Nueve de cada diez cámaras del Centro de Gestión de Tráfico de Levante, que abarca a la Comunidad Valenciana, Murcia, parte de Albacete y de Cuenca no funcionan. Y lo mismo sucede con el 90% de los radares. "Desde que en abril finalizó el contrato de mantenimiento la situación se ha ido agravando hasta quedarnos casi a oscuras", afirma el delegado del sindicato Central Sindical Independiente y de Funcionarios (CSIF), Ezequiel Archilla.
Una portavoz de la Dirección General de Tráfico admite el problema, que fue adelantado por Las Provincias, y afirma que se está trabajando en el restablecimiento del servicio. Pero remarca que aunque es cierto que los radares no pueden transmitir, los equipos sí están registrando las infracciones, que pueden ser descargadas manualmente, de modo que aunque con algo de retraso las multas correspondientes acabarán llegando.
"Ni vemos ni podemos advertir a los conductores a través de los paneles informativos sobre las obras, accidentes o retenciones"
Un trabajador de la sala del centro de tráfico de Valencia, desde la que se controlan todas las cámaras del área de Levante, consultado por este diario señala que en pleno puente, durante el que solo en la Comunidad Valenciana se esperan 2,7 millones de desplazamientos por carretera, su función y la de sus compañeros es "prácticamente testimonial".
"Ni vemos ni podemos advertir a los conductores a través de los paneles informativos sobre las obras, accidentes, retenciones y otros percances con los que se pueden encontrar", asegura el empleado de Tráfico.
Archilla, único representante sindical en el Centro de Gestión de Tráfico de Levante, señala, además, las dificultades que se derivan para la Guardia Civil. "Nosotros somos sus ojos en las carreteras. De noche, la patrulla de helicóptero no vuela. Y la única forma que tienen de saber lo que está pasando son las cámaras y lo que los agentes puedan ver circulando por la carretera".
Los fallos generalizados en las cámaras del centro de Levante pueden observarse en la página web de la DGT. En las ventanas de las tres provincias valencianas, Albacete y Murcia en vez de una instantánea sobre la situación del tráfico aparecen sobre fondo gris mensajes con la leyenda "imagen no disponible".
En la A3, la autovía que une Madrid y Valencia, la transmisión de imágenes se recupera al llegar a Cuenca. En el resto de España, las cámaras funcionan, en general, con normalidad, según puede verse en la web de la DGT, salvo en Granada y sobre todo en Almería, donde buena parte de ellas también aparecen caídas.
Una portavoz de la Dirección General de Tráfico achaca los problemas a un corte en el cableado de fibra óptica causado por unas obras, y resalta que tanto las cámaras como los radares funcionan, si bien la información no está llegando al centro de gestión. La misma fuente añade que "algunas cámaras" dependientes del centro de Levante pueden verse desde otras dependencias de la DGT.
Tanto el delegado del CSIF como el trabajador del centro de Valencia consultado mantienen, por su parte, que los fallos en las cámaras y el resto de equipos no se han producido de forma repentina, sino poco a poco a lo largo de los últimos meses, a partir de la finalización del contrato que la DGT tenía con Indra para el mantenimiento de las máquinas de la zona de Levante, y que expiró en abril.
La portavoz de la DGT asegura que en las próximas semanas se firmará un nuevo contrato, y que hasta entonces se está realizando un "mantenimiento básico" de los equipos del Centro de Gestión de Levante.
A Doppler radar is a specialized radar that uses the Doppler effect to produce velocity data about objects at a distance. It does this by bouncing a microwave signal off a desired target and analyzing how the object's motion has altered the frequency of the returned signal. This variation gives direct and highly accurate measurements of the radial component of a target's velocity relative to the radar. Doppler radars are used in aviation, sounding satellites, Major League Baseball's StatCast system, meteorology, radar guns,radiology and healthcare (fall detection and risk assessment, nursing or clinic purpose), and bistatic radar (surface-to-air missiles).
Partly because of its common use by television meteorologists in on-air weather reporting, the specific term "Doppler Radar" has erroneously become popularly synonymous with the type of radar used in meteorology. Most modern weather radars use the pulse-Doppler technique to examine the motion of precipitation, but it is only a part of the processing of their data. So, while these radars use a highly specialized form of Doppler radar, the term is much broader in its meaning and its applications.
The Doppler effect (or Doppler shift), named after Austrian physicist Christian Doppler who proposed it in 1842, is the difference between the observed frequency and the emitted frequency of a wave for an observer moving relative to the source of the waves. It is commonly heard when a vehicle sounding a siren approaches, passes and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession. This variation of frequency also depends on the direction the wave source is moving with respect to the observer; it is maximum when the source is moving directly toward or away from the observer and diminishes with increasing angle between the direction of motion and the direction of the waves, until when the source is moving at right angles to the observer, there is no shift.
Imagine a baseball pitcher throwing one ball every second to a catcher (a frequency of 1 ball per second). Assuming the balls travel at a constant velocity and the pitcher is stationary, the catcher catches one ball every second. However, if the pitcher is jogging towards the catcher, the catcher catches balls more frequently because the balls are less spaced out (the frequency increases). The inverse is true if the pitcher is moving away from the man. He catches balls less frequently because of the pitcher's backward motion (the frequency decreases). If the pitcher moves at an angle, but at the same speed, the frequency variation at which the receiver catches balls is less, as the distance between the two changes more slowly.
From the point of view of the pitcher, the frequency remains constant (whether he's throwing balls or transmitting microwaves). Since with electromagnetic radiation like microwaves frequency is inversely proportional to wavelength, the wavelength of the waves is also affected. Thus, the relative difference in velocity between a source and an observer is what gives rise to the doppler effect.
The formula for radar Doppler shift is the same as that for reflection of light by a moving mirror. There is no need to invoke Einstein's theory of special relativity, because all observations are made in the same frame of reference. The result derived with c as the speed of light and v as the target velocity gives the shifted frequency () as a function of the original frequency () :
which simplifies to
The "beat frequency", (Doppler frequency) (), is thus:
Since for most practical applications of radar, , so . We can then write:
There are four ways of producing the Doppler effect. Radars may be:
Doppler allows the use of narrow band receiver filters that reduce or eliminate signals from slow moving and stationary objects. This effectively eliminates false signals produced by trees, clouds, insects, birds, wind, and other environmental influences. Cheap hand held Doppler radar may produce erroneous measurements.
CW Doppler radar only provides a velocity output as the received signal from the target is compared in frequency with the original signal. Early Doppler radars included CW, but these quickly led to the development of frequency modulated continuous wave (FMCW) radar, which sweeps the transmitter frequency to encode and determine range.
With the advent of digital techniques, Pulse-Doppler radars (PD) became light enough for aircraft use, and Doppler processors for coherent pulse radars became more common. That provides Look-down/shoot-down capability. The advantage of combining Doppler processing with pulse radars is to provide accurate velocity information. This velocity is called range-rate. It describes the rate that a target moves toward or away from the radar. A target with no range-rate reflects a frequency near the transmitter frequency and cannot be detected. The classic zero doppler target is one which is on a heading that is tangential to the radar antenna beam. Basically, any target that is heading 90 degrees in relation to the antenna beam cannot be detected by its velocity (only by its conventional reflectivity).
In military airborne applications, the Doppler effect has 2 main advantages. Firstly, the radar is more robust against counter-measure. Return signals from weather, terrain, and countermeasures like chaff are filtered out before detection, which reduces computer and operator loading in hostile environments. Secondly, against a low altitude target, filtering on the radial speed is a very effective way to eliminate the ground clutter that always has a null speed. Low-flying military plane with countermeasure alert for hostile radar track acquisition can turn perpendicular to the hostile radar to nullify its Doppler frequency, which usually breaks the lock and drives the radar off by hiding against the ground return which is much larger.
Doppler radar tends to be lightweight because it eliminates heavy pulse hardware. The associated filtering removes stationary reflections while integrating signals over a longer time span, which improves range performance while reducing power. The military applied these advantages during the 1940s.
Continuous-broadcast, or FM, radar was developed during World War II for United States Navy aircraft, to support night combat operation. Most used the UHF spectrum and had a transmit Yagi antenna on the port wing and a receiver Yagi antenna on the starboard wing. This enabled bombers to fly an optimum speed when approaching ship targets, and let escort fighter aircraft train guns on enemy aircraft during night operation. These strategies were adapted to semi-active radar homing.
Modern Doppler systems are light enough for mobile ground surveillance associated with infantry and surface ships. These detect motion from vehicles and personnel for night and all weather combat operation. Modern police radar are a smaller, more portable version of these systems.
Early Doppler radar sets relied on large analog filters to achieve acceptable performance. Analog filters, waveguide, and amplifiers pick up vibration like microphones, so bulky vibration damping is required. That extra weight imposed unacceptable kinematic performance limitations that restricted aircraft use to night operation, heavy weather, and heavy jamming environments until the 1970s.
Digital fast Fourier transform (FFT) filtering became practical when modern microprocessors became available during the 1970s. This was immediately connected to coherent pulsed radars, where velocity information was extracted. This proved useful in both weather and air traffic control radars. The velocity information provided another input to the software tracker, and improved computer tracking. Because of the low pulse repetition frequency (PRF) of most coherent pulsed radars, which maximizes the coverage in range, the amount of Doppler processing is limited. The Doppler processor can only process velocities up to ±1/2 the PRF of the radar. This is not a problem for weather radars. Velocity information for aircraft cannot be extracted directly from low-PRF radar because sampling restricts measurements to about 75 miles per hour.
Specialized radars quickly were developed when digital techniques became lightweight and more affordable. Pulse-Doppler radars combine all the benefits of long range and high velocity capability. Pulse-Doppler radars use a medium to high PRF (on the order of 3 to 30 kHz), which allows for the detection of either high-speed targets or high-resolution velocity measurements. Normally it is one or the other; a radar designed for detecting targets from zero to Mach 2 does not have a high resolution in speed, while a radar designed for high-resolution velocity measurements does not have a wide range of speeds. Weather radars are high-resolution velocity radars, while air defense radars have a large range of velocity detection, but the accuracy in velocity is in the tens of knots.
Antenna designs for the CW and FM-CW started out as separate transmit and receive antennas before the advent of affordable microwave designs. In the late 1960s, traffic radars began being produced which used a single antenna. This was made possible by the use of circular polarization and a multi-port waveguide section operating at X band. By the late 1970s this changed to linear polarization and the use of ferrite circulators at both X and K bands. PD radars operate at too high a PRF to use a transmit-receive gas filled switch, and most use solid-state devices to protect the receiver low-noise amplifier when the transmitter is fired.
Main article: Radar navigation
Wind speed correction
Doppler radars were used as a navigation aid for aircraft and spacecraft. By directly measuring the movement of the ground with the radar, and then comparing this to the airspeed returned from the aircraft instruments, the wind speed could be accurately determined for the first time. This value was then used for highly accurate dead reckoning. One early example of such a system was the Green Satin radar used in the English Electric Canberra. This system sent a pulsed signal at a very low repetition rate so it could use a single antenna to transmit and receive. An oscillator held the reference frequency for comparison to the received signal. In practice, the initial "fix" was taken using a radio navigation system, normally Gee, and the Green Satin then provided accurate long-distance navigation beyond Gee's 350-mile range. Similar systems were used in a number of aircraft of the era, and were combined with the main search radars of fighter designs by the 1960s.
Doppler navigation was in common commercial aviation use in the 1960s until it was largely superseded by inertial navigation systems. The equipment consisted of a transmitter/receiver unit, a processing unit and a gyro stabilised antenna platform. The antenna generated four beams and was rotated by a servo mechanism to align with the aircraft's track by equalising the Doppler shift from the left and right hand antennas. A synchro transmitted the platform angle to the flight deck, thus providing a measure of 'drift angle'. The ground speed was determined from the Doppler shift between the forward and aft facing beams. These were displayed on the flight deck on single instrument. Some aircraft had an additional 'Doppler Computer'. This was a mechanical device containing a steel ball rotated by a motor whose speed was controlled by the Doppler determined ground speed. The angle of this motor was controlled by the 'drift angle'. Two fixed wheels, one 'fore and aft' the other 'left to right' drove counters to output distance along track and across track difference. The aircraft's compass was integrated into the computer so that a desired track could be set between two waypoints on an over water great circle route. It may seem surprising to 21st. century readers, but it actually worked rather well and was great improvement over other 'dead reckoning' methods available at the time. It was generally backed up with position fixes from Loran, or as a last resort sextant and chronometer. It was possible to cross the Atlantic with an error of a couple of miles when in range of a couple of VORs or NDBs. Its major shortcoming in practice was the sea state, as a calm sea gave poor radar returns and hence unreliable Doppler measurements. But this was infrequent on the North Atlantic
Location-based Doppler techniques were also used in the U.S. Navy's historical Transit satellite navigation system, with satellite transmitters and ground-based receivers, and are currently used in the civilian Argos system, which uses satellite receivers and ground-based transmitters. In these cases, the ground stations are either stationary or slow-moving, and the Doppler offset being measured is caused by the relative motion between the ground station and the fast-moving satellite. The combination of Doppler offset and reception time can be used to generate a locus of locations that would have the measured offset at that intersects the Earth's surface at that moment: by combining this with other loci from measurements at other times, the true location of the ground station can be determined accurately.
- Luck, David G. C. (1949). Frequency Modulated Radar. New York: McGraw-Hill.
- Liu, L; Popescu, M; Skubic, M; Rantz, M; Yardibi, T; Cuddihy, P (May 23–26, 2011). "Automatic Fall Detection Based on Doppler Radar Motion". Proceedings, 5th International Conference on Pervasive Computing Technologies for Healthcare. Dublin, Ireland. pp. 222–225. Lay summary.
|Acoustic, sound, vibration|
|Electric, magnetic, radio|
|Flow, fluid velocity|
|Optical, light, imaging|
|Force, density, level|
- ^CopRadar.com -- subsidiary of Sawicki Enterprises (1999–2000). "Police Traffic Radars". CopRadar.com -- subsidiary of Sawicki Enterprises. Retrieved July 17, 2009.
- ^L.L., M.P., M.S., M.R., etc (2011). "Automatic fall detection based on Doppler radar motion signature". IEEE PervasiveHealth.
- ^M. Mercuri, P. J. Soh, G. Pandey, P. Karsmakers, G. A. E. Vandenbosch, P. Leroux, and D. Schreurs, "Analysis of an indoor biomedical radar-based system for health monitoring," IEEE Trans. Microwave Theory Techn., vol. 61, no. 5, pp. 2061-2068, May 2013.
- ^CopRadar.com -- subsidiary of Sawicki Enterprises (1999–2000). "Doppler Principles (Police Traffic Radar Handbook)". CopRadar.com -- subsidiary of Sawicki Enterprises. Retrieved July 17, 2009.
- ^Ditchburn, R.W. "Light", 1961, 1991. Dover publications Inc., pp331-333
- ^Jaffe, Bernard M., "Forward Reflection of Light by a Moving Mirror," American Journal of Physics, Vol. 41, April 1973, p577-578
- ^Ridenour, "Radar System Engineering", MIT Radiation Lab series, vol 1, year 1947, page 629
- ^"Ground Surveillance Radar Section". 1st Battalion 50th Infantry Association.
- ^"AN/SPG-51 Gun and Missile Fire Control Radar". Jane's Information Group.
- ^John Barry, "Doppler Navigator Development", Friends of the CRC, 17 September 1973