The process of getting an aircraft safely and efficiently from its origin to destination requires effective air traffic management systems supported by three key functions using satellite technology: Communications, Navigation and Surveillance.
Communications is the exchange of voice and data information between the pilot and air traffic controllers or flight information centers. Navigation pinpoints the location of the aircraft for the air crew. Surveillance pinpoints the location of the aircraft for air traffic controllers.
It includes communication of navigation information from aircraft to air traffic control centers which facilitates the continuous mapping of the relative positions of aircraft. The International Civil Aviation Organization (ICAO) calls the three functions the CNS systems and regards them as forming the basic support services of air traffic management (ATM) systems.
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 Communications
The communication element of CNS/ATM systems provides for the exchange of aeronautical data and messages between aeronautical users and/or automated systems. Communication systems are also used in support of specific navigation and surveillance functions.
It is envisaged that most routine air-ground communications in the en-route phase of flight will be via digital data interchange. For this purpose, the user selects a particular message from a pre-constructed set of messages using a screen menu, adds some specific parameters (or free text) and then sends it. Some data transfers take place between automated airborne and ground systems without the need for manual intervention. Such data exchanges will greatly reduce the volume of voice communications and therefore reduce the work load of pilots and controllers. In busy terminal areas, however, the use of voice communications will likely still be preferred. For emergency or non-routine communications, voice will remain as the primary means of air-ground communications.
For ground-ground communications, it is envisage that most routine communications between ground-based aeronautical users and systems will be by data interchange. Such interchanges between entities such as meteorology offices, NOTAM offices, aeronautical data banks, ATS units, etc., may be in any of the following forms:
- free text messages;
- pre-selected data messages (with some manually added parts; and
- automated data interchange between computerized systems.
A variety of ground networks, implemented by States, a group of States or commercial networks which use packet switching techniques and are compatible with ISO’s OSI reference model will be able to use the internetworking services of the Aeronautical Telecommunication Network (ATN). With the gradual implementation of the ATN, the use of AFTN will diminish. However, during the transition period, interconnection of AFTN terminals to the ATN will be possible via special gateways.
The ATN and its associated application processes, has been specifically designed to provide, in a manner transparent to the end-user, a reliable and-to-end communications service over dissimilar networks in support of air traffic services. ATN can also carry other communication service types, such as aeronautical operational control (AOC) communications, aeronautical administrative communications (AAC) and aeronautical passenger communications (APC).
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 Navigation
The navigation element of CNS/ATM systems is meant to provide accurate, reliable and seamless position determination capability, worldwide, through the introduction of satellite-based aeronautical navigation or global navigation satellite system (GNSS).
The GNSS is a worldwide position and time determination system that includes one or more satellite constellations, aircraft receivers, and system integrity monitoring, augmented as necessary to support the Required Navigation Performance (RNP) for the actual phase of operation.
The satellite navigation systems in operation are the global positioning system (GPS) of the United States and the global (orbiting) navigation satellite system (GLONASS) of the Russian Federation. Both systems were offered to ICAO as a means to support the evolutionary development of GNSS. In 1994, the ICAO Council accepted the United States offer of the GPS (State letter LE 4/4.9.1-94/89 dated 11 December 1994), and in 1996, it accepted the Russian Federation offer of GLONASS (State letter LE 4/49/1-96/80 dated 20 September 1996).
The GPS segment is composed of twenty-four satellites in six orbital planes. The satellites operate near-circular 20,200 km (10,900 NM) orbits at an inclination angle of 55 degrees to the equator and each satellite completes an orbit in approximately 12 hours.
The GLONASS space segment consists of twenty-four operational satellites and several spares. GLONASS satellites orbit at an altitude of 19,100 km with an orbital period of 11 hours and 15 minutes. Eight evenly spaced satellites are arranged in each of the three orbital planes, inclined 64.8 degrees and spaced 120 degrees apart.
To overcome inherent system limitations and to meet the performance requirements (accuracy, integrity, availability and continuity) for all phases of flight, GPS and GLONASS require varying degrees of augmentation. Augmentations are classified in three broad categories: aircraft-based, ground-based and satellite-based.
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 Surveillance
The surveillance systems presently in use can be divided into two main types: dependent surveillance and independent surveillance. In dependent surveillance systems, aircraft position is determined on board and then transmitted to ATC. The current voice position reporting is a dependent surveillance systems in which the position of the aircraft is determined from on-board navigation equipment and then conveyed by the pilot to ATC by radiotelephony. Independent surveillance is a system which measures aircraft position from the ground. Current surveillance is either based on voice position reporting or based on radar (primary surveillance radar (PSR) or secondary surveillance radar (SSR)) which measures range and azimuth of aircraft from the ground station.
Voice position reporting. Surveillance through voice position reporting is mainly used in oceanic airspace and aerodrome control service or area control service outside radar coverage. Pilots report their position using VHF and/or HF radios.
Primary surveillance radar (PSR). The ground based PSR system provides information on the bearing and distance of the aircraft. PSR does require carriage of any equipment by aircraft and is capable of detecting almost any moving target. With increasing usage of more advanced surveillance systems, the use of PSR for international air traffic management will diminish. PSR will, however, continue to be used for national applications. Primary radars are currently used for surface movement detection as well as weather detection. Precision approach radars (PARs) are primary radars used for approach operations based on specific procedures for the pilot and the controller; however, use of PARs for civil applications is rapidly decreasing.
Secondary surveillance radar (SSR). The SSR interrogates transponder equipment installed in the aircraft. In Mode ‘A’, the aircraft transponder provides identification information, aircraft bearing and distance and in Mode ‘C’, it provides pressure-altitude information. The current SSR is in wide use in many parts of the world where terrestrial line-of-sight surveillance systems are appropriate. The accuracy, resolution and over-all performance of range and azimuth information is significantly improved by the application of monopulse (including large vertical aperture antennas) and other advanced processing techniques.
The beneficial role of SSR for surveillance purposes can be enhanced through the use of Mode S which is a technique that uses a unique address (the 24-bit address) for each aircraft. It permits the selective interrogation of Mode S transponder-equipped aircraft and therefore eliminates garbling. It also provides for a two-way data link capability between Modes S ground stations and Mode S transponders. SSR Mode S is the appropriate surveillance tool in high-density traffic areas. The interconnection of ground stations in clusters provide an enhanced surveillance and communication system.
Automatic dependent surveillance (ADS). The introduction of air-ground data links, together with sufficiently accurate and reliable aircraft navigation systems, presents the opportunity to provide surveillance services in areas which lack such services in the present infrastructure, in particular oceanic and other areas where the current systems prove difficult, uneconomic, or even impossible to implement. ADS is an application for use by ATS in which aircraft automatically transmit, via a data link, data derived from on-board navigation systems. As a minimum, the data include the four-dimensional position, but additional data may be provided as appropriate.
The ADS data would be used by the automated ATC system to present information to the controller. In addition to providing traffic position information in non-radar areas, ADS will find beneficial application in other areas, including high-density areas, where ADS may serve as an adjunct and/or back-up for SSR, thereby reducing the need for primary radar. In some circumstances, it may even substitute for secondary radar. As with current surveillance systems, the full benefit of ADS is obtained by supporting complementary two-way pilot/controller data and/or voice communication (voice for at least emergency and non-routine communication).
ADS-broadcast (ADS-B). ADS-B is an expansion of the ADS technique that involves broadcast of position information to multiple aircraft or multiple ATM units. Each ADS-B-equipped aircraft or ground vehicle periodically broadcasts its position and other relevant data derived from on-board equipment. Any user segment, either airborne or ground-based, within range of this broadcast, can process the information. ADS-B is currently defined only for line-of-sight operations (e.g. broadcast over VHF digital link or by SSR Mode S extended squitter). ADS-B is also envisaged to be applied for surface movement, thus being an alternative to surface radars such as airport surface detection equipment.
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