Locating Systems

Currently there are four different types of AVL locating systems which are in use. The use of these systems has evolved with the introduction of better products and newer systems. The types of systems in use include signpost, dead-reckoning, radio navigation and GPS. These types of AVL systems can be used as standalone or in combinations. For example, GPS technology is used in combination with dead reckoning. Normally GPS is used but it is substituted with dead reckoning when no GPS signal is available.

3.3.1.1 Signpost System

The signpost system consists of a series of sensors or beacons placed along the route of a vehicle. Each of the beacons emits a low-powered radio signal with a unique ID. The passing vehicle has a special receiver built in, which can decode the ID of the beacon. The vehicle's receiver uses the radio to transmit the code and time of the last signpost signal received. At the control station, the AVL system is updated to reflect the information that, at the time the vehicle received the signpost signal, the vehicle's location was approximately equal to the location of the signpost. Vehicles participating in this type of tracking system transmit a signal uniquely coded for each vehicle. (S., Riter, and McCoy, J., 1977). The late 1960s saw the first use of a signpost system with a transit fleet AVL system by the Chicago Transit Authority. In 1989, the Ottawa Canada ‘OC’ transit system used signpost systems to track their transit fleet. In the early stages of AVL development, the signpost technique was the

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most advantageous positioning technique in terms of both implementation and performance criteria. Due to the immediate advantages of signpost systems, many transit agencies began using signposts to implement their own AVL systems at that time. Besides the advantages that signposts can provide for tracking transit vehicles, there are some disadvantages, such as the expense of installing sensors wherever vehicles need to be located. In some instances, the signal cannot be detected when a signpost is damaged or a signal is blocked. (The vehicle may also be diverted around the signpost system and also the signpost system requires continual maintenance to keep it operating properly).

3.3.1.2 Dead Reckoning

Dead reckoning has been used in position location for a long time. It is a navigational term first used by seafarers charting oceans in the eighteenth century (Randell, C. et al., 2003). Navigators would determine their current position by calculating the ship's bearing and speed since the last known position. Dead reckoning techniques are also used to calculate the position of motor vehicles. The distance travelled is calculated by subtracting the odometer reading at the last known position from the current odometer reading. The bearing of the vehicle is determined by using some type of compass device or by knowing a pre-determined route. Locating vehicles with dead reckoning can be difficult because many uncontrollable factors can affect the validity of the position calculation. For better calculation, odometers must be very precise and initially calibrated, but other factors, such as tyre pressure and road conditions, can affect measurements. Since positions are calculated from the previous position, errors can accumulate rapidly and cause miscalculation. Dead reckoning can also be integrated with GPS for location determination when no GPS signal is available (Silvester and Adeline 2004; Ochieng, et al., 2010).

3.3.1.3 Radio Navigation

Radio Navigation Systems, like signpost systems, rely on fixed reference points. In radio navigation, the constant speed of light and the propagation delays of radio signals are used to find the ranges from a vehicle to fixed sites. Radio navigation has been used for many years to aid in aviation and marine navigation, but it was not considered for use in land AVLs until the 1970s, when it became economically

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feasible (Roth, H. 1977). There are several ways to use radio signals to determine the location of a vehicle. They can be used either at the vehicle or at fixed stations to calculate vehicle location. When a vehicle's position is determined at a fixed station, the system measures the absolute time the signal takes to travel from that fixed station to the vehicle and back. The absolute times found by a minimum of three stations are sent to a central control station, where the times are used to calculate the distance from each fixed station to the vehicle. These distances are treated as the radii of circles with the stations at the centre. The intersection of all the circles described by the stations gives the location of the vehicle. When measurements from just three stations are used, the technique is referred to as ‘tri-lateration’. If more than three fixed stations are used, the technique is called ‘multi-lateration’. Increasing the number of stations decreases the estimate error in the measurements (S., Riter, S. and McCoy, J., 1977).

One advantage of radio navigation is that it can be used for applications on land, in the air and on water. Another advantage of a national radio navigation system is that it offers automatic vehicle location for 24 hours each day. Thus, it eliminates restrictions that other systems might impose on where vehicles may travel and still be tracked. When GPS was still in the planning stages, radio navigation systems had already been implemented and were producing satisfactory results in aviation and marine applications.

3.3.1.4 Global Positioning System (GPS)

In real-time public transport systems, transit vehicles are mostly equipped with tracking devices that contain GPS systems and are connected through dedicated wireless networks (satellite, terrestrial radio or cellular networks) to a central server. The GPS is a satellite-based radio-navigation system developed and operated by the U.S. Department of Defence (DOD) (Zhong-Ren P. and Oliver J. 1999). GPS provides the user with their three-dimensional position, their current velocity and the exact time. The GPS system consists of twenty-four satellites orbiting the earth in six circular orbits. Figure (3-2) shows these twenty-four GPS satellites. The satellites are arranged so that at any given time, there are six satellites within range of a GPS receiver. The control segment of the GPS system consists of one master control station in Colorado USA, with five ground control stations and three ground

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antennae located around the world. The monitor stations passively track all satellites in view.

Each satellite’s broadcast signal is continuously sampled. These samples are then forwarded to the master control station, which calculates extremely precise satellite orbits. The orbit calculations are formatted into navigation instructions, which are uploaded to the individual satellites via the ground antennae. Simultaneously, each of the satellites continuously broadcasts an exact position and time signal. The GPS receiver receives messages from at least four satellites and measures the time delay for each signal. From these values, the GPS receiver can calculate the user position and velocity. GPS devices can identify locations with high accuracy up to 10 meters.

Figure 3-2 Twenty-four GPS satellites

The European Union (EU) introduced a free global positioning system that it claims is almost five times more accurate than the U.S. system currently in use. Called the European Geostationary Navigation Overlay Service (EGNOS) (Dragos C., and Andrei M., 2010; Z. Hunaiti., et al, 2006), the system uses three satellites and a ground network of about 40 ground positioning stations and four control centers. The U.S. military-run GPS system, in widespread use across the globe, offers a 10-meter accuracy level, but EGNOS promises to fine-tune this experience and deliver accuracy levels to around 2 meters. It is expected that the next generation of GPS- enabled smarptohones will benefit from the improved accuracy of EGNOS. Some standalone GPS devices might soon use EGNOS as well, given that the manufacturers release firmware updates to support the system.

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3.3.1.5 Differential GPS

Differential GPS (DGPS) is a technique used to increase the accuracy of GPS receivers to within one to three metres. The technique involves placing a GPS receiver (a reference receiver) in a known physical location. The reference receiver collects data from all the satellites in view and performs error corrections on the signals, checking the actual location against the broadcast location. These corrections can be either recorded (used for post-processing of signals) or broadcast in real-time via radio (Daniel J., 1996). Figure (3-3) highlights using DGPS for correcting GPS signals. As we can see from Figure (3-3) if a GPS (reference receiver) device is placed in a location with known coordinates, error corrections can be performed on the signals and then broadcasted in real-time via radio.

DGPS is used in the transportation industry for more accurate positioning of transit vehicles to help vehicles maintain their schedules regardless of traffic congestion.

Figure 3-3 DGPS for correcting GPS signals