Choosing the right technology for indoor vehicle location systems

  • 18-Aug-2011 04:48 EDT
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Ultra-wideband location systems are being used by the rail industry to simultaneously improve service and the bottom line for transit operators.

Automatic vehicle location (AVL) systems are becoming commonplace in the transit industry. An AVL system keeps track of trains while moving through the system providing real-time data to both operators and passengers. With real-time knowledge of train location, operators can improve on-time performance, improve safety, and increase track capacity. Passengers also benefit with up-to-the-minute information on when trains will arrive.

Increasingly, vehicle location systems are also being used in transit yards to manage rolling stock inside maintenance and marshaling areas. Here operators make significant savings due to improved maintenance efficiency, which results in reduced downtime and therefore higher utilization. Accurate train car location also provides benefits in marshaling areas where search time for individual cars is significantly reduced.

Maintenance operations and marshaling in the light-rail industry are typically performed in buildings where GPS cannot operate, requiring a different location technology to be used. Whereas GPS is the clear choice for outdoor location technology, ultra-wideband (UWB) is the clear winner for indoor location systems.

It’s worth taking a few moments to understand how a location system operates to help explain what makes one technology better than another, and why UWB is the winner. At the most basic level, a location system works by attaching a device to the item you want to find (we’ll call that device a “tag”) and having that device communicate with other devices to calculate its location (we’ll call these other devices “the infrastructure”). As a reference, in the case of GPS the tag is the chip in your phone or AVL transponder or jet liner, and the infrastructure consists of orbiting satellites.

Indoor location systems use a tag that emits a signal that is heard by an infrastructure of sensors usually mounted high up in the ceiling. If you can measure how far the tag is from multiple sensors and you know where the sensors are, then you can triangulate the precise tag location. The difficulty comes in measuring the distance between the tag and the sensors, especially in large indoor spaces such as a transit facility.

There are only two ways to measure how far away a tag is, and that is by measuring how loud the signal is (the louder, the closer) or by measuring when the signal arrives (the sooner, the closer). When discussing location technology, these are known as signal-strength systems and time-of-arrival systems, respectively.

Signal-strength systems suffer from poor accuracy in difficult environments such as maintenance buildings. When a sensor estimates the distance to a tag based on signal strength, it makes the assumption that low signal strength means long range. However, there are many other factors that affect signal strength inside a building; the worst are signals that echo off walls and other structures and cancel each other out.

Many of us have experienced poor FM radio reception when we stop at a red light, only to have the reception improve dramatically as we pull forward a few feet. This large swing in signal strength has nothing to do with the fact that we have altered our distance from the radio transmitter—we have hardly moved at all.

Rather, the signal from the transmitter has bounced off surrounding buildings so that multiple versions of the same signal (echoes) arrive at our radio simultaneously. Sometimes these signals reinforce each other, and sometimes they cancel each other out. These combining echoes fool a signal-strength sensor into large errors in the estimated distance to a tag.

Inside a metallic building, echoes are a real problem for radio transmitters. Radio waves bounce off anything made of metal, and in a rail maintenance facility just about everything is made of metal. The constant reinforcing and cancelling effect of echoes, as tags move only small distances, cause signal strength location systems to be rather inaccurate, finding tags to within only 20 to 50 ft (6 to 15 m) of their actual location. This level of accuracy is of no use when individual tracks may be spaced apart by only a few feet.

Echoes also affect time-of-arrival location systems but in a different and less damaging way. A time-of-arrival system measures how long it takes the signal to travel from a tag to a sensor, and since we know that the signal travels at the speed of light, we can calculate the range to the tag.

But again, the signal actually arrives at the sensor many times over: the one that traveled directly from the tag to the sensor and the ones that bounced off walls, locomotives, shelves, and so on—multiple versions of the original signal echoing off everything.

The one thing we know about echoes, however, is that they all arrive at the sensor after the original signal, since they all had to travel farther to get there (they had to go and bounce off something first). A signal-strength system that can distinguish the first arriving signal from all the others can therefore measure the distance to tags quite accurately.

The reason UWB is the ideal indoor location technology is because it is a time-of-arrival system, but one further subtlety sets it apart from the other time-of-arrival systems. Echoes do indeed arrive later than the direct signal from a tag, but they can still overlap with that signal and cause timing problems. This is exactly the same effect as being inside an empty room and listening to someone clap or sing a sustained note. If they clap, you hear a succession of clap echoes, but each is distinguishable from the others because the claps are of very short duration and do not overlap. A sustained note, however, appears to come from everywhere at once, with echoes arriving on top of each other.

Indoor location systems similarly vary in terms of the duration of the signal. Long duration signals can overlap, and the problem is compounded when multiple tags are in the same vicinity. Such long signal time-of-arrival systems may achieve 10 to 30 ft (3 to 9 m) accuracy, but once again this is not sufficient to reliably distinguish a locomotive on one track vs. another.

UWB is the time-of-arrival technology with the shortest pulse of all, meaning that it is the most accurate of all indoor location systems. UWB reliably achieves location accuracy of 3 ft (0.9 m), which provides the level of precision required for transit yard applications.

With reliable knowledge of which track a locomotive is on in a maintenance facility, for example, and pinpointing which work station along that track the locomotive is in, operators gain an unprecedented level of visibility into workflow that helps to improve efficiency and react to unexpected events. UWB location systems are being used today by the rail industry to simultaneously improve service and the bottom line for transit operators.

Adrian Jennings, Vice President of Technology, Ubisense, wrote this article for SAE Off-Highway Engineering.

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