GPS and other common positioning systems
By GB Blog Official 2018-04-23 9571 1
GPS: Global Positioning System
Global Positioning System is a positioning system based on Medium Earth Orbit satellites. It is developed by the United States' Department of Defense, aiming to provide precise position and high-accuracy time reference for most areas on the earth. It uses 24 satellites, at least 3 of which to determine your current location on the earth.
The application of GPS
GPS is often used by civilians as a navigation system. On the ground, any GPS receiver contains a computer that "triangulates" its own position by getting bearings from at least three satellites. The result is provided in the form of a geographic position - longitude and latitude - to, for most receivers, within an accuracy of 10 to 100 meters. Software applications can then use those coordinates to provide driving or walking instructions.
GPS receiver start
Getting a lock on by the GPS receivers on the ground usually takes some time especially where the receiver is in a moving vehicle or in dense urban areas. The initial time needed for a GPS lock is usually dependent on how the GPS receiver starts. There are three types of start - hot, warm and cold.
The hot start is when the GPS device remembers its last calculated position and the satellites in view, the almanac used (information about all the satellites in the constellation), the UTC Time and makes an attempt to lock onto the same satellites and calculate a new position based upon the previous information. This is the quickest GPS lock but it only works if you are generally in the same location as you were when the GPS was last turned off.
The warm start is when the GPS device remembers its last calculated position, almanac used, and UTC Time, but not which satellites were in view. It then performs a reset and attempts to obtain the satellite signals and calculates a new position.
The receiver has a general idea of which satellites to look for because it knows its last position and the almanac data helps identify which satellites are visible in the sky. This takes longer than a hot start but not as long as a cold start.
And finally - the cold start is when the GPS device dumps all the information, attempts to locate satellites and then calculates a GPS lock. This takes the longest because there is no known information.
The GPS receiver has to attempt to lock onto a satellite signal from any available satellites, basically like polling, which takes a lot longer than knowing which satellites to look for. This GPS lock takes the longest.
Assisted GPS technology
In an attempt to improve lock times, cellphone manufacturers and operators have introduced the Assisted GPS technology, which downloads the current ephemeris for a few days ahead via the wireless networks and helps triangulate the general user’s position with the cell towers thus allowing the GPS receiver to get a faster lock at the expense of several (kilo)bytes.
Assisted GPS (A-GPS) is a system that often significantly improves start-up performance of a GPS satellite-based positioning system. By combining the information of network base station and GPS, it can locate the position of mobile devices, good for speed up start-up times of GPS-based positioning systems.
A-GPS is extensively used with GPS-capable cellular phones, along with GSM, GPRS, WCDMA and CDMA2000 networks.
By employing A-GPS, receivers do not have to download and decode navigation data from GPS satellites, so they can have more time and processing capacity to track GPS signals, which can reduce first-time positioning time, increase sensitivity, and maximum availability.
While GPS may have problems getting a lock when the signal is weak and in such a case A-GPS would assist in getting a lock. This, however, is achieved by the use of an Assistance Server, so a data connection is required and charges may apply for the data transfer.
Other common positioning systems
GPS is the most widely used positioning system. Besides GPS, GLONASS and BDS positioning systems are also used in our lives.
GLONASS is an acronym for Globalnaya Navigazionnaya Sputnikovaya Sistema, or Global Navigation Satellite System. GLONASS is Russia’s version of GPS (Global Positioning System), is a space-based satellite navigation system operating in the radio navigation-satellite service and used by the Russian Aerospace Defence Forces. It provides an alternative to GPS and is the second alternative navigational system in operation with global coverage and of comparable precision.
GLONASS is developed by Russia originally started by Soviet Union in 1976. This has a network of 24 satellites covering the earth.
Since 2012, GLONASS was the second most used positioning system in mobile phones after GPS. The system has the advantage that smartphone users receive a more accurate reception identifying location to within 2 meters, but in overall, its accuracy is lower than that of GPS. For that, Russia is working on the improvement of GLONASS. By 2010, GLONASS had achieved 100% coverage of Russia's territory and in October 2011, the full orbital constellation of 24 satellites was restored, enabling full global coverage.
BDS, that is, BeiDou Navigation Satellite System, is China's self-developed global satellite navigation system. It is the third mature satellite navigation system after the United States Global Positioning System (GPS) and the Russian Global Navigation Satellite System (GLONASS).
It consists of three parts: space segment, ground segment and user segment. It provides high-precision positioning, navigation, timing service for all types of users in the global all weather and all day. It has equipped with capability of regional navigation, positioning and timing, with positioning accuracy of 10 meters, speed accuracy of 0.2 m/s, and timing accuracy of 10 nanoseconds.
Galileo is the global navigation satellite system (GNSS) that is being created by the European Union (EU) through the European Space Agency (ESA). The first Galileo test satellite, the GIOVE-A, was launched 28 December 2005, while the first satellite to be part of the operational system was launched on 21 October 2011. As of December 2017, 22 of the planned 30 active satellites are in orbit. The next launch, which will bring the system to operational completion, is planned in mid 2018. Galileo started offering Early Operational Capability (EOC) on 15 December 2016, providing initial services with a weak signal, and is expected to reach Full Operational Capability (FOC) in 2019. The complete 30-satellite Galileo system (24 operational and 6 active spares) is expected by 2020.
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