by Gurbir Singh
Just as IDSN cooperates with international partners for Earth to space radio communication, India’s SLR stations cooperate internationally through the US-based NASA-operated International Laser Ranging Service (ILRS). A worldwide network of SLR stations is coordinated through the ILRS. The timing signals received from the atomic clocks of navigation satellites are used as a source of high-precision time to synchronise clocks by terrestrial service providers, such as financial services systems (ATMs, stock trading and banks), traffic management (road, rail, marine and aviation) and utility providers (electricity and water). The services of navigation satellites are now considered a utility in modern societies. Modern satnav receivers are remarkably sensitive and can pick up the signal from a satellite orbiting 36,000 km away. However, that signal is weak and vulnerable to deliberate or unintentional interference, jamming or spoofing.[750] One of the critical services that IRNSS will eventually offer is a high precision location service for the civil aviation sector.[751]
GAGAN: GPS Aided GEO Augmented Navigation
The GAGAN system provides satellite-based location service similar to IRNSS but is currently independent of it. Named after the Hindi word for sky, GAGAN is a joint project between the Airports Authority of India and ISRO to bring modern communication, navigation, surveillance and air traffic management to Indian airspace. GAGAN is an example of a Satellite-Based Augmentation System (SBAS). The primary objective of SBAS systems is to provide information (position, velocity and time) with a high level of accuracy, availability, integrity and reliability to the civil aviation sector.[752] Satnav precision of about 10 m is sufficient for road vehicles, where a driver can clearly see the road and road signs and make a correction on the move. For commercial aviation, it is not sufficient.
GPS or IRNSS data alone is not sufficiently precise for the aviation industry’s internationally recognised Safety-of-Life Service criteria as defined by the International Civil Aviation Organisation (ICAO). Commercial airliners coming in to land at 200 km/h in poor visibility or at night require high accuracy positional information around 10 times a second. SBAS provides this enhanced positional accuracy by overlaying the existing GPS/IRNSS signals with dynamic and reliable corrections from the ground station whose position is known with high precision.[753]
GAGAN provides positional information of known accuracy to civilian aircraft while at the terminal, en-route and approach. On 30 December 2013, GAGAN was certified only for en-route operations by the Indian government’s regulator, Director General of Civil Aviation. On 21 April 2015, it was reclassified for precision approach. GAGAN provides Non-Precision Approach (positional information of horizontal accuracy of 18 m) over all India and a Precision Approach (with positional information of 16 m horizontal and 20 m vertical) over most of the Indian landmass on nominal days, determined primarily by the variability of the ionosphere through which the IRNSS signal must travel.
Figure 13‑6 GAGAN Ground Stations. Credit Airports Authority of India[754]
It is built with a level of integrity such that only one in 10 million approaches may receive misleading positional information.[755] It has a built-in self-assessment mechanism that alerts the pilot if the quality of the signal is poor and its calculations are not reliable. An SBAS system relies on three components, the US’s GPS network, a national network of ground stations and typically three satellites in GEO directly overhead the landmass in which it operates. GAGAN’s space segment consists of three Indian communication satellites, GSAT-8, GSAT-10 and GSAT-15, in GEO over India. GSAT-8 and GSAT-10 are active, while GSAT-15 is in stand-by mode. The extensive GAGAN infrastructure consists of three key elements.
Indian Reference Stations: that continuously determine the precise positions of the US’s GPS (26,000 km and moving at almost 4 km/s) and Indian NavIC (36,000 km and moving at 3 km/s) satellites. Since the exact positions of the 15 reference ground stations are known to an extremely high degree of precision. These measurements are used to determine the precise positions of the satellites in orbit dynamically.
Two Indian Master Control Centres (INMCC): in Bengaluru to which the reference stations forward their measurements. These data are processed to determine the dynamic error value in the satellites’ positions. The dynamic correction value calculated by the ground stations have two key attributes, range and integrity. The range provides the precise location of the GPS satellites as measured by each of the multiple ground stations. The integrity of the correction is provided through numerous repeated measurements conducted independently at each ground station and the INMCCs. It is these corrections that a GAGAN receiver uses to compensate for the GPS errors.
GSAT-8 and GSAT-10: The corrections calculated by the INMCC based on the data received from the reference stations are transmitted to GSAT-8 and GSAT-10 for retransmission back to Earth for GAGAN receivers. The two INMCCs in Bengaluru are connected to the Indian uplink stations, two in Bengaluru and one in New Delhi, using two fibre optic links operating at 2 MB, as well as two 128 kb VSAT terminals.
The GAGAN infrastructure relies on other elements to minimise the positional errors. The radio signals between the Earth and the satellites must pass through the ionosphere, which is subject to variability from space weather arising primarily from the variability in the solar wind, charged particles emitted by the Sun. On arrival, the charged particles interact with the atoms and molecules in the Earth’s upper atmosphere and strip electrons from them. The Total Electron Content (TEC) is a measure of the density of electrons in the Earth’s ionosphere. This electron density in the ionosphere has a direct influence on the quality of signal reception from the navigation satellites in orbit to receivers on Earth. Monitoring TEC helps determine the magnitude of the interference satnav signals are subjected to.
As with all other aspects of the Earth’s atmosphere, the TEC value is subject to random changes and is particularly influenced by solar activity. India is especially prone to TEC variations because the Equatorial Ionization Anomaly (EIA - a concentrated region of ionisation near the magnetic equator) and the EEJ happen to lie directly over India. A network of TEC stations around India continuously measure these variations and provide corrections for the positions of the navigation satellites to maintain the accuracy of the SBAS system. The TEC measurements are also used by the INMCC to process corrections, along with the data from the reference stations.
Facility
Number
Provision
Master Control Centre (INMCC)
2
Bengaluru × 2
Land Uplink Station (INLUS)
3
Bengaluru × 2
Delhi
Indian Reference Station (INRES)
15
Airports in the following cities Ahmedabad, Bengaluru, Bhubaneswar, Kolkata, Delhi, Dibrugarh, Gaya, Goa, Guwahati, Jaisalmer, Jammu, Nagpur, Porbandar, Port Blair and Trivandrum
Total Electron Content (TEC) Stations
18
TEC stations to monitor the total electron content over the Indian subcontinent.[756]
Data Communication Network (DCN)
4
Two optical fibre cable networks
Two VSATs
Geosynchronous Satellites
3
GSAT-8, GSAT-10 and GSAT-15
Table 13‑2 Overview of the GAGAN Architecture
SBAS can help replace the function of a traditional Instrument Landing Systems (ILS) used by pilots, particularly in complex terrains, difficult weather conditions or at night. Unlike ILS that has to be built around airports and individual runways, SBAS does not make a similar demand on an airport infrastructure. In addition to the benefits of fuel saving, reduction in delayed landings and take-offs, efficient use of airspace capacity, reduction in environmental impact and enhanced navigation, GAGAN improves safety in the aviation sector. Any aircraft equipped with a GAGAN receiver can provide a satellite-based location in real time to ground-based air traffic controllers.[757]
In addition to India’s GAGAN , other SBAS implementations include:
Japan – Multi-Functional Satellite Augmentation System (MSAS)
US – Wide Area Augmentation System (WAAS)
Canada – Canadian Wide Area Augmentation System (CWAAS)
Europe – European Geostationary Navigation Overlay Service (EGNOS)
Russia – System for Differential Correction and Monitoring (SDCM)
China – Satellite Navigation Augmentation System based on the BeiDou constellation
Airport Authority of India is widening the scope of GAGAN beyond aviation and looking to offer the service to additional sectors, such as vehicle tracking, train tracking, disaster management, marine and farming applications, as well as to surrounding nations that happen to fall in GAGAN’s footprint.[758]
SBAS relies on GEO satellites, which maintain their positions over the same points on Earth, and associated ground stations. Therefore, SBAS systems have evolved regionally. There are several SBAS networks around the world, but only three (WAAS, MSAS and EGNOS) are fully operational. Since most international flights operate globally, SBAS systems have been designed to be compatible with each other, and providers collaborate to ensure that all SBAS systems integrate into a seamless global navigation system. GAGAN is designed to interoperate with WAAS, EGNOS and MSAS. SBAS cooperation is coordinated via the Interoperability Working Groups EGNOS/MSAS and EGNOS/WAAS, and interoperability tests are regularly conducted. Countries that provide the SBAS service do not charge for it.
Global Navigation Satellite Systems
The US developed the first GPS in 1973, and it is the first of what is now called Global Navigation Satellite Systems (GNSS). GPS has been fully operational since the mid-1990s. A satnav receiver is able to determine its position, speed and time from a minimum of four navigation satellites in Earth orbit. Local topography, such as trees, high-rise buildings or mountains could reduce the field of view. With a constellation of at least 24 satellites in orbit at around 26,000 km altitude, a minimum of eight GPS satellites will be overhead at any one time, which would provide an adequate level of the signal in most inhabited parts of the world.
In the interest of maintaining control over critical national services independent of the US’s GPS and, to some measure, securing sovereignty, other nations are developing their own systems. Russia, China, Japan and the European Union (EU) have or are developing their own GNSS infrastructures, GLONASS, BeiDou, Quasi-Zenith Satellite System (QZSS) and Galileo, respectively.[759] India’s development of IRNSS, despite its not being global, is part of this growing international infrastructure. The Japanese QZSS has been under development for several years. Once complete, by 2018, the final configuration will consist of four satellites, three of which will be in a very unusual orbit. The QZSS satellites will be at GSO (35,786 km) inclined at 43°. The fourth satellite is for redundancy, an in-orbit spare. This unique orbit will allow at least one satellite to be overhead (elevation 60° or more) over Japan at any one time providing a navigational signal to receivers on Earth directly below.[760]
GPS
GLONASS
GALILEO
COMPASS/BeiDou-2
Number of satellites
32
27
24 (6 spares) by 2020
35 (by 2020) + 5 GEO
Number of orbital planes
6
3
3
3
Semi-major axis (km)
26,560
25, 508
29, 601
21,500
Orbital revolution period
11:58 H
11:15 H
14:07 H
12:35 H
Inclination
55°
64.8°
56°
55°
Satellite mass
1,100 kg (IIR)
1,400 kg
700 kg
2,200 kg
Table 13‑3‑ Summary of GNSS systems in operation and in development
By the end of the second decade of the 21st century, over a hundred navigational satellites (32 GPS, 27 GLONASS, 30 Galileo, 11 IRNSS, 4 QZSS and 35 BeiDou) will be orbiting the Earth. Larger constellations offer higher positional accuracy and inherently more reliable service, suggesting the need for interoperability and cooperation.
System
Status
The US’s GNSS GPS (Global Positioning System)
The US military established the GPS system in 1978, and in 1996, it was made available globally for civilian use but with limited accuracy. The restrictions were removed to its present form in 2000. GPS consists of 24 operational satellites in six orbital planes inclined at 55° at an altitude of 26,560 km and has two master control stations (primary and backup), eleven command and control antennas and fifteen monitoring stations. All ground stations are located around the world.
Russia’s GNSS GLONASS (Globalnaya Navigazionnaya Sputnikovaya Sistema)
By 2015, GLONASS consisted of 27 of the eventual 29 satellites in 3 orbital planes inclined at 64.8º at 19,000 km orbit. It was established during the Soviet Era and became operational in 1995 but was not maintained for several years. It has been developed by the Russian Federation in two phases 2002–2011 and New GLONASS 2012–2020. GLONASS has been operating as planned since 2011. It has 60 ground-based monitoring stations, with at least 25 located outside the Russian Federation.
China’s GNSS BeiDou Navigation Satellite System (BDS)
Currently, BeiDou consists of 20 satellites providing primarily regional coverage. The number of satellites in orbit is set to increase from the current 20 to 35 by 2020 making BeiDou a GNSS. Eight satellites in the BeiDou constellation are in GSO or GEO orbits providing IRNSS-like service over China. The remaining will be at medium Earth orbit of 21,000 km in three orbital planes inclined at 55º.
EU’s GNSS Galileo
Galileo (named after the 17th-century Italian astronomer) is a joint ESA and EU programme for a GNSS with 18 satellites already in orbit of an eventual 30. The first satellite was launched in 2005, and Galileo became partially operational in 2016. Galileo is due to become fully operational by 2020 with all 30 satellites (24 operational and 6 in-orbit spares) at 23,000 km orbit in 3 orbital planes. The ground segment consists of 16 sensor stations, 6 tracking and command facilities and 5 uplink stations.
India’s regional navigation system IRNSS (Indian Regional National Satellite System)
IRNSS is a regional (not global) satellite system consisting of 7 satellites (three in GEO and four in GSO orbits) with a footprint over India and 1,500 km beyond. The final number of satellites in the constellation is not defined but will increase beyond 7. It is designed to have an operational lifetime of 12 years.
Table 13‑4 Summary of National Satellite Navigation Systems
Just as there is a requirement for interoperability between the SBASs for use predominantly in civil aviation, there is also a desire for interoperability between the satellite navigation (regional and global) systems. The larger nations with larger investment now have operational GNSS programmes (if not yet fully mature) for global navigational systems. However, lack of cooperation between the US, Russia and China has hindered interoperability for reasons of political interest, the pursuit of commercial advantage and prestige of maintaining national self-sufficiency.
But this is changing. International cooperation, through the UN, EU and more recently BRICS, has demonstrated remarkable success in space-based search and rescue service COSPAS-SARSAT, SBAS systems in the US and Europe and collaboration in human spaceflight making the ISS viable.The UN-sponsored International Committee on Global Navigation Satellite Systems has been promoting cooperation for mutual benefit between nations on matters of civilian use of GNSS resources since 2005. Although it has not yet achieved its vision of the “best satellite-based positioning, navigation and timing for peaceful uses for everybody, anywhere, anytime”, there has been progress.[761]
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Interoperability between nations is to the benefit of all the nations that participate. These benefits include minimisation of the risk of in-orbit collisions between satellites and between satellites and space debris, the prevention of radio interference and maximisation of the density of satellites in constellations. Collaboration also simplifies the receiver design that can work with multiple constellations, increase in signal strength and improvement in location accuracy for end users.
The US established bilateral agreements with Russia and ESA in 2004, with India in 2005, with Japan in 2008 and with China in 2014 to support interoperability with the US’s GPS. To help deliver a global, compatible and interoperable GNSS system, cooperative agreements are facilitated through international organisations, such as the ICAO, UN’s International Telecommunications Union, the Radio Technical Commission Aeronautics and the European Organisation for Civil Aviation Equipment.
While India is exploiting both IRNSS and GAGAN for national programmes, such as farming, fishing, aviation, town planning and new business opportunities, it has also been in consultation with the US on GPS-related cooperation since 2005. India’s IRNSS functionality, except the RS, fits naturally with its space programme's raison d'être to support India's social and economic development. To facilitate the development of commercial applications and international cooperation, ISRO released the Signal in Space Interface Control Document (ICD) in June 2014. The document details the technical parameters that developers require to build systems and applications using the navigation signals from the IRNSS constellation.[762] ICDs have been released by Russia for GLONASS (version 5.1 in 2008), by the US for GPS (version H in 2013), by China for BeiDou (v2.0 in 2013), by India for IRNSS (v1.1 in 2017[763]) and by the EU for Galileo (v1.2 in 2015).