Radar Flashcards
What type of energy is transmitted by he silver eagle II
A continuous wave of electromagnetic energy down the roadway at a frequency of 34.9GHz at the speed of light
What speed does electromagnetic energy travel at
The speed of light - 300 million metres per second
What is the wavelength of the police radar unit
They transmit 34.9GHz therefore the wavelength is 8.59 millimetres
Explain Doppler shift
It is the actual frequency change which takes place as a result of movement between the antenna and target.
The Doppler shift could be called the difference frequency because it is the difference between the transmitted and received frequencies
How do you calculate Doppler shift frequency
Multiply the speed of the target vehicle in km/hr by 64.6
What does an approaching target do to the wavelengths transmitted by the radar
It causes the wavelengths to compress. A shorter wavelength gives a higher frequency
What does a reversing target do to the wavelengths transmitted by the radar
It causes the wavelengths to expand. Therefore a lower frequency
What is the Doppler tone
It is the Doppler shift frequency divided down for listening comfort
What is the importance of a clear tone during a valid speed check
It indicates the radar is monitoring only one target and is not being subjected to any interference
What evidence must changing pitch be accompanied by
Evidence of altering speed
List the behaviour of electromagnetic energy
Reflection Refraction Diffraction Absorption Re-transmitted
Explain reflection and give an example
It is an abrupt change in the direction of a wave. When energy strikes a surface at an angle other than 90 degrees much of it is reflected in directions away from the receiver and is lost.
Eg: a motor vehicle due to its irregular shape can reflect an amount of energy away from the source.
Explain refraction and give an example
It’s when a ray of light meets the surface of separation between two transparent media and it is sharply bent or refracted. The Ray is bent toward the normal as it enters a denser medium and away from the normal as it enters a less dense medium
Eg:
Explain diffraction and give an example
This is when electromagnetic waves are restricted by an aperture or by the edge of an obstacle. Some energy spreads into the region not directly in line with the source.
Eg:
Explain absorption and give an example
Electromagnetic energy at the frequency used by police radar will be absorbed by some compounds. The most common is water. Things with a high water content can also absorb such as grass leaves and shrubs can absorb the energy to varying degrees
The amount of energy returned to the radar antenna from a target will be determined by what two main factors
The reflective area of the target
Distance between the radar antenna and target
How does interference commonly affect the silver eagle II
Display of a speed in the absence of a target
No speed displayed when a target is present
Name the types of interference
Natural
And man made
List possible natural interferences
Large bushes and trees can reflect and diffuse the radar energy
Heavy rain ca add or subtract from the patrol speed
Birds
Random movement can cause a reduction in effective range
List man made interferences
Large advertising signs
Large broad bladed fans
Electronic emissions from fluorescent lights
Power transformers
These may cause a speed reading or a reduction in effective range
How is electrical inference from a vehicles electrical system overcome
The police vehicle is fitted with a shielded cable to ground any spurious emissions and negate the adverse effects of electrical interference
What is interference
Any external event which influences the operation of the radar
What angle does the radar beam leave the antenna
12 degrees plus or minus one degree
How does the width of the beam increase
It increases about 21 units for each 100 units away from the antenna
What is the effective beams nominal length
600 metres
Explain the rules for proper antenna placement
- The antenna should be mounted externally on the vehicle
- The antenna must be minted not more than 2m and not less than 1m from the ground
- The antenna must face directly to the front and parallel with the roadway in moving mode
- The antenna must face front and rear and parallel with the roadway in stationary mode
Radar can be mounted internally in approved mounting bracket
How is patrol speed obtained
From energy reflected off the roadway. Ideally a minimum distance of seven metres to the front of the patrol vehicle
What are the five main effects which an operator could encounter
Cosine
Split speed
Differential
Double bounce
Add on speed
Explain cosine effect
It is an angle between the direction in which the beam is transmitted and the direction of travel of the target.
To avoid errors due to cosine effect what should police do
Apply the 7:1 ratio
Correlate patrol speed with checked speedometer
Why is reflection important to the operation of the silver eagle II radar
The radar relies upon energy from the target to operate. When energy strikes a surface at an angle other than 90 degrees much of it is reflected and lost
What test does the operator perform to check for segment faults in the displays of the silver eagle II radar
A ‘manual test’
Press and hold the test button to observe all LED segments are illuminated and performs an internal calibration test
Is the silver eagle II radar capable of registering the speed of a motorbike when a truck is in sight.
It is possible however the radar obeys inverse square law. Both the reflective area of the target and the distance between the radar antenna and the target need to be considered.
The truck reflects more energy at a greater distance and the motorbike would have to be closer
Why is it important to correlate the patrol speed with the checked speedometer during speed checks
To ensure the radar isn’t being subjected to any interference.
An operator will be able to negate the possibility of an incorrect speed being displayed as a result of an effect simply by way of correlation
What is the transmitted frequency of the silver eagle II
34.9GHz
Explain differential effect and give an example
This may occur in moving mode when a target is checked while a large reflective vehicle is immediately in front of the police vehicle. The radar would great the vehicle as if it were stationary and display an erroneous patrol speed.
Eg: a caravan in front of the police vehicle
Explain double bounce and give an example
When the radar is allowed to transmit a beam when there is no target vehicle present it is possible to obtain target speed from stationary objects like overhead bridges, billboards and signs.
Eg: beam is released and hits an overhead bridges before bouncing back with same energy to police vehicle which then registers energy movement and then hits the bridge again and is returned to the radar antenna
Explain split speed and give an example
When the police vehicle is travelling in excess of 120km/hr the instrument is unable to obtain a patrol speed from the roadway in front of the vehicle so it obtains it from some angle at the side.
As a result of the cosine effect the displayed speed is lower than the true patrol speed. The radar then obtains the true patrol speed but because it already has a patrol speed treats the latter as the closing rate speed.
Explain add on speed
This usually occurs at low speeds when the police vehicle is travelling below the radars threshold (16km/hr) and the instrument must establish patrol speed.
Any oncoming vehicle within about 50m of the radar is treated as a stationary object.
As a result the radar will display the closing are speed between the radar and the oncoming vehicle the two speeds are added together and displayed as the patrol speed
What does the tuning fork test establish
- That the transmitter/receiver is functioning and
2. The accuracy of the Logic Control unit
Explain inverse square law and how it affects operation
There is a minimum amount of energy required before the radar will register a speed. The larger the surface (reflective area) the more energy will be captured by the spread of the beam and returned to the antenna. The amount of energy returned is determined by two factors
- The size and reflective surface of the target
- The distance between the target and the antenna
What are the requirements of a valid speed check
- Visual observation and estimation of the targets speed
- Observe the numerals in the patrol and target displays
- Clear and continuous tone (however pitch may vary)
- Correlation between patrol speed and checked speedometer
- Duration of the check (Steps 2, 3 and 4) must not be less than three seconds
- Minimum of 200m between targets of similar size
What responsibility do traffic enforcement commanders, highway supervisors and radar lidar operations have in regard to the use of radar equipment
To ensure that the radar equipment is utilised in the most effective manner.
To make sure they are not used
- Within 50m of a speed restriction or de-restriction sign creating a change to the speed zone (unless speed is excessive, area subject to complaint, high accident history) this does not apply to school zones
- At any location that would engender legitimate criticism or give rise to the complaint that they are government revenue raising
