GPS satellite

Virtualchargingzone

D ev e l o p m e n t o f t h e N ex t
Generation Road Pricing
System with GPS Technology

Central system

Wide area
communication
network

HIDEKAZU OHNO* 1
TOHRU SUZUKI* 1
YASUHIRO YAMAGUCHI* 1
SHIGEO OKAMOTO* 2
KENJI IIZUKA* 2

On-board GPS unit

Enforcement camera system

With the aim of realizing the next generation road pricing system, we have developed a system consisting of anon-board unit which is capable of highly accurate positioning. In this system, the on-board unit can measure thetravel distance and detect the charging zone without matching map information, by using a high performance GPS(global positioning system) which combines an on-board unit and a dead reckoning navigation function. The reports so far released on road pricing systems using GPS use a method in which measurement of the travel distanceand calculation of the charges are performed by the central system by matching the location on map information.
This system necessitates the analysis of the travel locus and matching with a map for all vehicles at the centralsystem, requiring a huge amount of data processing. This system, by employing a GPS using highly accurate deadreckoning navigation, has realized the positional accuracy necessary for charging and has established a distancebased charging system to calculate the charge in real time in the vehicle without matching map information.
Mitsubishi Heavy Industries, Ltd. (MHI) has conducted evaluation tests by selecting test routes in Singapore,including the high-rise building district (urban canyon) of Singapore which offers a very severe operational environment for GPS. As a result, a distance measurement error within 2% was attained, making a good start for therealization of a GPS pricing system.

MHI has developed and delivered the roadside equipment and on-board units for an ERP (electronic roadpricing) system using DSRC (dedicated short range communication) which started service in Singapore in 1998.
Since the start of the service, the ERP system has been
operating stably, gaining customers' trust.
Following the DSRC type ERP system, a new systemusing GPS which has been put into practical use by targeting heavy goods vehicles on autobahns in Germany isconsidered to be promising and movement toward standardization of the system is now in progress.
When utilizing GPS, we need at least three GPS satellites to receive signals for positional data and at least
four satellites to receive signals for position and time
data. The four or more satellites needed for GPS positioning can be traced in a stable manner on highways,
with an open sky and a clear view, and the positional
accuracy necessary for practical use can be assured.
However, when introducing the positional measurement
method using GPS in city areas with tall buildings, which
are the ERP target areas, problems such as GPS signals
being blocked by buildings and flyovers and reflection(multi-path) of signals are bound to arise. Therefore, ingeneral, it is difficult to secure the positioning performance of GPS in city areas.
When using GPS as the vehicle position measurement
method in the ERP system, it is necessary for us to address these problems in city areas. Using a highly
sensitive GPS to receive weak GPS signals in city areas
could be one possible way. However, as susceptibility to
the effect of multi-path signals increases, we should face
the other issues.
In recent years, Road User Charging has become recognized as a concept based on the idea that a car driverreceives the benefits of road services and is subject tocharging according to their travel distance.
Establishing a road pricing system by using GPS incity areas was also studied in London and a demonstration experiment was done. In the efforts toward roaduser charges, the measurement of the actual travel distance and the duration as well as a charge calculation
based on these measurement results are necessary.
There have been discussions as to whether the travel
distance based on distance and duration should be calculated in the vehicle or by the central system. There
have been reports on the method in which the central*1 Kobe Shipyard & Machinery Works*2 Takasago Research & Development Center, Technical Headquarters

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 2 (Jul. 2007)

1. Introduction

1system accumulates vehicle position data transmitted
from the on-board units with a GPS and matches themagainst map information for processing.
In the course of matching map information and travel
locus by the central system, the travel path and time
duration need to be obtained for each individual vehicle.
This raises problems concerning the processing load fora massive number of vehicle and protection of road users' privacy. On the other hand, for on-board processing,road matching processing and charge calculation would
be performed by the on-board unit, and increases in theprocessing load and cost of the on-board unit need to beaddressed.

pulse signal. As for the output from the GPS receiver,the measurement results of the GPS single unit and themeasurement results by the dead reckoning navigation
are weighted in accordance with the input signals fromthe satellite and input to a Kalman filter, where the calculated results are output.
The virtual charging zones surrounded by four points
are set beforehand in the on-board unit.
The on-board unit receives positional data from theGPS receiver at intervals of one second, and calculates
whether the vehicle has passed the border of a virtualcharging zone. When entry to a zone is detected, thetravel distance is accumulated from that entry time until the vehicle is detected leaving the charge zone. Assoon as leaving the zone is detected, the accumulation oftravel distance is stopped and the charge based on thedistance is calculated by the CPU in the on-board unit.
The travel distance in the zone and the charge amountare then transmitted to the central system via the GPRS
communication network.
The on-board unit can store the position data received
from the GPS, the accumulated travel distance andcharge amount as log data in its internal memory.
The central system receives the distance and thecharge amount from the on-board unit, and stores themin its database. In this system, as zone detection anddistance accumulation are performed by the on-board
unit of each car, there is no need for the central system
to generate locus data or match map data. Therefore,calculation and communication by the central system
will be less affected even when the number of on-board
units increases.

2. Outline of Next-Generation Pricing System
2.1 Objectives
To explore the possibilities of on-board processing, wedeveloped a prototype on-board unit which detected virtual charging zones, calculated the travel distance inside
the zones, and calculated the charge amount on the vehicle by using a combination of GPS and highly accurate
dead reckoning (DR) navigation. Using this prototype,
we conducted dynamic tests and evaluated its performance.
To evaluate the performance, three virtual charging
zones in Singapore, which well represented urban environments, were used for the dynamic tests.
2.2 System configuration
The system consisted of an on-board unit equippedwith a GPS receiver and a GPRS (general packet radioservice) communication modem, and a central system toreceive the charges amount transmitted from the onboard unit. The virtual charging zones were set in theon-board unit. The on-board unit acquired the current
position data from the GPS receiver and judged whether
the vehicle had entered or left a virtual charging zone.
While the vehicle was in a virtual charging zone, thetravel distance was calculated and, upon leaving thezone, the charge amount was transmitted from the onboard unit to the central system via the GPRS
communication network. The central system displayed
current the vehicle position transmitted from the onboard unit at intervals of 5 seconds on a map so that thetravel locus of the test vehicle could be monitored.
2.3 On-board unit
Figure 1 shows the configuration of the on-board unit.
It consists of a GPS receiver, a CPU board, and a GPRS
communication modem to communicate with the centralsystem.
The antenna receiving signals from the GPS satellite
is connected to the GPS receiver, and the odometer pulse
signals from the vehicle are input. The GPS receiver isequipped with a uni-axial turn-rate sensor to obtain thedirection of travel of the vehicle which is input to thedead reckoning navigation system along with odometer

Real-time locus display,
Locus history display,
Charge display

Central system

GPS signal
Charge amountCurrent position

GPS antenna

On-board unit

GPRS
communicationmodem

CPU board

GPSreceiver

Charging zoneposition
coordinates

Odometer pulse

Travel distance accumulation,
Charge calculation,
GPRS communication processing

Reverse signal
Fig. 1 Processing diagram of on-board unit

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 2 (Jul. 2007)2

3. Results of Dynamic Tests
Dynamic tests were conducted in business district
with many high-rise buildings (urban canyon), commercial districts with many shopping centers, and on aFig. 2suburban road (Fig.
2).
As an example of the test runs, Fig. 3 shows the results of runs round the commercial district, asillustrated by overlapping. The section enclosed bydotted lines in the figure shows the virtual charging
zone registered in the on-board unit.
The travel loci from the runs overlap each other, indicating that the GPS located positions stably.

(a) Urban canyon area(c) Suburban district(b) Commercial district
Fig. 2 Dynamic tests

800

Virtual charging zone

Distance (m)600

40020000200400600

800

1 000

1 200

1 400

1 600

Distance (m)Fig. 3 Dynamic test loci including virtual charging zone (commercial district)Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 2 (Jul. 2007)

3

Distance (latitudinal direction) (m)3.2 Result(1) Cordon line detection accuracy
Figure 4 shows the borders of the virtual charging zones and the actual border detection positions.
In the illustrations, the virtual charging zones
were located on roads in urban canyon, commercial districts, and suburban districts. In each of
the illustrations, the arrows show directions oftravel and the circles indicate the positions at
which an entry or leaving was detected by the onboard unit.

280

260

2406080100120140120100120140160

180200(a) Urban canyon area: Entry to zone(b) Urban canyon area: Leave from zone

280

260

1 180

1 200

1 220

1 240

1 260

180160

140
1 120

1 140

1 160

1 180

1 200

Distance (longitudinal direction) (m)(c) Commercial district: Entry to zone(d) Commercial district: Leave from zone

1 170

1 150

1 890

1 910

1 930

1 950

1 970

220

200

Distance (longitudinal direction) (m)

1 100

1 130
1 870

140

Distance (longitudinal direction) (m)

300

240
1 160

180

Distance (longitudinal direction) (m)Distance (latitudinal direction) (m)22040Distance (latitudinal direction) (m)Distance (latitudinal direction) (m)Distance (latitudinal direction) (m)Distance (latitudinal direction) (m)3.1 Evaluation method
Dynamic tests (including entries and leavings) wereconducted for the virtual charging zones which were set
for each travel route. The travel distances inside thezones and the detection accuracies at the cordon line wereevaluated.
For the evaluation the coordinates of the detected borders of the virtual charging zones and the travel distances
inside the charging zones were retrieved from the log file,and then the cordon line detection accuracies as well as
the travel distance measurement accuracy were calculated.

1 220

3 020

3 000

2 980

2 960
2 220

2 220

2 240

2 260

2 280

Distance (longitudinal direction) (m)Distance (longitudinal direction) (m)(e) Suburban district: Entry to zone(f) Suburban district: Leave from zone

2 300Fig. 4 Cordon line detection of virtual charging zones

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 2 (Jul. 2007)4

Occurrence frequency (%)

Occurrence frequency (%)

2520151050

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Error (m)25201510501 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Error (m)Occurrence frequency (%)(a) Cordon line detection error: Urban Canyon area(b) Cordon line detection error: Commercial district

25Fig. 5 Detection errors of virtual charging zone borders
20151050

Table 1 Accuracy of distance measurement
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Error (m)Reference
Maximum
Maximum
distance (m) distance error (m) error (%)(c) Cordon line detection error: Suburban district

Figure 5 shows the distances between the cordonlines and each zone detection position for the virtual
charging zones. In all zones, all errors in cordon line
detection fall within 20 m. As the measured GPS
points were plotted after the vehicle had passed overthe border, a buffer zone will have to be provided ateach charging cordon line. In consideration of the testresults, the buffer zone should be about 20 m long in
the direction of travel. For the purpose of practicaloperation, further evaluation will be necessary withvarying parameters such as vehicle speed and satellite receiving conditions.
(2) Distance measurement accuracy
Table 1 shows the accuracies of distance measurement taken inside the virtual charging zones. As shownin Table 1, the maximum error in the accumulated travel
distance remains within 2% for all of the districts.

Hidekazu Ohno

Tohru Suzuki

Urban canyon area587

111.9

Commercial district

725

91.2

Suburban district

1 889

80.4

4. Conclusion
The method which is most commonly used today forroad user charges necessitates the calculation of a travel
locus, the detection of the cordon line, and the calculation of charges to be performed by the central system.
However, our new system allows processing in the vehicle, and its accuracy has been tested and evaluatedthrough dynamic tests. As a result, it was verified thatan on-board unit alone could achieve the practical use
level for cordon line detection and distance measurement
without position correction based on map information.
In the future, an evaluation test of the entire systemis scheduled in association with actual charges made tovehicles and enforcement technology.

Yasuhiro Yamaguchi

Shigeo Okamoto

Kenji Iizuka

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 2 (Jul. 2007)5