May 18, 2020
The federal government is calling “All aboard” to the United States railroad industry in a louder voice and with greater urgency, as the industry speeds towards the December 31, 2020 deadline for full compliance and functionality of Positive Train Control (PTC) Advanced Civil Speed Enforcement System (ACSES) systems. For those railways, much work remains – particularly testing – before the calendar flips to 2021.
Forty-two Class 1 railroads are required to implement PTC systems, ranging from large passenger lines to freight railroads. The technology will cover approximately 60,000 miles of the national railroad network, and affect an estimated 20,000 locomotives, 24,000 waypoints (each with a unique digital address), and back office servers. All of this will be done utilizing a new interoperable secure wireless network with more than 400,000 components.
That’s a tall order in and of itself, but there is more. Three criteria must be met for railroads to have their PTC ACSES systems meet government approval. They have to:
- Receive Federal Railroad Administration (FRA) approval of their PTC Safety Plan for system certification
- Install full PTC operations on all mainline route miles
- Verify interoperability so tenant locomotives operating on the same main line can communicate to PTC and seamlessly cross property boundaries
PTC = Increased Safety
Undoubtedly, PTC is worth all of the effort. PTC is a processor-based/communication-based train control system that can automatically control train speeds and movements in the event a rail operator fails to take appropriate action. It can prevent overspeed derailments, incursion into an established work zone, passing through a main line switch in the improper position, among other accidents. All of this will save countless lives.
Another benefit of PTC is interoperability. It allows for equipped locomotives traversing another railroad’s PTC-equipped territories to communicate with and respond to that respective railroad’s PTC system.
ACSES is one of three PTC systems. It utilizes sets of wayside transponders installed at home signals, distant signals, pre-distance signals, block points, or cut section locations to communicate to the on-board components. PTC ACSES provides railway trains with positive enforcement of speed restrictions based on the physical characteristics of the line, such as bridges, curves, and tilting. The on-board components keep track of a train’s position and continuously calculate a maximum safe braking curve for upcoming speed restrictions. If the train exceeds the safe braking curve, the brakes are automatically applied.
Testing to Ensure Operation, Compliance
Several stages of testing must be completed to receive government approval. Among the most challenging is testing a PTC ACSES implementation during field and revenue service demonstration (RSD) to ensure that the new deployment is operating properly.
Most PTC ACSES signals shown in the block diagram of figure 1 are in a proprietary protocol. Over-the-Air (OTA) transmission enables this message flow to be captured. With the correct modulation decode capabilities, testing of the PTC ACSES system quality and accuracy can be made.
Several tests are important to verify that signals are being transmitted in accordance with the specification. Among the most critical are:
- Receive Signal Strength Indicator (RSSI) to test the RF received signal strength
- Error Vector Magnitude (EVM) to ascertain the quality of the received ACSES data transmitted by the system
- Per Message Bit Error Rate (BER) to ensure the data quality received per message
- Cumulative Packet Error Rate (PER) to verify signal/data quality at the packet level
To perform these tests, field engineers and technicians responsible for the safety systems need a high-performance receiver/spectrum analyzer, such as the Anritsu LMR Master™ S412E analyzer (figure 2). It can be used to conduct several critical functions during field testing:
- Decoding the message type, such as source of message (i.e., wayside) and destination type (i.e., office or central control)
- Capturing raw message payload data in a familiar hex format
- Leveraging hex also enables other information to be captured and decoded, such as source and destination ATCS addresses, as well as time slot in frame and epoch
Value of OTA Testing PTC ACSES
OTA testing is preferred because it captures a signal and makes a measurement between the base radio unit (waypoint) and mobile radio unit (locomotive/railcar). This is critical, as this is the only way a high-performance receiver/signal analyzer can access and analyze the otherwise secure PTC ACSES transmission.
To perform these tests accurately and efficiently, a high-performance receiver/spectrum analyzer needs PTC ACSES demodulation capabilities. The configuration also includes the appropriate antenna (and possibly a filter) tuned in to the PTC ACSES transmit frequency. Field engineers and technicians can measure and analyze the PTC ACSES signal quality and demodulate the messages to help validate proper PTC ACSES functionality, as well as spot potential problems for troubleshooting. You can learn more about PTC ACSES OTA testing by watching this educational video.
PTC ACSES Coverage Mapping
Coverage mapping is another essential tool in assessing a PTC ACSES signal environment and ensuring proper deployment, installation, and operation. Field engineers and technicians can use coverage mapping to verify signal strength and quality over a specific area by effectively monitoring RSSI and ACPR levels, as well as to conduct BER mapping. For PTC ACSES systems, coverage mapping should be conducted over the various rail lines used for passenger rail as well as integrated lines with Class 1 commercial traffic.
Coverage mapping can be performed with a high-performance receiver/signal analyzer with GPS connected to the PTC ACSES radio’s antenna system. The high-performance receiver/spectrum analyzer with mapping capabilities can also perform coverage mapping by accurately and simultaneously collecting key PTC ACSES signal quality measurements, such as EVM, BER, and RSSI data, for the PTC ACSES frequency under test. Parameters from each measurement is plotted on a map of the area/route being tested.
All the information saved is in .kml format so it is compatible with Google maps. All three values can then be displayed on Google Maps to gain a clear insight into any areas that may need improved coverage to ensure seamless operation (Figure 3).
A PTC technology page has been published that outlines PTC and how to properly test it, as well as valuable resources, including videos and application notes. You can also download an Anritsu application note on Testing PTC ACSES deployments to meet mandated implementation standards.