GPS (applications)
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[edit] 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.
Due to the potential for GPS receiver technology to be used in improvised weaponry, the US Government has classified civilian receivers for controlled export. This means that, in general, a US-based manufacturer can not export a receiver unless it has limits on the velocities and altitudes at which it will report position and speed information.[1]
[edit] Navigation
GPS is used by people around the world for navigation in vehicles of all types and when moving about on foot.
[edit] Automobiles
Many automobiles are equipped with GPS receivers at the time of manufacture, or as after-market equipment. Units often display moving maps and information about location, speed, direction, and nearby streets and landmarks.
[edit] Ships
Boats and ships are important users of GPS, which allows them to find their way anywhere on the world's seas and oceans, even in the absence of any other reference points. Maritime GPS units include functions specifically useful for use on water, such as “man overboard” (MOB) functions that allow instant recording and retrieval of the location at which a person has fallen overboard, which simplifies rescue efforts. GPS capabilities may be integrated into larger systems for navigational or other use aboard the boat or ship (e.g. Chartplotters). GPS also allows to increase the security of shipping traffic by enabling AIS. Further applications of GPS is tracking vessels on the globe. Usually, GPS will be integrated using the NMEA 0183 interface.
[edit] Aircraft
Many aircraft use GPS for en-route, terminal, and final approach navigation. Aviation-certified units usually provide moving map displays and are often integrated with autopilot systems to allow for completely automated en-route navigation. Aviation GPS receivers may be built into aircraft, but handheld aviation receivers are available and in common use for general aviation.
Aviation GPS receivers may use augmentation technologies such as WAAS or LAAS to increase accuracy and permit the use of GPS for approach and landing operations.
The relatively poor accuracy of altitude data in standard GPS (as compared to lateral navigation measurements or traditional barometric altimeters) has made GPS inappropriate as the sole means of altitude measurement in aircraft, but continuing development in GPS and augmentation technologies may lift this restriction in the future, particularly for certain critical phases of flight such as instrument landings.
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 the glide path to waypoints stored in its database, such as alternate airports or mountain passes, as an aid to enroute decision making for cross-country soaring. Most glider flight computers can also compute the winds aloft using GPS velocity information when circling or flying different headings.
[edit] Bicycle
GPS is used by Cyclists for racing and touring. Cyclists often prefer to use quieter narrower streets. GPS navigation allows cyclists to plot their course in advance and follow this course, without having to stop frequently to refer to separate maps. Some GPS receivers are specifically adapted for cycling with special mounts and housings.
[edit] On foot
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). Many models of inexpensive handheld GPS receivers are available for these uses, and they are one type of personal navigation device used for these applications. (See also geocaching under "Location-based games" below.)
[edit] Other uses
Low-cost GPS receivers are often combined with PDAs, cell phones, car computers, or vehicle tracking systems. Examples of GPS-based services are MapQuest Mobile, Veripath Navigator and TomTom digital maps. The system can be used to automate harvesters, mine trucks, and other vehicles. GPS equipment for the visually impaired is available.
[edit] 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.
[edit] 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.
[edit] Location-based games
The availability of hand-held GPS receivers for a cost of about US$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.
[edit] 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.[2]
[edit] Surveying
More costly and precise receivers are used by land surveyors to position boundaries, structures, and survey markers, and for road construction. Survey grade GPS receivers use the carrier wave signal from both the L1 and L2 GPS frequencies. The second frequency enables correction of some ionospheric errors, even though the L2 code data is encrypted for military use only. 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.
[edit] Mapping and GIS
Mapping of resources and other less precise applications typically used with Geographic Information Systems often require greater precision than is possible with autonomous GPS receivers, but do not justify the expense of a survey grade receiver. 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.
[edit] Machine Guidance
GPS can be used by systems that automatically control the blades and buckets of construction equipment. GPS is also used in precision agriculture and can be coupled to an aircraft autopilot or self-steering gear for a ship. GPS can be used by Agricultural equipment 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.
[edit] Geophysics and geology
High precision measurements of crustal strain can be made with differential 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.
[edit] 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.
[edit] Heading information
The GPS system can be used to determine heading information, even though it was not designed for this purpose. "GPS compasses" [1] use a pair of antennas separated by about 50 cm to detect the phase difference in the carrier signal from a particular GPS satellite. Given the positions of the satellite, the position of the antenna, and the phase difference, the orientation of the two antennas can be computed. More expensive GPS compass systems use three antennas in a triangle to get three separate readings with respect to each satellite. A GPS compass is not subject to magnetic declination as a regular magnetic compass is, and it does not need to be reset periodically like a gyrocompass. It is, however, subject to multipath effects.
[edit] 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.
