>> Satellite navigation systems are revolutionising the way the world manoeuvres its way around. Here is a sneak peak at some of these systems… <<
Global navigation satellite system or GNSS: Global navigation satellite system can be defined as a constellation of active satellites which are equipped with electronic devices continuously transmitting signals to users anywhere on the Earth. The satellites provide Position, Navigation and Timing (PNT) services, round-the-clock, in all types of weather. The signals are free of charge.
There are usually three classes of satellite navigation systems. These are:
- Global navigation satellite systems such as the United States’ GPS and Russia’s Glonass; GNSS GEOINTELLIGENCE Jan – Feb 2012 36
- Regional Navigation Satellite Systems (RNSS) such as India’s IRNSS; and
- Satellite Based Augmentation Systems (SBAS) such as the US’ WAAS, European EGNOS and India’s GAGAN
While GNSS covers the entire Earth, the area of operation under RNSS is limited to a particular region. SBAS, on the other hand, supports these satellite systems by providing an integrity signal which alerts the receiver in case GNSS or RNSS develops a fault. SBAS is designed primarily for enhancing safe navigation in the civilian domain but finds applications in other domains as well, for example, maritime, agriculture, land transport and so on.
While GNSS comprises 24-32 Mid- Earth Orbiting (MEO) satellites, RNSS constellations include both MEO and Geostationary (GEO) satellites in the range of 3-7 satellites. SBAS, however, consists of geostationary satellites.
Global Positioning System (GPS)
GPS is the first and best known global navigation satellite system in the world. It has been fully operational since 1995 and consists of the following three segments: the space segment, the control segment, and the user segment. The US Air Force develops, maintains and operates the space and control segments.
Space segment
It consists of a constellation of satellites (at present there are about 31 operational GPS satellites, plus 3-4 decommissioned satellites (called residuals) which can be reactivated if required) transmitting radio signals to users. The satellites fly at an altitude of about 20,200 km. Each satellite circles the earth twice a day, and contains a computer, an atomic clock and a radio. These are essential for calculating a satellite’s changing position and time.
The US Air Force manages the constellation and ensures the availability of at least 24 GPS satellites all the time. These satellites are arranged into six equally-spaced orbital planes surrounding the Earth. The 24-slot arrangement ensures there are at least four satellites in view from any point on the planet.
Control segment
It consists of a global network of ground facilities that track the satellites, monitor their transmissions, perform analyses and sends data. The current operational control segment comprises a master control station, an alternate master control station, 12 command and control antennas and 16 monitoring sites.
User Segment
It consists of a GPS receiver which receives signals from these satellites and uses the transmitted information to calculate the time and 3-D position of the user.
A GPS receiver triangulates its position by obtaining data from three of the four available satellites. This helps in determining the position of an object – longitude as well as latitude. If the receiver is capable of obtaining signals from the fourth satellite, it can then determine the altitude of an object; and can also calculate the speed and direction of the moving object.
GLONASS
It is a radio-based global satellite navigation system operated by Russia. It provides real-time position and velocity determination for military and civilian users and is meant to complement as well as provide an alternative to GPS. The satellites are located at 19,100 km altitude, in middle circular orbit, with a 64.8 degree inclination. Its orbit makes it suitable for usage in high latitudes (north or south). A fully operational constellation consists of 24 satellites. Eighteen satellites are required for covering the entire territory of Russia. The Glonass constellation operates in three orbital planes with eight evenly spaced satellites located in each orbit. Its ground control segment is located entirely within former USSR’s territory. According to Space and Tech, the ground control center and time standards is located in Moscow and the telemetry and tracking stations are in St. Petersburg, Ternopol, Eniseisk, Komsomolsk-na-Amure.
Compass system (Beidou-2 or BD2)
Developed by China, this new GNSS is similar in principle to GPS and GLONASS. The system became operational in December 2011 with coverage of China and surrounding areas. It is expected to provide complete global coverage by 2020. Compass is not an extension to the previously deployed Beidou-1 which was an experimental regional navigation system and consisted of four satellites (three working satellites and one backup satellite). The new system will have a constellation of 35 satellites (30 non-geostationary satellites (27 in medium earth orbit and 3 in inclined geostationary orbit) and five geostationary orbit satellites for backward compatibility with BeiDou-1 system). Also, the system will provide two levels of service:
- Free service to civilians: It will involve 10 meter locationtracking accuracy, accuracy of 10 ns for synchronising clocks, and will help measure speeds within 0.2 m/s.
- Licensed service to Chinese government and military users: This will be more accurate than the free service and can be used for communication purposes.
So far, about ten satellites for BeiDou-2 have been launched.
Galileo
Galileo is a satellite navigation system currently being built by the European Union (EU) and European Space Agency (ESA) to provide a high-precision positioning system to European nations. The system will be interoperable with other navigation systems like GPS and Glonass.
By offering dual frequencies as standard, the system is expected to provide real-time positioning accuracy down to the metre range. According to European Space Agency, the system will guarantee availability of the service under all but the most extreme circumstances and will inform GEOINTELLIGENCE Jan – Feb 2012 38 users within seconds of any satellite failure, thus making it suitable for safety-critical applications such as landing aircraft, running trains and guiding cars. The first two of the four operational satellites designed to validate its concept in space as well as on Earth were launched in 2011. Two more are expected to follow in 2012. Once the In-Orbit Validation (IOV) phase is completed, additional satellites will be launched around mid-decade to reach Initial Operational Capability (IOC). The system is expected to become fully operational by 2020. When in operation, the system will have two ground operation centres – in Italy and Germany.
Galileo will comprise 30 satellites (27 operational and three active spares) which will be positioned at 23,222 km altitude above the Earth at an inclination of 56 degrees to the equator. It will provide both horizontal and vertical positions measurements with metre-class precision. It will also provide a global Search and Rescue (SAR) function. For this, each satellite will be equipped with a transponder which will transfer distress signals from the user’s transmitter to the Rescue Coordination Centre. The system will also provide a signal to the user, informing him that his message has been conveyed to the centre and the help is on his way. This feature is new and a major upgrade compared to the existing navigation systems. The basic (lowprecision) Galileo services will be free and open to everyone. However, the high-precision capabilities will be available for fee and for military use.
Indian Regional Navigational Satellite System (IRNSS)
IRNSS is an autonomous regional satellite navigation system being developed by Indian Space Research Organisation (ISRO). The system is being developed to reduce the country’s dependency on GPS.
The indigenously developed IRNSS would be under total control of Indian government. As such, the space segment, ground segment and user receivers will be built in the country. IRNSS is expected to provide an absolute position accuracy of around 20 meter over India and the region extending to about 1,500 to 2,000 km around it. The system is expected to provide :
- Highly accurate information in real-time about position, velocity and time for authorised users
- 24*7 services in all weather conditions
The proposed system would consist of seven satellites and a support ground segment. The ground segment would include a Master Control Centre (MCC), ground stations that would enable accurate estimation of satellite orbits and monitoring the health of these satellites. The centre would also estimate and predict the position of satellites, calculate integrity, make necessary ionospheric and clock corrections and run the navigation software. The satellite payloads would consist of atomic clocks and electronic equipment to generate the navigation signals.
The first satellite of the system is scheduled to be launched during 2012-2013, while the constellation is expected to be completed by 2014.
Wide Area Augmentation System (WAAS)
Developed by Federal Aviation Administration (FAA), WAAS is an air navigation aid developed to augment the GPS, thus improving its accuracy, integrity and availability. It uses a network of ground-based reference stations to measure small variations in GPS satellites’ signals. The measurements are then routed to master stations, which queue the received Deviation Correction (DC) and send the correction to WAAS satellites in less than five seconds. The correction messages are then sent back to Earth.
Accuracy: WAAS provides a position accuracy of 7.6 metres or better (for both lateral and vertical measurements). In fact, actual performance measurements at some locations in the US, Canada and Alaska, have shown that it can provide accuracy better than 1metre laterally and 1.5 metres vertically.
Integrity: The WAAS specification requires it to detect errors in its network or in the GPS and notify users within 6.2 seconds.
Availability: Its specification mandates its availability at all times (almost 99.9 per cent) throughout the service area.
European Geostationary Navigation Overlay Service (EGNOS)
Meant to augment the GPS system, EGNOS is the first pan-European satellite navigation system. It consists of three geostationary satellites and a network of ground stations. Its data is freely available in the region to anyone who has an EGNOS-enabled GPS receiver. It provides horizontal position accuracy of better than 1.5 metres to its users in Europe.
EGNOS is a joint project of ESA, the European Commission and Eurocontrol, the European Organisation for the Safety of Air Navigation, and began its operations in July 2005. It was certified for use in safety of life applications in March 2011.
GPS Aided GEO Augmented Navigation (GAGAN)
GAGAN is a part of the Satellite- Based Communications, Navigation and Surveillance (CNS)/Air Traffic Management (ATM) plan for civil aviation in India. It consists of GEOINTELLIGENCE Jan – Feb 2012 40 reference stations (15 stations spread across the entire Indian sub-continent), Indian Master Control Centre for carrying out computing corrections, Indian Navigation Land Uplink Station (three stations which receive messages from the control centre and format those messages in GPS compatibility mode and transmits them for broadcast to user platforms), and space-based segment (navigation payload onboard satellites GSAT-8 and G-SAT-10 that use the bend pipe transponder with uplink in C-Band, downlinks in L1 frequencies).
GAGAN is expected to provide a civil aero-nautical navigation signal consistent with International Civil Aviation Organistaion (ICAO) Standards and Recommended Practices (SARPs) as established by the Global Navigation Satellite System Panel (GNSSP). It has already established its capability to meet ICAO-SARPS requirements through its Technology Demonstration System (TDS) Phase. TDS was successfully implemented over the Indian air space in 2007 with the installation of eight Indian Reference Stations (INRESs) at eight airports and linked them to the Master Control Center (MCC) located near Bangalore.
Preliminary System Acceptance Testing (PSAT) was also successfully completed in 2010. The first GAGAN navigation payload was launched in 2011 on GSAT-8. The second GAGAN payload is scheduled to be launched on GSAT-10 in the first quarter of 2012.
Apart from the civilian sector, the system is also likely to be used in non-civil aviation sectors like maritime applications, improved surveying and cartography, scientific research etc.
COURTESY: searchmobilecomputing.techtarget. com, www.space.gov.au, www.spaceandtech. com, ESA, ISRO, AAI, wikipedia
