Reference Points

1Peter Gutierrez outlines the navigation backbone of space-based air traffic management and the deployment issues involved
Global Navigation Satellite Systems (GNSS), which include most famously the US Global Positioning System (GPS) but also Russia’s GLONASS, China’s Beidou and the European Union’s Galileo, are changing the face of aviation, increasing the safety and efficiency of air transport all over the world.
Space-based navigation signals enable three-dimensional position determination in all flight phases, from departure to arrival, and during taxiing.
Space-Based Augmentation Systems (SBAS) are being used to improve GPS signals, for specific aviation applications such as landing aircraft under conditions of poor visibility. Improved approaches to airports, with significant operational benefits, can now be implemented even at remote locations, where traditional ground-based navigation aids are unavailable.
With a number of satellite navigation systems now deployed or in various phases of development, and with newer systems on the horizon, here’s a brief overview of who’s doing what.
Operated by the US Air Force Space Command, the Global Positioning System, or GPS, is the granddaddy of all GNSS. Initiated as a military navigation project in 1973, it was the first GNSS to become fully operational in 1995.
GPS can be divided into three basic segments. First, the satellite constellation itself is made up of low-earth-orbit satellites that broadcast ranging signals and navigation data messages.
Second, a ground control network monitors satellite health, orbital configuration and the integrity of the signal. Ground control also carries out tasks such as clock correction and updating of other parameters essential to determining position, velocity and time (PVT).
Finally, the third segment encompasses the user equipment – the GPS receiver in your car, your smartphone, or in the aircraft overhead. All GNSS, from GPS to GLONASS, to Beidou and Galileo, are essentially configured in this same way, split into the same three segments.
Over the decades, GPS has been improved and upgraded, and, it is widely seen as a key component of tomorrow’s air traffic management system, not just in the US but also around the world.
The GNSS-based ‘Next Generation’ (NextGen) programme in the US will see ground ATC facilities, airports, and private aircraft equipped to generate, display and transmit precise and real-time positions of all aircraft operating in a given air space. The result, according to the Federal Aviation Administration (FAA), will be a 35 per cent reduction in total delays, in flight and on the ground, compared to what would happen without NextGen.
Russia’s Global Navigation Satellite System, GLONASS, is operated by the Russian Aerospace Defence Forces. The system has traditionally been advertised as an alternative to GPS, with global coverage and comparable precision.
But it is the ability to use both GPS and GLONASS, combining signals from both constellations, essentially doubling the number of available satellites in the sky, that really brings home the advantages of ‘multi-constellation’ GNSS.
With GPS+GLONASS, faster and more accurate position fixes are possible, particularly in built-up areas, the so-called ‘urban canyons’, where buildings can obscure line-of-sight GNSS satellites.
The Soviet Union began developing GLONASS in 1976, with satellites launches from the early 1980s until the constellation was completed in 1995. In the later 1990s the system fell into disrepair.
Then, starting in 2001 under Vladimir Putin, the restoration of the system was made a top priority. GLONASS still accounts for a large share of the Russian Federal Space Agency’s annual budget.
GLONASS had achieved 100 per cent coverage of Russia’s territory by 2010, and in 2011 the complete constellation of 24 orbiting satellites was restored, along with full global coverage. The satellites have been repeatedly upgraded. GLONASS-K is the latest and most advanced version.
In 2012, the Russian Ministry of Transport published a decision to make GLONASS or GLONASS+GPS satellite navigation equipment standard for most civil aircraft and helicopters in Russia. The measure, said the Ministry, was intended to boost safety and improve air traffic control, and included specific requirements for hazardous goods transporters as well as commercial aircraft, general aviation aircraft and helicopters of specific weight classes.
Foreign manufactured aircraft, flown commercially in Russia by both domestic and foreign airlines are expected to meet the same requirements by the end of 2017.
The decision is only one in a series of decrees the Russian transportation regulator has issued since 2008 in pursuance of legislation mandating the use of GLONASS or GLONASS+GPS by all Russian passenger carriers, haulers and shippers, whether on land, on the sea or in the air.
There has been some confusion among non-Russian parties concerning how exactly to satisfy the GLONASS mandate, although most will agree that a second full GNSS constellation is of certain benefit to global air navigation.
On the other hand, it is worth remembering that GPS and GLONASS satellites do not necessarily display identical performance characteristics, so it is essential to understand these differences when attempting to combine the positioning capabilities of the two systems.
Although China was initially seen as a potential partner in the EU’s Galileo programme, the two sides ultimately fell out over security and financial issues. China subsequently decided to go it alone, taking steps to develop its own full-fledged and fully independent satellite navigation system.
The BeiDou Satellite Navigation System comprises two separate satellite constellations. The first, the BeiDou Satellite Navigation Experimental System, sometimes referred to as BeiDou-1, is essentially a test system. Operating since 2000, it is comprised of three satellites offering limited coverage and applications. Users mainly in China and neighbouring regions have been receiving BeiDou-1 navigation services since 2000.
The second generation of the system, referred to as COMPASS or BeiDou-2, became operational in China in 2011 with a partial constellation of 10 satellites in orbit. Since 2012 it has been offering services to customers in the Asia-Pacific region. BeiDou-2 will eventually consist of 35 satellites and is expected to begin global services upon completion in 2020.
BeiDou will provide global positioning, navigation and timing services, including two service modes: an open service and an authorised service. The open service will feature a positioning accuracy of 10 metres, velocity accuracy of 0.2 metres per second and timing accuracy under 10 nanoseconds.
In the early 1990s the European Union started seriously discussing the need for a European GNSS. A defining characteristic of Galileo would to be that, unlike GPS and GLONASS, it is and will remain under non-military, civilian control.
While European independence has been a driving motivation behind the creation of the new system, Galileo is nevertheless interoperable with GPS and GLONASS. Once fully operational, probably in 2020, Galileo will include 30 orbiting satellites and will deliver an array of services, including a free open-access navigation service, an encrypted commercial service, a safety-of-life service, an encrypted public regulated service, and a search and rescue service.
The issue of governance has been a sticky one for Galileo. Currently, the European Space Agency (ESA) is in charge of Galileo technical development and deployment, with the European Commission retaining overall responsibility for the programme and the Commission’s subsidiary European GNSS Agency (GSA), based in Prague, covering operational management.
The Galileo programme works closely with a number of key partners in the aviation sector, including Eurocontrol, the European air navigation safety organisation. The common objective is to further the integration of GNSS within a seamless, pan-European air traffic management (ATM) system.
SBAS – an ever-growing circle
One thing is certain – if we want to use GNSS as a primary navigation tool for applications such as aviation, the most rigorous safety requirements have to be met. Specifically, we have to be certain about the level of trustworthiness, i.e. the ‘integrity’ of the satellite signal.
One way to provide integrity information is via a space-based augmentation system (SBAS). The typical SBAS setup involves monitoring stations distributed across a particular geographical region, along with a central control station.
The true positions of the monitoring stations are known, so these positions can be compared against the computed position based on GNSS signals from space. Any potential errors can thus be detected and quantified. A set of uplink stations then transmits the integrity and correction information to users via geostationary satellites.
A number of SBASs have already been deployed, and while they all work to improve GPS signals, plans are in the works to move to a multi-constellation design, adding Europe’s
Galileo, China’s Beidou and Russia’s GLONASS systems post-2020.
Indeed, over the past few years, SBAS operators have worked to define a standard SBAS methodology for the next generation of SBAS systems, employing dual-frequency, multi-constellation signals on the L5 frequency.
The ultimate aim is to have a worldwide system of linked SBASs, with standardised signalling, allowing aviators to fly anywhere without losing SBAS-augmented GNSS coverage.
In the US, the Wide Area Augmentation System (WAAS); the world’s first operational SBAS, covers the United States and most parts of Alaska, Canada, and Mexico.
Jointly developed by the US Department of Transportation and the FAA starting in 1994, WAAS was intended to enable aircraft to rely on GPS for all phases of flight, including precision approaches to any airport within its coverage area.
The WAAS signal was activated for safety-of-life aviation in July 2003. At present, WAAS supports en-route, terminal and approach operations down to a full LPV-200 for the contiguous US, Canada, and Mexico.
In Europe, the European GNSS Navigation Overlay System (EGNOS) provides integrity information and improved accuracy for the European continent and parts of North Africa. EGNOS was the first operational European GNSS and the world’s second active SBAS.
The European Commission has said it intends to maintain EGNOS services for a minimum of 20 years, with six years advance notice in case of significant changes in the provided services.
Moreover, EGNOS is specifically intended to be part of the future inter-regional network of SBAS services, with a multi frequency and multi constellation configuration and with full operational capability by 2020, when GPS, GLONASS, Beidou and Galileo will all be operational.
The third operational SBAS is Japan’s Multi-functional Satellite Augmentation System (MSAS), sometimes referred to as the MTSAT Satellite Augmentation System. MSAS was manufactured and delivered by NEC, under contract with the Civil Aviation Bureau, Ministry of Land, Infrastructure, Transport and Tourism.
Currently owned by the same ministry and the Japan Meteorological Agency (JMA), MSAS has been operational since 2007, supporting en-route, terminal and non-precision approach operations (RNP 0.1). Successful LPV flight trials have also been completed.
India’s GPS Aided Geo Augmented Navigation (GAGAN) system is the fourth certified SBAS to enter service worldwide. Jointly developed by the Airports Authority of India and the Indian Space Research Organisation, GAGAN provides improved the accuracy, availability and integrity of GPS. Although still under development, GAGAN can already be used for en route navigation and non-precision approaches without vertical guidance, as it is certified to RNP 0.1 service level.
The Indian government has said it will use the GAGAN experience in the creation of an autonomous navigation system called the Indian Regional Navigational Satellite System (IRNSS).
One of the main concerns about SBAS operations in India is the equatorial ionospheric anomaly belt, where certain physical process and features such as plasma bubbles, scintillations and the Appleton geomagnetic anomaly can adversely affect GNSS signals.
Russia is currently putting in place the WAAS-compatible System for Differential Corrections and Monitoring (SDCM). Unlike other current SBASs, SDCM will augment both GPS and GLONASS signals.
Expected to be certified in the coming years, SDCM is likely to be just the first step in a strategy that will eventually encompass other broadcast means, possibly including a polar medium earth orbit, allowing service provision to the extreme northern parts of Russia.
Finally, China has also entered the SBAS game with its Satellite Navigation Augmentation System (SNAS), although little information about the system has been made available to the public. We do know that, as long ago as 2002, the Novatel company was awarded a contract for the provision of 12 receivers for a second phase of SNAS development.