Defining location has become important to our technology and daily life. With devices that are static there are ground based methods in accomplishing this such as through interconnected telecommunications systems with methods including IP address mapping, cell towers, or other ground based sensors. When you are on the move using the systems can become a challenge. Accuracy and availability need to rely on systems with greater coverage and precision. The term Global Navigation Satellite Systems (GNSS) is used to describe a constellation of orbiting satellites working together with a network of ground control stations and receivers that calculate ground positions through an adapted version of trilateration.  We commonly just reference this location system as GPS. Most modern vehicles and mobile devices rely on some form of location service to include that which comes from GNSS. The performance of a satellite navigation system is assessed according to four criteria :
Accuracy is difference between the measured and the real position, speed or time of the receiver.
Integrity refers to a system’s capacity to provide confidence thresholds as well as alarms in the event that anomalies occur in the positioning data.
Continuity refers to a navigation system’s ability to function without interruption.
Availability refers to the percentage of time during which the signal fulfills the accuracy, integrity and continuity criteria.
Here is a overview of the major GNSS systems that operate on a global level:
U.S. Global Positioning System (GPS)
The first US Global Positioning System (GPS) satellites went up in 1974 and initially called NAVSTAR, The formal block I GPS system went up in 1978. However, it was not made available for civilian use until 1983 as President Reagan responded to giving civilian air travel accurate and safer ways to determine where they are at after a soviet interceptor aircraft shot down the civilian airliner KAL 007 in restricted Soviet airspace, killing all 269 people on board. Over the years the constellation has been updated with greater accuracy and reliability. As of April 24, 2019, there are a total of 31 operational satellites distributed in six orbital planes inclined 55° from the equator in a Medium Earth Orbit (MEO) at about 20,200 kilometers (12,550 miles) and circling the Earth every 12 hours.  with availability 95% of the time. Accuracy of GPS ranges from 500-30cm and depends on a variety of factors.
Russian Global Navigation System (GLONASS)
The first Russian block I GPS satellites were launched in 1982 and the GLONASS system became fully operational in 1993 and with 24 satellites in its constellation in 1995 it rivaled the precision of the US GPS system. The constellation experienced a period of decline until the early 2000s in which the Russian government committed to standards. Currently, GLONASS has a full deployment of 24 satellites in the constellation.  As with GPS, GLONASS transmits the high-precision code on both L1 and L2. But, unlike the GPS satellites, the GLONASS standard-precision code has also been transmitted on the L2 frequencies beginning with the GLONASS-M satellites. What is unique about this constellation is that they have polar coverage as well. No doubt supporting Russian increased interests in the North Sea. 
European Union Galileo positioning system
The European’s Galileo constellation had its first launch in August 2014 with GSAT 0201 and GSAT0202. Today the constellation consists of 22 operational satellites and will be completed in 2020 with 30 satellites. Satellites of Galileo will be spread across three orbital planes inclines at an angle of 56 degrees to the equator with each satellite taking 14 hours to orbit the earth. Most locations will have visibility by 6-8 satellites at all times. Galileo is designed to be compatible with all existing and planned GNSS and interoperable with GPS and GLONASS therefore it is meant to enhance the coverage currently available. 
BeiDou/Compass Navigation Satellite System (BDS)
The Compass Navigation satellites started going into orbit in the early 2000s. The system was originally a regional dedicated constellation until the launch of the BeiDou satellites in 2007 designed to provide global coverage. They operate in three orbital planes at a 55 degree inclination with an orbital period of 13 hrs. There are currently 27 satellites in orbit with an expected 35 to be in the fully operational constellation by 2020. Due to its expanded frequency range and coverage BeiDou is advertised to to provide better service to building interiors, underground areas, and underwater. 
With all of the major space organizations having GNSS constellations the global coverage for GPS devices, vehicles, and equipment is well saturated. They all are interoperable with each other offering increased coverage, accuracy, and precision to its users. These global constellations overlap with regional constellations such as by Japan and India as well. The future of GNSS systems will have greater emphasis on supporting technology beyond just traditional location-based services related to roads, aviation, rail, maritime, and agriculture.
In future webcasts and articles we plan to discuss some of the use cases of GNSS in more detail. To read more about Global Navigation Satellite Systems please check out our references.
 Global Navigation Satellite System (GNSS)(2019). Retrieved from https://www.techopedia.com/definition/16847/global-navigation-satellite-system-gnss
 Ramawickrama, Y., Wijesekera, J., & Wijesooriya, H. (2016, November). The Future of GNSS in the next Ten years. Research Gate.
 NASA. (2019). Global Positioning System. Retrieved from https://www.nasa.gov/directorates/heo/scan/communications/policy/GPS.html
 NovAtel. (2019). GLONASS. Retrieved from https://www.novatel.com/an-introduction-to-gnss/chapter-3-satellite-systems/glonass/
 Langley, R. B. (2019). Innovation: GLONASS â€” past, present and future. Retrieved from https://www.gpsworld.com/innovation-glonass-past-present-and-future/
 European Global Navigation Satellite Systems Agency. (2019). Galileo. Retrieved from https://www.gsa.europa.eu/galileo/programme
 Earth Observation Portal. (2019). CNSS (Compass/BeiDou Navigation Satellite System). Retrieved from https://directory.eoportal.org/web/eoportal/satellite-missions/content/-/article/cnss