Global Positioning System: Difference between revisions

==== Space segment ====

The orbital planes are centered on the Earth, not rotating with respect to the distant stars.<ref>[http://metaresearch.org/cosmology/gps-relativity.asp What the Global Positioning System Tells Us about Relativity]. Accessed [[January 2]], [[2007]].</ref> The six planes have approximately 55° [[inclination]] (tilt relative to Earth's [[equator]]) and are separated by 60° [[right ascension]] of the [[orbital node|ascending node]] (angle along the equator from a reference point to the orbit's intersection).<ref name="GPS overview from JPO">[http://gps.losangeles.af.mil/jpo/gpsoverview.htm GPS Overview from the NAVSTAR Joint Program Office]. Accessed [[December 15]], [[2006]].</ref>

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.  As of 2006, even low-cost units commonly include [[Wide Area Augmentation System|WAAS]] receivers.

=== Navigation signals ===

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, civil receivers are highly vulnerable to spoofing since correctly formatted C/A signals can be generated using readily available signal generators.  [[RAIM]] features will not help, since RAIM only checks the signals from a navigational perspective.

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.

One interesting side effect of the Selective Availability hardware is the capability to correct the frequency of the GPS Cs and Rb clocks to approximately two parts in ten to the thirteenth frequency accuracy. This represented a significant improvement over the raw accuracy of the clocks.{{fact|date=March 2007}}

{{further|[[SAASM|Selective Availability / Anti-Spoofing Module]]}}

[[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<ref>[[Phrack]]. [http://www.phrack.org/archives/60/p60-0x0d.txt Issue 0x3c (60), article 13]. [[December 28]], [[2002]].</ref> 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.<ref>GPS World. [http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=43404&&pageID=1 The hunt for an unintentional GPS jammer]. [[January 1]], [[2003]].</ref> 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".<ref>Ruley, John. AVweb. [http://www.avweb.com/news/avionics/182754-1.html GPS jamming]. [[February 12]], [[2003]].</ref> [[RAIM|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 storm]]s, predominantly at high latitudes.<ref>[[Space Environment Center]]. [http://www.sec.noaa.gov/nav/gps.html SEC Navigation Systems GPS Page]. [[August 26]], [[1996]].</ref>

'''Relative Kinematic Positioning''' (RKP) is another approach for a precise GPS-based positioning system. In this approach, determination of range signal can be resolved to an accuracy of less than 10 [[centimeters]] (4 in). This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver. This can be accomplished by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests—possibly with processing in real-time ([[Real Time Kinematic|real-time kinematic positioning]], RTK).

While most clocks are synchronized to [[Coordinated Universal Time]] (UTC), the [[Atomic clock]]s on the satellites are set to '''GPS time.''' The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain [[leap seconds]] or other corrections which are periodically added to UTC. GPS time was set to match [[Coordinated Universal Time]] (UTC) in 1980, but has since diverged. The lack of corrections means that GPS time remains synchronized with the [[International Atomic Time]] (TAI).  

The project aims to improve the accuracy and availability for all users and involves new ground stations, new satellites, and four additional navigation signals. New civilian signals are called '''L2C''', '''L5''' and '''L1C'''; the new military code is called '''M-Code'''. A goal of 2013 has been established with incentives offered to the contractors if they can complete it by 2011.

GPS allows accurate targeting of various military weapons including [[cruise missile]]s and [[precision-guided munition]]s. 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 simultaneously (1) at an altitude above 18 kilometers (60,000ft) and (2) traveling at over 515 m/s (1,000 knots).<ref>Arms Control Association. [http://www.armscontrol.org/documents/mtcr.asp Missile Technology Control Regime]. Accessed [[May 17]], [[2006]].</ref>

The GPS satellites also carry nuclear detonation detectors, which form a major portion of the [[United States Nuclear Detonation Detection System]].<ref>Sandia National Laboratory's [http://www.sandia.gov/LabNews/LN03-07-03/LA2003/la03/arms_story.htm Nonproliferation programs and arms control technology].</ref>

[[Image:TomTom Screenshot.jpg|right|thumb|GPS Navigation System using [[TomTom (company)|TomTom]] software]]

[[Image:GPS receiver (mouse).jpg|thumb|right|A modern SiRF Star III chip based 20-channel GPS receiver with WAAS/EGNOS support.]]

*'''[[Surveying]]''' — Survey-Grade GPS receivers can be used to position [[survey marker]]s, 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.

*'''Geophysics and geology''' — High precision measurements of [[crust (geology)|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.

[[Image:GPS roof antenna dsc06160.jpg|thumb|right|This antenna is mounted on the roof of a hut containing a scientific experiment needing precise timing.]]

*'''Precise time reference''' — Many systems that must be accurately [[synchronization|synchronized]] use GPS as a source of accurate time. GPS can be used as a [[Radio clock|reference clock]] for [[time code]] generators or [[Network Time Protocol|NTP]] clocks. [[Sensor]]s (for [[seismology]] or other monitoring application), can use GPS as a precise time source, so events may be timed accurately. [[Time division multiple access|TDMA]] communications networks often rely on this precise timing to synchronize RF generating equipment, network equipment, and [[multiplexer]]s.

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