GNSS GPS and SBAS So what do these acronyms stand for?
Handy to know if your headed out to buy a new satellite navigation system.
GNSS stands for Global Navigation Satellite System, and is an overarching term that encompasses all of the global satellite positioning systems. This includes constellations of satellites orbiting over the earth’s surface continuously transmitting signals that enable users to determine their position. These satellite constellations are owned and controlled by the governments of different countries. Due to design and operation, the signals and frequency's from the different systems are individual. Purchasing a system that is purported to be capable of processing the signals from a variety of systems must be backed up with an antenna capable of receiving the signals different frequencies. Keep in mind there are few receivers capable of using more than two constellations at the one time. In a lot of cases the constellations can be selected using the receivers software, however as I have noted your unit must be connected to antenna capable of receiving the signals.
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| Constellations of satellites orbiting over the earth’s surface continuously transmitting signals that enable users to determine their position. At a minimum, four satellites must be in view of the receiver for it to compute four unknown quantities (three position coordinates and clock deviation from satellite time), this is also why more satellites equal more precision. |
GPS Global Positioning System is but one component of GNSS (the Global Navigation Satellite System). GPS specifically refers to the NAVSTAR Global Positioning System, a constellation of satellites developed by the United States Department of Defence. Originally GPS was developed for military use, but was later made accessible to civilians and industry. GPS is now the most widely used component of GNSS in the world, and provides continuous positioning and timing information globally even under the most arduous weather conditions. This is the system that started the satellite positioning revolution. The term GPS does get used a lot to explain satellite navigation, like the term iPad is used to describe a tablet computer.
In addition to GPS, other systems are in use or under development. These systems make up the other orbiting constellations of satellites. Several governments are developing or have operational their own systems due to the fact the US government can selectively deny access to the GPS system or degrade the system performance at any time. This has happened in the past and it is possible it could happen again in the future.
GLONASS the Russian Global Navigation Satellite System (GLONASS) was developed at the same time as GPS, but suffered from incomplete coverage of the globe until recent years. GLONASS signals can be added to GNSS devices, making more satellites available and enabling positions to be fixed more quickly and accurately, sometimes to within two meters.
Galileo is Europe’s Satellite Navigation System constellation, providing improved positioning and timing information with a significant positive impact for many European services and users. Until now, GNSS users have had to depend on American non-civilian GPS or Russian GLONASS signals. With Galileo, users now have a, reliable alternative that, unlike these other programs, remains under civilian control.
BDS is China's BeiDou Navigation Satellite System. In 2015, China started the latest generation BeiDou system for a global coverage constellation. The first satellite of the latest generation was launched in March 2015. As of October 2018, fifteen BDS satellites had been launched. The constellation will eventually consist of 35 satellites and is expected to provide global services upon completion in 2020. When fully completed, BeiDou will provide an alternative global navigation satellite system to the United States owned Global Positioning System the Russian GLONASS or European Galileo systems and is expected to be more accurate than these.
NAVIC The Indian Regional Navigation Satellite System (IRNSS), with an operational name of NAVIC. NAVIC is an independent regional satellite navigation system that provides accurate real-time positioning and timing services. It covers India and a region extending 1,500 km around it, with plans for further extension. The system currently consists of a constellation of seven satellites. NAVIC will provide two levels of service, the "standard positioning service", which will be open for civilian use, and a "restricted service" (an encrypted one) for authorised users (including the military). NAVIC is planned to become available for civilian use in the first half of 2020. There are plans to expand the NAVIC system by increasing its constellation size from 7 to 11.
To sum up the main satellite constellations are GPS (USA), GLONASS (Russia), Galileo (EU), BeiDou (China) NAVIC (India). These five satellite systems are the major players under the umbrella term GNSS. So a global navigation satellite system (GNSS) is a group of synchronized satellite constellations working collectively transmitting radio signals used for position navigation and time solutions. The position navigation and time solutions provided by these GNSS are used for a wide and growing variety of applications covering most industry sectors including agriculture, aviation, construction, consumer, resources, road, rail, maritime, mining and water utilities. Time solutions for the synchronisation of communication including cell phones, and electrical networks to name a few.
Fundamentals
The GNSS concept is based on time and the known position of GNSS satellites. The satellites carry very stable atomic clocks that are synchronized with one another and with the ground station clocks. Any drift from true time maintained on the ground is corrected daily. In the same manner, the satellite locations are known with great precision. GNSS receivers have clocks, but they are less stable and less precise.
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| At a glance its easy to see the benefits of having a number of available sources of data. However that said most receivers use GPS as the primary source of positioning, so that has to be saying some thing about the robustness of the system as a whole. |
GNSS Receivers
GNSS satellites continuously transmit data about their current time and position. A GNSS receiver monitors multiple satellites and solves equations to determine the precise position of the receiver and its deviation from true time. How many satellites a receiver can track or monitor, I have seen up to 99 listed however this varies with brand. Another technique becoming common is to deliver position aiding data to the GNSS receiver via wireless networks or the Internet. Supplying information such as ephemeris, almanac, approximate last position, time and satellite status and an optional time synchronisation signal significantly reduces Time to First Fix (TTFF) and improves acquisition sensitivity.
At a minimum, four satellites must be in view of the receiver for it to compute four unknown quantities (three position coordinates and clock deviation from satellite time), this is also why more satellites equal more precision. The information provided by a generic GNSS receiver can be used by a wide range of applications. Most systems use the receiver's solution, the receivers computed position, velocity and time to run the task assigned.
Keep in mind most GNSS receivers can pick up GPS signals (if configured), however a GPS receiver can not pick up signals from the other constellations of orbiting satellites. This can be a hard to comprehend due to the way most systems are loosely referred to as GPS, I have even seen systems that use triangulation of phone towers to find a positioning solution called GPS.
If one system is down, most of the newer GNSS receivers will already be using signals from all the other systems. Just look for the number of satellites your receiver will track, fifty six is common but more is now the norm. And of course, the more satellites your receiver is looking at and acquiring data from the more likely that if your line of sight to one satellite is obstructed by a mast, boom, Bimini frame or other obstacle, it can receive signals from another satellite.
GNSS Accuracy
In open sky conditions, standard accuracy GNSS receivers are accurate to around two meters, however, because GNSS receivers rely on the time it takes a satellite signal to reach them, even the slightest errors like a millionth of a second can impact accuracy.
Errors in satellite orbit position can lead to around 2.5 meters loss of accuracy. Satellite clock errors can add another 1.5 meters. Atmospheric disturbances can add another five meters, plus throw in the occasional intense burst of solar activity or multi path effects like signals bouncing off the sea surface, mast, boom or spray dodger frames, and this accuracy can easily bump the error out to 10 meters or more.
GNSS systems in Australia
Australia is one of few countries in the world with high visibility to six GNSS due to our geographical location. These include not only the main global systems of GPS, GLONASS, Galileo, BeiDou, NAVIC but also Japan's Quasi Zenith Satellite System (QZSS).
RTK
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| A simple RTK system the rover can be backpack or vehicle mounted |
Real-time kinematic (RTK) positioning technique is used to enhance the precision of positioning data received from global navigation satellite systems such as GPS, GLONASS, Galileo, NAVIC and BeiDou. Luckily, high precision GNSS systems dramatically improve precision using GNSS correction data to cancel out the errors. One way to do this involves monitoring GNSS signals at a base station set up on a known location (surveyed). Deviations from the base station’s position are observed and sent via radio link to the mobile vehicle (AKA rover). The rover is equipped with a GNSS receiver and radio link receiver for the deviation corrections. When the deviation corrections are received at the rover they are applied in real time to the GNSS position to obtain a more accurate position reading. In favourable conditions, this approach can be used to achieve centimetre level accuracy, provided that the base station and the rover are not too far apart. Real-time kinematic (RTK) positioning technique is widely used by the geospatial industry for increased accuracy in surveying and mapping (navigation).
Welcome to the wonderful world of Satellite Based Augmentation Systems
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| This coverage map may mean more after reading the description below. |
SBAS
We hear the term *augmentation system or SBAS thrown around a lot these day so what does SBAS stand for?
SBAS stands for satellite-based augmentation system. A Satellite Based Augmentation System is a wide area differential Global Navigation Satellite System (DGNSS) signal augmentation system which uses a number of geostationary satellites, able to cover vast regional areas. These geostationary satellites don’t move, in other words they stay in sync with the rotation of the earth as opposed to the GNSS satellites that orbit the earth continuously.
To calculate GNSS position errors, GNSS data received from satellites is compared against the precise location of each land based ground station. Discrepancies are measured and the corrections, called deviation corrections, are transmitted to the geostationary satellites. These satellites then broadcast primary GNSS data which has been provided with the received integrity, ranging, and deviation correction information sent from the regional SBAS ground stations.
While the primary purpose of SBAS is to provide integrity assurance, use of the system also increases accuracy and can reduce positional errors to less than 1 meter.
So in a nut shell SBAS is a regional network of ground stations and satellites that work together to boost the accuracy and dependability GNSS data. The increased accuracy is critical for aviation and is another technique widely used by the geospatial industry for increased accuracy in navigation and mapping.
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| Here is a drawing of a basic SBAS set up. On the left bottom is a number of receivers collecting positioning data from the orbiting satellites. The signals are then passed to the central processing unit that calculates what corrections are required for what satellite. That information is then available via the internet or it is transmitted to one of the GeoStationary satellites. The GeoStationary satellites then transmits these corrections to our mobile (ship car airplane) GNSS receiver to correct our position making for very a accurate location (lets hope the map is as accurate to make it of use) . |
While all this sounds great, how do we benefit. The GNSS receiver you purchase will hopefully be able to use the correction data that has been sent to it from the regional geostationary satellites. Your GNSS receiver may then display that you are navigating with DGNSS. I think most of us have heard of WAAS and how it will transform our GPS in to a super accurate navigation device. Well WAAS can do wonderful things, however for those of us who live in Australia or sail around the Pacific it won’t help give us any better positioning accuracy.
Now the bad news, while we here in Australia are lucky enough have high visibility of up to six GNSS constellations due to our geographical location. We do not have the use of a Satellite Based Augmentation System at present. The tests have been done and hopefully it will become fully operational in the near future. If the documentation I have researched is correct our corrections will be broadcast by Japan's Quasi Zenith Satellite System (QZSS). I have seen this pop up on my GNSS from time to time and my receiver does indeed switch to DGNSS.
So if the sales man trying to sell you a GNSS receiver starts letting you know about the added accuracy of the various systems like WAAS, EGNOS, MSAS and GAGAN. You have the heads up that none are any good to you unless your headed off around the world, and really you don’t want to be paying extra for systems you will never need or use. Its nice to know those in other parts of the world can enjoy dynamic positioning however in Australia its getting there. SBAS is regional and the regions and systems are listed below.
Now for some good news. The Australia test (not cricket) transmission has been extended and will be available until 31 July 2020. This will provide continuity of SBAS signals to support R&D, industry testing and encourage early adoption. Ya gota hope so.
Regions that have SBAS coverage, for a better visual look at the coverage map at the begining of this section
WAAS United States, Canada, and Mexico, Wide Area Augmentation System, or WAAS, is operated by the United States Federal Aviation Administration. Development for WAAS began in 1994.
EGNOS European Union: European Geostationary Navigation Overlay Service, or EGNOS, was developed by the European Space Agency.
MSAS Japan: The Multi-Functional Satellite Augmentation System, or MSAS, is operated by the Japan Civil Aviation Bureau, a division of the Ministry of Land, Infrastructure, and Transport. The QZSS service area covers East Asia and Oceania region and its platform is multi-constellation GNSS. The QZSS system is not required to work in a stand-alone mode, but together with data from other GNSS satellites.
GAGAN India: GPS-Aided GEO Augmented Navigation, or GAGAN, was developed by the Indian Space Research Organization and Airport Authority of India.
MTSAT Satellite Augmentation System (MSAS) is the Japanese Satellite Based Augmentation System System a GPS Augmentation system with the goal of improving its accuracy, integrity, and availability, and that uses the Multifunctional Transport Satellites (MTSAT) owned and operated by the Japanese Ministry of Land, Infrastructure and Transport and the Japan Meteorological Agency. First tests were accomplished successfully, and MSAS system for aviation use was declared operational in September 27, 2007, providing a service of horizontal guidance for en route through Non-Precision Approach.
SBAS systems under development
SNAS China: Satellite Navigation Augmentation System, or SNAS, is in development.
SDCM Russia: System for Differential Corrections and Monitoring, or SDCM, is in development. When completed, SDCM will offer corrections for GPS and GLONASS, the Russian satellite navigation system.
WADGPS South Korea: Wide Area Differential Global Positioning System, or WADGPS.
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