SubscribeTIME is an essential element in Airport Systems particularly in mission critical applications such as Air Traffic Control. So how do we explain the varieties of clocks and unsynchronised time displayed around an Airport?
During projects, when departmental project teams are concerned only with delivering their intermediate products, it is all too easy to neglect the procurement of a Centralised Timing System that could be integrated to all elements within a modern Air Traffic Control System.
In most cases the control and display of time is left to individual system suppliers. Although each piece of equipment can be set to a standard reference and modern computers are supplied with fairly accurate clocks, every system will have its own tolerances and will drift at different rates. Over the past 20 years Time & Frequency Solutions have worked with the individual systems suppliers to develop a centralised timing concept particularly suited to Air Traffic Control applications.
This guide will cover the main components of a typical Air Traffic Control System and outline the advantages of a centralised time source driven by an accurate time standard
ATC Facilities
In a typical ATC System, as illustrated above, individual departments often supply their own timing solutions leading to a multitude of diverse “ Standards”.
Although each standard can then be synchronised to an agreed time reference this process must be repeated at regular intervals to ensure that a constant time-of-day indication is maintained throughout the System. As we can see from Table 1.1.1, below, a large number of facilities and possible “standards” can exist within an Air Traffic Control System leading to major synchronisation problems.
Table 1.1.1
Facilities |
Time Source |
Sync |
Sync Source | |
| Navaid Department | ILS, VOR/DME, Precision Radar, Monitoring Systems | Internal Internal Internal Internal | No No No No | |
| Radar Department | External Radar Sites, Airport Radar Sites, Transponder Sites Radar Processing Displays-Radar | Internal Internal Internal Internal | No No No No Yes |
Radar Processor Server |
| Data Department | Message Switch Displays-MS Information Database Displays-ID
Volmet Displays-Volmet Atis Displays- Atis | Internal
Internal
Internal
Internal | No Yes No Yes
No Yes No Yes |
Message Switch Server
Information Database Server
Volmet Server
Atis Server |
| Communications Department | Speech Processing Operator Displays Radio Comms. External Sites Telephones TMR | Internal
Internal Internal Internal Internal | No Yes No No No No |
Speech Server
|
| Technical Department | Recording Monitoring TOD Clocks Simulators | Internal Internal Internal Internal | No No No No |
Modern processing and working techniques have also brought many visual interfaces, and their associated clocks, directly to the Operators desks. Although the displays are normally synchronised with their associated system servers there is no common time source driving the servers with the result that even minor discrepancies between System times are immediately apparent to the Operators.
In addition to the obvious visual discrepancies many current processing techniques rely on precise timing for both internal and external functions. Attempts to “correct time” within these Systems can lead to major problems as the processing cannot tolerate an abrupt, significant change in time. This problem is aggravated when different systems are integrated together and attempts to change the time on one piece of equipment can lead to problems surfacing on another part of the ATC System.
Centralised Timing
The figure below illustrates an Air traffic Control System utilising a Central Master Clock integrated to all of the major Systems and sub-systems. This type of configuration ensures that all Systems are automatically synchronised to a common source.
It is now a logical step to drive the clock source from an accurate input signal. The choice of available interface drivers is large covering GPS, VHF, HF, MF, LF or locally available standards. In most cases GPS is the preferred standard as it offers superior accuracy coupled with proven, off-the-shelf modules and small discreet antennas.
Ideally this type of configuration should be used at the Project design phase but there is no reason why existing Systems cannot be retroactively fitted to emulate this design philosophy. Many equipment suppliers now offer external clock interfaces on their processors or can offer the facility as an accessory. If software is locally produced, off-the-shelf clock synchronisation modules can be obtained for most types of operating systems.
The use of GPS also allows easy synchronisation of remote, external sites as illustrated above on the radar and communication facilities.
Systems & Subsystems
This section will examine the clock interfaces of the systems and sub-systems used in typical ATC applications.
Radar/Flight Data System Interfaces:-
Accurate, synchronised timing can bring major advantages to Radar System Interfaces particularly in Multi-Radar Tracking Systems.
Modern Radar systems, combined with current signal protocols, allow the use of time stamping of the radar data. This process provides an improved accuracy factor in the radar signal as we can virtually eliminate positional errors introduced by ambiguities in the processing and transit times of the signals. The advantages are particularly noticeable in Multi-radar systems where sites are distributed over a wide physical area and signal transfer times can be significant.
Precise timing is also gaining increased importance in the Flight Data processing as manufacturers are now offering displays systems that can provide synthetic tracks produced by positional data derived directly from the aircraft in areas not covered by Radar signals.
Accurate timing is an added confidence factor when applied to recordings of the radar and flight-plan data and users can be confident that the replayed data is a precise representation of the original event.
Data Network Department: -
An accurate time source is invaluable in Message switching applications both for message time stamping at the origin and transit time measurements for through-traffic applications.
The current trend for Airport Information databases is likely to continue as Operators and users discover the benefits of a single information source. This popularity can itself cause problems as the volume of data increases and processing times are impacted. Time-stamping of the data is an essential tool and provides a positive confidence factor by providing, to the user, a precise validity time of the viewed information.
Time-sensitive applications, such as Volmet, gain accuracy with the use of a precise timing source. Most data sources are now recorded for audit purposes and precise timing information ensures a faithful and accurate playback of the data.
Communications Department: -
Communication Departments can utilise an accurate, centralised timing system for message stamping and high integrity recording facilities covering speech and data circuits.
The precise frequency standards available can be used as source reference oscillators on radio Systems in widely separated sites and the high stability available can make possible synchronous operation of VHF transmitters thus avoiding the problems associated with offset-carrier application
Navaids Department: -
Although Navigational Aids such as ILS, VOR & DME do not require TOD inputs their stability may benefit from the availability of a high stability, frequency standard.
Accurate timing signals are of assistance in the monitoring of the transmitted signals and can be utilised, during calibration periods, for detailed practical comparisons between the radiated signal, as seen on the ground, and the recordings, using a GPS controlled time-line, taken on the Flight Check aircraft.
Technical & Maintenance Department: -
As we have already explained, a centralised Timing System eliminates the time-consuming problems associated with un-synchronised systems
Many Technical Departments are moving towards Centralised Fault Reporting Systems. The ability to receive time-stamped messages from individual systems removes many possible ambiguous situations in the fault-reporting centre and provides a true sequence of events for monitoring and recording purposes.
Many new Simulator systems can be integrated, when required, into the parent System allowing additional positions and facilities. A centralised timing system allows the Simulator components to be easily merged, when required, into the operational on-line System
System Solutions
Time & Frequency Solutions have now provided turnkey centralised timing systems to a number of Air Traffic Control networks world-wide. These include the UK National Air Traffic Service, Hong Kong & Zhu Hai ATC centres and centres in Finland, Ireland, Brunei, Hungary, Kuwait and Saudi Arabia.
These systems all utilise the M210 or M211 Modular Timing System concept, which allows the selection of particular interface modules, and time message formats depending upon the equipment to be installed. This solution also allows enhanced features, such as dual redundancy and the use of high precision oscillators either for improved performance or to facilitate frequency outputs for the synchronisation of communication systems.
At Time & Frequency Solutions we have made it our business to understand the individual requirements of the Air Traffic Control and Airport Passenger Terminal industry and this enables us to provide accurate advice regarding your particular ATC timing application. We look forward to working with you.
Acronym
| AFTN | : Aeronautical Fixed Telecommunication Network |
| ATIS | : Automatic Terminal Information Service |
| ATC | : Air Traffic Control |
| AVBS | : Automatic Volmet Broadcast Service |
| DME | : Distance Measuring Equipment |
| GPS | : Global Positioning System |
| ILS | : Instrument Landing System |
| ISO | : International Organisation for Standardisation |
| Message Switch | : AFTN Switch |
| METAR | : Meteorological Report |
| MCS | : Master Clock System |
| PAX | : Private Automatic Exchange |
| PABX | : Private Automatic Branch Exchange |
| PSTN | : Public Switched Telephone Network |
| SMR | : Surface Movement Radar |
| SPE | : Speech Processing Equipment |
| SPECI | : Aviation selected special weather report |
| TMR | : Trunk Mobile Radio |
| TOD | : Time of Day |
| TWR | : Aerodrome Control Tower |
| UTC | : Universal Coordinated Time |
| VOLMET | : Meteorological information for aircraft in flight |
| VOR | : VHF Omni Range |