Doppler radar

Doppler radar uses the Doppler effect to return additional information from a radar system. The Doppler effect shifts the frequency of the radar beam due to movement of the "target", allowing for the direct and highly accurate measurement of speeds. Doppler radars were originally developed for military radar systems, but have since become a part of almost all radar systems, including weather radar and radar guns for traffic police and sports.

Basic concept

Early radar systems sent out powerful radio pulses that were reflected off "targets"; the reflected signal was then detected on a separate antenna. Systems soon evolved to use the same antenna to act as both a broadcaster and receiver, with electronics — a duplexer — switching between the two modes. These pulse radar systems had several drawbacks, however. Since the system couldn't broadcast and receive at the same time, the pulses had to 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 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 being introduced. The target is "seen" because the returned signal will be frequency shifted due to the Doppler effect, allowing it to be picked out of the outbound signal by filtering. 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.

In meteorology, the Doppler effect becomes especially useful. While Doppler radar can still detect reflectivity, other information is collected from the returning microwave signal's Doppler shift. The information is then used by computers to derive wind velocity in real time. The velocities that can be detected by a single dish are velocities directed away from the dish or toward the dish (see vector mechanics).

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. Also, 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. Doppler radar can also detect radial velocity.