PROJECT NAME: MULTILATERATION

Introduction

With the current drive by ATNS to enforce the carriage of transponders, ATNS started investigating more cost effective alternatives to rotating monopulse secondary surveillance radar (MSSR) systems.

Wide Area Multilateration surveillance systems (WAM) are based on the well known concept of triangulation used extensively in the communications industry. WAM is a technique whereby 1090MHz transponder signals from the aircraft are detected by ground receivers and by applying a Time Difference of Arrival (TDOA) calculation, the originating point of the signals can be calculated. This gives the estimated position of the aircraft in question. Altitude, identity and other data can be extracted from the 1090MHz signal, which will usually be a Mode A, Mode C or Mode S reply. In fact, any 1090MHz signal such as ADS-B 1090 Extended Squitters or TCAS Acquisition Squitters can be used for the multilateration process, even without taking advantage of the signal content.

Several other components like a central processing unit, independent time reference (GPS) and interrogators as necessary as well as other elements to provide an independent integrity check of performance normally form part of a WAM system.

One of the major advantages of a WAM system is the fact that there are no rotating mechanical components such as the LVA antenna, being a single point of failure, as is normally found in radar systems.  The antennas used in WAM systems are small and stationary making it easy and cost effective to install at remote sites with existing infrastructure.  The soft fail feature of the system is also a benefit. In addition no specially prepared sites are required as already established infrastructure in safer areas are normally utilized.

Due to the cost and compactness of the equipment, many such receivers and interrogators can be installed, thus overcoming the problem of obstructions that is normally found on airfields and the surrounding terrain.

The accuracies achieved with this technology are also very attractive.  The accuracy depends on various factors but is mostly determined by the terrain, the number of receivers and their layout and the position of the aircraft relative to the deployed receivers. Performance is better or equal to that normally achieved with MSSR and can easily be improved by adding more receivers. Sufficient and optimally spaced receivers and interrogators are deployed with the required N-1 or N-2 redundancy for both receivers and interrogators. These stated accuracies were achieved during the trials held in June 2008 at FACT even when various receivers and interrogators were removed one at a time to illustrate the fail safe characteristics and the WAM's inherent robustness in design.

It is important to bear in mind that whilst one is deploying a WAM system the use of ADS-B is automatically prepared for as the WAM system also receives and decodes data content contained in the extended squitter transmissions received from aircraft.

FACT WAM Installation

In 2007, a trial WAM system was installed in the Cape Town TMA on a trial basis, alongside the already existing dual MSSR environment. The aim of this installation was to learn about Multilateration technology, and assess its performance in the context of the Cape Town TMA. The final objective was to have WAM certified as an alternative surveillance method. It should be noted that at this point, the data from the system has not been used operationally.

The trial WAM system was installed at and around Cape Town International Airport as the surrounding terrain conditions lend itself very well to determine the impact of terrain conditions on the accuracy and failure modes of WAM surveillance systems, bearing in mind that Cape Town has sufficient Mode-S and Mode A/C traffic in order to perform a reasonable number of tests in a short time.

Flight Trials

The first set of flight trials was successfully completed during February 2006.  However it was decided that the existing WAM trial system be expanded, by adding two additional receivers, relocating the current interrogator as well as adding an additional remote interrogator with a view to its potential operational use. ATNS view was that the new expanded configuration with the remote interrogator and additional sites could be deployed operationally in the en-route environment, thereby extending the coverage in difficult terrain, improved operational tracking performance and enhanced availability.

A second set of flight trials was held on 27 June 2008. The WAM and ADS-B were integrated into the ATNS Eurocat-X system by means of the new prototype Thales Eurocat-X Multi Sensor Tracking System. Standardized interface protocols like Asterix 20 and Asterix 21 were used for WAM and ADS-B respectively.

The aim of the flight trials was not to show how good the performance of WAM was but to evaluate the performance under worst case conditions. The accuracies recorded most probably would have been much better had the flight trial been performed inside the predicted high accuracy area dictated by the deployed receivers and interrogator.

The results from the trials proved that WAM is as good or better than MSSR.

WAM Safety Assessment Report

The results from the flight trials were then supplied to Austro Control who was tasked in 2008 to conduct a safety assessment for WAM in the FACT TMA.

This safety case had to be in line with the ATNS Safety Policy in that safety is paramount, non-negotiable, and one of the cornerstones of the ATNS service delivery model.

In particular, the safety case clearly defined responsibilities and accountabilities for the safety performance, and established the system safety performance against the ICAO Target Level of Safety and related measures. It also sought to reduce the risk of an aircraft incident to "as low as reasonably practicable".

This safety case provided the evidence that the deployment of the WAM system, and quality of the resultant data for operational use, was acceptably safe. It was written in the context of the surrounding operational and technical environment, and applicable as long as the assumptions made in the safety case remain valid.

A specific safety assessment exercise was carried out to identify potential hazards and risks associated with the change in the air traffic system, and specific mitigating Safety Requirements were defined. Implementation of the Safety Requirements has to be overseen by designated responsible people in ATNS, identified in the safety case.

The safety case covered the deployment, transition into operations, and ongoing monitoring of the WAM system, and resultant modified technical surveillance service.

In terms of process, the safety case was based on a EUROCONTROL generic WAM safety assessment.

Conclusion

The safety assessment report was submitted to the SACAA in October 2009 and WAM was certified on 22 December 2008 for use as a surveillance method in South Africa in accordance with Part 171 of the Civil Aviation Regulations, 1997, as amended.

ATNS is very proud of the achievement as it maintains its high global standing in terms of using modern technology in achieving its objectives. ATNS is specifically considered a world leader in the WAM environment because of the FACT WAM activities.

The use of WAM in the FACT is however still being delayed because the ATNS Eurocat-X will not be able to interface with WAM until it has been upgraded to a multisensory configuration.