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Advanced Automatic Train Control pioneered in San Francisco

01 Jun 2002

This month Bay Area Rapid Transit will put its Advanced Automatic Train Control moving block control into revenue service. Radio-based technology helps keep down installation costs in the $40m project to reduce headways and increase capacity on the busy A and M lines


Jeffrey K BakerProduct Manager, GE Transportation Systems Global Signaling

FORMING PART of an upgrade of Bay Area Rapid Transit's A and M lines, the busiest routes on the 153 km network, Advanced Automatic Train Control is expected to obtain its safety case approval this month, paving the way for revenue service.

BART already handles 90 million passenger journeys a year, and in the mid-1990s projections for traffic growth suggested that BART urgently needed to increase line capacity. Critical points on the network are the Oakland Wye, used by every train on the network, and the Trans-Bay Tube. Building new lines under the Bay would cost many billions of dollars, and the search for a more cost-effective alternative fell on resignalling to permit shorter headways on the existing network.

BART began developing AATC in 1994, working with Hughes Aircraft and Morrison Knudsen Corp. At that time $19·5m of government defence conversion funding was provided to support the adaptation of Enhanced Position Location Reporting System military radio technology for civilian use. Eplrs has now been in service with the US military for over 15 years, and it is well suited for use in busy radio traffic and noisy electromagnetic environments.

AATC was installed on BART's Hayward test track, and trials were carried out for several months in 1996 (RG 7.98 p463).

Shortly afterwards, however, the launch of a pilot service on operational lines was delayed while the project alliance was restructured. Morrison Knudsen withdrew, and Hughes (later Raytheon) licensed Eplrs to Harmon Industries. Harmon was awarded a $45m development and implementation contract in 1998, with Raytheon as a subcontractor. Harmon was acquired by GE Transportation Systems, and the production version of AATC was developed by GETS Global Signaling and BART.

AATC is a full moving-block control system designed to keep installation costs low. Bob Miller, BART's Group Manager, Systems Capital Program, says AATC will cut headways and shorten end-to-end journey times, improving the ability to recover after delays and allowing BART to run its existing service with one fewer trainset. With fewer brake-to-power transitions, energy consumption will be reduced.


The backbone of AATC is a robust radio network providing data communication and radio-ranging determination of train location. AATC communicates vital location data using the radio network rather than inductive loops or balises.

The 'brains' of the network are computers installed at stations or other convenient points. These collate location and status messages from trains, calculate train location, control speeds, generate movement authorities and control the moving blocks. The calculations can be modified from the BART control centre to enforce temporary speed restrictions or regulate traffic.

Each computer is connected to two station radios, which serve as master radios in the network. These station radios form part of a trackside network, communicating with other radios positioned alongside the railway on each side of the station. The network uses store-and-forward (bucket brigade) architecture, providing trains with multiple copies of every message for reliability.

On-train radios listen to the lineside communications, receiving messages that are outbound from the station computer and transmitting status messages back. Trains receive commands from the four closest lineside radios, so even in tunnels the train has multiple opportunities to hear the instructions. Use of lineside equipment permits simple, low-cost hardware to be installed on the trains.

Spread-spectrum and time division multiplexing are used to communicate with trains every 0·5sec. Messages to trains are synchronised, and time-of-arrival measurements used to determine the location, speed and direction of each train. Head-end and tail-end radios provide redundant on-board communication and monitor train integrity and length. By updating the speed command for each train every 0·5sec, the Station Computer provides basic Automatic Train Operation within a vital closed-loop control.

Testing in San Francisco and New York since 1999 has demonstrated message delivery reliability above 99·9%, with an ability to calculate train location to within 5m for 99·9% of the time, and speed to within 2·5 km/h.

The plug-and-play nature of the network virtually eliminates the maintenance requirements and exacting specifications of loops and balises. If any radio in the network fails, it is removed from the network automatically. Communication then continues normally, skipping over the failed unit until corrective action can be taken. The maintenance department is automatically alerted to the failure so that a new radio can be deployed when convenient. On power-up, the new radio joins the existing network and seamlessly begins transferring messages to and from neighbouring radios without requiring programming or special software. All radios contain the same software, so there is no need to hold an extensive range of spares.

The vital station computer calculates the location, direction and speed of each train, monitors train integrity, and sends speed and acceleration commands to all trains. The vital station computer calculates the status of fixed obstacles and the position of the rear of trains ahead in a true moving-block fashion to enforce the correct safe speed for the train.

The computers have off-the-shelf Motorola PowerPC processors, with checked-redundant architecture for safety. Diagnostic and logging functions are provided, along with local displays of the status of trains in the area. Errors in lineside or on-train equipment are reported for use in maintenance planning.

A non-vital processor can supply speed request information to the vital computer to co-ordinate the movements of trains, implementing schedule recovery or energy management algorithms. Each computer has a set of processors configured as a hot standby unit, able to take over control on the fly.

Mixed operation possible

GETS Global Signaling developed AATC to meet the requirements of the urban transport industry, rather than for a single customer. AATC can be integrated with traditional signalling and legacy on-board equipment, meaning the control system does not need to change significantly when AATC is installed.

With AATC, the only lineside installations are the radios, power supplies and station computers. Configuration and programming are automatically carried out by the system.

When new train control technology is deployed on an existing railway, phased installation can reduce disruption. AATC can overlay existing in-cab signalling, providing AATC-equipped trains with speed commands by radio and using existing track circuits to track unequipped trains. Trains enter AATC territory at line speed, seamlessly reverting to cab-signalling control as they leave.

Station computers are designed to interface to the existing signalling for tracking unequipped trains, but can also be operated as a stand-alone control system without underlying track circuits. AATC allows for operation without track circuits, but is compatible with installations requiring broken rail detection.

Not every section of a metro needs to be equipped with AATC, which can be installed incrementally according to traffic and budget constraints. Mixed-mode operation is possible, with both AATC equipped and unequipped trains operating in the same area. Instead of equipping the entire fleet with AATC technology at one time, a migration path is available for installation, with commissioning of lines and trains as the need arises.


  • CAPTION: Top: The AATC system architecture uses a 'bucket brigade' radio network to send commands to and from the on-train computers; at any moment each train should be in contact with four lineside radios
  • CAPTION: Due to start this year, Phase 3 of the project will see the installation of AATC through the Trans-Bay Tube and the Oakland Wye, providing capacity to accommodate extra services to and from San Francisco International Airport

Advanced Automatic Train Control pioneered in San Francisco

This month Bay Area Rapid Transit will begin using Advanced Automatic Train Control to improve capacity on its two busiest lines. AATC makes use of lineside radios for communication with on-train systems, reducing installation costs compared with traditional balises and track circuiting. Developed with GE Transportation Systems Global Signaling, this radio-based full moving-block control system will permit shorter headways, faster journeys and lower energy costs.

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