Communication Navigation Surveillance / Air Traffic Management (Cns / Atm) Beyond 2012

Communication Navigation Surveillance / Air Traffic Management (Cns / Atm) Beyond 2012

v. Free Flight/Autonomous Flight:
It is developed in the United States and originally conceived to give aircrafts more maneuvering capacity, with the support of available new technologies. It also emphasized the need for users to decide their own schedules, routes, and altitudes, thus reducing delays and cost involved.

1.0 Communication System:
In future, voice communication will be used for critical messages, such as vectoring to avoid traffic and landing clearance at airports with heavy traffic. It will serve as back up for data link. VHF analogue radios available today are not compatible with upcoming new technology. The VHF Data Link (VDL) operation requires a VHF digital radio (VDR). VDL is essential for ATN implementation and consequently, for greater use of ATS data links. The availability of airborne equipment manufactured in series, including digital radios, will make it possible to equip a larger number of aircraft. The VDL formats specify a protocol for delivering data packets between airborne equipment and ground systems similar to that used in the aircraft communication addressing and reporting system (ACARS). The difference is that the VDL provides a capacity 10 times greater than the equivalent of 25 KHz VHF channel.

VDL Mode 1: The use of VHF analogue radios for data exchange was started by air lines in the late 70s. Current air borne radios have been used to transmit data between operators and their aircrafts by means of special ground-based stations and interconnection networks. The so-called ACARS system has been developed and has grown considerably, with limited use for ATC communication. This mode has been especially designed to use ACARS modulation equipment and radio. ACARS and VDL mode-1 is a low speed bit oriented data transfer system. It uses carrier sense multiple access (CSMA) methodology. The new development has overtaken VDL mode-1 and is no longer in use.

VDL Mode -2: It is improved version of the VDL- Mode 1 and also uses same technology, but is not capable of handling voice communication. Average data transmission is 31.5 kbps. It employs a globally dedicated common signaling channel of 136.975 MHz. Limited commercial service are available at this time, as aircraft operators and service providers are able to introduce new equipment.

VDL Mode 3: It is an integrated digital data and communication system that makes it possible to use four radio channels on a carrier (with a 25 KHz spacing). It uses a data link technology called TDMA. The data capability provides a mobile sub network i.e. compliant to aeronautic traffic network (ATN). It is presently is not available for operational use.

VDL Mode 4: It also has navigation and surveillance capabilities and uses a data link technology called self-organizing time division multiple access (STDMA). In this mode station, stations transmit their geographical position together with data message in time slots that are dynamically modified at frequent intervals.

Before starting a transmission using the STDMA technique, the aircraft keeps listening watch on the frequency to be used and establish a track and a table of time slots of all other aircrafts. An algorithm in the aircraft transceiver selects a free slot or takes the slot of the most distant aircraft. This modulation system allows distant stations to transmit in the same slot with a minimum of interference. STDMA does not have voice communication capability. In this mode aircraft is not involved in any manual frequency tuning for any station change. Even so, reception of the geographic position of other aircraft gives a surveillance capability, which is candidate technology for ADS-B operations.

1.1 Data Link:
Data link is the basic component of communication between air traffic controller (ATC) units and aircraft. Data links were initially used only for ACARS communications but new equipments are compatible with ATN communications, which will improve the efficiency and compatibility of aeronautical VHF data channels. There are two types of ATS data links: one for the exchange of text message and the other for more complex and computer processed message for future air navigation system (FANS) uses, which employs ACARS data links.

MODE-S data link: It allows for an air-ground data link whose use is particularly indicated for airspace with traffic of high density. It can also operate in a mixed environment, in which aircrafts are equipped with transponders of different data link capabilities to fly.
HF data link (HFDL): The feasibility of using HF data links for ATC communication has been demonstrated The propagation anomalies rarely affect the entire HF frequency band, it is possible with carefully sited system of well connected ground stations with a number of adequate frequencies available in the HF band, to find best frequency for transmission of data packages anywhere and at any time. HF data link is also an excellent stand by system for aeronautical mobile satellite system (AMSS) in oceanic / remote areas. In this mode aircraft can contact three or more HFDL ground station constantly and its hub can become ATN routers.
Controller-Pilot Data Link Communication (CPDLC): It is the means of communication between the controller and the pilot that uses data links for ATC communication. In the area where CNS/ATM routes are built and where airspaces exist, that are outside HF communication range, CPDLC is the primary means of communication supplemented by HF and voice satellite links. Messages may be composed through individual utilization or a combination of upto five message elements for clearance, pre-departure clearance, and message related to ATC. The CPDLC will resolve number of flaws in the existing system e.g. it will provide an automatic data entry capabilities, which will permit ground systems and airborne flight management computes (FMC) to enter critical information, such as the flight routes etc. This will cut down on errors caused by manual data entry. It will also permit a significant reduction in transmission time, thus reducing the congestions and will eliminate misunderstanding due to a deficient quality of the voice received, propagation problems, dialects and the possibility of having instant access to previous voice transmission recording.

In future the CPDLC may be the primary means of communication, all aircraft should be also be advised of appropriate voice communication frequencies. Aircraft communication via CPDLC should only be done with the appropriate ATC unit for it route segment, otherwise request may get rejected due to absence of corresponding flight plan. The pilot initiates the CPDLC procedure by sending a contract message containing the four-letter ICAO site designator of the ATC unit. The latter will respond with an acknowledgement message. When an aircraft enters into airspace where CPDLC is used, the pilot has to send a contract message between 15 and 45 minutes before entering. An automated ATS system will not consider log-on attempts if the flight number or registration used for contract are not exactly the same as those indicated in the flight plan. Under normal conditions, the relevant ATC unit initiates the CPDLC disconnect sequence, sending an uplink end of service message. In response to this the airborne equipment sends a downlink disconnect message. Now next ATC unit can exchange the CPDLC message with the aircraft.

1.2 Data Links between ATS Units (ADIC)
The ADIC offers the means for exchanging data during the reporting, co-ordination, and transfer of control phases. ADIC use will largely reduce the need for voice co-ordination. ADIC message format and procedures are designed for use through any ground-ground circuit, including the AFTN and the future ATN. The means of communication for transmission of AIDC messages can be AFTN, ATN and Dedicated data network.

1.3 Aeronautical Mobile Satellite Services (AMSS)
The radar surveillance system and VHF communication equipment are limited to line-of-sight, they are not usable for surveillance and communication over oceanic or desert regions; further, they require many means of support, such as electric power and maintenance. In addition, HF communication is not fully acceptable because it is unsafe, of medium quality and requires too much technical support. Satellite communication on the other hand can provide high-quality voice and data communication services instantaneously, irrespective of the type of air space involved.

The AMSS provides digital voice and data services using geo-stationary satellites to aeronautical users and operates in the mobile satellites service portions of L-band from 1545 to 1555 MHz and from 1646.5 to 1656.5 MHz. AMSS is also designed to be a sub-network of the ATN and can support ACARS messages. The digital voice component of AMSS is designed to interface with terrestrial public switched telephone network (PSTN) and to provide high quality telephone service both for aeronautical passenger communications (APC), ATS & Aeronautical operational control (AOC). Geo-stationary communication satellite is designed especially for mobile communication offers near-global coverage for both voice and data communication channels.

1.4 Aeronautical Telecommunication Network (ATN)
The various communication sub-networks (AMSS, VHF data, Mode S data link etc) will be interconnected through ATN. The satellite assisted air navigation system concept supported by ICAO allows for more efficient use of the CNS in assisting the migration towards an ATC that is fully integrated with the ATM concept. In computer data interoperation terminology, the necessary infrastructure for supporting the interconnection of automated ATM system is called inter-network. An inter-network involves the interconnection of computers with gateways or routers through actual sub-networks. This make possible to build a virtually homogenous data network in a common environment from both the administrative and technical viewpoints.

The ATN has been defined as an inter-network architecture shown in Fig-1 that allows for the interoperation of the ground, air-ground and avionics data sub-networks through the adoption of common interface and protocol services based on the International Standardization Organization (ISO) open system inter-connection (OSI) reference model. The ATN is designed in such a way that it can offer communication services to different groups of users such as: ATS, AOC, aeronautical administrative communication (AAC) and passenger aeronautical communication (PAC).

In designing the ATN it is essential to understand how data link communication can interconnect with end systems, both airborne and ground based. It is therefore necessary to define the operational utilization of data messages. Although as stated above before, different system user groups can be identified, priority should be given mainly to ATS service users. Use of data communications for ATS purposes can vary significantly.

Figure – 1 ATN end-to-end relationship
2.0 Navigation:
Future navigation technology will definitely improve the accuracy of position determination and to provide better predictions of future position to enable aircraft to fly more accurate and well-defined profiles. Improvement in position accuracy is also a prerequisite for the introduction of reduced separation minima. The GNSS is the solution for seam less navigation.

2.1 Global Navigation Satellites System (GNSS)
The term GNSS is generic name used by ICAO to define any world-wide positioning and time-determining system that includes one or more satellite constellations, aircraft receivers and various integrity monitoring systems including the required augmentation system for meeting operational performance requirements. The core constellation of the GNSS is GPS, GLONASS and Galileo (in future).

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