Boulder HOPs Aboard GPS Tracking

January 1, 2002 By: Tom Foley, Steve Blacksher GPS World

In Boulder, Colorado, buses are the only viable alternative for short-route public transit systems. Introduction of a GPS-based automated vehicle tracking system has helped the local transportation provider to keep its route on schedule.

Communities across the country are expanding their public transportation systems to reduce the environmental effects of vehicle emissions, traffic congestion, and time spent commuting, as well as to provide transportation alternatives for those who cannot drive.

In Colorado, Special Transit, a not-for-profit provider of door-to-door transit throughout Boulder County, operates the HOP, a high-frequency, fixed-route circulator shuttle serving the city of Boulder. (HOP is the bus fleet/route name and corresponds with the other routes named SKIP, JUMP, LEAP and BOUND,) All of these routes fall under the authority and funding of Rapid Transit of Denver. HOP is a circular loop approximately 5.5 miles around with 50 stops along the route and a coverage area of approximately 12 square miles. (See Figure 1.)

Funded by the City of Boulder and the Regional Transportation District (RTD), Special Transit recently implemented an advanced automatic vehicle location (AVL) system incorporating GPS technology on the HOP. This high-tech "make-over" is designed to improve system efficiency, scheduling, customer service, and communication through more efficient use of resources and better on-time performance. This article describes the technical development, implementation, and operation of the HOP AVL.

HOP On

Public transit is important to Boulder. City agencies and residents are very environmentally conscious and recognize that reliable public transit is critical to offsetting increased traffic pressure. Currently comprised of 14 buses that serve downtown Boulder, the University of Colorado at Colorado (CU), and Crossroads Shop-ping Mall, the HOP system is designed to provide a bus at each bus stop on an average of every 10 to12 minutes.

Figure 1 The 5.5-mile, 50-stop HOP route links the University of Colorado at Boulder campus with the downtown and other high-traffic areas. A GPS-based vehicle tracking system helps dispatchers and drivers to maintain a schedule designed to bring a bus to each stop every 10-12 minutes.

As the centerpiece of Boulder's network of bus systems, the HOP not only deposits riders into high-use areas of Boulder but also feeds passengers from the city center into other systems serving the corners of the community. The high frequency and regular timing of bus stops is critical to retaining customer confidence and HOP's repeated use by commuting workers and students.

By all measures, the HOP has proved to be very successful. Since the service's launch in 1995, average daily volume has grown from 1,500 to 3,500 passengers, with levels often exceeding 4,500 passengers a day during CU's academic year. With both Boulder and CU populations growing steadily, traffic during peak commuter hours frequently approaches gridlock conditions. The high volume of construction work and traffic accidents have also begun to interfere with the predictability and consistency of HOP service.

In the fall of 2000, Special Transit and the City of Boulder began to evaluate AVL systems as a more cost-effective and environmentally attractive solution than continuing to increase HOP fleet size. AVL systems have become available and received considerable publicity over the past few years. At their most basic, they provide a communications link between the dispatcher and a set of vehicles being tracked via GPS.

Special Transit managers believed that, if HOP dispatchers knew bus locations at all times, they could eliminate bunching and ensure schedule consistency. Special Transit also saw that in the future a GPS-enhanced communications infrastructure could support other more advanced functions to improve operations such as automatically sensing and collecting vehicle performance data, fare and passenger count data, and messaging support between drivers and dispatchers. Furthermore, if the public could have access to real-time bus location information, riders could better control their transportation needs and would respond to better service with growing ridership. As a first step, however, Special Transit focused on improving service predictability and dispatcher effectiveness while maintaining driver service levels.

Use of the HOP route has more than doubled since 1995, with an average of 3,500 passengers trips taking place each day and more than 4,500 during the University of Colorado academic year.

Weighing Trade-Offs

From a technical perspective Special Transit's driving consideration was to support the critical data needs of the HOP fleet, which meant optimizing the system to reliably provide frequent, real-time location updates simultaneously from a number of remote vehicles operating within a defined area. In developing the system architecture around this primary goal, the AVL investigation uncovered some interesting trade-offs among such factors as frequency of information updates, GPS accuracy, wireless communications alternatives, and dispatcher and driver control requirements. We targeted these issues for further examination.

Location Update Frequency. Given the small footprint of the HOP route and the desire to interactively reroute buses around obstructions to maintain schedule conformity, the dispatchers needed a real-time flow of location-based information to provide an accurate overview of the entire route and fleet. Special Transit determined that they needed position updates of buses as frequently as every few seconds. This update rate provides dispatchers with a real-time situation awareness that enables them to manage the entire fleet of buses on a minute-by-minute basis and to effectively respond to blockage situations without negatively affecting service along the route. Moreover, frequent location updates reduce need for predictions modeling of vehicle flow.

GPS Accuracy. When considering location accuracy requirements for the system, Special Transit determined that the frequency of position updates is more important than high accuracy for the HOP system's current needs.(The current update rate is two seconds.) Since the elimination of selective availability (SA), GPS has provided HOP buses with an accuracy of about three meters and often better.

This could not have been accomplished a few years ago with SA active; a correction of some kind would have been required. This level of accuracy allows us to determine where each bus is on the route. Special Transit does not anticipate a need for higher accuracy in the future. However, with a firmware upgrade the GPS receivers installed on the HOP buses could use the Wide Area Augmentation System (WAAS) being developed by the Federal Aviation Administration. The WAAS signal includes differential corrections, and activating the WAAS capability would increase the accuracy to better than one meter CEP (circular error probable).

Figure 2 Installation of repeater stations has established redundant coverage for HOP's wireless data communications system.

Wireless Communications. Many AVL systems use cellular digital packet data (CDPD), a data transmission technology developed for cellular phone systems, to connect their central monitoring software with the vehicles tracked by the system. CDPD has proven acceptable for supporting expanded routes spanning large distances. However, it poses problems for short-route circulators that, in order to be effective, require guaranteed network access and coverage within very specific areas and frequent position updates delivered in near??????real time.

The pervasive nature of CDPD initially attracted Special Transit. The populated Boulder metropolitan area already had a network of cellular towers in place. Moreover, although cellular network capacity and service availability can pose problems, they can usually accommodate systems needing only periodic communications and position updates. However, Special Transit had concerns with CDPD's ongoing service costs and the issue of reliable coverage throughout the entire HOP route. It also appeared likely that CDPD capacity limitations would occur at the very times - during rush hours - that the HOP most needed frequent and reliable communications. These factors caused us to investigate other wireless solutions.

In Boulder, several alternatives to cellular-based systems were available for consideration as part of the HOP AVL. These included commercially operated specialized mobile radio (SMR) services, Boulder County's trunked system, commercial 802.11b service, satellite-based service, or a private UHF/VHF solution. Special Transit ultimately chose to implement a private wireless network solution based on frequency-hopping spread spectrum technology.

The wireless network and the in-vehicle units operate in the 900 MHz frequency band (902??????928 Mhz). No license is required to operate in this band, and the nature of the technology offers a high level of security of the data. The wireless network uses Time Division Multiple Access (TDMA) technology, a method of digital wireless communications transmission that enables a large number of users to access a radio-frequency channel without interference. In a TDMA tracking system, each user - in this case, individual HOP buses - is given a unique time slot within the channel in which to report its position and other data.

The system that we selected offers several advantages over the alternatives, including:

1. Real-time data
2. A cost-effective, one-time expense without
3. monthly fees
4. Good transmission range and bandwidth performance (In the Boulder area, the tested performance in the 900 MHz frequency band has shown a range of about 20 miles with line of sight at 1 Watt output, the maximum permitted for 900MHz in United States. The channel bandwidth of 115Kb that Special Transit uses provides an excess amount of spectrum to handle the data demands for the AVL system today and to support future expansion.)
5. Resistance to interference
6. License-free operation
7. Integrated GPS
8. Control, coverage, and access (as can only be available with a private solution). The radio system covers more than 200 square miles, positioning Special Transit well for expansion of HOP and other services.

Historically, dispatchers have relied upon a large wall map of the city with clearly marked routes and a voice-only radio link to communicate with all bus operators. With these rudimentary tools, a single dispatcher on each shift managed the timeliness of all HOP buses in the system and handled all other issues. Drivers assisted this process by providing the dispatcher with their position when prompted and when they reached particular stops along their routes.

Eventually , this logistical process became unmanageable during daily peak traffic periods. The additional challenges stemmed from bus delays due to increased traffic, heavier rider loads that filled buses and required the addition of buses to some routes, and buses becoming backed up due to central stops (where passengers exit or enter the bus for connections with other routes), which affects Special Transit's ability to ensure the 10??????12 minute frequency of buses on the HOP route. These challenges and other unpredictable issues, such as accidents and breakdowns, placed a heavy burden on the dispatcher.

Figure 4 Real-time GPS positioning provides Special Transit dispatchers with a systemwide view of bus locations.

In responding to the increased passenger load and traffic conditions, Special Transit was not willing to cut back on the individualized service its drivers provide, such as helping to load a wheelchair passenger or assisting passengers with other disabilities or special needs. Therefore, they were attracted to GPS-enhanced transceivers' ability to automate routine communications, freeing operators to serve customers and address onboard emergencies. Special Transit believed that automating routine tracking via the AVL system would allow bus drivers to avoid communication clutter and free up their radios for problem solving.

System Description

Taking these issues into consideration, Special Transit specified and ultimately selected an AVL system consisting of four primary subsystems:

1. network infrastructure hardware
2. network management software
3. in-vehicle system, and
4. application used by the dispatch center, which will eventually be made available to the public through City of Boulder's web site. This application currently runs on a PC that receives a data feed from the network management software, which resides on a server. An applet of this application can be served over the Internet displaying real-time bus positions to viewers over the web and at the kiosks. This is currently planned but not implemented at HOP.

Infrastructure Hardware. To provide the continuous coverage over the entire HOP route required building up a network of wireless transceivers or repeater stations throughout downtown Boulder. The repeater network provides bidirectional communications between the mobile systems mounted on the HOP buses and Special Transit?????????s dispatch center, where the network is controlled and operated through a PC connected to a master transceiver. Each repeater station consists of a highly compact, low-power wireless data transceiver capable of being covertly mounted, directional or omnidirectional antenna, AC or DC power system, and appropriate mounting hardware for the site conditions.

Currently, two repeater stations are sited. (See Figure 2.) One is located on a hillside above Boulder and provides full coverage of the downtown area. A second repeater station has been installed on the south side of town atop a building at the National Center for Atmospheric Research. This second location has added redundancy to the coverage and helps cover some areas challenged by structural or topographical shadowing of the RF signal. In particular this second location resolved much of the coverage issue associated with Pearl Street Mall, where the signal is obstructed by buildings for approximately 150 meters along the route. A third repeater station will be permanently installed in the near future to provide redundant coverage throughout most of the route and expanded network coverage in and around the downtown Boulder area.

Network Software. The network uses control software running at the dispatch center that allows a Special Transit staff member to configure the wireless network and manage its operation remotely. Special Transit can configure parameters such as GPS sample rate and message type. Network bandwidth allocation is accomplished through TDMA techniques to ensure that frequent GPS locations are delivered to the dispatch center as quickly as possible from the entire fleet simultaneously. GPS and other data are interfaced through standard Internet protocol (IP) formats, providing local or networked application level access to the wireless network and data.

In-vehicle System. The in-vehicle system on each of the 14 HOP buses incorporates GPS-enhanced, wireless data transceivers to provide location and communication (See Figure 4). A 12-channel, single-frequency GPS board set uses the C/A-code signal for position determination. The in-vehicle unit is hidden in a compartment above the windshield area and is powered through the vehicle's 12-VDC current, typically drawing 320 mA. Dual-mode GPS/wireless radio antennas on each bus provide GPS signal reception and data transmission.

Dispatch Application. The first phase of the HOP AVL implementation involved a simple tracking application to give dispatchers a view of the entire circulator fleet. (See Figure 4.) The application component employs a custom mapping interface with geographic information system (GIS) functionality that provides a real-time graphical view of all the buses, their direction of travel, and speed. These images can be stored for playback and external access. Update rates are user selectable and are currently configured to update every 2 seconds. Future functionality can easily be enhanced as the system is interfaced with several dispatch applications.

Application Demonstration

Special Transit conducted a trial of the AVL system on two buses in October 2000. During this trial, GPS performance was found to be within the less-than-five-meter specification for the HOP system. The test helped determine sections of the routes that needed improved wireless communication coverage due to structural shadowing. This led to the addition of the second repeater site, which was located to the south of the first repeater. GPS performance demonstrated some challenges in only a few areas and with limited effect on performance - lack of position fixes for no more than several seconds. We evaluated the accuracy and performance of the system until June 2001, when the in-vehicle system was extended to include the remaining 11 buses.

Since going live, the AVL system has improved both the public and internal perception of the HOP route. The system has positively affected Special Transit's ability to deliver quality and timely transportation services to the Boulder community. With increased efficiency in routing, passengers spend less time waiting for buses, which is the bottom line for measuring the success of such a system. Additional benefits include overall improvement in fleet management, better use of dispatchers' time to manage pressing operational needs (such as breakdowns, accidents, and so forth), and a significant decrease in the amount of time operators and dispatchers spend on the radio.

Freed from routine communications tasks, bus operators can better focus on customer service and improve their ability to adapt and change their routes to deal with traffic backups, accidents, and other issues. And, because drivers are no longer being inundated with position information requests from the dispatch center, they can focus on safely operating their buses. Special Transit believes that, over the coming months, it will see safety record improvements and enhanced passenger service experiences.

Future Considerations

Special Transit and the City of Boulder Transportation Department are collaborating to plan the availability of a real-time bus location map for the HOP on the city's Internet Web site. A future step will implement kiosks displaying up-to-the-second bus locations at all major stops.

From there, possibilities range from expanding the AVL system onto other bus systems within Boulder and to Special Transit's "Para-Transit" fleet. More than two dozen vehicles comprise this fleet and respond to on-demand transportation requests across the county from low-income, elderly, and medically challenged persons. Special Transit believes these vehicles could be better utilized and service improved with a similar AVL system.

We also have the option of upgrading the GPS-enhanced transceivers on the buses to automatically collect vehicle usage information. This real-time data can assist in bus maintenance scheduling and deployment decisions. Additionally, Special Transit can analyze historical data to improve operational efficiency. Special Transit is excited by the benefits it has experienced during Phase One and believes these improvements will continue to strengthen the system efficiency and performance in the months and years to come.

Manufacturers

The Special Transit AVL System uses SmartGPS Tracker radio transceivers, and WiGate wireless network management software from Intuicom, Inc. (Boulder, Colorado). CMC Electronics (formerly BAE Systems Canada; Montreal, Quebec, Canada) Superstar II GPS receiver boards are integrated within the SmartGPS Trackers and use dual-mode antennas. The Intuicom TargeTracker application software in the dispatch center uses an industry standard Microsoft Windows operating system and an Intel-based personal computer platform.