What is a satellite navigation system and how does it work?
A satellite navigation system (also known as a sat nav system) is a system of satellites, usually managed by one company or country that provides geo-spatial positioning, which is a technical term for a specific location on or above the Earth in 3 dimensions. A sat nav system receiver can be used to locate latitude, longitude, altitude, velocity and time information. Commercial systems are accurate to within a few meters. High-end systems are accurate to within centimeters. The satellites broadcast a signal that contains orbital data and the exact time the signal is transmitted. The orbital data is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an atomic clock (most accurate time and frequency standards known) to maintain synchronization of all the satellites in the constellation. The receiver compares the time of broadcast encoded in the transmission with the time of reception measured by an internal clock, thereby measuring the time-of-flight to the satellite.
The receiver measures signals from several satellites at the same time so that it can use triangulation to determine its location. Triangulation is the process of determining the location of a point by measuring the angles to it from two known points. The precise satellite locations are included in the transmission and the time-of-flight of the signal is used to calculate the distance to each satellite. The receiver then does some math and calculates its location on the Earth. The more satellites the receiver can track, the more accurate the location calculation.
The receiver calculates 4 parameters; latitude, longitude, altitude and time. As a result, the receiver generally needs to see at least 4 satellites to calculate the 4 unknowns. It can give estimates for the values with fewer satellites, but the potential error increases.
The basic triangulation math is not that complicated, but the fact that the known points, the satellites, are moving very fast and the fact that the Earth is a curved surface adds quite a bit of complexity. In addition, the Earth is not a perfect sphere and is not uniformly shaped or curved. This adds some error depending on how far off the average curvature a specific location is. For this reason, local augmentation systems are used. The receiver can also use regional data sets that better describe the local geography and ultimately give a more accurate position.
More details on frequency and antenna applications can be found here.
Why are there different satellite navigation systems?
In the early days of the U.S. run NAVSTAR GPS system, an error code was transmitted with the satellite signal. This error decreased the accuracy of the system so that it was not as effective for those outside the United States military. Commercial organizations began to use terrestrial beacons on the Earth to augment the system and account for the error. These beacons were built along the coast and waterways by the United States Coast Guard and similar organizations in other countries to help ships navigate local coastlines and waterways. This required a separate receiver, which increased the cost and really prevented the systems from becoming commercially viable.
Other agencies in the United States and around the world created their own augmentation systems to improve accuracy. This includes the Federal Aviation Administration for commercial aircraft navigation and the Coast Guard for maritime navigation. Similar systems were created in Europe, Russia, Japan, India and others.
The error code was deactivated in 2000 and GPS has since become very widely used. However, the fact that the United States could turn the error code back on at any time or even turn off the signals entirely has prompted other countries to begin to develop satellite navigation systems of their own.
Did you know that “GPS” is NOT an umbrella term for satellite navigation systems?
Contrary to popular belief, GPS (Global Positioning System) is NOT an umbrella term for all satellite navigation systems. It used to be, but the term GPS has become associated with the United States-owned NAVSTAR system. GNSS (Global Navigation Satellite System) is an umbrella term used today for global systems. We hope to leave you with a basic understanding of these various systems so you can determine which type of satellite navigation system would be best for your application.
Global Navigation Satellite Systems
Global navigation satellite systems (GNSS) provide coverage all over the world.
Global Positioning System (GPS)
The NAVSTAR GPS system is composed of 24 satellites, and was created by the U.S. Department of Defense. It can be accessed anywhere on or near the Earth where there is an unobstructed line-of-sight to four or more GPS satellites. The system provides critical capabilities to military, civil, and commercial users worldwide and is freely accessible to anyone with a GPS receiver.
Galileo is a global navigation system developed by the European Union and European Space Agency intended primarily for civilian use. Named after the Italian astronomer Galileo Galilei, one of the goals was to provide a high-precision positioning system for European nations that would be independent from the Russian GLONASS, U.S. GPS, Indian IRNSS and Chinese Compass systems. The 30-satellite system first launched in 2011 with expected completion at the end of 2020. The use of the basic services is free and open to everyone, while the high-precision capabilities will be available for paying commercial users and for military use.
BeiDou Navigation Satellite System (BDS)
The BeiDou Navigation Satellite System (BDS) is a Chinese satellite navigation system, consisting of two separate satellite constellations. The first, BeiDou-1, was launched in 2000 and decommissioned in 2012, offered limited coverage and navigation services, mainly for users in China and neighboring regions. The second system, or BeiDou-2, was launched in 2011 with a partial constellation of 10 satellites in orbit and serves customers in the Asia-Pacific region. China’s third generation system, or BeiDou-3, was launched in 2015.
Quasi-Zenith Satellite System (QZSS)
The Quasi-Zenith Satellite System (QZSS) is a four-satellite regional time transfer system and Satellite Based Augmentation System for the Global Positioning System developed by Japan, serving the Asia-Oceanic region. QZSS is targeted at mobile applications to provide communications-based services and positioning information. Its three satellites, each 120° apart, are in highly-inclined, slightly elliptical, geosynchronous orbits. Because of this, they do not remain in the same place in the sky. Their ground traces are asymmetrical figure-8 patterns, created to ensure that one is almost directly over Japan at all times.
Indian Regional Navigational Satellite System (IRNSS)
The Indian Regional Navigational Satellite System (IRNSS), also known as NavIC (Navigation with Indian Constellation), is an autonomous regional satellite navigation system developed by the Indian Space Research Organization to provide standard service for civilian use and an encrypted restricted service for authorized users (military), covering India and the surrounding region with plans for further expansion.
When deciding on which module to use for your design, the desired coverage is just one factor. There are definite advantages to using global systems and also multiple systems in terms of coverage and accuracy.
Hopefully, we’ve helped you gain some insight into the complex world of satellite navigation systems. If you have any questions or need help figuring out what type of satellite navigation system to use for your application, let us know. We have a variety of GPS and GNSS receiver modules and GPS and GNSS antennas that work with a variety of applications and satellite navigation systems.