Communication Navigation Surveillance / Air Traffic Management (Cns / Atm) Beyond 2012
2.1.a Global Positioning System (GPS): The GPS is a satellite-based radio navigation system that provides its users with high precision position and time information over the entire globe. The space segment is composed of 24 satellites with a useful life of approximately 7.5 years, arranged in 6 orbits of four satellites each at an altitude of 20,200Km. The system is divided into three segment i.e user segment, ground segment and space segment. The user segment consists of the antenna and the processor receiver for receiving and processing the navigation solutions used to provide it with the precision position and time. The GPS satellites are positioned in very precise and predictable orbits. The GPS satellites orbit the earth every 11 Hrs 58 Minutes and pass over some of the monitoring station at least twice a day. These stations are equipped to calculate satellite positions with precision and to uplink the corrected information to them. GPS satellites send information to the receiver of their position with respect to the center of the earth, together with the time signal.
The airborne receiver uses this information to calculate a position with respect to the surface of the earth, which will be presented to the user in terms of latitude and longitude. The GPS satellites use 1984 geodetic coordinate system (WGS-84) i.e. this coordinate system was adopted by US in 1984.
2.1.b Global Orbiting Navigation Satellite System (GLONASS)
The Russian Federation has implemented the GLONASS, its concept quite similar to the GPS with different signal processing techniques. It provides for space signals to be sent to properly equipped users for precision determination of position, speed and time. The space segment consists of 24 satellites orbiting at an altitude of 19,100 Km with an orbit period of 11Hrs and 15 minutes. They are distributed into 3 orbits of 8 satellites each with an operational life of 3 years (5 years improved version GLONASS-M)
The ground segment fulfils satellites monitoring and control functions at same time it selects the data to be modulated in the encoded signals sent for navigation purposes. This segment includes the master station monitoring and information delivery stations. The measurement data from each monitoring station are processed at the master station and used to compute the navigation data down linked to satellites by relay stations. The operation of the system requires precise synchronization of satellites clocks with the time of the GLONASS system.
The user segment automatically receives navigation signals from at least four satellites and measures their speed. Simultaneously, it selects and processes all input data and calculates 3 coordinates, 3 speed components and the precise time. The geodetic coordinate system used in the system is called Earth Parameters 90 (PE-90).
2.1.c Galileo:
Galileo is European’s contribution to the next generation Global Navigation Satellite System (GNSS) intended to provide the European Nations with greater independence by delivering a civil controlled satellite based navigation system. Implementation of Galileo is expected to stimulate growth in the use of GNSS technology in intermodal transport systems thereby improving mobility safety and quality of life whilst also stimulating economic growth in the areas of receiver manufactures and application development. When considering the possibility of a failure of GPS to maintain a service, the implications for commercial customer in the fields such as telecom, location based services and financial community are clearly not acceptable. For these reasons and in consideration of the potential impacts on European business, the European Commission (EC) and the European Space Agency (ESA) has embarked on the definition phase of Galileo.
The current base line for Galileo calls for a constellation of around 30 middle Earth orbiting (MEO) satellites. The option of including some GEOS in the constellation is not rule out and indeed. Some interesting options could become available if GEOS are used to complement the system. The option of delivering a search and rescue service is also being analyzed for both technical and economic viability.
2.1.d GPS –L5:
As part of GPS modernization process many civil user group have advocated the need for second and third civil GPS signal. The proposed signals are a C/A code signal at the L2 frequency, starting on Block IIR-M satellite Vehicle (SVs) and a new signal in an existing Aeronautical Radio Navigation Services band at 1176.45 MHz called L5, starting with Block IIF SVs. The signal at L5 will occupy a new bandwidth of at least 24 MHz.
2.1.e Errors: Like all other conventional navigation systems, the GPS is subject to errors that can degrade the precision of the system. The errors are Ionospheric error, Troposheric error, Selective Availability, Satellite clock error, Receiver clock error, Multi path error, Receiver error, Satellites Ephemeris error and Geometrical error
The most significant error occurs when the satellites signal goes through the earth atmosphere. This is a layer of electrically charged particles located approximately between 130 and 190 Km above the surface of the earth. As the GPS signal travels through the ionosphere, it is slowed down in a proportion that varies according to time of days, solar activity and series of the other elements. Ionospheric delays may be forecast and an average correction applied to the GPS position. Another error is caused by water vapour in the atmosphere which delays the GPS signal and also contributes to degrade the position of the system.
Lastly the error existing in the system can be significantly increased, depending upon the geometry of the satellites used to determine a position. When the position dilution of precision (PDOP) is a factor in errors of between 30 and 300 meters can occur, depending upon receiver type, relative satellite position and extent of other errors.
Note: DOP factors are expressed how geometry affects to yield position accuracy & scale ranging accuracy. The optimum geometry for four satellites is achieved when three satellites are equally spaced on the horizon and one satellite directly on overhead. The geometry can be said to “dilute” the range domain accuracy by the DOP factors.
3.0 Geodetic Reference
There are many geodetic reference datum’s in use throughout the world providing reference for the charting of particular area. Each datum has been produced by fitting a particular mathematical earth model (ellipsoid) to the true shape of the earth grid ( Geoid) in such a way as to minimize the difference between the ellipsoid and the geoid over the area of the interest. Most ellipsoids in the use were derived in the last center and were normally referenced to a local observatory. These different datum’s and ellipsoids produce different latitude and longitude grids and hence different sets of geographical coordinates. Implementation of CNS/ATM systems require a global geodetic frame of reference to avoid errors in geographic coordinates that might be caused by the location of references in more than one datum.
3.1 World Geodetic System –1984 (WGS-84)
The WGS-84 was developed to provide for more precision and continuing updating of geodetic gravitational data also to offer means for interrelating positions based on various geodetic systems or datum through a system of coordinates that consider a single earth center as its fixed system. The WGS-84 represents the model of geocentric, geodetic and gravitational earth that uses data and technology available as of 1984. Such system allows the user to relate geographic data, such as coordinates obtained from a source based on a local datum, with another source. The WGS-84 is an ideal system for global navigation applications such as international air operations. In a static survey modality, the precision of geodetic latitude and longitude and geodic height of WGS-84 is within ± one meter.
3.2 Navigation System Performance Requirements: Navigation system performance requirements are defined in ICAO’s manual on RNP (DOC 9613) for single aircraft and for the total system that includes the Signal-In –Space (SIS) the airborne equipment and the ability of the aircraft to fly the desired trajectory. All the navigation aids must fulfill four basic performance requirements in order to be certified i.e. continuity, availability, integrity and accuracy.
- Continuity: It is the ability of the entire system to carry out its function without interruption during planned operating period.
- Availability: It is the ability of the system to transmit signals of the required quality most of the time. This is a critical requirement in landing guidance and for this reason stand by equipment is added to ground-based aids.
- Integrity: It is the ability of the navigational aid (s) to warn the pilot that it has failed or giving incorrect message.
- Accuracy: It is the ability of the navigational aid(s) is to guide the path of an aircraft within pre- defined tolerances.
The GPS component of the GNSS has a precision of 100 meters on the horizontal plane 95% of the time. The signals available for civil users are degraded signal-in-space (SIS) due security reason. It is estimated that GLONASS signal can be manipulated in a similar fashion. It is available as intended for civil users. It should also be borne in mind that a satellite fix in space is an ellipsoid in which the vertical axis of error is almost 50% larger than the horizontal axis error.
3.3 Navigation System: There are three type of navigation system.
- Supplementary Navigation System: The navigation system must meet the precision and integrity requirements but not the availability and continuity requirement.
- Primary Navigation System: The navigation system that must meet the precision and integrity requirements on approval for a given operation of flight phase but not the availability and continuity. Safety is achieved by limiting flights to specific periods of time and establishing certain procedural restrictions.
- Sole Means Navigation System: The navigation system is approved for a given operation or flight phase which must meet, the four navigation system performance requirements i.e. continuity, availability, integrity and precision for that operation of flight phase
3.4 Augmentations:
The GPS fails to provide continuity, availability, integrity and accuracy to allow for its use as the sole means of navigation for all phases of flight. In order to meet operational requirements, augmentation must be applied to basic GPS signals to eliminates the errors. Three basic categories of augmentation have been proposed i.e. Air-borne-based augmentation system (ABAS), Ground-based augmentation system (GBAS) and Satellite–based augmentation system (SBAS). The brief of the augmentation systems are given below.
3.4.a Air-borne based augmentation system (ABAS): It contains two types.
- Receiver autonomous integrity monitoring (RAIM): this technique can be used if there are more than 4 satellites with the appropriate geometry within the range of receiver; with 5 satellites, 5 independent positions can be computed. If these do not match, the receiver infers that one or more of the satellites are supplying incorrect information and a warning light turn ON on equipment panel. If there are six or more satellites within range, more independent position positions can be calculated and receiver will be able to identify the defective satellites and exclude it from positioning calculation. A process known as barometric aiding may assist the RAIM technique. Aircraft barometric altitude information is taken from the GPS receiver, which can simulate a satellites placed directly over the user.
- Aircraft Autonomous integrity monitoring (AAIM): in this method on-board augmentation can be used. An Inertial Navigation System (INS) can replace the GNSS at times when their antennas are shielded (during maneuvering of aircraft) or when numbers of satellites within range of the receiver are inadequate.
3.4.b Ground- Based Augmentation System (GBAS):
This system is used to enhance the continuity, availability, integrity and precision of GNSS signals within a reduced geographic area. It consists of a ground monitoring station whose locations known with precision. This station evaluates the information received from GNSS satellites, detects clock and other errors and sends a correction signal to airborne receivers through a VHF data links. Precision of the order of 5 meters can be achieved with ground-based augmentation systems, which makes them suitable for Cat-II/III instrument approaches. The advantages of the GBAS lie in the fact, that it can serve all airport runways within a range of 25 nautical miles (NM) from the ground monitoring station.