The RADAR (RAdio Detection And Ranging)
Police radar (RAdio Detection And Ranging) guns determine vehicle speed by sending microwaves towards a vehicle and determining their frequency shift upon reflection.
A police speed enforcement radar gun works by sending microwaves towards a vehicle. The vehicle reflects the microwaves and sends them back to be received by the gun detector. We can expect that as the vehicle moves towards the radar, it reflects/pushes off more waves in the unit of time than what the gun had sent in the first place. It can help to refer to the mechanical wave analog of ripples in a pond generated by a pebble and us walking towards it or to the sound wave analog. As a result the frequency of the received waves will be shifted. This effect is called the Doppler effect.
As mentioned at https://en.wikipedia.org/wiki/Radar_gun, modern radar speed guns normally operate at the X band (8 to 12 GHz), Ku band (12-18 GHz) in Europe, K band (18 to 27 GHz) and Ka band (27 to 40 GHz).
Video: https://youtu.be/R2dgDedg9BE
The LIDAR (LIght Detection And Ranging)
Gradually, radar guns (RAdio Detection And Ranging) are being replaced by lidar guns (LIght Detection And Ranging) which use lasers.
In accordance with what is mentioned at https://en.wikipedia.org/wiki/LIDAR_traffic_enforcement
A typical US Dept of Transportation approved lidar device emits 30 ns pulses of laser light with a 905 nm wavelength and 50 milliwatts of power. These parameters ensure that no ocular damage occurs. "Light travels approximately 30 cm per ns so each pulse has a length of about nine metres. At a target distance of 300 meters the light pulses take 2,000 ns to complete the round trip. The time interval between pulses is no less than one million ns, providing time to make a distance estimation from each pulse. Up to several hundred pulse readings are taken over a period less than half a second and used to estimate the change in distance over time, thereby estimating vehicle speed. Returning light is filtered to exclude light not in the wavelength range 899 nm to 909 nm. An internal proprietary algorithm rejects inaccurate readings; detection avoidance methods usually attempt to overload the filter and persuade the error rejection algorithm to incorrectly reject a reading."
Video: https://youtu.be/GB16prlxz8A
Weather radars and Air route surveillance radars
Phased array radars to replace weather radars and Air route surveillance radars
https://upload.wikimedia.org/wikipedia/commons/6/6d/Polarimetric_Radar.gif
Phased Array Radars (Phased Arrays)
https://eiscat3d.se/drupal/sites/default/files/EISCAT_Radar_School_2012/11_Chau_Phased_Arrays.pdf
> Definition
A phased array is a group of antennas whose effective (summed) radiation pattern can be altered by phasing the signals of the individual elements.
> By varying the phasing of the different elements, the radiation pattern can be modified to be maximized/suppressed in given directions, within limits determined by the radiation pattern of the elements, the size and the configuration of the array.
> History / Technology
Originally developed during WWII for aircraft landing.
Now used for a plethora of military applications.
Applied to radio astronomy in 1950’s.
> Some benefits of phased arrays
>> Impact on ionospheric research: (…)
> Non-ionospheric scientific benefits
Radio astronomy - affordable way to achieve spatial resolutions of a few arc minutes or better. Aperture & real-estate directly associated with cost of system. E.g., consider a square kilometer dish versus a square kilometer array.
> Non-scientific benefits
Conformity of a phased array to the “skin” of a vehicle/ aircraft
Surveillance/tracking - can both survey and track 1000s of objects
Communication/downlink? - small satellites
Images:
“Phased Array, λ/2 spacing” Image on slide 8
> Phased Array Radars
•Jicamarca (Peru, built by the CRPL lab that became part of NOAA)- Phased array with very large collecting area, but (a) “Passive”, (b) Modular but not portable, (c) Fixed pointing
(The Jicamarca Radio Observatory (JRO) is the equatorial anchor of the Western Hemisphere chain of Incoherent Scatter Radar (ISR) observatories extending from Lima, Peru to Søndre Strømfjord, Greenland).
•MU Radar (Japan)- Active phased array, but not good for IS
“known as the most capable atmospheric radar in the world” http://www.rish.kyoto-u.ac.jp/English/MU/
•AMISR - Advanced Modular Incoherent Scatter Radar
“Modern” Incoherent Scatter Radar constructed by the NSF
PFISR, RISR-N, RISR-C
http://amisr.com/amisr/about/amisr-overview/
“The first AMISR face was deployed in Poker Flat, Alaska in 2006 (the Poker Flat ISR, or PFISR), and the remaining two faces have been deployed in Resolute Bay, Nunavut, Canada (RISR-N and RISR-C).”
The Advanced Modular Incoherent Scatter Radar (AMISR) Historical Perspectives
https://www.nsf.gov/bfa/lfo/seminars/pub/ROBINSONLFWAMISRtalk.pdf
The Doppler Effect for Sound Waves
A. Let us consider this case: Stationary Source, Observer (Detector) Moving (towards the Source)
The source emits sound wavefronts similar to concentric circles or ripples in a pond with a frequency f as shown in the figure.
When we move towards the source, how frequently are wavefronts intercepted by our ear?
In other words what is the frequency or rate by which wavefronts are intercepted by our ear during our displacement?
It corresponds to the number of wavefronts intercepted in the unit of time.
It also corresponds to the number of wavefronts represented in our displacement.
The wavefronts move and we move as well. We have to add up the numbers that we would get from the two independent displacements.
Let us add up the displacements: S+SD= vt+vDt
Let us divide by the wavelength to find the number of wavelengths in the combined displacement: (vt+vDt)/l
In order to reply to the question “what is the frequency by which wavefronts are intercepted by our ear?” consider the above result in the unit of time:
f'=[(vt+vDt)/l]/t=(v+vD)/l
Because l=v/f
f'=f (v+vD)/v
We conclude that the frequency by which our ear intercepts the waves is shifted. This shift is called Doppler shift.
Figure: Stationary Source (S), Observer (Detector) represented by "ear" moving towards the source (to the left)
B. Let us consider this case: Moving Source, Stationary Observer (Detector)
When the source moves towards us, how frequently are wavefronts intercepted by our ear?
In other words what is the frequency or rate by which wavefronts are intercepted by our ear during the displacement of the source?
It corresponds to the number of wavefronts intercepted in the unit of time.
It also corresponds to the number of wavefronts represented in the displacement of the source s=vt.
Let us divide by the wavelength to find the number of wavelengths represented in the displacement: vt/l'.
In order to reply to the question “what is the frequency by which wavefronts are intercepted by our ear” consider the above result in the unit of time:
f'=(vt/l)/t=v/l'
In the above equation the wavelength is, as we know, the distance between the concentric circles shown in the figure that follows.
When the source was at position S1, it emitted wavefront W1.
When at S2, it emitted W2.
(When at S3, it emitted W3.)
Let us consider that at t=0 the source is at S1 and it emits W1.
At t=T, the source is at S2 and it emits W2.
At t=T, wavefront W1 has moved by vT “to the right”. That would be the wavelength we would measure for a stationary source as shown in the previous image.
At t=T, the source has moved by vST “to the right”. It is as if it got closer to the recently emitted wavefront W1. At this time it also emits W2 and therefore the W2 will be closer to W1 than it would have been if the source was not moving. The distance between W2 and W1 constitutes the new wavelength. We can see that it is smaller than the previous one if we compare the two wavelengths in the figure below.
To find the resultant wavelength we have to substract one displacement from the other.
f'= v/l’=v/(vt-vST)=v/(v/f’-vS/f’)=f’[v/(v+vS)]
We conclude that the frequency by which our ear intercepts the waves is shifted. This shift is called Doppler shift.
Figure: Moving Source (moving to the right), Stationary Observer (Detector) represented by "ear"
Figure: The two images shown above are provided here side by side for comparison.
The Doppler Effect for Electromagnetic Waves
The Doppler Effect for Light Waves
« Principles of Physics » 10th International Edition by David Halliday, Robert Resnick, Jearl Walker Module 37-5 The Doppler Effect (page 1026)
What is Geiger Mode LIDAR? (non-linear LIDAR)
https://www.pobonline.com/articles/100949-what-is-geiger-mode-lidar
0min35s] "In conventional (linear) LIDAR you take a large energy pulse (flash) and you send quadrillions of photons and you measure what percentage of photons comes back in that one flash.
Geiger mode LIDAR is based on very low energy pulses, at very high repetition rates. So instead of measuring what percent of photons come back in a single flash, you are measuring what percent of
flashes register return."
The notion of "Low per-pulse probability of detection" in Geiger-mode LIDARS (non-linear LIDARS)
1. Use of little energy per pulse, or few photons: that means there is a low probability that you are detecting accurately where they are returning from (target)
2. Use of multiple pulses: obtaining high probabilities of detection of a target surface
The Relative Nature of Low Probability of Detection Radar (LPD Radar)
p.14 [3.3.4 Noise Based Transmissions]
"Radars which use noise based waveforms transmit aperiodic signals that are as spectrally similar to white noise as possible. Their noise-like signals are typically spread evenly across their
entire frequency bandwidth and, due to their random transmission scheduling, there are not easily detected by standard spectral analysis methods. Since each pulse is unique, standard pulse
compression techniques will not work. Instead, the radar must retain a copy of each pulse for correlation tests against each returned signal. Noise based waveforms can be thought of as ultra-long
coded pulses composed of very short bit lengths which are unique to each transmitted pulse."
p.19 [3.1.3 Parasitic Radars]
"Parasitic radars (which include bi-static and tripwire systems) use the emissions of non-associated transmitters such as radio and television broadcasts to detect and track targets [5]."
Scientists collaborate to increase the accuracy of optical radar
Scientists established an international consortium to implement new approach to increase the accuracy of optical radar's function
https://www.eurekalert.org/pub_releases/2017-01/ptgs-sct012617.php
facebook post by @spbstu.official
https://www.facebook.com/spbstu.official/posts/1539228959421361:0
Key technologies for millimeter-wave distributed RADAR system over a radio over fiber network
Feasibility analysis of WDM links for radar applications
http://www.sciencedirect.com/science/article/pii/S2214914714000981
Excerpts
"Active phased array antennas enhances the performance of modern radars by using multiple low power transmit/receive modules in place of a high power transmitter in conventional radars."
"This also includes performance evaluation of non-linear frequency modulation (NLFM) signals, known for better signal to noise ratio (SNR) to specific side lobe levels. NLFM waveforms are further
analysed using pulse compression (PC) technique. Link evaluation is also carried out using a standard simulation environment and is then experimentally verified with other waveforms like RF
continuous wave (CW), pulsed RF and digital signals. "
BAE Systems - Remote Sensing
Resources
Remoting sensing courses by ESA (ESA Climate Office)
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