In fact, they expressly chose a frequency which was audible on a normal transistor radio. Listening to it, you would clearly hear the Doppler Effect, unmistakable proof that the Russians had launched the first man-made satellite into orbit. This led to the development of the United States' TRANSIT system, which was immediately followed by several other projects such as Nova and Timation. These were primarily experiments that eventually led to the research of GPS in 1969. Russia has also played its part in satellite navigation, but constantly was one step behind. They followed the American's TRANSIT with the Russian Tsikada, and answered GPS with the exclusively military GLONASS system.
The Global Positioning System (GPS) is currently the only fully-functional Global Navigation Satellite System (GNSS). More than two dozen GPS satellites are in medium Earth orbit, transmitting signals allowing GPS receivers to determine the receiver's location, speed and direction.
1. Introduction
2. History of Navigation Systems
3. NAVSTAR GPS
Since the first experimental satellite was launched in 1978, GPS has become an indispensable aid to navigation around the world, and an important tool for map-making and land surveying. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.
Developed by the United States Department of Defense, it is officially named NAVSTAR GPS (NAVigation Satellite Timing And Ranging Global Positioning System). The satellite constellation is managed by the United States Air Force 50th Space Wing. Although the cost of maintaining the system is approximately US$400 million per year, including the replacement of aging satellites, GPS is free for civilian use as a public good.
3.1 Working principles
As it turns out, working of navigation satellite system is a fairly elaborate process.
At a particular time (let's say midnight), the satellite begins transmitting a long, digital pattern called a pseudo-random code. The receiver begins running the same digital pattern also exactly at midnight. When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern.
The length of the delay is equal to the signal's travel time. The receiver multiplies this time by the speed of light to determine how far the signal traveled. Assuming the signal traveled in a straight line, this is the distance from receiver to satellite.
To make the position determination precise enough, satellites' and the receiver's clocks must be highly synchronized. Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary quartz clock, which it constantly resets. In a nutshell, the receiver looks at incoming signals from four or more satellites and gauges its own inaccuracy. In other words, there is only one value for the "current time" that the receiver can use. The correct time value will cause all of the signals that the receiver is receiving to align at a single point in space. That time value is the time value held by the atomic clocks in all of the satellites. So the receiver sets its clock to that time value, and it then has the same time value that all the atomic clocks in all of the satellites have. The GPS receiver gets atomic clock accuracy "for free".
When you measure the distance to four located satellites, you can draw four spheres that all intersect at one point. Three spheres will intersect even if your numbers are way off, but four spheres will not intersect at one point if you've measured incorrectly.
In order for the distance information to be of any use, the receiver also has to know where the satellites actually are. This isn't particularly difficult because the satellites travel in very high and predictable orbits. The GPS receiver simply stores an almanac that tells it where every satellite should be at any given time. Things like the pull of the moon and the sun do change the satellites' orbits very slightly, but the Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellites' signals.
The GPS design calls for 24 space vehicles (GPS satellites) to be distributed equally among six circular orbital planes. The orbital planes are centered on the Earth, not rotating with respect to the distant stars. The six planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection).
Orbiting at an altitude of approximately 20,200 kilometers (orbital radius of 26,600 km), each SV makes two complete orbits each sidereal day, so it 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 from almost anywhere on Earth.
The satellites broadcast two forms of clock information, the Coarse / Acquisition code, or C/A which is freely available to the public, and the restricted Precise code, or P-code, usually reserved for military applications.
3.2 Accuracy and error sources
The position calculated by a GPS receiver requires the current time, the position of the satellite and the measured delay of the received signal. The position accuracy is primarily dependent on the satellite position and signal delay.
To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version. By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset to within about 1% of 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 higher-speed P(Y) signal. Assuming the same 1% accuracy, the faster P(Y) signal results in an accuracy of about 30 centimeters.
Electronics errors are one of several accuracy-degrading effects outlined in the table below. When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters. These effects also reduce the more precise P(Y) code's accuracy.
Source Effect
Ionospheric effects ± 5 meter
Ephemeris errors ± 2.5 meter
Satellite clock errors ± 2 meter
Multipath distortion ± 1 meter
Tropospheric effects ± 0.5 meter
Numerical errors ± 1 meter or less
Table 1. Sources of errors
3.2.1 Atmospheric effects
Changing atmospheric conditions change the speed of the GPS signals as they pass through the Earth's atmosphere and ionosphere. Correcting these errors is a significant challenge to improving GPS position accuracy. These effects are minimized when the satellite is directly overhead, and become greater for satellites nearer the horizon, since the signal is affected for a longer time. Once the receiver's approximate location is known, a mathematical model can be used to estimate and compensate for these errors.
Picture 1. Influenced propagation of radio waves through the Earth's atmosphere [12]
Because ionospheric delay affects the speed of radio waves differently based on 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 more precise correction. This can be done in civilian receivers without decrypting the P(Y) signal carried on L2, by tracking the carrier wave instead of the modulated code. To facilitate this on
lower cost receivers, a new civilian code signal on L2, called L2C, was added to the Block IIR-M satellites, first launched in 2005. It allows a direct comparison of the L1 and L2 signals using the coded signal instead of the carrier wave.
Picture 2. Position determination without and with atmospheric corrections by using the second frequency on a dual-frequency receiver [12]
The effects of the ionosphere are generally slow-moving, and can be averaged over time. The effects for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location. This correction is also valid for other receivers in the same general location. Several systems send this information over radio or other links to allow L1 only receivers to make ionospheric corrections. The ionospheric data are transmitted via satellite in Satellite Based Augmentation Systems such as WAAS, which transmits it on the GPS frequency using a special pseudo-random number (PRN), so 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 the ionospheric effects, making precise compensation for humidity more difficult. Altitude also causes a variable delay, as the signal passes through less atmosphere at higher elevations. Since the GPS receiver measures altitude directly, this is a much simpler correction to apply.
3.2.2 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. These delayed signals can cause inaccuracy. A variety of 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. Short delay reflections are harder to filter out since they are only slightly delayed, causing effects almost indistinguishable from routine fluctuations in atmospheric delay.
Multipath effects are much less severe in moving vehicles. 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.
Picture 3. Interference caused by reflection of the signals
3.2.3 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 tend 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. The onboard clocks are extremely accurate, but they do suffer from some clock drift. This problem tends to be very small, but may add up to 2 meters of inaccuracy.
This class of error is more "stable" than ionospheric problems and tends to change over days or weeks rather than minutes. This makes correction fairly simple by sending out a more accurate almanac on a separate channel.
V. I L- I.
" I - "J r-r ' jm —T* I .
,, L, . , . 1
Picture 4. Plot of the position determination with and without SA [12] 3.2.4 Selective availability
The GPS includes a feature called Selective Availability (SA) that introduces intentional errors between 0 meters and up to a hundred meters 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 horizontally and 30 meters 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 or setting it to an error of 0 meters (effectively the same thing) presented a clear benefit.
Under the pressure of Federal Aviation Administration during the 1990s Selective Availability was eventually "discontinued" in 2000 [5] following an announcement by U.S. President Bill Clinton allowing users access to an undegraded L1 signal. However, Selective Availability is still a system capability of GPS, and error could be in theory reintroduced at any time. In practice, in view of the hazards and costs this would induce for US and foreign shipping, it is unlikely to be reintroduced, and various government agencies, including the FAA [6], have stated that it is not intended to be reintroduced.
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.
3.2.5 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 [7] 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 [8]. 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.
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. During the Iraq War, the U.S. military claimed to destroy a GPS jammer with a GPS-guided bomb [9].
3.2.6 Relativity
According to the theory of relativity, due to 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). For the GPS satellites, general relativity predicts that the atomic clocks at GPS orbital altitudes will tick faster by about 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 will tick slower by about 7,200 ns per day than stationary
ground clocks. When combined, the discrepancy is 38 microseconds per day. To account for this, the frequency standard on-board each satellite is given a rate offset prior to launch, making it run slightly slower than the desired frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz.
Another relativistic effect to be compensated for in GPS observation processing is the Sagnac effect. The GPS time scale is defined in an inertial system but observations are processed in an Earth-centered, Earth-fixed (co-rotating) system; a system in which simultaneity is not uniquely defined. The Lorentz transformation between the two systems modifies the signal run time, a correction having opposite algebraic signs for satellites in the Eastern and Western celestial hemispheres. Ignoring this effect will produce an East-West error on the order of hundreds of nanoseconds, or tens of meters in position.
3.3 Error correction
3.3.1 Augmentation
Augmentation methods of improving accuracy rely on external information being integrated into the calculation process. There are many such systems in place and they are generally named or described based on how the GPS sensor receives the information. Some systems transmit additional information about sources of error (such as clock drift, ephemeris, or ionospheric delay), others provide direct measurements of how much the signal was off in the past, while a third group provide additional navigational or vehicle information to be integrated in the calculation process.
Examples of augmentation systems include the Wide Area Augmentation System, Differential GPS, and Inertial Navigation Systems.
The Wide Area Augmentation System (WAAS) is an extremely accurate navigation system developed for civil aviation by the Federal Aviation Administration (FAA), a division of the United States Department of Transportation (DOT). The system augments the Global Positioning System (or GPS) to provide the additional accuracy, integrity, and availability necessary to enable users to rely on GPS for all phases of flight, from en route through GLS approach for all qualified airports within the WAAS coverage area
Differential Global Positioning System (DGPS) is an enhancement to Global Positioning System that uses a network of fixed ground based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions. These stations broadcast the difference between the measured satellite pseudoranges and actual (internally computed) pseudoranges, and receiver stations may correct their pseudoranges by the same amount.
An inertial navigation system (INS) provides the position, velocity, orientation, and angular velocity of a vehicle by measuring the linear and angular accelerations applied to the system in an inertial reference frame. It is widely used because it refers to no real-world item beyond itself (other than the earth's magnetic field). It is therefore immune to jamming and deception.
3.3.2 Precise monitoring
The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways.
The first is called Dual Frequency monitoring, and refers to systems that can compare two or more signals, such as the L1 frequency to the L2 frequency. Since these are two different frequencies, they are affected in different, yet predictable ways by the atmosphere and objects around the receiver. After monitoring these signals, it is possible to calculate how much error is being introduced and then nullify that error.
Receivers that have the correct decryption key can relatively easily decode the P(Y)-code transmitted on both L1 and L2 to measure the error. Receivers that do not possess the key can still use a process called codeless to compare the encrypted information on L1 and L2 to gain much of the same error information. However, this technique is currently limited to
specialized surveying equipment. In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies. When these become operational, non-encrypted users will be able to make the same comparison and directly measure some errors.
A second form of precise monitoring is called Carrier-Phase Enhancement (CPGPS). The CPGPS approach utilizes 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 of satellite-receiver sequence matching.
3.4 Applications
3.4.1 Military
GPS allows accurate targeting of various military weapons including cruise missiles and precision-guided munitions. To help prevent GPS guidance from being used in enemy or improvised weaponry, the US Government controls the export of civilian receivers. A US-based manufacturer cannot generally export a receiver unless the receiver contains limits restricting it from functioning when it is at an altitude above 18 kilometers and traveling at over 515 m/s (1,000 knots).
The GPS satellites also carry nuclear detonation detectors, which form a major portion of the United States Nuclear Detonation Detection System.
3.4.2 Navigation
Automobiles can be equipped with GPS receivers at the factory or as after-market equipment. Units often display moving maps and information about location, speed, direction, and nearby streets and landmarks.
Picture 5. GPS Navigation System using TomTom software
Aircraft navigation systems usually display a "moving map" and are often connected to the autopilot for en-route navigation. Cockpit-mounted GPS receivers and glass cockpits are appearing in general aviation aircraft of all sizes, using technologies such as WAAS or LAAS to increase accuracy. Many of these systems may be certified for instrument flight rules navigation, and some can also be used for final approach and landing operations. Glider pilots use GNSS Flight Recorders to log GPS data verifying their arrival at turn points in gliding competitions. Flight computers installed in many gliders also use GPS to compute wind speed aloft, and glide paths to waypoints such as alternate airports or mountain passes, to aid en route decision making for cross-country soaring.
Boats and ships can use GPS to navigate all of the world's lakes, seas and oceans. Maritime GPS units include functions useful on water, such as "man overboard" (MOB) functions that allow instantly marking the location where a person has fallen overboard, which simplifies rescue efforts. GPS may be connected to the ships self-steering gear and chartplotters using the NMEA 0183 interface. GPS can also improve the security of shipping traffic by enabling AIS.
Heavy Equipment can use GPS in construction, mining and precision agriculture. The blades and buckets of construction equipment are controlled automatically in GPS-based machine guidance systems. Agricultural equipment may use GPS to steer automatically, or as a visual aid displayed on a screen for the driver. This is very useful for controlled traffic and
row crop operations and when spraying. Harvesters with yield monitors can also use GPS to create a yield map of the paddock being harvested.
Bicycles often use GPS in racing and touring. GPS navigation allows cyclists to plot their course in advance and follow this course, which may include quieter, narrower streets, without having to stop frequently to refer to separate maps. Some GPS receivers are specifically adapted for cycling with special mounts and housings.
Hikers, climbers, and even ordinary pedestrians in urban or rural environments can use GPS to determine their position, with or without reference to separate maps. In isolated areas, the ability of GPS to provide a precise position can greatly enhance the chances of rescue when climbers or hikers are disabled or lost (if they have a means of communication with rescue workers).
3.4.3 Surveying and mapping
Surveying — Survey-Grade GPS receivers can be used to position survey markers, buildings, and road construction. These units use the signal from both the L1 and L2 GPS frequencies. Even though the L2 code data are encrypted, the signal's carrier wave enables correction of some ionospheric errors. These dual-frequency GPS receivers typically cost US$10,000 or more, but can have positioning errors on the order of one centimeter or less when used in carrier phase differential GPS mode.
Mapping and GIS (geographic information system) — Most mapping grade GPS receivers use the carrier wave data from only the L1 frequency, but have a precise crystal oscillator which reduces errors related to receiver clock jitter. This allows positioning errors on the order of one meter or less in real-time, with a differential GPS signal received using a separate radio receiver. By storing the carrier phase measurements and differentially post-processing the data, positioning errors on the order of 10 centimeters are possible with these receivers.
Geophysics and geology — High precision measurements of crustal strain can be made with differential GPS by finding the relative displacement between GPS sensors. Multiple stations situated around an actively deforming area (such as a volcano or fault zone) can be used to find strain and ground movement. These measurements can then be used to interpret the cause of the deformation, such as a dike or sill beneath the surface of an active volcano.
3.4.4 Other uses
GPS is also helpful in precise time reference, mobile satellite communications (to help orienting the antenna), emergency and location-based services, location-based games, in-flight positioning service for aircraft passengers, heading information (instead of the good old compass), GPS tracking systems, weather prediction improvements, skydiving and other sport activities.
4. Other Global Navigation Satellite Systems
There are also other satellite navigation systems. Among them is GLONASS, Russian counterpart to GPS, which used to be fully functional, but then deteriorated due to funding shortage. A next-generation Galileo Positioning System is going to be built by the European Union and its partners. The China's local navigation satellite system Beidou-1 is supposed to be expanded to cover the whole globe and offer services similar to other positioning systems.
No wonder that the working principles and other aspects of these systems are very similar to those of GPS, therefore we'll just dwell on some of their technical characteristics.
4.1 GLONASS
A characteristic of the GLONASS constellation is that the satellite orbits repeat after 8 days. As each orbit plane contains 8 satellites, there is a non-identical repeat (i.e., another satellite will occupy the same place in the sky) after one sidereal day. This differs from the GPS identical repeat period of one sidereal day.
At peak efficiency the system offered a standard (coarse-acquisition or C/A) positioning and timing service giving horizontal positioning accuracy within 57-70 meters, vertical positioning
within 70 meters, velocity vector measuring within 15 cm/s and timing within 1 |is, all based on measurements from four satellite signals simultaneously. A more accurate signal (precision or P(Y)) was available for Russian military use. In November 2006, Defense Minister Sergei Ivanov announced that the military signal will become available for civilian use in early 2007.
Like GPS, the complete nominal GLONASS constellation consists of 24 satellites, 21 operating and three on-orbit 'spares' placed in three orbital planes. Each plane contains eight satellites identified by "slot" number, which defines the corresponding orbital plane and the location within the plane: 1-8, 9-16, 17-24. The three orbital planes are separated by 120°, and the satellites are equally spaced within the same orbital plane, 45° apart. The GLONASS orbits are roughly circular, with an inclination of about 64.8° and a semi-major axis of 25,440 km. The planes themselves have an argument of latitude displacement of 15°.
The GLONASS constellation orbits the Earth at an altitude of 19,100 km (slightly lower than that of the GPS satellites). Each satellite orbits the Earth approximately every 11 hours, 15 minutes. The satellites' orbital spacing is arranged so that, if the constellation was fully populated, a minimum of 5 satellites are in view from any given point at any given time.
GLONASS satellites transmit two types of signal: standard precision (SP) and high precision (HP). The SP signal on L1 has a frequency division multiple access scheme: L1 = 1602 MHz + 0.5625n MHz, where n is a satellite's frequency channel number (n=0,1,2...).
4.2 Galileo
The Galileo positioning system, referred to simply as Galileo, is a proposed Global Navigation Satellite System, to be built by the European Satellite Navigation Industries for the European Union and European Space Agency (ESA) as an alternative to the United States operated Global Positioning System and the Russian GLONASS.
The Galileo's space segment will consist of 30 satellites orbiting at an altitude of 23 222 km. Three orbital planes will have 56° inclination (9 operational satellites and one active spare per orbital plane)
There will be four different navigation services available:
The Open Service (OS) will be free for anyone to access. The OS signals will be broadcast in two bands, at 1164-1214 MHz and at 1563-1591 MHz. Receivers will achieve an accuracy of <4 m horizontally and <8 m vertically if they use both OS bands. Receivers that use only a single band will still achieve <15 m horizontally and <35 m vertically, comparable to what the civilian GPS C/A service provides today. It is expected that most future mass market receivers, such as automotive navigation systems, will process both the GPS C/A and the Galileo OS signals, for maximum coverage.
The encrypted Commercial Service (CS) will be available for a fee and will offer an accuracy of better than 1 m. The CS can also be complemented by ground stations to bring the accuracy down to less than 10 cm. This signal will be broadcast in three frequency bands, the two used for the OS signals, as well as at 1260-1300 MHz.
The encrypted Public Regulated Service (PRS) and Safety of Life Service (SoL) will both provide accuracy comparable to the Open Service. Their main aim is robustness against jamming and the reliable detection of problems within 10 seconds. They will be targeted at security authorities (police, military, etc.) and safety-critical transport applications (air-traffic control, automated aircraft landing, etc.), respectively.
In addition, the Galileo satellites will be able to detect and report signals from Cospas-Sarsat search-and-rescue beacons in the 406.0-406.1 MHz band, which makes them a part of the Global Maritime Distress Safety System.
4.3 Beidou
Unlike the GPS, GLONASS, and Galileo systems, which use intermediate circular-orbiting satellites, Beidou uses satellites in geostationary orbit. This means that the system does not require a large constellation of satellites, but it also limits the coverage to areas on
Earth where the satellites are visible. The area that can be serviced is from 70°E to 140°E, and from 5°N to 55°N.
However, China has planned to develop a truly global satellite navigation system consisting of 35 satellites (known as Compass or Beidou-2).
The new system will include 5 geostationary orbit (GEO) satellites and 30 medium Earth orbit (MEO) satellites, that will offer complete coverage of the globe. There will be two levels of service provided; free service for those in China, and licensed service for the military.
The free service will have a 10 meter location-tracking accuracy, will synchronize clocks with an accuracy of 50 ns, and measure speeds within 0.2 m/s.
The licensed service will be more accurate than the free service, can be used for communication, and will supply information about the system status to the users.
5. The Future of GNSS
Who are likely to be GNSS users? Without a doubt many countries and users will embrace GNSS due to its high accuracy and independency. This could also result in many more users in higher latitudes for which the system is designed if GALILEO is incorporated. Along with NAVSTAR, applications will be augmented, particularly for regions where satellite visibility in valleys and inner cities are involved resulting in more real-time applications. NAVSTAR will continue to dominate the GPS scene due to its already existing and installed infrastructure that supports the system in hardware, software and technical know how. But more importantly is the issue of interoperability and capitalizing upon all three systems.
Also, currently static GPS sampling involves occupying a position for a longer time period. Using GNSS, static samples would likely have higher levels of accuracy and may even require shorter occupation times.
When should we expect for the changes in satellite navigation? NAVSTAR is the only fully operating system at the moment and major improvements in the system are planned by the U.S. Department of Defense. We can't still say certainly whether the recent launching of GLONASS satellites is a trend, which will continue until a full complement of navigation satellites is available. GALILEO itself is not planned to be fully operational until 2010. However, in the long term we will be hearing more about GNSS, and the benefits as new products and technologies evolve in the marketplace.
6. Conclusion
Recent launches of new satellites in the GLONASS GPS system may lead to the establishment of a second fully functional GPS system in the near future. Coupled with the already existing U.S. NAVSTAR system, GNSS applications may grow in the future. A third European based satellite system called GALILEO has been approved in principle but is not expected to be fully operational before 2010. Designed with interoperability in mind, GALILEO in addition to the others could result in GPS users having access to almost 75 satellites for highly accurate navigation and positioning. These developments have many potential advantages for not only the data collection aspects for GPS users, but also for the development of new applications using GIS worldwide. Potential areas of growth using these coupled systems will be associated with new applications, hardware and software.
7. Literature
1. http://paul.luminos.nl/go_file.php?t=1&f=satellite_navigation.pdf ("Satellite Navigation" P. F. Lammertsma, Institute of Information and Computing Sciences, Utrecht University, February 2, 2005)
2. http://www.radioshack.com/ (RadioShack Corporation)
3. http://www,wikipedia.org/ (Wikipedia, the free encyclopedia)
4. http://www.howstuffworks.com/gps.htm (By Marshall Brain and Tom Harris, "How GPS Receivers Work")
5. http://www.ostp.gov/html/0053 2.html (Office of Science and Technology Policy. Presidential statement to stop degrading GPS. May 1, 2000)
6. http://gps.faa.gov/gpsbasics/SA-text.htm (FAA, Selective Availability. Retrieved Jan. 6, 2007)
7. http://www.phrack.org/archives/60/p60-0x0d.txt (Phrack. Issue 0x3c (60), article 13. December 28, 2002)
8. http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=43404&&pageID=1 (GPS World. The hunt for an unintentional GPS jammer. January 1, 2003)
9. http://www.defenselink.mil/news/Mar2003/n03252003 200303254.html (American Forces Press Service. CENTCOM charts progress. March 25, 2003)
10. http://www.ipgp.jussieu.fr/~tarantola/Files/Professional/GPS/Neil Ashby Relativity GPS .pdf (Ashby, Neil Relativity and GPS. Physics Today, May 2002)
11. http://www.GISCafe.com (GISCafe.com, March 2002)
12. http://www.kowoma.de/en/gps/errors.htm (Michael WoBner)
