Satellite Navigation Systems
Satellite navigation systems have become so widespread that they are almost taken for granted. However, the fact that they are so useful is leading to an expansion of available systems. Several countries are working on systems so product selection may become more complicated. To start, one must have a solid understanding of how satellite navigation systems work, as well as the terminology that is commonly used when referring to the technology.
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.
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.
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, but only two global systems exist at this time. There are also regional satellite navigation systems, and some of those regional systems are in the process of becoming global. Are you confused yet? That’s okay. 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.
As stated in the prior paragraph, there are two types of satellite navigation systems: global and regional. Global navigation satellite systems (GNSS) provide coverage all over the world. Regional satellite navigation systems provide coverage just to one area. Regional systems typically augment a global system, but some can be used as stand-alone systems. First, we will cover GNSS.
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.
GLONASS is also composed of 24 satellites but was developed in the Soviet Union and is operated by the Russian Aerospace Defense Forces. This sat nav system is the only other navigational system in operation with global coverage and of comparable precision.
Galileo is a global navigation system currently being 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 is to provide a high-precision positioning system for European nations that is independent from the Russian GLONASS, U.S. GPS, Indian IRNSS and Chinese Compass systems. The 30-satellite system is expected to be completed in 2019. The use of the basic services will be free and open to everyone, while the high-precision capabilities will be available for paying commercial users and for military use.
Compass is a global navigation system currently being developed in China. It is the second generation of its regional BeiDou Satellite Navigation System (BDS), also known as BeiDou-2. Its completion is expected in 2020 and will consist of 35 satellites.
The Quasi-Zenith Satellite System (QZSS) is a proposed three-satellite regional time transfer system and Satellite Based Augmentation System for the Global Positioning System that would be receivable within Japan and Australia. 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.
The BeiDou Navigation Satellite System (BDS) consists of two separate satellite constellations. The first is a limited test system that has been operating since 2000 known as BeiDou-1. The BeiDou-1 consists of three satellites and offers limited coverage and applications. Its navigation services have been mainly for customers in China and neighboring regions. As mentioned previously, the second generation is a full-scale global navigation system that is currently under construction and will be known as Compass, or BeiDou-2.
The Indian Regional Navigational Satellite System (IRNSS) is a regional satellite navigation system being developed by the Indian Space Research Organization. When complete, it will be under control of the Indian government. IRNSS will provide standard service for civilian use and an encrypted restricted service for authorized users (military).
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. However, there are case-by-case scenarios where a regional system may be more beneficial, but that is beyond the scope of this article.
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 receiver modules and GNSS receiver modules that work with a variety of applications and satellite navigation systems.