With the current drive 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.

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.

Advantages

• One of the major advantages of a WAM system is the fact that there are no rotating mechanical components such as the LVA antenna 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 is normally utilised.

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

• Accuracy of this technology depends on various factors but is mostly determined by the terrain, the number of receivers, 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 advantages were achieved during the trials held in June 2008 at FACT. 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, alongside the already existing dual MSSR environment. The aim of this installation was to learn about multilateration technology, and assess its performance. 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 had not been used operationally.

The trial WAM system was installed at and around Cape Town International Airport as the surrounding terrain conditions effectively lent themselves to determining the impact of terrain conditions on the accuracy and failure modes of WAM surveillance systems (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 in 2006. However it was decided that the existing WAM trial system be expanded through two additional receivers, by relocating the current interrogator as well as adding an additional remote interrogator with a view to its potential operational use. ATNS’s 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 coverage in difficult terrain;
• improving operational tracking performance; and
• enhancing availability.

A second set of flight trials was held in 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. Standardised 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 would have been 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 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 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 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 remained 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 people at ATNS, as 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 this 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.

r4sa, r4sa ext I & II

Project Objectives

The CAPEX plan submitted to and approved by the Regulator and the users in 2003 identified the need for the replacement of the L Band en-route radar at FAJS ATSU as well as the Sutherland en-route secondary surveillance radar. This was due to the normal replacement cycle and age of these surveillance radars.

The replacement of the en-route radar at FAJS with approach radar co-mounted primary and secondary mono-pulse radar allowed for the reduction in separation within the FAJS ATSU terminal area from 5nm to 3nm with the associated capacity increase and a fully redundant radar service. Upon a successful commissioning and flight trial, the radar was attached for operational use.

The Sutherland primary radar was withdrawn from service. It has not been replaced, as the primary radar service is not required for en-route surveillance. Upon successful commissioning and flight trial, the radar has been attached for operational use.

A need was also identified for a second approach radar at FACT, in order to provide improved radar redundancy and reduced radar separation with a resulting increase in capacity. Upon successful commissioning and flight trial, the radar has been attached for operational use.

The primary radars at FAEL and FAPE ATSU have been replaced with new approach radars as part of an extension of the project r4sa contract. With the extension of the r4sa contract, an approach radar for the ATNS ATA, secondary radar for FABL have been installed and commissioned. The FAGG secondary radar and an approach radar for the new airport at La Mercy have been procured and civil works/installation is in progress.

A TCU has also been installed and commissioned at FAEL ATSU. In addition, two TCU installations and commissioning are planned for George and the new airport at La Mercy.

Project Scope

New approach radars consisting of co-mounted primary and secondary Mode S surveillance radars have been installed at FAJS, FACT, FAEL, FAPE and the ATNS ATA.

En-route mono-pulse secondary radars with a Mode S capability have been installed at Sutherland and FABL.

The installation of an approach radar and secondary radar – together with associated TCU installations- is currently in progress at La Mercy and FAGG respectively.

A TCU has been installed and commissioned at FAEL ATSU to display the tracks from the radar sensor. The scope includes/included additional works like:

• civil works site preparation and construction of a concrete tower
• electrical provision and installation
• air conditioning
• fire protection and intruder alarm
• communications infrastructure
• radar dome works
• hoisting equipment
• standby generator and UPS systems
• Professional fees
• Flight trials
• Emergency tower for FACT

The table below shows the r4sa and its extensions installation schedule:

TeRNS

Description

As a result of the GNSS delays the operational life of the existing terrestrial DVOR/CVOR/DME network must be extended to 2015 and will most likely remain in service beyond that date.

The existing DVOR/CVOR/DME equipment (with the exception of those replaced under PRONAV1) has exceeded its operational life and is becoming increasingly difficult to maintain due to equipment obsolescence.

The TeRNS project was initiated in 2004 and encompasses the:

• replacement of certain elements of the existing DVOR/VOR/DME network;
• relocation of existing facilities where necessary; and
• establishment of new facilities as required by ATM operations to maintain the terrestrial navigation and conflict management service.

The contract for this project was signed in July 2005 and the following facilities are now operational:

Training facility

A completely new DVOR/CVOR/DME training system was installed at the ATNS ATA. Training of students will include this equipment as well as the Maintenance Control System. A new CBT computer-based training system is designed to support both trainers and trainees.

Sites to be completed

Ceres (CEV)
Ceres is a new DVOR/DME installation and the commissioning of this facility is scheduled for 19 November 2009.

Lanseria (LIV)
The Monitor System Upgrade must be done to get the facility on the same standard as TeRNS and commissioning is scheduled for 24 September 2009.

Hofmeyer (HMV)
A new DVOR was installed and commissioned. The facility will be operational soon, but the date will be determined depending on the AIRAC (Aeronautical Information Regulation and Control) cycle.

La Mercy (TGV)
As this is a new airport, a new DVOR/DME will be installed and the commissioning is scheduled for 11 March 2010.

Start date: 1st of July 2005
End date: 11 March 2010