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GPS problems bedevil Denver light rail

Denver’s RTD light-rail system has bragged that “RTD is one of the few transit agencies, nationally, to meet Congress’ original Dec. 31, 2015 deadline” for instituting Positive Train Control (PTC).

Yet reception of the GPS signals upon which their PTC system depend continues to be a significant problem.
PTC is designed to prevent train-to-train collisions, derailments caused by excessive train speed, train movements through misaligned track switches, and unauthorized train entry into work zones.

This means that the location, speed and time information derived from GPS are critical elements.

Photo: RTD

Photo: Regional Transportation District of Denver (RTD).

Lack of reliable GPS reception has meant significant reductions in the efficiency and performance of the light-rail system overall, and much more work for its operators. Coping mechanisms have included manual overrides of PTC safety features and human flaggers at “automated” crossings.

This has undoubtedly greatly increased operating costs, not to mention the price of attorneys to deal with regulatory issues and litigation.

The upshot of all of this is that Denver RTD continues to struggle to make its operational PTC system work.

This has been a concern for the Federal Railroad Administration (FRA), the agency within the Department of Transportation responsible for overseeing and regulating such systems.

In a December 2018 submission to the FRA, Denver’s RTD outlined the steps it was taking to address this problem. In the document and public statements, RTD has blamed high-rise buildings in the vicinity of lines and terminals for its GPS problems.

“Specifically at Denver Union Station, there has been significant high-rise building development in the surrounding area which has impacted reception of GPS signal (sic) in the platform area. There are a few PTC initialization issues each day due to poor reception of GPS signal (sic). This increases PTC sut outs for the first section of the train trip and in some cases cause a longer waiting time at York crossing once the train initializes at 38th/Blake station.”

One of the solutions to be evaluated in January 2019 was installation of GPS signal repeaters at the stations to improve reception at the platform.

This was likely not successful. In October 2019 RTD responded to a complaint on Twitter with ”We are in the middle of installing hardware on our light-rail trains so they will be able to use GPS also.”

When asked this week about the status of their GPS problems, RTD replied simply “We are still working on it.”

GPS expert and Colorado resident Logan Scott opines that high-rise interference might not be as problematic if we can learn how to safely use all of the GNSS signals available. “Depending on the time of day, there are from 5 to 12 healthy GPS satellites visible in the open sky over that city. If we include all healthy GNSS satellites, the numbers rise to between 22 and 35 navigation satellites visible. As the nation looks for backups and augmentations to GPS for critical applications, the possibility and benefits of using GNSS systems beyond GPS is often overlooked.”

Scott will be teaching a course titled “Towards the Safe Use of GNSS in Critical Applications” at June’s ION Joint Navigation Conference in Cincinnati.

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