Short time constant

Schematic diagram of a Rainreject circuit: The red arrow points to a precipitation area: the front edge is very bright due to the reaction time of the circuit, the rest of the precipitation area is suppressed with the moving target indication (moving target detection, MTI).

Short Time Constant (STC) is a component in an analog radar device for suppressing interference signals, in particular interference from weather phenomena. This function is also called “Fast Time Constant” (FTC) or “Rainreject” in some American publications. In Russian radars this circuit was called Automatic Band Impulse ( автоматический строб ). The module compares the length of the echo pulse with the length of the transmitted pulse.

Working method

If the radar device has a transmission pulse duration of, for example, 1 μs, then it can be expected that the echoes from objects with a geometrically limited extent, including from aircraft, are also around 1 μs long. Even if several aircraft are flying in close association, the sum of the lengths of the echoes of more than 10 µs is then rather improbable. It can be assumed that echoes with a multiple pulse duration of the transmission pulse are interference. This type of disturbance usually occurs when heavy rain areas create an area of ​​diffuse reflection. An extensive bright spot would appear on the viewing device , representing the rain cloud. Aircraft inside the rain cloud would not be shown.

In this STC circuit shown in the picture, it is checked, for example, whether the echo signal is longer than 4 µs. If so, then a switch is made to the coherent channel , which suppresses the interference with the aid of the Doppler frequency . The phase is measured from pulse to pulse in the coherent channel. Echo signals with a constant phase position are suppressed, echo signals with a rapidly changing phase position are amplified. Aircraft within a rain cloud are now displayed on the display device again, as they move much faster than the cloud.