be fairly short so the transmitter could be switched off, and the receiver switched on, before the transmitted pulse returned from its trip out to the target and back (see Radar—Transit time
for a more detailed explanation). This meant that the total energy sent and reflecting off the target was reduced. The pulses could be extended to return more energy, but this reduced the range. Another problem was that the pulses would reflect off of any solid object, including the ground, so they had to be pointed up in order to detect airborne targets — allowing aircraft to escape detection close to the ground. While this was only a minor problem for ground-based radars, aircraft radars could not see targets below them. Using the Doppler
effect allows both of these problems to be avoided. Instead of sending out pulses, the radio signal is continuous, thereby maximizing the amount of energy returned from the target. For this reason the system was often referred to as continuous-wave radar when it was first introduced. The target is "seen" because the returned signal will be frequency shifted due to the Doppler effect, allowing it to produce a target frequency (usually audio) after being mixed with a sample of the transmit frequency. Since the amount of shifting is dependent on the relative speed of the target, the minimum detectable speed is a function of the narrowness of the filtering the equipment is capable of.
In aircraft use, the filters can be set to filter out any signal with the exact same speed as the aircraft, thereby filtering out the reflection from the ground. This allows the radar to look straight down, detecting aircraft that were formerly invisible. As with pulse radar systems, many Doppler systems also pulse their signal to allow the use of a single antenna in these roles.
Since the Doppler system requires a speed difference between the antenna and target in order for there to be a phase shift to detect, it is possible to "spoof" them by flying parallel to the radar, or laterally "across the front". For this reason most aircraft radar systems use both the returned pulse and the Doppler shift to detect targets.
Doppler radar as weather radar
A simple weather radar can detect precipitation or objects just by the reflection of microwaves. Most weather radars employ pulsed microwave signals. With more precipitation or a bigger object, there is more reflectivity. As in the case with heavy rain or hail, more signal is reflected back to the radar dish. The reflectivity of an object is measured in dBZ, or decibels of Z.
, the Doppler effect becomes especially useful. However, strickly speaking, the Doppler shift is too small with meteorological targets' speed to note the Doppler shift itself of a continous emitting beam. Instead, what is used is a comparision of the phase between successive pulses, from a pulsed radar, coming back from the same targets. In wave mechanics
, the intensity of the returned signal is:
A = A0Sin ( (distance ΔTime) + ΔΘ)
So if the rain dropplets in the scanned volume move ever so slightly between pulses the ΔΘ will be greater than zero and will give their mouvement. The velocities that can be detected by a single dish are then velocities directed away from the dish or toward the dish (see vector mechanics). This product is known as radial velocity. Another derived product uses radial data to calculate the storm cell's propagation speed which is then subtracted from the radial velocity. This yields storm relative velocity which is useful for identifying rotation in supercell thunderstorms.
Even though most weather radar has the ability to collect Doppler wind velocities, it is usually not used for display to the public since it is difficult for even the most experienced meteorologist to quickly understand. Typically, research meteorologists depend more heavily on the Doppler data for wind vector retrieval. Doppler radars collect reflectivity too and, for example, some products from Doppler data are used to indicate (on the reflectivity display) regions of wind shear. Most TV meteorologists refer to their radar products as "Doppler", when in reality their displays are just reflectivity. Software packages used by many TV stations make use of the radial velocity product to generate storm tracks and cell motion forecasts...