See also the brilliant wu by Yurei under RADAR
Ever been caught speeding by a Police radar gun? Ever wanted to know how fast you can pitch a cricket ball or how fast you can ride your bike? Ever watched those moving maps of the rainfall across the country?
If you answered “yes” to any of the above, then you know something about Doppler radar.
Basically, Doppler radar is a good way of measuring the speed of motion of an object as it moves toward and away from the radar source. It works by combining the Doppler effect with a fairly standard pulsed radar unit. Like most radar systems, it works best on objects larger than 5-10 centimetres in size, as the most effective radio frequencies tend to be between 1 and 10 cm in wavelength (see band definitions below).
The chief applications of Doppler radar fall into two main categories: large, bulky professional systems for measuring the speed of clouds and precipitation, used in weather forecasting and analysis, and smaller, hand-held systems used by Police and sports coaches to measure the speed of objects such as cars and trucks, or sports balls. Many of the cheaper systems in the latter category are not true Doppler radar systems at all—more on this later.
Advances in radar techniques, such as phased array systems, polarised radar and more accurately focussed beams, have improved weather analysis to the point where modern forecasters can identify small tornados forming within a large cloud mass and obtain data on precipitation motion (and hence wind speeds), both in the direction of the radar pulse, and perpendicular to it.
Doppler radar is used on a national scale to obtain wind speed data in the regions (typically a few hundreds of km) surrounding each of a number of static weather stations. Each station gives only the wind speeds in the radial direction. This information is used routinely in weather forecasting and monitoring, and is the source of those animated weather maps shown on TV forecasts. Although individual raindrops are usually quite small, radar can detect large sheets of rain (or snow) quite easily, because a lot of small radar targets behave more or less the same as one large one.
Measuring the speed of cars or sports balls
For a few hundred dollars you can buy a radar gun, which is designed to measure the speed of a sports ball: a tennis ball during serve, or a baseballwhen it is pitched. These are used routinely by sports coaches to assess the improvements in performance by their athletes.
The gun is a basic system, based on pulsed radar technology. The gun sends out a stream of radar pulses, each of which might be 1 microsecond long, repeated at 1 millisecond intervals. Thus, the gun only emits radar energy for about 0.1 percent of the time it is being used. Each pulse flies out of the gun, encounters an obstacle (such as a tennis ball) and is reflected back, as an echo. The gun measures the time difference between the outgoing pulse and receipt of the echo. This gives a measure of the range of the ball.
There are two ways to get at the speed of the ball. The easiest is to take consecutive pulses, and see how the distance changes with time.
The more sophisticated way is to measure the change in frequency in the reflected radar pulse as it arrives back at the gun.
Cheaper radar systems tend to use the former method. It is reasonably accurate, as a lot of pulses can be sent out in a short time and the differences can be averaged to get a good measure of speed.
More sophisticated systems use the latter method—which is the true Doppler-radar technique. As the radar pulse is fired, it has a known frequency. When it is reflected from a stationary object, then the pulse comes back with the same frequency, but if it the target is moving away from the gun, then the reflected wave is at a slightly longer wavelength (lower frequency) than the outgoing pulse. Of course, if the target is moving toward the gun, then the reflected pulse is at a higher frequency than the outbound pulse.
Early Police radar guns tended to use the true Doppler-radar technique, but newer guns use a laser system. These are not Doppler-based, but calculate speed by measuring the range of the object at a series of intervals. These laser-based systems are more accurate than the radar-based technologies, as the laser beam remains compact, even at a distance, whereas the radar beam tends to spread out, allowing objects apart from the target to confuse the measurement.
Looking at the weather
The trouble with radar in terms of weather systems is that radar can only measure motion in the line joining the radar source with the target. If there is any motion across that direction, either vertically or horizontally, then it simply does not register on the radar picture. What radar is very good at is in getting a very broad field view of the weather conditions, and this more than makes up for its limitations in other areas.
However, this ability only to see motion in the radial direction was a severe limitation in first-generation radar systems and the scientists and weather forecasters have thought hard about how to get a better idea of the total wind field. There are three basic approaches to this.
The first is relatively simplistic and uses simple weather models to guess what the other (horizontal) component of velocity is doing. There are two main standard models Volume Velocity Processing (VVP) and (extended) Velocity-Azimuth Display (E)VAD. These can give an estimate of the kinds of wind patterns that are likely to occur, based on the radial data from a single Doppler radar station.
The second method uses a second or third Doppler radar station and some heavy computing power to give actual speeds over the ground. These methods are called double-Doppler (from 2 stations) and triple-Doppler (three stations). If you probe any given patch of sky with two radar beams from different locations, then you can work out exactly how the rain is moving in two horizontal axes with relatively simple geometrical methods. A third beam will improve overall accuracy, and may give data in the vertical direction as well.
The third method is to use very sophisticated radars, such as phased-array systems and other modern techniques to get information about the circumferential speeds from a single weather radar. These tend to be expensive and experimental at present. One of the most promising approaches is to use a series of passive transponders at geographically separate locations to pick up multiple echos from a single transmitted pulse.
Where it is important to measure small, localised wind speeds (such as on an airfield), laser-doppler anemometry offers greater accuracy and higher resolution. Laser-Doppler systems use exactly the same principles of operation as Doppler-radar, but are usually based on CO2 lasers operating in the infra-red region. Infra-red light passes through clouds better than visible light, but is absorbed by water vapour, so laser-based systems do not have the same range as microwave radars. Lasers, operating at very short wavelengths rely on reflections from tiny aerosols: dust particles floating in the air.
Radar bands and an explanation of where they are usedCut and pasted from http://www-cmpo.mit.edu/Radar_Lab/FAQ.html
L-Band: 1-2 GHz, 15-30 cm wavelength. Mostly used for clear-air turbulence studies.
S-Band: 2-4 GHz, 08-15 cm wavelength. Used for long- and short- range weather surveillance. The WSR-88D's (Nexrad) are S-band. Not easily attenuated but require large dishes and motors. /p>
C-Band: 4-8 GHz, 04-08 cm wavelength. Used for short-range weather surveillance (e.g., near airports). Portability means they're often used in research field programs. Nice tradeoff between X- and S-Bands. Nearby bands are often used for microwave communications links. /p>
X-Band: 8-12 GHz, 2.5-4 cm wavelength. Used for very short-range work; very sensitive to smaller particles and thus useful for studies of early cloud development. However, attenuated rapidly as they pass through storms. Share some space with police speed radar. /p>
K-Band: 12-18, 27-40 GHz, 1.7-2.5, .75-1.2 cm wavelength. Actually two bands, split down the middle by a strong water vapor absorption line. Similar comments as with X-bands, above. Also share space with police radar.
Sources / further reading