Satellite navigation device
A satellite navigation device, satnav device or satellite navigation receiver is a user equipment that uses one or more of several global navigation satellite systems (GNSS) to calculate the device's geographical position and provide navigational advice. Depending on the software used, the satnav device may display the position on a map, as geographic coordinates, or may offer routing directions.
"GPS navigation" redirects here. For navigation provided by a GPS, see Turn-by-turn navigation.
As of 2023, four GNSS systems are operational: the original United States' Global Positioning System (GPS), the European Union's Galileo, Russia's GLONASS,[1][2] and China's BeiDou Navigation Satellite System. The Indian Regional Navigation Satellite System (IRNSS) will follow and Japan's Quasi-Zenith Satellite System (QZSS) scheduled for 2023 will augment the accuracy of a number of GNSS.
A satellite navigation device can retrieve location and time information from one or more GNSS systems in all weather conditions, anywhere on or near the Earth's surface. Satnav reception requires an unobstructed line of sight to four or more GNSS satellites,[3] and is subject to poor satellite signal conditions. In exceptionally poor signal conditions, for example in urban areas, satellite signals may exhibit multipath propagation where signals bounce off structures, or are weakened by meteorological conditions. Obstructed lines of sight may arise from a tree canopy or inside a structure, such as in a building, garage or tunnel. Today, most standalone Satnav receivers are used in automobiles. The Satnav capability of smartphones may use assisted GNSS (A-GNSS) technology, which can use the base station or cell towers to provide a faster Time to First Fix (TTFF), especially when satellite signals are poor or unavailable. However, the mobile network part of the A-GNSS technology would not be available when the smartphone is outside the range of the mobile reception network, while the satnav aspect would otherwise continue to be available.
An automotive navigation system takes its location from a GNSS system and, depending on the installed software, may offer the following services:
Sensitivity[edit]
GNSS devices vary in sensitivity, speed, vulnerability to multipath propagation, and other performance parameters. High-sensitivity receivers use large banks of correlators and digital signal processing to search for signals very quickly. This results in very fast times to first fix when the signals are at their normal levels, for example, outdoors. When signals are weak, for example, indoors, the extra processing power can be used to integrate weak signals to the point where they can be used to provide a position or timing solution.
GNSS signals are already very weak when they arrive at the Earth's surface. The GPS satellites only transmit 27 W (14.3 dBW) from a distance of 20,200 km in orbit above the Earth. By the time the signals arrive at the user's receiver, they are typically as weak as −160 dBW, equivalent to 100 attowatts (10−16 W). This is well below the thermal noise level in its bandwidth. Outdoors, GPS signals are typically around the −155 dBW level (−125 dBm).
Conventional GPS receivers integrate the received GPS signals for the same amount of time as the duration of a complete C/A code cycle which is 1 ms. This results in the ability to acquire and track signals down to around the −160 dBW level. High-sensitivity GPS receivers are able to integrate the incoming signals for up to 1,000 times longer than this and therefore acquire signals up to 1,000 times weaker, resulting in an integration gain of 30 dB. A good high-sensitivity GPS receiver can acquire signals down to −185 dBW, and tracking can be continued down to levels approaching −190 dBW.
High-sensitivity GPS can provide positioning in many but not all indoor locations. Signals are either heavily attenuated by the building materials or reflected as in multipath. Given that high-sensitivity GPS receivers may be up to 30 dB more sensitive, this is sufficient to track through 3 layers of dry bricks, or up to 20 cm (8 inches) of steel-reinforced concrete, for example.
Examples of high-sensitivity receiver chips include SiRFstarIII and MediaTekʼs MTK II.[11]
Aviators[edit]
Aviators use Satnav to navigate and to improve safety and the efficiency of the flight. This may allow pilots to be independent of ground-based navigational aids, enable more efficient routes and provide navigation into airports that lack ground-based navigation and surveillance equipment. There are now some GPS units that allow aviators to get a clearer look in areas where the satellite is augmented to be able to have safe landings in bad visibility conditions. There have now been two new signals made for GPS, the first being made to help in critical conditions in the sky and the other will make GPS more of a robust navigation service. Many aviator services have now made it a required service to use a GPS.[26] Commercial aviation applications include GNSS devices that calculate location and feed that information to large multi-input navigational computers for autopilot, course information and correction displays to the pilots, and course tracking and recording devices.
Military[edit]
Military applications include devices similar to consumer sport products for foot soldiers (commanders and regular soldiers), small vehicles and ships, and devices similar to commercial aviation applications for aircraft and missiles. Examples are the United States military's Commander's Digital Assistant and the Soldier Digital Assistant.[27][28][29][30] Prior to May 2000 only the military had access to the full accuracy of GPS. Consumer devices were restricted by selective availability (SA), which was scheduled to be phased out but was removed abruptly by President Clinton.[31] Differential GPS is a method of cancelling out the error of SA and improving GPS accuracy, and has been routinely available in commercial applications such as for golf carts.[32] GPS is limited to about 15 meter accuracy even without SA. DGPS can be within a few centimeters.[33]
Privacy concerns[edit]
User privacy may be compromised if Satnav equipped handheld devices such as mobile phones upload user geo-location data through associated software installed on the device. User geo-location is currently the basis for navigational apps such as Google Maps, location-based advertising, which can promote nearby shops and may allow an advertising agency to track user movements and habits for future use. Regulatory bodies differ between countries regarding the treatment of geo-location data as privileged or not. Privileged data cannot be stored, or otherwise used, without the user's consent.[41]
Vehicle tracking systems allow employers to track their employees' location raising questions regarding violation of employee privacy. There are cases where employers continued to collect geo-location data when an employee was off duty in private time.[42]
Rental car services may use the same technique to geo-fence their customers to the areas they have paid for, charging additional fees for violations.[43] In 2010, New York Civil Liberties Union filed a case against the Labor Department for firing Michael Cunningham after tracking his daily activity and locations using a Satnav device attached to his car.[44] Private investigators use planted GPS devices to provide information to their clients on a target's movements.