The term GNSS stands for Global Navigation Satellite System(s). A GNSS typically consists of three segments: the satellites orbiting the Earth, stations on the ground to track and monitor the satellites, and users who rely on the satellites to compute their position and motion. There are several independent GNSS in operation today, including GPS, GLONASS, BeiDou, Galileo, and QZSS. All GNSS devices make use of two or more of these constellations. Many GNSS receivers use the signals from all the constellations, although some will use a combination of only a few. GNSS systems offer increased robustness, faster fix times, and greater accuracy compared to single constellation technology.
GPS is based on a constellation of 24 to 32 satellites operated by the U.S. Department of Defense (DoD). They circle 22,000 km (14,000 miles) above the Earth, twice a day in precise orbits. The satellites continuously transmit coded information in the UHF band (1.575 GHz) back to GPS receivers on the ground. There are currently two GPS services – the Precise Positioning Service (PPS), operated by the DoD, and the Standard Positioning Service (SPS), available to individuals worldwide. The Global Positioning Service has gone through several upgrades since its introduction. The most significant of these is the addition of the L2C and the L5 carrier signal. L2C gives dual-frequency capable receivers the ability to incorporate more ionospheric correction. The L5 signal is transmitted at a higher power with greater bandwidth and using an advanced signal design. The most recent GPS upgrade is the addition of the L1C signal. Broadcasting on the same frequency as the Legacy L1 C/A, the L1C exists to enhance interoperability with the other GNSS constellations.
GLONASS is the satellite navigation system operated by the Russian Aerospace Defense Forces. There are approximately 24 GLONASS satellites in operation, orbiting at a similar altitude as GPS satellites. However, GLONASS satellites orbit in a way that provides better coverage at higher latitudes compared to the other GNSS. GLONASS currently broadcasts on two frequencies but will be expanding to three frequencies with future satellite launches. GLONASS is currently modernizing by deploying the GLONASS K-1 and K-2 satellites. These signals incorporate triple-frequency technology, similar to GPS. Also similar to GPS, GLONASS offers two services, an enhanced military positioning service and a civilian band. Unlike GPS, GLONASS does not use encryption to secure its military service. Instead, GLONASS relies on “security by obscurity.” The format of the information encoded in the military GLONASS signal is not released to the public, so civilian manufacturers are unable to make use of the signal.
The BeiDou Navigation Satellite System operates under the China National Space Administration. Sometimes called COMPASS, BeiDou has both regional and global satellites in space. The regional satellites are visible over the Eastern hemisphere while the global satellites orbit Earth similar to other GNSS. BeiDou is still being deployed but provides operational coverage in areas like Asia, Australia & New Zealand, India, Russia, Africa, and Europe. The completed system will have 5 – 10 regional satellites and 25 – 30 global satellites. As with GPS and GLONASS, BeiDou provides two tiers of service, one military and one civilian.
The European Space Agency operates the Galileo system. The Galileo constellation, when complete, will consist of approximately 30 satellites, transmitting signals on several frequencies that overlap those used in other GNSS. Galileo is still in the deployment phase but has 22 satellites broadcasting, which gives extensive worldwide coverage. Like the other major constellations, Galileo offers two tiers of service. Instead of a secondary military signal, Galileo provides a service for critical infrastructures, like the power grid or telecommunications. In addition to positioning service, Galileo also provides a search and rescue function with beacons that communicate with a central rescue operations centre. Because Galileo has worldwide coverage and high availability, the Galileo search and rescue beacons are a perfect tool for sailors, mountaineers, and other extreme sports enthusiasts. Galileo makes use of highly accurate hydrogen clocks. This timekeeping precision increases the baseline accuracy of the Galileo constellation, although Galileo still suffers from atmospheric effects, as do all GNSS constellations.
The Quasi-Zenith Satellite System, or QZSS, is a constellation developed by the Government of Japan. QZSS fulfills a unique need in Japan for additional satellite coverage due to the large number of high-rise buildings in the country which cause an “urban canyon” effect and the performance of GNSS. QZSS bridges this gap by providing a constellation of satellites which orbit directly over Japan at an ultra-high orbit, allowing them to penetrate the urban canyon of Japan’s large cities. QZSS is functionally a cross between a satellite-based augmentation system and a positioning system. QZSS provides independent time-keeping and positioning, but also SBAS type localized corrections.
A GNSS receiver knows where each satellite is in its orbit and compares it with the time required to receive each satellite’s signal. The receiver uses these measurements to calculate its specific position on Earth. A GNSS receiver can only track satellites orbiting above the horizon. Typically, there are between six to twelve satellites visible above the horizon at any one time, although with GNSS, this number increases significantly. If some satellites become blocked or “shaded” by tall buildings or other obstacles, the receiver will try to reacquire the blocked signals. Although a GNSS receiver needs at least four satellites to provide a three-dimensional solution (latitude, longitude, and altitude), it can maintain a (latitude-longitude) position using three satellites. Ephemeris data is the precise location of the GNSS satellite; this data is useful for only four hours. A receiver will download the ephemeris for each satellite, and this download is updated as necessary. GNSS constellations are designed to provide global positioning services with an accuracy ranging from 5 to 15 meters. While more precises accuracies are not possible with standard GNSS, there are ways to improve GNSS accuracy using additional services. There are four primary services available, each capable of improving position accuracies to within one meter: