electroteam__@hotmail.com

Sports Radar GunSports Engineering

Author

Mohamed Saad Aly

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Contents

1. Abstract………………………………………….2

2. Introduction…………………………………......3

3. How radar speed guns work…………………....4

4. Doppler effect……………………………………5

5. Moving' radar speed guns……………………...6

5.1Physical Limitations…………………………………...7

5.2 Cosine Effect and Radar Accuracy…………………..7

6. Size……………………………………………….9

7. Distance……………………………………...…10

8. Environment…………………………………...11

9. Sports radar applications……………………..11

9.1 Position………………………………………………..…11

Conclusion…………………………………..……15

References……………………………………..…16

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1. Abstract

Sports Radar displays are a valuable complement to radar guns for

continuous unattended operation, solo practice, and competitions.

Displays allow the MPH or Km/H speed to be viewed from a distance

so they are perfect for fast pitch competitions, tennis courts, auto

races, and other multiple viewer events. Sports Radar displays feature

large bright digits using either LCD or bright LED technology. All

displays can be powered by either batteries or an optional AC adapter.

Optional stands and wall mount brackets are available.

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2. Introduction

The Sports Radar Gun is a microprocessor based computing device

that uses a low power doppler radar transceiver. The radar gun sends

out a signal, which bounces off the object you are tracking and is

reflected back to the radar gun. A mixer provides the difference in the

frequencies of the original sent signal and the reflected signal that

bounced off the object. From this difference signal, which is

proportional to the speed of the object, a microprocessor calculates

speed and displays it in miles per hour (MPH) or kilometers per hour

(KM/H).

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3. How radar speed guns work

Speed guns use doppler radar to perform speed measurements.

Radar speed guns, like other types of radar, consist of a radio

transmitter and receiver. They send out a radio signal in a narrow

beam, then receive the same signal back after it bounces off the target

object. Due to a phenomenon called the Doppler effect, if the object is

moving toward or away from the gun, the frequency of the reflected

radio waves when they come back is different from the transmitted

waves, and from that difference the radar speed gun can calculate the

object's speed.

The target object's speed v is proportional to the difference in

frequency Δf between the outgoing and the reflected radio waves:

where f is the frequency of the outgoing radio waves, and c is the

speed of light.

After the returning waves are received, a signal with a frequency

equal to this difference is created by mixing the received radio signal

with a little of the transmitted signal. Just as when two different

musical notes are played together they create a "beat note" at the

difference in frequency between them, when the two radio signals are

mixed they create a "beat" signal (called a heterodyne) at the

difference in frequency between the outgoing and reflected waves.

The circuit then converts this frequency to a number by counting the

number of cycles of the signal in a fixed time interval using a digital

counter, and displays the number on a digital display as the object's

speed. It is important that the radio waves leave the gun in a narrow

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beam that doesn't spread out much, so that the gun will get a return

only from the vehicle it is aimed at, with no chance of receiving a

false return from nearby objects or vehicles. To create a narrow beam

with an antenna small enough to fit in a handheld gun, radar speed

guns use high frequency radio waves in the microwave range. X band

(8 to 12 GHz) guns are becoming less common due to the fact the

beam is strong and easily detectable. Also, most automatic doors

utilize radio waves on X band and can possibly affect the readings of

police radar. As a result K band (18 to 27 GHz) and Ka band (27 to

40 GHz) are most commonly used by police agencies.

4. Doppler effect

The Doppler effect is the change in frequency of a wave for an

observer moving relative to the source of the wave. It is commonly

heard when a vehicle sounding a siren or horn approaches, passes, and

recedes from an observer. The received frequency is higher (compared

to the emitted frequency) during the approach, it is identical at the

instant of passing by, and it is lower during the recession.

The relative changes in frequency can be explained as follows. When

the source of the waves is moving toward the observer, each

successive wave crest is emitted from a position closer to the observer

than the previous wave. Therefore each wave takes slightly less time

to reach the observer than the previous wave. Therefore the time

between the arrival of successive wave crests at the observer is

reduced, causing an increase in the frequency. While they are

traveling, the distance between successive wave fronts is reduced; so

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the waves "bunch together". Conversely, if the source of waves is

moving away from the observer, each wave is emitted from a position

farther from the observer than the previous wave, so the arrival time

between successive waves is increased, reducing the frequency. The

distance between successive wave fronts is increased, so the waves

"spread out".

5. Moving' radar speed guns

The above-described system measures the difference in speed between

the target and the radar speed gun itself. The gun must be stationary to

give a correct reading; if the gun is used from a moving car it just

gives the difference in speed between the two vehicles. So a different

system is used in radar speed guns designed to work from moving

vehicles. In so-called "moving radar", the gun receives reflected

signals from both the target vehicle and stationary background

objects, such as the road, road signs, guard rails, streetlight poles, etc.

Instead of comparing the frequency of the signal reflected from the

target with the transmitted signal, it compares the target signal with

the background signal. The difference in frequency of these two

signals gives the true speed of the target vehicle.

Traffic radar comes in many models. There are hand held, stationary

and moving radar instruments. Hand held units are mostly battery

powered, and for the most part are used as stationary speed

enforcement tools. Stationary radar is mounted in police vehicles, and

may have one or two antennae. These are employed when the vehicle

is parked. Moving radar is employed, as the name implies, when the

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police vehicle is in motion. These devices are very sophisticated, able

to track vehicles approaching and receding both in front of and behind

the patrol vehicle. They can also track the fastest vehicle in the

selected radar beam, front or rear.

5.1Physical Limitations

Mobile or hand-held radar are only reliable in a sterile

environment with one moving object in the field of view and no

other moving objects nearby.

Mobile traffic enforcement radar must occupy a location above

or to the side of the road, except when the roadway is occupied

by only one vehicle. The user must understand trigonometry to

"guess" vehicle speed as the direction changes while a single

vehicle moves within the field of view when positioned

adjacent to the roadway. Vehicle speed and radar measurement

are rarely the same for this reason.

5.2 Cosine Effect and Radar Accuracy

If the target is in a direct line (collision course) with the police

radar or sports radar gun the measured speed will be exact. As

the angle of incidence increases, if you move either right or left

of this direct line, the accuracy of radar guns will decrease. The

measured speed will decrease as you move off this centerline.

This phenomenon is called the Cosine Effect. It is called this

because the measured speed is directly related to the cosine of

the angle between the radar gun and the target’s direction of

travel. As a quick reference to radar accuracy, remember to

keep your targets direction of travel in a direct line with you,

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and not perpendicular. The Cosine Effect refers to the angle of

the target vehicle in relation to the patrol vehicle where the

radar is mounted. The traffic radar should be operated as

parallel as possible to the targets, although it is hardly possible

to do perfectly. When the angle between the radar beam and

target becomes too significant, the relative speed will be less

than the true speed producing a lesser speed reading than what

the vehicle is actually traveling. Thus, the cosine effect is

always in the favor of the motorist. The greater the angle the

lesser the speed will be recorded compared to the actual speed

of the moving target. In the radar moving mode of operation

care needs to be taken to make sure the radar antenna is pointed

at a less than 100 degree angle to the roadway. Since the Target

Speed is calculated by taking the Closing Speed and subtracting

the Patrol Speed, if the patrol speed is incorrect, then an

incorrect Target speed reading could occur. Proper aiming and

positioning of the antenna, proper tracking of the target vehicle,

proper observation and proper checking of the patrol vehicle's

speed displayed in the radar 's Patrol Speed window against the

Patrol Vehicle's speedometer should make it apparent to the

operator if the reading is incorrect.

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6. Size

The primary limitation of hand held and mobile radar is size. Antenna

diameter less than several feet limits directionality, which can be

improved with higher transmit frequency. This limitation is imposed

by antenna aperture and radiation pattern determined by antenna

geometry. Mobile weather radar is mounted on semi-trailer truck for

this reason.

As an example, the antenna on some of the most common hand-held

radar is 2 inches, while the wavelength at X band is about 1 inch. That

kind of antenna is 2 wavelengths across, and the "beam" of RF energy

produced by that antenna occupies a cone that extends about 22

degrees surrounding the line of site in the direction where the radar is

pointed (44 degrees wide). This beam is called the main lobe. There is

also a side lobe extending from 22 to 66 degrees away from the line of

sight, which surrounds the main beam like a donut. There are other

side lobes, including some that point backward behind the user. Side

lobes are about 20 times less sensitive than the main lobe (13dB), but

side lobes produce detection when the object in the side lobe is close

or large. The primary field of view is about 130 degrees wide, but the

total field of view actually extends 360 degrees in all directions for

large objects and nearby objects.

Compare this radar geometry with the anatomy of the human eye. We

see accurately within a small region about 5 degrees wide. The fovea

determines direction we are looking, which is called our visual line of

sight. Accurate vision extends about 20 degrees. Our total field of

view is about 100 degrees horizontally and 60 degrees vertically. The

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field of view for small hand-held and mobile radar devices may

exceed the visual field of the user because of side-lobe detections.

Size limitations cause hand-held and mobile radar to produce

measurements from multiple objects within the field of view of the

user.

7. Distance

The second limitation is that hand-held devices are limited to

continuous-wave radar to make them light enough to be mobile.

Speed measurements are only reliable when evaluated at a specific

distance, and distance measurements require pulsed operation or

cameras when more than one moving object is within the field of

view. Continuous-wave radar produces only a steady tone and not

pulses. The frequency shift of this tone is used to measure speed.

Continuous-wave radar may be pointed directly at a vehicle 100 yards

away but produce a speed measurement from a second vehicle 1 mile

away when pointed down a straight roadway. Users cannot tell which

object is being measured within the field of view without knowing the

distance, which is impossible with continuous wave radar.

Some sophisticated devices may produce two different speed

measurements from two objects within the field of view. This is used

to allow the speed-gun to be used from a moving vehicle and not to

discriminate between multiple vehicles within the field of view.

Reliable operation cannot be achieved as more moving objects are

added to the environment. Portable hand-held or vehicle-mounted

radar can never produce a reliable measurements when 2 or more

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moving vehicles occupy the field of view if no distance measurement

is produced by the radar.

8. Environment

Environmental influences also play a role. Using a hand-held radar to

scan traffic on an empty road while occupying the shade of a large

tree renders the hand-held radar sensitive to detecting the motion of

the leaves if the wind is blowing hard (sidelobe detection). Airports

cause a similar phenomenon. Hand-held radar is only reliable on

single vehicles when the location has been certified to be free of

environmental influences that will cause false readings. Site survey

must be repeated periodically for reliable operation.

9. Sports radar applications

9.1 Position.

To reduce or eliminate the cosine effect (in doppler radar applications)

and achieve maximum accuracy, align the radar unit in the line of

travel of the intended target. If the boresight of the radar unit is not in

the direct line of target travel, the recorded speed will be less than the

actual ball speed by the cosine of the angle between the boresight of

the radar unit and the line of travel of the target.

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The radar unit will read target speeds accurately for both targets

moving towards the boresight, or moving away from the boresight of

the radar unit. For optimum performance and accuracy the radar unit

should be no more than 12 feet from either the release point, or end

point of the target.

Instructions for positioning the sr3600 radar unit for tennis serve

speed applications. To reduce or eliminate the cosine effect (in

doppler radar theory) and achieve maximum accuracy, align the

Radar boersight in the direct line of travel of the ball. If the boresight

of the radar unit is not in the direct

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Line of ball travel, the recorded speed will be less than the actual ball

speed by the cosine of the angle between the boresight of the radar

unit and the line of travel of the ball.

It is very important to align so the center line, or boresight, of the

Radar unit is pointing directly in line with the ball flight path.

Positioning the radar gun for optimum performance for reading a

volleyball serve speed. To reduce or eliminate the cosine effect (in

doppler radar theory) and achieve maximum accuracy, align the

Radar unit on a tri-pod in the direct line of travel of the ball. If the

bore sight of the radar unit is not in the direct line of ball travel, the

recorded speed will be less than the actual ball speed by the cosine of

the angle between the bore sight of the radar unit and the line of travel

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of the ball. Mount the radar unit on a sturdy tri-pod, no more than 12

feet behind the base line (figure 1) and about 5-6 feet

High. Move the radar unit from right to left depending on which side

of the court is being served to.

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10. Conclusion

The Speedtrac is a very easy to use radar gun. The Speedtrac is a very

practical radar device that unfortunately most people don't know

about. Sporting goods stores can make a killing selling these. it will

not disappoint if it is used within it's means. Just be sure to read the

manual beforehand to get an understanding of its features.

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11. References

http://www.epinions.com

http://www.sportsradargun.com

http://www.hssports.co.uk

http://en.wikipedia.org

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