GPS World - May 2008 - (Page 30)

AVIONICS & TRANSPORTATION | Vehicle Integration reducing the processing load of the system and helping minimize unnecessary alerts. Å FIGURE 2 Straight-forward warning zone to determine accurate differential position and velocity vectors, and calculates the expected relative position vector at each of the next eight seconds. If the ped end of the vector is calculated to be within the warning zone (represented in the Å FIGURE 3 Right-turn warning zone figure by a rectangle), determined based on the vehicle and ped velocities, a warning is given to the driver and transmitted to the at-risk ped. If the ped is determined to be out of the warning zone, they are judged to be a low priority and may be tracked at a lower update rate. Since peds can be near the road but not in danger, warnings are not issued based on proximity. A warning is only issued if the ped is actively moving on a collision course with the vehicle (or vice versa). As shown in the figure, the system works regardless of the visual screen formed by the shrubbery. At the instant depicted in the figure, ped B is outside the wake-up signal range so his receiver and transmitter remain asleep to conserve battery power. In the figure, the warning zone has a rectangular shape but in practice it is shaped like a triangle for a vehicle going straight forward or backward. FIGURE 2 shows the warning zone for straight-forward movement. The warning zone width corresponds to the maximum orthogonal distance that the ped can travel before the car reaches its point of closest approach; peds outside the zone cannot get in front of the vehicle if their speed remains below the assumed or observed maximum. The warning zone is not only adapted to the ped and vehicle velocities but to the detected state of the vehicle controls. If the car in Figure 2 were in reverse, the warning zone shown would flip to the rear of the car. FIGURE 3 shows how the warning zone is adapted when a right-turn signal is activated. The zone naturally widens at low vehicle speeds. The concept of the warning zone provides the ability for the system to focus on the most at-risk peds while paying minimal attention to peds well removed from the zone. An average warning zone is only about five degrees, so peds in 98.6 percent of the angular space around the vehicle are assigned a low priority, 30 GPS World | May 2008 Architecture The PAS consist of both a ped unit (P-unit) and vehicle unit (V-unit). The GPS unit in both has 16 channels and is WAAS-capable. Its sensitivity is speciﬁed as 140 dBm acquisition/ 149 dBm tracking, which provides tracking under heavy foliage and better performance Å FIGURE 4 Ped unit packaging. in urban canyons. It further provides sufﬁcient processing power and memory to allow us to embed the PAS application software; no separate microprocessor is required to control the receiver and transceiver, which saves power. The GPS antenna has a broad reception pattern and is resistant to collocated transmitters. The radio transceiver is a frequency-hopping spread spectrum (FHSS) radio with seven hop sequences on 25 frequencies between 902± 928 MHz. It emits 1 mW transmit power so it is compliant with unlicensed FCC Part 15 operation. It provides the capability to turn the receiver and transmitter off/on very quickly, allowing sophisticated power-saving operations. A rechargeable lithium-polymer battery is sandwiched between the GPS and transceiver board (FIGURE 4). A 5-mm thick battery corresponding to the dimensions of the transceiver will provide about 900 mAh of current. We estimate 578 mAh/ day are needed for the unit. The size of the unit is about 2 3 1 inches, with a weight of about 3 ounces including battery. In the future, the size can be further reduced by using smaller commercial off-the-shelf (COTS) transceivers or a custom design. It also could be incorporated into other devices such as cell phones and MP3 players. The V-unit has two transceivers, both of which are higher power versions of the one used in the P-unit. The primary transceiver continually monitors for ped position and velocity data, while the secondary transceiver broadcasts the wakeup signals to any P-units in the vicinity. The additional range provided by the increased 100 mW power provides time for the P-unit GPS receiver to begin producing navigation solutions before it is in range to transmit to the vehicle, further reducing the P-unit' s power requirements and the probability of interference between P-units. A challenge in the system design is to provide the capability for the system to work in an area where there are numerous peds. The PAS design uses a time-division multiple access (TDMA) that allows scores of P-units to be tracked simultaneously. Each second is divided into time slices that are synchronized for all vehicles and peds by precise GPS time. Once receiving a wakeup signal in its fixed time slice, each P-unit randomly selects a transmission www.gpsworld.com http://www.gpsworld.com

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GPS World - May 2008

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