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Global Positioning System
The Global Positioning System, usually called GPS, is the only fully-functional satellite navigation system. A constellation of more than two dozen GPS satellites broadcasts precise timing signals by radio to GPS receivers, allowing them to accurately determine their location (longitude, latitude, and altitude) in any weather, day or night, anywhere on Earth.
GPS has become a vital global utility, indispensable for modern navigation on land, sea, and air around the world, as well as an important tool for map-making, and land surveying. GPS also provides an extremely precise time reference, required for telecommunications and some scientific research, including the study of earthquakes.
ApplicationsMilitaryGPS 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. [2]
Navigation
This taxi in Kyoto, equipped with GPS navigation, is an example of how GPS technology can be applied in routine activities.
GPS is used by people around the world as a navigation aid in cars, airplanes, and ships. The system can also be used by computer controlled harvesters, mine trucks and other vehicles. Hand-held GPS receivers can be used by mountain climbers and hikers. Glider pilots use the logged signal to verify their arrival at turnpoints in competitions. Low cost GPS receivers are often combined in a bundle with a PDA, car computer, or vehicle tracking system. GPS equipment is even available for the visually impaired. SurveyingMore 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 Automatic Grade Control systems that use GPS positions and 3D site plans to automatically control the blades and buckets of construction equipment.
Geocaching
GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such those shown here from manufacturers Trimble, Garmin and Leica (respectively, left to right).
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 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. Aircraft passengersMost 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.[3]
Precise time referenceMany 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.
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 UTC time adjust 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. 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.
Geophysics and GeologyHigh 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.
Location-based servicesGPS functionality is opening up new and innovative ways to use search in mobile phones with location-based service. In the future mobile units could be equipped with GPS technology, such as Assisted GPS, which could be coupled with web search tools to allow users to search keywords and have their results be returned that are specific to their geographic locations.
Technical descriptionNavigation signals
GPS broadcast signal
GPS satellites broadcast three different types of data in the primary navigation signals. The first is the almanac which sends coarse time information with second precision, along with status information about the satellites. The second is the ephemeris, which contains orbital information that allows the receiver to calculate the position of the satellite at any point in time. These bits of data are folded into the 37,500 bit Navigation Message, or NM, which takes 12.5 minutes to send at 50 Hz.
The satellites also broadcast two forms of accurate clock information, the Coarse Acquisition code, or C/A, and the Precise code, or P-code. The former is normally used for most civilian navigation. It consists of a 1,023 bit long pseudo-random code broadcast at 1.023 MHz, repeating every millisecond. Each satellite sends a distinct C/A code, which allows them to be identified. The P-code is a similar code broadcast at 10.23 MHz, but it repeats only once per week. In normal operation, the so-called "anti-spoofing mode", the P code is first encrypted into the Y-code, or P(Y), which can only be decrypted by units with a valid decryption key. All three signals, NM, C/A and P(Y), are mixed together and sent on the primary radio channel, L1, at 1575.42 MHz. The P(Y) signal is also broadcast alone on the L2 channel, 1227.60 MHz. Several additional frequencies are used for unrelated purposes.
Calculating positionsGPS allows receivers to accurately calculate their distance from the GPS satellites. The receivers do this by measuring the time delay between when the satellite sent the signal and the local time when the signal was received. This delay, multiplied by the speed of light, gives the distance to that satellite. The receiver also calculates the position of the satellite based on information periodically sent in the same signal. By comparing the two, position and range, the receiver can discover its own location.
Pseudorange
To calculate its position, a receiver first needs to know the precise time. To do this, it uses an internal crystal oscillator-based clock that is continually updated by the signals being sent in L1 from various satellites. At that point the receiver identifies the visible satellites by the distinct pattern in their C/A codes. It then looks up the ephemeris data for each satellite, which was captured from the NM and stored in memory. This data is used in a formula that calculates the precise location of the satellites at that point in time.
Finally the receiver must calculate the time delay to each satellite. To do this, it produces an identical C/A sequence from a known seed number. The time delay is calculated by increasingly delaying the local signal and comparing it to the one received from the satellite; at some point the two signals will match up, and that delay is the time needed for the signal to reach the receiver. The delay is generally between 65 and 85 milliseconds. The distance to that satellite can then be calculated directly, the so-called pseudorange.
The receiver now has two key pieces of information: an accurate estimate of the position of the satellite, and an accurate measurement of the distance to that satellite. This tells the receiver that it lies on the surface of an imaginary sphere whose radius is that distance. To calculate the precise position, at least four such measurements are taken simultaneously. This places the receiver at the intersection of the four imaginary spheres. Since the C/A pattern repeats every millisecond, it can only be used to place the user within 300 kilometers (180 mi). Thus the multiple measurements are also needed to determine whether the receiver has lined up its internal C/A code properly, or is "one off".
The calculation of the position of the satellite, and thus the time delay and range to it, all depend on the accuracy of the local clock. The satellites themselves are equipped with extremely accurate atomic clocks, but this is not economically feasible for a receiver. Instead, the system takes redundant measurements to re-capture the correct clock information.
To understand how this works, consider a local clock that is off by .1 microseconds, or about 30 meters (100 ft) when converted to distance. When the position is calculated using this clock, the range measurements to each of the satellites will read 30 meters too long. In this case the four spheres will not overlap at a point, instead each sphere will intersect at a different point, resulting in several potential positions about 30 meters apart. The receiver then uses a mathematical technique to calculate the clock error that would produce this offset, in this case .1 microseconds, adjusts the range measurements by this amount, and then updates the internal clock to make it more accurate.
This technique can be applied with any four satellites. Commercial receivers therefore attempt to "tune in" to as many satellites as possible, and repeatedly make this correction. In doing so, clock errors can be reduced almost to zero. In practice, anywhere from six to ten measurements are taken in order to round out errors, and civilian receivers generally have 10 to 12 channels in total.
Calculating a position with the P(Y) signal is generally similar in concept, assuming one can decrypt it. The encryption is essentially a safety mechanism; if a signal can be successfully decrypted, it is reasonable to assume it is a real signal being sent by a GPS satellite. In comparison, the C/A signal can be generated fairly easily, allowing an unscrupulous user to send out their own fake signal, which would be difficult to distinguish from the original. Mathematical techniques can be used here as well, making spoofing of the C/A signal a very difficult prospect for any modern receiver equipped with some sort of RAIM system.
AccuracyBest caseThe 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 300,000,000 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 course 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 effectsOne of 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 military is considering broadcasting the C/A signal on L2 starting with the Block III-R satellites. This would allow a direct comparison of the L1 and L2 signals using the same circuitry that already decodes the C/A on L1.
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 metres 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 that 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 effectsGPS 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 errorsThe 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 accuracyThe accuracy of GPS can be improved several ways:
Selective availabilityWhen 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 disabling the Selective Availability. This was, perhaps, 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[8] 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
GPS satellite on test rack
The GPS system uses a satellite constellation of at least 24 active satellites 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 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. [9] 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. [10]
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. [11]
Frequencies usedSeveral frequencies make up the GPS electromagnetic spectrum:
Two new signals are also being studied:
A modern SiRF Star III chip based 20-channel GPS receiver with WAAS/EGNOS support.
ReceiversGPS 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 effected by improved chip making, allowing even low cost modern receivers to outperform much more expensive earlier models.
Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. NMEA 2000[12] 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.
Likewise, many receivers include an input for differential corrections, using the RTCM SC-104 format. This is typical 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.
Some receivers can also use Bluetooth for a wireless connection.
RelativityThe clocks on the satellites are affected by both special and general relativity, which causes them to run at a slightly faster rate than do clocks on the Earth's surface. This amounts to a discrepancy of around 38 microseconds per day. To account for this, the frequency standard on-board the satellites runs slightly slower than its desired speed on Earth, at 10.22999999543 MHz instead of 10.23 MHz—a difference of 0.00457 Hz.[13] This offset is a practical demonstration of the theory of relativity in a real-world system; it is exactly what has been predicted by the theory, within the limits of accuracy of measurement.
Neil Ashby presented a good account of how these relativistic corrections are applied, why, and their orders of magnitude, in Physics Today (May 2002).[14] Whether relativity must be considered as a mere correction to a Newtonian GPS theory, or, rather, as the necessary foundation of a cleaner (and more fundamental) GPS theory, is currently under debate. Bartolomé Coll has recently developed the basic notions necessary for a fully relativistic theory of Positioning Systems.[15]
References1. Federal Aviation Administration. FAA WAAS fact-sheet. Accessed May 14, 2006
2. Arms Control Association. Missile Technology Control Regime. Accessed May 17, 2006.
3. Joe Mehaffey. Is it Safe to use a handheld GPS Receiver on a Commercial Aircraft?. Accessed May 15, 2006.
4. Hydrographic Journal. Developments in Global Navigation Satellite Systems. April 2002. Accessed May 14, 2006.
5. United States Department of Defense. Announcement of Initial Operational Capability. December 8, 1993.
6. National Archives and Records Administration. U.S. GLOBAL POSITIONING SYSTEM POLICY. March 29, 1996.
7. Federal Highway Administration. Nationwide DGPS Program Fact Sheet. Accessed May 14, 2006
8. Office of Science and Technology Policy. Presidential statement to stop degrading GPS. May 1, 2000.
9. HowStuffWorks. How GPS Receivers Work. Accessed May 14, 2006.
10. Dana, Peter H. GPS Orbital Planes. August 8, 1996.
11. USNO. NAVSTAR Global Positioning System. Accessed May 14, 2006.
12. NMEA. NMEA 2000
13. Rizos, Chris. University of New South Wales. GPS Satellite Signals. 1999.
14. Physics Today. Relativity and GPS. May 2002.
15. Bartolomé Coll. Coll on relativity.
16. United States Naval Research Laboratory. National Medal of Technology for GPS. November 21, 2005
17. Phrack. Issue 0x3c (60), article 13. December 28, 2002.
18. GPS World. The hunt for an unintentional GPS jammer. January 1, 2003.
19. Ruley, John. AVweb. GPS jamming. February 12, 2003.
20. Space Environment Center. SEC Navigation Systems GPS Page. August 26, 1996.
21. American Forces Press Service. CENTCOM charts progress. March 25, 2003.
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