How Does Doppler Radar Work?

The Doppler effect, is a scientific discovery that is utilized in a variety of ways. Despite the fact that at first sight the scientific breakthrough appears to be somewhat impractical, it is still used in many ways.

The Doppler effect is all about waves, which are caused by things that generate them (sources), as well as the things that receive them (observers). It basically says that if the source and observer are moving with respect to each other, the frequency of the wave will be different for both of them. This implies it’s a type of scientific relativity.

Doppler radar works by emitting a pulse of radio waves and then measuring the Doppler effect caused by the object or person it is detecting. This method is not only accurate in detecting surface movement of the human body, but can also wirelessly sense heart rate, respiration, and other vitals. Additionally, larger movements can be detected with this technology, as well as speed and direction.

There are two areas where the Doppler radar idea has been used practically. The first is law enforcement use of “radar guns” to measure the speed of a car.

The Pulse-Doppler radar is probably most commonly known from hearing about it during weather reports. It tracks the speed of precipitation, giving people up-to-date information on current conditions.

The use of Pulse-Doppler is showing practical use in the medical field as a way to conduct no-contact vital sensing.

Doppler Radar Used in Police Radar Gun

By sending a fine-tuned beam of electromagnetic radiation waves at a moving target, Doppler radar is able to take readings. Although you’re technically able to use Doppler radar on stationary objects, the results are not very interesting unless there’s movement.

When electromagnetic radiation is reflected off of a moving object, it “bounces” back to the source, which includes a receiver as well as the original transmitter. Because the wave was altered by the reflecting object, according to relativistic Doppler effects.

The returning wave is treated as an entirely new wave, emitted by the target it bounced off of. In other words, the target acts as a new source for this incoming wave. When received back at the gun, though, this wave has a frequency different from when it was originally sent out.

Because the electromagnetic radiation was at a specific wavelength when it was emitted and is back to that same frequency upon return, this can be used to calculate the target’s velocity, v.

This is the deciding factor on if you get a ticket or not, aside from not speeding of course.

Weather Doppler Radar or Pulse-Doppler Radar

The phrase “Doppler radar” has become popularly linked to the kind of radar utilized in meteorology because it is frequently employed by television weather forecasters in on-air weather reporting.

Although modern weather radars largely rely on Pulse-Doppler technology to study precipitation movement, there is much more that goes into processing their data. Thus, while these types of radars certainly use Doppler radar principles, the term “Doppler radar” encompasses a far broader range of applications and meaning.

The Pulse-Doppler radar system can not only calculate linear velocity, as a traditional radar gun would, but also radial velocities. It does this by sending pulses instead of beams of radiation. The shift in frequency and carrier cycles allows for the calculation of radial velocities.

It’s necessary to exercise tight control over the radar system in order to do so. The system must be in a coherent state, which allows for the maintenance of the phases of radiation pulses. This has a disadvantage in that the Pulse-Doppler method can’t detect radial velocity above a certain speed.

To grasp this idea, let’s look at a situation where the measurement shifts the phase of the pulse by 500 degrees. This is mathematically equivalent to a shift of 50 degrees because it has circled all the way around (a full 360°). The speeds that cause these types of shifts are called “blind speed.” Its function depends on altering the signal’s pulse repetition frequency, which meteorologists can control to some extent.

The US Weather Bureau’s first experimental Doppler weather radar unit was obtained from the US Navy in the 1950s.

Development of the Doppler function for weather radar has been underway in many countries for quite some time. In June 1958, researchers David Holmes and Robert Smith from America discovered they could detect rotation of a tornado using mobile continuous-wave radar.

The National Severe Storms Laboratory (NSSL) which was formerly Norman’s laboratory, optimized this radar by making it a pulsed Doppler radar. This allowed for easier detection of echo positions and increased power overall.

After the 1974 Super Outbreak in the United States, which saw 148 twisters strike thirteen states, their work was accelerated.

The reflectivity-only radar of the time could only locate precipitation within thunderclouds, but not mesocyclonic rotation (a large scale circulation of winds) or downbursts (a strong downdraft that ushers in a microburst). The NSSL Doppler became operational in 1971 and as a result, the NEXRAD network was deployed by the end of the 1980s.

Long range and high speed capability are combined in Pulse-Doppler radars to create a unique advantage. Pulse-Doppler radars use a medium to high PRF (on the order of 3 to 30 kHz), which enables them to detect either high-speed targets or precise velocity readings.

Most radars are created to serve a specific purpose–either detecting targets from zero to Mach 2 or measuring velocity with high resolution.

How Accurate Are Doppler Radars?

The amount of energy an object reflects back to the radar reveals the intensity of precipitation. The time it takes for the pulse to hit an object and come back to the radar helps to determine where that object is located.

After measuring the intensity and location of an object, you can then map it. What sets a Doppler radar apart is its ability to tell which direction the object is moving. To track this movement, you can look at the phase of the radio wave (its shape, position and form). By clocking how long it takes for the pulse sent out to return back tells you if an object is coming closer or going further away from the radar.

Meteorologists use Doppler radar to track severe thunderstorms or rotation in supercells that could lead to tornadoes. In recent years, National Weather Service Doppler radars have been upgraded to include dual-polarization which transmits and receives pulses in both a horizontal and vertical orientation.

The radio waves frequencies hit the objects and provide their horizontal and vertical dimensions.

There is multiple benefits to be gained from dual-pol radars:

  • Differentiate between precipitation types – hail, snow, sleet and even heavy rain
  • Debris detection from tornados
  • Overall improved accuracy of the estimated accumulation from precipitation

The National Weather Service uses Doppler radar to scan the sky at various angles. By doing this, they can determine the location and intensity of precipitation within different levels of the atmosphere.

On average, this process is completed every 4-6 minutes.

From a weather standpoint, the earth’s curvature limits radar’s effectiveness. The pulse flies higher in the sky as it travels farther.

Most precipitation can be detected by Doppler radar within 100 miles of its location.

There are 159 National Weather Service radars located across the United States to provide as much coverage as possible. Even with all of these, there are still some areas without proper coverage. The power emitted by Doppler radar pulses is 450,000 watts on average. For comparison, a typical home microwave emits 1,000 watts.

Although each pulse is extremely quick, the total accumulated time the radar is transmitting per hour is only 7 seconds.

Doppler Radar Used for Noncontact Sensing

Doppler radar sensors operating at 24 GHz have the potential to pave the way for wireless, non-invasive sensing devices that could be used in a number of healthcare and wellness applications related to vital sign monitoring.

The Doppler method is not only accurate in detecting surface movement of the human body, but it can wirelessly sense heart rate, respiration, and other vitals. Additionally, larger movements can be detected with this technology, as well as speed and direction. All of these factors make the Doppler method an excellent choice for fall detection systems.

A Doppler radar sensor can detect minute bodily motions. Although a heart beat takes place internally, blood pressure changes at the body’s surface may be seen as a heartbeat occurs. These movements, while only a few millimeters per heartbeat on average, are an involuntary action that translates to a greater Doppler effect, which is received by an outside circuit and host system software.

Respiration can also be detected at the body’s surface with movements larger than a heartbeat. Respiration is primarily a slow motion, and it may be identified using the Doppler effect, but it might also need to use another parametric feature of the reflecting signal.

Doppler radar sensors can penetrate clothing, curtains, glass, and most wooden structures more readily than other types of detection technologies such as infrared, laser scan, and ultrasound. Doppler radar sensors are less susceptible to external factors like weather conditions, noise levels, dirt particles in the air or on people’s skin, temperature extremes or low illumination levels.

Designing an interference tolerant capability from nearby electronic equipment is simple for engineers using a 24 GHz Doppler radar sensor that supports multiple channels and has a sufficient output signal range (~1Hz to 1 MHz) with very low I/F noise. With these capabilities, they can achieve high detection sensitivity.

The detection signal by default captures and reveals physical radio wave reflections, but no personal information. This is to address patient privacy and security issues, which are concerns that have been increasing in our connected society.

Conclusion

Doppler radar is a powerful tool that can be used for a variety of purposes, from weather forecasting to non-invasive sensing in healthcare. Utilizing the Doppler effect has provided it many advantages over other types of detection methods including other radars, making it an excellent choice for a variety of applications.

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