**Final Report**

Application of DGPS for Collision Avoidance

in Intelligent Transportation Systems

In a Wireless Environment

**(Contract No. BC096 -- Research Project Work Order #6; FM
4059251B201)**

Submitted to

Mr. John Nicholson, P.E.

Florida Department of Transportation

FDOT Research Center, M.S. 30

605 Suwannee Street

Tallahassee, FL 32399-0450

By

Dr. Amr A. Oloufa, P.E., Principal Investigator

Dr. Mohamed A. Aty, P.E., Co-Principal Investigator

University of Central Florida

Department of Civil and Environmental Engineering

P.O. Box 162450

Orlando, FL 32816-2450

February 19^th^, 2001

**Application of DGPS for Collision Avoidance**

**in Intelligent Transportation Systems**

**in a Wireless Environment**

# Executive Summary

This project evaluated the application of Global Positing System (GPS)
technology in collision avoidance. This system tested in this research
utilized both GPS and wireless communications.

The Global Positioning System (GPS) is a satellite based
radio-navigation system with satellites orbiting the Earth, and
transmitting radio signals to ground receivers (i.e. GPS devices). Based
on measurements of the amount of time that the radio signals travel from
a satellite to a receiver, GPS receivers calculate the distance and
determine with great accuracy the location of their antennae in terms of
longitude, latitude, and altitude. GPS can be used in various areas such
as air, land, and sea navigation, mapping, surveying and other
applications where precise positioning is required.

A relatively large number of vehicles is already equipped with GPS
devices whose accuracy is about 20 meters. Position accuracies obtained
with GPS can be as good as 1 cm in real-time, however the cost of such
devices is still very expensive to be incorporated as a standard item in
vehicles.

The system implemented in this research consists of a GPS device on a
vehicle that is capable of receiving differential processing signals.
Using wireless communications, the vehicle radio sends the vehicle\'s
position to a central traffic server, which evaluates potential
collision scenarios with another simulated vehicle. If a collision is
imminent, the server radios back a cautionary message to the roving
vehicle.

While GPS technology is superior to other technologies, it also has its
own limitation. Namely, the technology works only in situations where it
receives signals from all other objects in its vicinity. This means that
other objects in the surrounding area have to either have a fixed and
known location, or be equipped with GPS devices and wireless
communications.

The wireless communication routines for communicating with the server
and receiving caution messages from the server were tested extensively
with extremely fast response times.

This project proves the viability of using GPS technology for collision
detection avoidance. As the majority of the components used for this
research are expected to be standard items in vehicles, the main cost
and challenge will be the computational infrastructure needed to detect
collisions and warn vehicles in an extremely short period of time. For
this reason, more research is needed to evaluate whether collision
avoidance calculations should be implemented on a central traffic server
or within each vehicle, where vehicle computers only evaluate traffic in
each vehicle\'s vicinity.

**Application of DGPS for Collision Avoidance in Intelligent
Transportation Systems in a Wireless Environment**

# Table of Contents

Executive Summary [2](#executive-summary)

Table of Contents [3](#table-of-contents)

I. Abstract: 4

II\. Collision Avoidance Technologies 5

III\. The Global Positioning System 6

1\) GPS Satellite Signals 7

2\) GPS Receivers 7

3\) Basic Concept of GPS 8

4\) GPS Position Accuracy and Error Sources 9

5\) Differential Correction of GPS Positions 10

IV\. Hardware Architecture 12

V. Software Architecture 12

1\) Server Software 12

2\) Vehicle Software 13

VI\. Collision Detection Algorithm: 14

VII\. System Testing 19

VIII\. Future Challenges 19

1\) System Architecture 19

2\) System Response Time 20

3\) System Integration 20

IX\. Conclusion 20

**Application of DGPS for Collision Avoidance in Intelligent
Transportation Systems in a Wireless Environment**

# I. Abstract:

Traffic accidents that occur as a direct results of collision represents
the majority of accidents occurring on your highways and streets, and
lead to hundreds of thousands of fatalities and injury to people and
destruction to property. The aim of this project is to reduce the number
of collisions through the application of the Global Positing System
(GPS) and wireless communications. Even a small reduction in the number
of these incidents yields very large savings and improvement to the
quality of life.

In this project, the researchers developed and implemented a study where
GPS technology was used in tracking a single vehicle and relaying its
information to a central server. Using another simulated vehicle, the
server evaluated collision scenarios and sent cautionary messages to the
roving vehicle if a collision is expected.

# II. Collision Avoidance Technologies

Several technologies exist for collision detection and avoidance. They
differ in their cost, size, response time, reliability, and effective
operation range. Ultrasonic technologies rely on high frequency devices.
They have a low implementation cost, small size, and have a fast
response time. They are however not reliable under some conditions, and
their range is only a few feet.

Infrared technologies have been used for a while. They have a very small
implementation cost and are small in size. However, they exhibit a high
response time, are not very reliable and their range is also very short.

Radar technologies are perhaps the most effective for collision
detection. Improvements have led to size reductions and high
reliability. However, their cost is relatively high.

Vision technologies have also been used for collision detection.
However, their cost is extremely high due to heavy computational
requirements. They also suffer from low reliability in some lighting
conditions.

GPS technologies have not been used for collision detection. These
technologies offer a multitude of benefits and their costs have been
continuously decreasing. The major benefits of using GPS is that these
technologies are not dependent on line-of-sight issues (to other
vehicles) which is one of the major limitations of all the technologies
listed above. This is explained in the diagram below when two opposing
vehicles are traveling on a divided highway. GPS technology coupled with
Geographic Information Systems (GIS) can determine that while both
vehicles are on a theoretical collision course, a collision cannot occur
due to the existence of a median or a barrier on this divided highway.

While GPS technology is superior to other technologies, it also has its
own limitation. Namely, the technology works only in situations where it
receives signals from all other objects in its vicinity. This means that
other objects in the surrounding area have to either have a fixed and
known location, or be equipped with GPS devices and wireless
communications.

![](media/image1.wmf)

# III. The Global Positioning System:

Global Positioning System (GPS) is a satellite based radio-navigation
system. There are 24 GPS satellites orbiting the Earth and transmitting
radio signals. Based on measurements of the amount of time that the
radio signals travel from a satellite to a receiver, GPS receivers
calculate the distance and determine the locations in terms of
longitude, latitude, and altitude, with great accuracy. GPS was created,
and is controlled by the U.S. Department of Defense (DOD) for military
purpose, but is available to civilian users worldwide free of charge.

GPS can be used in various areas such as air, land, and sea navigation,
mapping, surveying and other applications where precise positioning is
required. The system inherently has no limitation in speed or altitude,
but U.S. DOD requires that commercial receivers be limited to operate
below about 900 knots and 60,000 feet (18,000 meters).

## 1) GPS Satellite Signals

GPS satellites transmit two carrier signals, L1 and L2. The L1 frequency
carries the P-Code (Precise Code), C/A (Coarse Acquisition) Code, and
navigation message. The L2 frequency carries a P-Code and navigation
message and is used to measure the ionospheric delay by PPS equipped
receivers.

The Navigation Message also modulates the L1-C/A code signal. The
Navigation Message consists of data bits that describe the GPS satellite
orbits, clock corrections, and other system parameters (Dana, 1996).

## 2) GPS Receivers

GPS receivers can be categorized broadly into three types based on
accuracy: C/A code, carrier phase and dual frequency receivers. Each of
the three types offers different levels of accuracy, and the price of
the receiver is dependent on its accuracy.

C/A code receivers typically provide 1 \~ 5 meter accuracy with
differential correction, with an occupation time of 1 second. Longer
occupation time (up to 3 minutes) will provide accuracy consistently
within 1 \~ 3 meter and can be reduced to 30 centimeter.

Carrier phase receivers typically provide 10 \~ 30 cm accuracy with
differential correction. Distance to satellites from the receiver is
determined by counting the number of waves that carry the C/A code
signal (referred to as ambiguity resolution). This method is much more
accurate but requires a substantially higher occupation time to attain
10 \~ 30 cm accuracy.

Dual-frequency receivers are capable of providing sub-centimeter
accuracy with differential correction. Dual-frequency receivers receive
signals from the satellites on two frequencies simultaneously.

## 3) Basic Concept of GPS

Using GPS satellites as reference points, a GPS receiver determines its
location based on the distances between the receiver and GPS satellites.
The satellites are used as reference points because their orbital motion
is constantly monitored by ground control stations so that their
instantaneous positions are always know with great precision (Hurn,
1993).

The distance between a receiver and a satellite is calculated using a
simple equation of Distance = Speed x Time, where *Speed* is the speed
of the signal which is transmitting at the speed of light (186,000
mile/sec). *Time* is the time for the signal travels from the satellite
to the receiver. Time can be calculated by measuring the departure time
of the signal at the satellite and the arrival time at the receiver.
(Hurn, 1989).

For example, a receiver determines that the distance to a satellite, S1,
is 23,000 kilometers. This one measurement indicates that the receiver
is somewhere on the surface of an imaginary sphere that is centered on
satellite S1 with a radius of 23,000 kilometers. If the receiver
measures that its distance to a second satellite, S2, is 26,000
kilometers, then it narrows down where it could be in space. The only
places that are both 23,000 km from the satellite S1 and 26,000 km from
the satellite S2 are where those two imaginary spheres intersect. That
intersection is a circle of points. A third measurement adds a third
sphere, which will intersect the circle formed by the other two. The
intersection occurs at two points and the receiver has narrowed down its
position to two points with three measurements.

A fourth measurement would go through one of these two points and
pinpoint the position. However, it is not necessary to have a fourth
measurement to decide the position because one of two points will be
unreasonable, for example, thousands of kilometers from the surface of
the earth.

However, a fourth measurement is needed for another reason. It helps the
receiver to ensure that its clock is truly synchronized with the atomic
clocks on board the satellites (Hurn, 1993). This is the reason that a
minimum of four satellites are needed for accurate position
determination.

## 4) GPS Position Accuracy and Error Sources

Accuracy of GPS is the degree of conformance between the estimated or
measured position, time, and/or velocity of a GPS receiver and its true
time, position, and/or velocity as compared with a constant standard.
Radio navigation system accuracy is usually presented as a statistical
measure of system error and is characterized as predicable, repeatable,
and relative accuracy (*Fundamentals of GPS*, 1996).

The accuracy of GPS receiver is affected by errors caused by natural
phenomena, mechanical failure of elements in the system, or intentional
disturbance. The Selective Availability (SA) Error is intentionally
introduced to the GPS signals and then is broadcast by the satellites.
It effectively is a 30 meter range error that varies over time. This
error, the largest contributor of overall errors in the system has been
turned off permanently as of May 2000.

The Ionospheric Error is a function of the local time of day and
latitude. It is largest in the tropics in the afternoon. Mathematical
models have been introduced to reduce ionospheric errors.

The Troposhperic Error is a function of the weight of the atmosphere
above the GPS antenna and is modeled using the atmospheric pressure. It
is usually not a factor in position accuracy. GPS receivers can model
this effect to a few centimeters knowing only the altitude.

The Orbit and Satellite Clock Error occurs because there are slight
variations in the orbits of GPS satellites. Monitoring stations track
this error and broadcast these corrections to the satellites. Because of
the delay in sending these corrections, orbit errors exist. In addition
to the satellite position, the atomic clocks drift off causing another
error in time measurement and therefore in position.

The Multipath Error occurs when strong signals from satellites are not
along the direct line of sight between the user\'s antenna and the
satellite. GPS antennas can receive signals from anywhere above the
horizon and some of these signals may have been strengthened due to
reflections from other objects. When the direct signal from the
satellite is not considered in the solution, range errors to the
satellites occur leading to measurement errors. The GPS antenna location
is therefore extremely important. Multipath is often the dominant error
source in DGPS applications on mobile applications.

The Receiver Noise Error occurs as electronic devices emit
electromagnetic energy, some at the GPS frequencies, which contributes a
range error to the measurement. Newer GPS technology has been successful
in reducing this error.

## 5) Differential Correction of GPS Positions

Differential GPS (DGPS) is a means of correcting for some system errors
by using the errors observed at a known location to correct the readings
of another receiver (rover). A reference receiver, or base station,
computes corrections for each satellite signal. Most of the errors
caused by the sources discussed earlier are eliminated by differential
correction. Differential corrections may be used in real time or
post-processed (Dana, 1996).

The most important consideration in DGPS is that the base station and
rover have to be tracking the same satellites and taking data at the
same time. For this reason, the base station and rover should be within
150-km limit. The quality of the corrections is a function of the
distance between the base station and the rover.

**Error**\...\...\...\...\....................\...\...\.... **DGPS
Cancellation**

Selective Availability\...........\...\...\....Cancelled Completely

Ionosphere\...\...\...\...\...\...............\...\...Function of
Distance to Base

Troposphere\...\...\...\...\...\..............\...\...Function of
Distance to Base

Orbit & Satellite Clock\...\........\...\...\...Cancelled Completely

Multipath\...\...\...\...\...\...................\....Does Not Cancel

Receiver Noise\...\...................\...\.....Does Not Cancel

# IV. Hardware Architecture

The hardware is comprised of two subsystems, a server subsystem, and a
mobile (rover) subsystem. The server subsystem consists of a PC-based
server connected to a radio. The rover consists of a differential GPS
system receiver, GPS, a laptop, and a radio. The system architecture is
shown in Figure 2.

# ![](media/image2.wmf)

Figure 2

# V. Software Architecture

The software applications are shown in Figure 3. The software consists
of:

## 1) Server Software

This software receives position information from the rover and
calculates potential collision scenarios with the simulated vehicle. The
position of all vehicles is shown on the UCF campus aerial map. If a
potential collision is detected, the vehicle is alerted through wireless
communications.

## 2) Vehicle Software

This software receives the DGPS message and then sends it to the server.
The position is formatted according to the \$GPRMC sentence which is
decoded as follows:

\$GPRMC,221846,A,4916.45,N,12311.12,W,050.5,054.7,191199,020.3,E\*68

221846 Time of fix 22:54:46 UTC

A Navigation warning A = OK, V = warning

4916.45,N Latitude 49 degrees 16.45 min North

12311.12,W Longitude 123 degrees 11.12 min West

050.5 Speed over ground, Knots

054.7 Course Made Good, True

191194 Date of fix 19 November 1999

020.3,E Magnetic variation 20.3 degrees East

The vehicle program broadcasts that location to the Traffic Server which
will also decode and display "Caution" signals from the Traffic Server.

If the server software detects a potential collision, a message is
broadcast to the rover (vehicle) with the speed and direction of the
simulated vehicle.

# VI. Collision Detection Algorithm

The collision detection algorithm works by calculating the intersection
point of the two vectors representing two moving vehicles. Each vector
is defined by a point And a direction. In this case, The GPS position of
the vehicle (i.e. vehicle location), and the vehicle bearing (also from
GPS input) define each vector.

After the intersection point is computed, and knowing the vehicles\'
speeds from GPS, the program calculates the distance from the potential
collision point to each vehicle location. The program also calculates
the braking distance required for each vehicle in its operational
scenario. If the braking distance required approaches the distances
above (within a specified tolerance value), the server then issues
potential collision alerts to the vehicles in question transmitting an
alert message, along with the direction and distance of the vehicle in
question.

VII\. Software Implementation

The software was implemented using Visual Basic and a host of other
custom controls. The rover software is shown in Figure 4. Figure 5 shows
a screen dump of the rover software when a potential collision is
detected.

The server software screen is shown in Figure 6. Figure 7 shows an
aerial photograph of part of the UCF campus where the system was tested.
Figure 8 shows an enlarged section of Figure 7 with simulated vehicle
tracks.

The software code for the server and rover packages is included in the
Appendix.

![](media/image3.wmf)

![](media/image4.wmf){width="5.0in" height="3.8493055555555555in"}

![](media/image6.wmf){width="5.397916666666666in" height="3.01875in"}

![](media/image7.wmf){width="5.9944444444444445in"
height="4.347916666666666in"}

# VII. System Testing

The wireless communication routines for communicating with the server
and receiving caution messages from the server were tested extensively
with extremely fast response times.

The entire system including map display and continuous vehicle tracking
were tested on the UCF campus. The researchers evaluated the system
using radios operating in the 900 Mhz. Frequency range. Figures 9 & 10
show a picture of the vehicle and system used in testing. Figure 11
shows a screen of the real-time tracking on the UCF campus using the
vehicle above.

# VII. Future Challenges

As mentioned earlier, this project was a limited study of the potential
of GPS in Collision avoidance. The study was limited to a single vehicle
in potential collision with a simulated vehicle

Future work will need to evaluate the scalability impacts of this work
if several thousand vehicles are involved. In this case, several
important issues need to be evaluated:

## 1) System Architecture: 

Should evaluation be done on a Central Server or within each vehicle?
This is an elusive problem as local computing in the vehicle may be more
efficient, however, it will potentially increase the cost of the
computational hardware needed. In this project, all processing was done
on a central server.

## 2) System Response Time: 

Collision Avoidance requires immediate and reliable feedback. Internet
delays in the case of busy servers of a 2-3 seconds may not be of any
consequence for business traffic, however, in collision detection, this
could be the difference between life and death.

## 3) System Integration: 

Under normal driving, any collision avoidance system will always flag
several potential collisions until drivers slow down their vehicles by
braking. If the tolerance margin for server notification of potential
collisions is reduced, there may not be sufficient reaction time for the
drivers to apply the brakes. In the case of a large tolerance, the
server will notify the drivers of too may collision possibilities that
will turn driving into a potential nightmare! The solution is in the
future integration of this technology with other collision detection
technologies, and in connecting these systems to vehicle braking and
control systems. In this case, the cars can be programmed to execute
evasive maneuvers on their own without operator intervention.

# IX. Conclusion:

This project proves the viability of using GPS technology for collision
detection avoidance. As the majority of the components used for this
research are expected to be standard items in vehicles, the main cost
and challenge will be attributed to the computational infrastructure
needed to detect collisions and warn vehicles in an extremely short
period of time. For this reason, more research is needed to evaluate
whether collision avoidance calculations should be implemented on a
central traffic server or within each vehicle, where vehicle computers
only evaluate traffic in its vicinity.

![](media/image9.png)

Figure 9

![](media/image10.png)

Figure 10

![](media/image11.png){width="6.194444444444445in"
height="7.338888888888889in"}

Figure 11

# X. References

-   Dana, P. H. *Global Positioning System Overview* TX, 1995

-   *Fundamentals of GPS*. Arlington: VA, Navtech Seminars, Inc., 1996.

-   Hurn, J. *Differential GPS Explained*. Sunnyvale: CA, Trimble
    Navigation, Ltd., 1993

-   Hurn, J. *GPS: A Guide to the Next Utility*. Sunnyvale: CA, Trimble
    Navigation, Ltd., 1989

-   Kennedy, M. *The Global Positioning System and GIS: An
    Introduction*. Chelsea: MI, Ann Arbor Press, Inc., 1996.

-   Oloufa, A., W. Do and R. Thomas, Oct. 1997. Automated Monitoring
    Compaction Using GPS, *Proceedings of the ASCE Annual Convention and
    Construction Congress*.

-   *Using Visual Basic 6.* Indianapolis: IN, Que Cooperation, 1999