Radar Theory Flashcards
What radar band does Eagle 3 use
Ka band
What is Doppler effect (definition)
“The apparent difference between the frequency at which sound or light waves leave a source and that at which they reach the observer.”
Eagle 3 frequency and wavelength
Eagle 3 radar transmits a continuous transverse wave of electromagnetic energy down the roadway at a frequency of 34.9GHz (34900 million Hertz) at the speed of light.
Wavelength is 8.59mm
Describe Doppler shift
The Doppler shift is the actual frequency change which takes place as a result of movement between antenna and target. The dipole shift could be called the “difference” frequency because it is the difference between transmitted and received frequencies l.
Doppler shift formula
Fd = 2VFt
C
Fd = Doppler shift frequency (in hertz)
V = speed of target (in m/s)
Ft = transmitted frequency
C = Speed of light (in m/s)
Speed of light
300,000,000 m/s
Frequency change per km per hour
64.6 Hz
Doppler tone
The Doppler tone emitted by the radar is the Doppler Shift Frequency (Fd) divided down for listening comfort.
In stationary mode divided by 6
In moving mode divided by 12
Minimum distance of separation between vehicles of similar size
200m
Minimum speed threshold of Eagle 3
Minimum speed threshold is 16km/h however device may display speeds as low as 9km/h
Audio tone during check
Audio tone must be clear and unbroken. Evidence of changing pitch must be accompanied by evidence of altering speed.
Reflection
Reflection is an abrupt change in a direction of propagation of a wave which strikes the boundary between mediums.
Radar relies on energy being reflected from the target (echo) to operate therefore the radar beam should ideally strike the target perpendicular to its surface.
Refraction
When a ray of light meets the surface of separation between two transparent media, it is sharply bent or refracted. The ray is bent towards the normal as it enters a denser medium and away from the normal as it enters a less dense medium.
Think water refraction
Diffraction
When electromagnetic waves are restricted by edge of obstacle, some energy spreads into region not directly in line with source. Bending of EM waves is called diffraction.
Think wave bending over crest of hill or shadow behind a vehicle
Inverse square law
The amount of energy returned to the radar antenna from a target will be determined by two main factors:
1. The reflective area of the target
2. Distance between radar antenna and target
Absorption
Radar energy can be absorbed by some compounds including: fibre or rubber/plastic based sponge.
Most common for police radar is water and material with a high water content eg grass, leaves, shrubs resulting in loss of range.
Effects of interference
Interference does not add to or subtract from any target speed reading. Interference has two main effects.
1. Display of a speed in the absence of a target
2. No displayed speed when a target is present
Two fundamental categories of interference
Natural
Man-made
Types of natural interference
Natural interference: large trees, bushes, signs moved by wind rain, snow will diffuse radar energy or mask low level signals resulting in decreasing effective range
Heavy rain: heavy rain falling non perpendicular to roadway can add to or subtract from patrol speed.
Random movement
Man-made interference
Large advertising signs of rotated or swinging may reflect the beam. Large broad-bladed fans may also reflect sufficient energy to cause a speed reading. Electronic emissions from fluorescent lights, power transformers, x-ray and medical diathermy machines, high voltage transmission lines with leaky insulators, radio transmitters, mobile phones, automobile inverters etc may cause a speed reading or reduction in effective range
Shielded cable
Interference from vehicles electrical system can enter the radar through the connecting cables. To overcome, Highway patrol vehicles are fitted with shielded cable to ground any spurious emissions.
Radio frequency interference
Eagle radar 3 has an RF detector which when activated by excessive RFI causes display to blank. rFi will be displayed in target area
Multipath interference
This may occur on long straight roads where Armco railings are used. It is the arrival of two energies at different time intervals at the receiver. Can cause cancellation of readings or poor tone but does not affect accuracy.
Radar beam angle
The radar beam leaves the antenna and spreads out at an angle of 12 degrees (plus or minus 1 degree)
Width of the beam between half power points increases 21 units for each 100 units away.
The effective beam has a nominal length of 600 metres
Antenna mounting
- should be externally mounted
- must be mounted not more than 2m and not less than 1m from the ground
- must face directly to the front and parallel with roadway
- antenna must not be exposed to or used in inclement weather
Three functions in moving mode
- Determine and display patrol speed
- Determine combined speed of radar and target
- Determine and display target speed
Low Doppler vs High Doppler
Low Doppler is the patrol speed
High Doppler is the combined speed
5 Types of electromagnetic wave distortion
Reflected, refracted, diffracted, absorbed or retransmitted
What is gross error
Gross error could be defined as an error greater than 4km/h.
Obtained by incorrect patrol speed due to energy being reflected from a stationary object.
Digital speedometer check
- Visual check of speedometer and wiring for any damage
- Ensure the two tamper proof seals are intact and not damaged in any way
- Turn on digital speedometer and observe all segments on the 7 segment displays. All segments must illuminate
- At commencement of your patrol a correlation of the digital speedometer and the patrol speed of the radar must be performed. Should be done on relatively straight, flat portion with speed of vehicle constant. Accuracy is within +/- 2km/h
5 potential effects during moving mode
- Cosine
- Split speed
- Differential effect
- Double bounce
- Add-on speed
Cosine effect
Cosine effect is probably most common effect. Radar measures the target speed directly towards radar (relative velocity). When target approaches or recedes at an angle the relative velocity to the radar is slower than the actual forward speed of the vehicle.
Means angle towards radar
Why can cosine effect cause error (moving mode)
If patrol speed is picked up by billboard with cosine, the patrol speed will be lower than actual speed causing higher speed for target. However patrol speed will not correlate with digital speedometer and therefore operator will be aware of error.
7:1 ratio
Distance between radar and path of target multiplied by 7. Used to determine the range at which any radar speed check must cease.
Eg distance between radar and target path is 10m, 3 second check must be completed before target is within 70m (8 degrees)
Differential effect
Differential may occur in moving mode when a target is checked whilst a large reflective vehicle is immediately in front of police vehicle. While acquiring patrol speed the large reflective vehicle in front may cause the difference in speed between the two vehicles to be displayed as the patrol speed.
1)Do not check speed of target when close to rear of large moving object.
2)Verify displayed patrol speed with digital speedometer
Double bounce
When approaching stationary objects like bridges and when no target is present, may cause police vehicle to display a target speed when none are present. The wave is reflected off the bridge, strikes the police vehicle causing compression, reflects again off the bridge and reaches the radar unit.
Split speed
Can occur if police vehicle in excess of 120kmh and is unable to obtain patrol speed from road surface and instead from angles to the side. The cosine effect causes displayed speed to be lower than true patrol speed. If the radar than picks up the true ground speed but already has the incorrect lower ground speed, it treats the true ground speed as a closing speed and the difference displays as the target speed.
Add-on speed
Usually occurs when the police vehicle is moving at speeds below threshold (16kmh) and instrument must still establish patrol speed. Any oncoming vehicle within about 50m will return better signal than roadway and radar treats it as a stationary object. Radar will display the closing rate between the radar and oncoming vehicle. Two speeds are added together.
Eagle 3 is no longer susceptible to this effect.
Instrument errors
Stationary mode: plus or minus 1km/h
Moving mode: plus 1 or minus 2km/h
Speed is always rounded down to near while kilometre per hour
Tuning fork speeds
45 and 80kmh
Tuning fork tolerance
+ or - 1 km/h
Care of tuning forks
1: the mechanical tuning forks must never be struck against any material which is harder than the fork itself or against each other
2: when not in use the mechanical tuning forks should be carried in the police vehicle and
3: when sent to the radar engineering unit for periodic testing or for repair, both the mechanical and electronic tuning forks allocated to the Eagle 3 must accompany the instrument.
Remote LED light meanings
(Red, Green, Blue)
Red = trying to connect
Green = connection or test pass
Blue = electronic fork signal transmit
Eagle 3 testing procedure
1: visually check (antenna placement and alignment
2: Press power (visually observe all segments illuminate followed by internal test, certification days remaining, front/rear port connection. Press check button to clear tuning fork prompt
3: check and adjust audio (press menu button to access audio adjustment menu
4: check and adjust range (press menu button to access range adjustment menu. Press check button to exit menu display
5: carry out tuning fork test (using either mechanical or electronic tuning fork method
Following tuning fork procedure
Commence patrolling and correlate patrol speed with checked speedometer to ensure correct antenna alignment.
Technical specs
1. Frequency
2. 3dB Beamwidth
3. Operating voltage
4. Operating temperature
5. Target speed
6. Patrol speed
7. Max combined speed
1: 34.9 GHz plus or minus 0.1 GHz
2: 12 degrees plus or minus 1 degree
3: 9.0 to 16.5 volts dc
4: -30 to +60 degrees Celsius
5: typically 16 to 330 km/h
6: typically 16 to 160 km/h
7: 330 km/h
Operational guidelines (18)
- Radar/lidar is to be setup and tested as per instructions
- Mobile radars in moving mode are to be used only in rural areas or in areas where traffic is sparse. This will ensure ease of target identification.
- The minimum detection time is three seconds for a valid radar speed check.
- At all times the instrument is to be in hold mode until a target is visually observed
- Whilst patrolling, police vehicles must maintain a reasonable, constant patrol speed
- Never check an oncoming vehicle in moving mode when travelling close to the rear of a truck, caravan or other large vehicle
- In moving mode always compare the checked speedometer of the police vehicle with the patrol speed to ensure correlation during the period of the check.
- ensure a clear audio tone is heard throughout the period of the check
- Radar/Lidar instruments are to be used on relatively straight portions of roadway
- The radar antenna is to be aimed straight ahead or directly to the rear of the police vehicle
- Accuracy of the Eagle 3 radar is plus or minus 1 km/h in stationary mode and plus 1 km/h, minus 2km/h in moving mode. A general tolerance of 2 km/h should be borne in mind when completing traffic infringement notices.
- Never attempt any repairs or allow any other person to perform any repairs to the radar/lidar instruments. In the case of faulty equipment or if the seals are broken, cease operation and forward the device to the leader, radar engineering unit.
- All instruments must be sent to the radar engineering unit for periodic calibration
- Testing at intervals not exceeding 12 months
- The radar/lidar operator at the time of the offence shall be responsible for notifying the radar engineering unit when a radar expert is required at court
- When a radar equipped vehicle is to be left unattended the antenna and mounting bracket is to be detached and placed inside the vehicle. Replace antenna prior to resuming patrol
- Radar/lidar must not be used in inclement weather
- Avoid subjecting the radar display unit to direct sunlight for extended period
When radar is not to be used
1: within 50m of a speed restriction or de-restriction sign creating a change to the speed zone being enforced unless;
a) speed is excessive (at least 20kmh in excess)
b) subject of complaint
c) high accident history
Does not apply to school zones
2: At any location or deploy a vehicle that would engender legitimate criticism or give rise to the complaint that they are a means of raising government revenue
3: when operating speed measuring instruments police should be aware of the Australian design rules pertaining to motor vehicle speedometers
Requirements for a valid speed check
Know verbatim and in order
1: visual observation and estimation of the target’s speed.
2: observe the numerals in the patrol and target displays.
3: clear and continuous tone (however pitch may vary).
4: correlation between patrol speed and checked speedometer.
5: duration of the check (steps two, three and four) must not be less than three seconds.
6: minimum of 200 metres between targets or similar size.
