This article dives into the critical role of zero-velocity filters in air traffic control, highlighting how they improve radar accuracy by eliminating stationary objects from the radar display. A video shows an implementation with SkyRadar FreeScopes.

Zero-Velocity Filters in Air Traffic Control

In air traffic control, a zero-velocity filter plays a crucial role in improving the accuracy of radar systems by filtering out stationary or near-stationary objects, such as buildings or trees, from the radar's display. The primary aim is to focus solely on objects with a velocity indicative of aircraft, thereby increasing operational efficiency and reducing the risk of false positives.

Reflections from Stationary Targets are a Challenge

Conventional radar systems emit radio waves that bounce off objects and return to the radar. The time it takes for the radio wave to return is proportional to the distance to the object. While this information is valuable, it also includes reflections from stationary objects that are not relevant to air traffic control. This is where the zero-velocity filter comes into play.

How Zero Velocity Filters Work

Technically, a zero velocity filter is a low-pass filter with the task to keep only clutter. It is a recursive filter. These filters, also called Infinite Impulse Response (IIR) filters,  are an efficient way of achieving a long impulse response, without having to perform a long convolution. They execute very rapidly, but have less performance and flexibility than other digital filters. Objects that remain in the same position over multiple scans are presumed to be stationary. The zero velocity filter keeps these objects. In contrast, objects that change position are considered to be in motion and are displayed on the radar screen for air traffic controllers to monitor. They are filtered out. 


Sometimes, the zero velocity filter is defined as the algorithm which eliminates stationary clutter. In we use the positive definition. Meaning: the zero velocity filter keeps the stationary clutter and removes everything which moves. 

It hence gives us an image of all stationary objects within the range of the radar.

Eliminating Clutter

The inclusion of a zero-velocity filter is an essential feature in modern radar systems used for air traffic control. By eliminating stationary clutter and focusing on moving aircraft, it significantly enhances situational awareness, allowing for safer and more efficient airspace management. Read the article on clutter-map-subtraction to see how Zero Velocity Filters work together with Clutter Maps.

Zero Velocity Filter in FreeScopes - a Video

In the following short video, you see the implementation of a zero-velocity filter in FreeScopes.

It only shows the clutter - meaning the static data. Moving objects are filtered out.

Comparing MTI and the Zero Velocity Filter

The Zero Velocity Filter method detects the signal pulses coming from standing targets. This filter implements an inverse MTI method. Since the MTI extracts the moving targets, the MTI-1 detects the standing targets.


Learn more about how zero velocity filter and how it is used in an MTD.

Zero Velocity Filter in FreeScopes

The zero velocity filter is part of FreeScopes ATC II.  A-Scope and PPI are part of FreeScopes Basic I

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