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, 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).






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


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."





Image from video (


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Weather radars and Air route surveillance radars 

“The reason that the whole United States is covered with s-band radar, it’s probably safe to say, [is] because of Polly Austin. In Europe, there are many networks of c-band radars, but Polly knew that if you wanted accurate [rain] measurements, you had to go with s-band,” said Williams.
(Excerpt from:
"Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the intensity of the precipitation."
NEXRAD or Nexrad (Next-Generation Radar) is a network of 159 high-resolution S-band Doppler weather radars operated by the National Weather Service (NWS).
Its technical name is WSR-88D, which stands for Weather Surveillance Radar, 1988, Doppler.
NEXRAD sites in the US
Dual polarization mode of operation
Interesting infographic


Phased array radars to replace weather radars and Air route surveillance radars 

Multi-function Phased Array Radar (MPAR)
"Beyond dual-polarization, the advent of phased array radar will probably be the next major improvement in severe weather detection. Its ability to rapidly scan large areas would give an enormous advantage to radar meteorologists[30]. Its additional ability to track both known and unknown aircraft in three dimensions would allow a phased array network to simultaneously replace the current Air Route Surveillance Radar network, saving the United States government billions of dollars in maintenance costs [30][31]. Any large-scale installation by the NWS and Department of Defense is unlikely to occur before 2020. The National Severe Storms Laboratory predicts that a phased array system will eventually replace the current network of WSR-88D radar transmitters [32]."
"The Air Route Surveillance Radar is used by the United States Air Force and the Federal Aviation Administration to control airspace within and around the borders of the United States."
Type General Surveillance: 3D radar
Frequency: L band




Phased Array Radars (Phased Arrays)


> 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




“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”


•AMISR - Advanced Modular Incoherent Scatter Radar

 “Modern” Incoherent Scatter Radar constructed by the NSF


“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




The Doppler Effect for Sound Waves

Why a police car siren sounds differently (different frequency or pitch) when it comes towards us, when it is stationary, and when it goes away from us.
Frequency change termed Doppler shift.
Example at this video (time point 1:13 selected)


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:



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:



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.




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Relevant resource

YouTube Video



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)


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
facebook post by @spbstu.official



Key technologies for millimeter-wave distributed RADAR system over a radio over fiber network




Feasibility analysis of WDM links for radar applications

"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

Sonar system and “Laser Developed Atmospheric Lens (LDAL)"

Sonar system
From National Defense Magazine:
DARPA: “Sound that is projected will be scattered, producing reverberation and signal loss. Scattered sound may inadvertently illuminate the host submarine and possibly compromise stealth. For this reason, detailed and accurate predictions of the acoustic environment are important to manage the sonar and potential exposures.”
“Laser Developed Atmospheric Lens (LDAL)"
Why do we see a (green) tree in sunlight? Because light rays (electromagnetic waves) fall on the tree, the tree reflects them and they travel towards our eyes. (The tree actually absorbs everything but green).
Why can’t we see the tree well in dim light? Because there is not enough reflected light arriving at our eyes.
Desert mirage or why do we see blue lakes in the desert? Because the light from the blue sky is “bent” or “refracted” by the hot air near the surface and travels towards our eyes.
Similarly a sensor on a plane or a satellite cannot see/detect well because the light or electromagnetic signals in general do not arrive in good quality or quantity.
BAE Systems has developed a way to heat locally an area in the atmosphere/ionosphere using lasers so that light/electromagnetic radiation is well focused or mirrored and allows remote detection/sensing of objects. We are talking about suitable focusing of rays, or reflection, refraction and diffraction and therefore the concept is equivalent to creating an atmospheric lens with lasers.
Refer to video of the link below at time point 0:30
Similarly, the concept can be used to deflect a surveillance laser of this kind.
Another video:








Remoting sensing courses by ESA (ESA Climate Office)