Clutter is a significant risk to radar performance and the safety of warfighters around the world. Luckily, technological innovations allow us to build better, more modern radars that are less impacted by clutter. Additionally, there are several strategies and radar clutter reduction techniques to help modern warfighters get greater accuracy from the radars they use to navigate and stay safe.
What is Clutter?
Clutter is any reflected energy from the environment that can be mistaken for the detection of a true target in the radar system. Typical sources of clutter are birds, trees, bodies of water, and weather (rain, snow, fog). Clutter can cause false alarms in a radar system and limit the ability to process true target detections, posing a significant threat to warfighters.
What are Noise-Limited Radar and Clutter-Limited Radar?
In addition to clutter, the detected radar signal must also compete with noise, or randomly generated energy that is present in electronic devices in a radar. When the clutter is minimal, a radar may be referred to as a noise-limited radar, because the power level of the noise is greater than the clutter. When there is significant clutter, the power level of the clutter is greater than the noise, and the radar may be referred to as a clutter-limited radar.
Types of Clutter
Clutter is caused by a variety of objects and phenomena and can be categorized into three major categories: surface clutter, volume clutter, and point clutter. Radar clutter detection can help eliminate these distractions from the true signal.
Surface clutter is caused by the geography of the surrounding area. This radar ground clutter can be caused by uneven terrain, trees and other greenery, and even the texture or groundcover of the area (e.g. long grass vs. stone).
Volume clutter is typically caused by weather or chaff. Weather phenomena like rainstorms or snowstorms can be substantial enough that radar signals bounce back off of them due to their size and density. This can add to already difficult navigational circumstances due to weather.
Chaff is a unique form of volume clutter because it is man-made. Chaff was developed during World War II as a countermeasure against radars, which were a new and major threat at the time. Chaff is made of tiny pieces of aluminum or other metallic materials that are dropped from aircraft to create “clouds” that reflect radar signals and cause clutter. Radar clutter reduction techniques are crucial in these situations, as they can help to separate clutter from the true signal, bringing warfighters greater situational awareness.
Unlike volume clutter, point clutter refers to false signals created by single points of interest, like birds, insects, or even infrastructure (like tall buildings or utility towers). While some of these stationary threats can be accounted for with GPS systems, small, moving point clutter like birds and insects can be especially tricky to identify.
Radar Clutter Reduction Techniques
While clutter continues to pose a threat to radar performance (and defensive techniques like chaff continue to evolve) there are also plenty of strategies to counteract its effects.
Doppler Shift Separation
Radars that employ pulse-Doppler processing are able to distinguish between targets and clutter based upon their different velocities. For both stationary and moving clutter, the Doppler signature of the desired target(s) can be separated from the clutter, improving performance and awareness for the warfighter.
Sensitivity Time Control
Sensitivity time control (STC) is commonly used to attenuate the nearby radar ground clutter which has an extremely large signal strength due to its close proximity to the radar. Without STC, a majority of received target detections would be masked by the strong clutter signal in these situations.
Moving Target Indication
Moving target indication (MTI) uses signal processing to filter stationary and quasi-stationary clutter. This radar clutter reduction technique is extremely effective when the desired target is moving with a Doppler velocity greater than zero. This technique is used in both air- and ground-based radars and is referred to as AMTI and GMTI.
Space-Time Adaptive Processing
Space-time adaptive processing (STAP) is an advanced signal processing technique that applies adaptive weighting to the detected energy (signal, clutter, and/or noise), to discern the signal from clutter. The advanced algorithms can be tailored to the clutter presented to the particular radar, aiding in improved target detection.
Constant False Alarm Rate
Constant False Alarm Rate (CFAR) is an adaptive signal processing technique to help mitigate clutter. CFAR algorithms use a time-varying detection threshold that provides a constant false alarm rate to ensure that only the signals of interest are detected.