Pulse-Doppler Radar: Meteorological Application
The radar (a term derived from the English acronym “radio detection and ranging”) is a system that uses electromagnetic waves to measure distances, altitudes, directions and speeds of static or mobile objects such as aircraft, ships, motorized vehicles, meteorological formations and the terrain itself. Its operation is based on the emission of a radio impulse, which is reflected in the target and is typically received in the same position as the emitter (Long, Mooney & Skillman, 1990).
The use of electromagnetic waves allows detecting objects beyond the range of other types of emissions (visible light, sound, etc.). In meteorology, the radar known as “Doppler” is used. “The meteorological Doppler radar provides estimates of radar reflectivity, mean radial velocity, and the distribution of radial velocities of the scatterers in each radar resolution cell” (Trotter, Strauch, & Frush, 1980).
Its location system is based on emitting trains of pulses at a specific frequency and using the Doppler Effect to determine the relative transverse velocity of the objects. This type of radars present ambiguity in the measurement of distances, therefore, they are not very useful for location tasks (Zrinc & Mahapatra, 1985).
The Doppler Effect is a phenomenon in which an apparent change occurs in the frequency of sound, or in the wavelength of light, which is due to the relative movement of the entity that emits it in relation to an observer (Chan, 1970). “Doppler radar systems can provide information regarding the movement of targets as well as their position. When the radar transmits pulses of radio waves, the system keeps track of the phase (shape, position, and form) of those pulses. By measuring the shift (or change) in phase between a transmitted pulse and a received echo, the target's movement directly toward or away from the radar is calculated” (National Weather Service, 2007).
The magnitude and direction of the displacement allow us to determine the movement of the targets, which may be approaching or moving away from the radar. “This ability to detect motion has greatly improved the meteorologist's ability to peer inside thunderstorms and determine if there is rotation in the cloud, often a precursor to the development of tornadoes” (National Weather Service, 2017).
The Doppler Effect has a lot of uses in different fields, in the case of sound, it is very useful in the detection of objects that are out of the visual field as it happens with the radar and in the GPS satellite location. It is also used to locate objects submerged in seas and oceans or to determine the course and speed of military targets (Bech & Chau, 2012).
Pulse-Doppler radar operation
The Doppler radar is based on the fact that in objects in which there is movement respecting to the radar, right in the component that is placed perpendicular to the direction of illumination, produce an alteration in the frequency of the electromagnetic wave when it affects them (Zrnic, 2012).
The operating principle of the weather radar is to emit, through an antenna, a pulse of electromagnetic energy of certain duration and wavelength. This energy is concentrated in a beam that when emitted to the outside and by the effect of diffraction takes a conical shape. Inside that cone, the energy is not distributed uniformly but in the form of a lobe: it is much larger in the center and decreases rapidly as it moves away from it (Green, Gage, & Van Zandt, 1979).
Due to the impossibility to concentrate all the energy in the said cone, a part of it escapes out of it. As a result, the energy emitted is distributed in the form of a central lobe (which is the one that contains most of the energy) and a series of secondary lobes of lower energy. It is similar to the reception of electromagnetic signals received by a terrestrial TV antenna. (Trotter, Strauch, & Frush, 1980).
When that energy is intercepted by a 'target' (for example, a drop of water) it is scattered in all directions, so that a fraction is returned in the direction of the radar and picked up by the receiver (usually located on the same antenna). The distance to the 'target' is determined by recording the time elapsed between the emission and reception of the energy and knowing that this energy is transmitted at the speed of light. (National Weather Service, 2017).
Objects that come close to the source will positively influence the frequency of the echo they produce, and objects that move away will have a negative influence on this relationship, with the frequency of the wave emitted by the source being greater than that of the echo. The difference between the frequencies emitted and received allow us to calculate the speed of the moving object. The calculation of the speed is based on prior knowledge of the frequency of the radar, the speed of light, the number of pulses emitted per second and the difference between the frequency emitted and the one produced by its echo (Hildebrand & Mueller, 1984).
The number that a radar measure is the return power that is converted to a quantity called "reflectivity". The magnitude of the reflectivity is related to the number and size of a water drop found by an electromagnetic pulse (National Weather Service, 2017). The reflectivity factor is obtained by summing the pulse powers, subtracting the noise power, and using the radar equation (Doviak & Zrnic, 2006).
For this reason, a high reflectivity usually implies strong precipitation, while a low reflectivity implies light precipitation. “Surveillance range is limited to about 460 km because storms beyond this range are usually below the horizon. Without beam blockage, the horizon’s altitude at 460 km is 12.5 km; thus only the tops of strong convective storms are intercepted. Quantitative measurements of precipitation are required for storms at ranges less than 230 km” (Zrnic, 2012).
Recent advances in meteorological radars have made some of their products go from the field of highly specialized personnel in electronics and meteorology, to the field of the television public where it is commonly presented in the weather reports as simplified mosaics of weather measurements coming from radar networks, such as Pulse-Doppler Radars.
Now, meteorological radars are potentially useful for innumerable professionals, businesses, and activities in a direct way, without the intermediate interpretation of a meteorologist. Pulse-Doppler radars measurements represent a great resource to facilitate the task of operational hydrologists, civil protection personnel, and hydraulic engineers associated with activities that somehow involve rainfall or any kind of weather.
Of course, its increase in utility for meteorology is undeniable, to the point of becoming one of the main tools for very short-term forecasts. The main advances that have had an impact on meteorological radars go from their incorporation of digital processing and control through generic computers instead of specialized analog circuits, to the incorporation of the ability to measure the speeds of targets through the Doppler Effect.
These features, their form of operation, their advantages, and possible applications as well as certain limitations that must be taken into account for their use.
Trotter, B. L., Strauch, R. G., & Frush, C. L. (1980). Preliminary tests of an airborne meteorological pulse Doppler radar. Geophysical Research Letters, 7(5), 361-364.
Long, W. H., Mooney, D. H., & Skillman, W. A. (1990). Pulse-Doppler radar. Radar Handbook, 2.
Zrinc, D., & Mahapatra, P. (1985). Two methods of ambiguity resolution in pulse Doppler weather radars. IEEE Transactions on Aerospace and Electronic Systems, (4).
Chan, Y. W. (1970). Equivalence of the Doppler effect, relativistic Doppler effect, and scattering effect. Physics Letters A, 33(7), 445-446.
Bech, J., & Chau, J. L. (2012). Doppler radar observations-weather radar, wind profiler, ionospheric radar, and other advanced applications. INTECH.
Zrnic, D. S. (2012). Doppler radar for USA weather surveillance. In Doppler Radar Observations-Weather Radar, Wind Profiler, Ionospheric Radar, and Other Advanced Applications. InTech.
Hildebrand, P. H., & Mueller, C. K. (1984). Evaluation of meteorological airborne Doppler radar.
Green, J. L., Gage, K. S., & Van Zandt, T. E. (1979). Atmospheric measurements by VHF pulsed Doppler radar. IEEE Transactions on Geoscience Electronics, 17(4), 262-280.