What is GPS, GLONASS and GPS Locking



GPS stands for global positioning system which is a location detection technology developed by the United States government and operated by the United States, Air Force. It is a global navigation satellite system that gives location and time information to GPS receivers on or near Earth where there’s in clear view of a minimum of four GPS satellites. GPS technology is independent of any other technology like Mobile communication or internet, however, these technologies have the ability to further enhance the functioning of GPS.

The United States government maintains this system and has made it freely accessible to anyone with a GPS receiver, however, they have the right to selectively deny access to specific receivers like they did to India during the 1999 Kargil war. This was first launched in 1973 to overcome the limitations set by then existing systems, initially, the system had a total of 24 location providing satellites. GLONASS is another similar technology developed by Russia which was imperfect till the 2000’s due to lack of global coverage.

GPS on Phone
Photo by Enrique Alarcon on Unsplash

This technology can be added to any GPS receiver so that they make use of more satellites and improve the accuracy of location to up to 2 meters. Currently, the GPS system has somewhere between 24 and 32 medium earth orbit satellites transmitting precise microwave signals which let the GPS receiver find the location, time and velocity.

The GPS receivers work by triangulating the location once it has a lock from at least 3 GPS satellites, this information is received in the form of GPS coordinates, and this data can be interpreted by the software for further applications. Triangulation is a method of plotting circles based on the distance from each satellite and then finding their intersection thereby finding the location of the receiving device. The initial time taken by the GPS receiver to get the signal is generally more in moving vehicles or dense urban areas, the time is taken for the fix also depends on how the device starts. They can be classified mainly into 3 types.

Hot Start

The Hot start is when the device still remembers the device location and the satellite position. The GPS receiver can get the GPS Fix very quickly in this state, however, this state is rare and is applicable only if the device has remained in a position close to the position it was in when the device GPS was switched off.

Warm Start

Warm start is when the device remembers the last found location and the UTC time but forgets the Almanac or the Satellite position. In this case, the device takes considerably longer time than the Hot start time but is much lesser than a cold start.

Cold Start

This is when the device doesn’t remember any data or if it dumps all the stored data. In this case, the time taken for the fix is considerably longer than the above two. In this case, the device tries for a fix with the satellites and then recalculates the device location. Here the device has to try for a lock on the available satellites via a method like polling which takes considerably longer than trying to acquire a fix when the satellites are known.

What is AGPS?

AGPS stands for Assisted GPS, an aiding technology which helps the GPS receiver acquire a quicker fix. This technology works by downloading the Satellite position for different times of the day, over a week. Then the wireless networks help triangulate the user location using cell towers so that the satellites in that position can be found and contacted directly.

How does GPS actually work?

Once a GPS receiver gets a fix with the satellite, the satellite keeps transmitting the satellite location and time at regular intervals, these waves travel at the speed of light and when they finally reach the GPS receiver device, the device interprets this data to find out how far the satellite is from the device. Similarly, once the device has this data from at least 3 devices, the data is plotted on the map to come up with a location by the triangulation method. The satellites use an atomic clock on board to keep track of the time accurately, no matter how accurate these clocks are, due to the difference in general relativity and special relativity, there arises a significant difference in the time measured by the clock on Earth and the clock on these satellites.

GPS satellite constellation

Photo source: GPS.gov

This is because due to a stronger gravitational pull on the earth, these clocks run slower than those in the satellites, however since the time on the satellites in relative and are compared with the clocks on earth, it turns out in a way that the clocks on the satellites run slower than those on Earth. Thus, the satellites should keep these accounts in mind and keep adjusting the clocks accordingly so that they’re in sync with the clocks on the Earth. Also, atomic clocks can be really expensive, and normal clocks cannot work for GPS.

This is because of the fact that both the receiver and the satellite need to be in sync with an accuracy of up to a nanosecond, this is not possible with the regular quartz clocks being used in the phone, so the receiver smartly acquired the time from the GPS satellites itself. The time is taken from at least 4 different satellites and gauges its own accuracy. This way the receiver gets the atomic clock accuracy fro “free” just using the quartz clock. This time obtained from the Satellites is again used to determine the distance of the receiver from the satellite. The GPS receiver keeps trying to get a fix from as many satellites as possible and keeps improving the accuracy based on it. GLONASS works in a similar way but has much better accuracy because of the placement of their satellites, however, GLONASS doesn’t have as good a coverage as GPS and there are many parts of the world that are not covered by GLONASS.

Featured image: Photo by Alvaro Reyes on Unsplash





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