What is a Passive Radar?

Mary McMahon
Mary McMahon

A passive radar is a radar system which receives only, instead of alternating transmission and reception. These systems are used in a variety of settings, and they have a number of benefits which make them targets of interest for many military technology developers. Essentially, a passive radar is a very finely tuned pair of ears, sensitive to high frequency radio waves rather than sounds which can be heard by the human ear.

Radar devices can be used to identify the distance, direction and speed of physical objects that may not be visible to the naked eye.
Radar devices can be used to identify the distance, direction and speed of physical objects that may not be visible to the naked eye.

Many people are familiar with the way in which radar works: A signal is transmitted, and a receiver waits for the signal to return, drawing inferences from the returned signal about what kind of objects the signal interacted with, and how far away they are. For example, a ship could use radar to look for potential enemy ships in the area. Unlike sonar, which uses sound, radar uses high frequency radio waves.

A ship could use radar to look for potential enemy ships in the area.
A ship could use radar to look for potential enemy ships in the area.

Passive radar has the same ability to pick up microwave energy, but it does not transmit it. Instead, is uses reflections from other objects and the original source to gather information about targets, working passively rather than actively to identify objects in its vicinity. Using this information, the device can provide information about range, velocity, and location which can be used to make decisions. Passive radar on an aircraft, for instance, might be used to identify other aircraft in the area for the purpose of avoiding them.

From a military perspective, the clear advantage to passive radar is that it is stealthy. Since it does not transmit, it can be difficult for an enemy to locate, because there is no signal to track. Instead, passive radar simply sits and waits for signals of interest. Even if a passive radar array is identified, it can be difficult to jam, because there isn't a way to know which frequencies it is using, although tactics may be used to spoof it for the purpose of confusing it. Using passive covert radar can improve mission safety and allow people to have access to radar data continuously.

Passive radar detection of targets of interest is also less costly and complex than working with an active radar system. This makes it less expensive to implement, and can make it appealing in settings where radar needs to get operational quickly. Many different things can be used as a passive radar reflector, allowing the technology to utilize a variety of objects in its vicinity to collect information.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


I think I can try to answer some of these questions. I'm not a credible source just a person interested in the technology.

So the question on the logical first question is from JimmyT. He asked what sort of signal passive radars typically used. Passive radar system don't send out any signal, they are like a submarine in silent running mode, just listening. It utilizes signals already present in the area it is set-up in. From my understanding the passive radar likes steady signals in the VHF and UHF band like TV and FM radio signals. TV is preferred because radio signals get weaker during moments of silence, and TV signals almost always are broadcasting something. The radar can also use waves typically found from cell phones and GPS.

The other question I can answer is whether or not stealth aircraft can be detected with passive radars. Simple put yes, when a stealth aircraft flies around within range of the passive radar it affects the radio waves around it. The passive radar picks up on those changes and tracks it like any other aircraft.

And the radar jammer question, they could work but it depends on what kind of medium the speed gun is using. For example a radio signal jammer won't work on a infrared laser gun. plus police use a variety of signals, to make a device to jam so many signals sounds like a terrible idea. Personally, I'd take a speeding ticket or a FCC violation for jamming the radios in planes or to an ambulance.


@jmc88 - Interesting question, I would be interested to know. My guess is that the planes or other stealth equipment wouldn't be affected by passive signals. I'm not sure exactly what frequencies radars use, but I would assume they are all different, so the planes would have to be made to eliminate all kinds of different signals.

I was wondering if anyone knew whether police used passive radar systems when they are checking speeds. I was on a website looking at radar jammers the other day, and some people claimed that they didn't work. I was just curious if the passive radar they were talking about is the same type of passive radar this article talks about.


I know that when I think of submarines, I always think of them using sonar. I've always figured they used sonar over radar because of some effect the water has on the receivers. I doubt radio waves travel very well underwater. Does anyone know whether submarines are able to use radar? Are they also susceptible to passive radar that can be picked up by other vessels in the area?

I was also wondering, are stealth planes and other stealth equipment still vulnerable to passive radar? If I remember right, the planes are "invisible" because all of the jagged edges on the planes are meant to trap the radar signal so that it never returns back to the ground. Do the planes do this with passive signals already in the air, or are they specially modified just for regular active radar technology?


@kentuckycat - I think you have the understanding right. I'm not positive, but I can't imagine there being anywhere on Earth where you couldn't at least find a radio signal of some type. I am curious though what types of signals are used.

Can they use regular broadcast signals from things like radio or TV stations, or are there special other signals that have to be used instead? What are the other signals, and where do they come from if that is the case?


Wow, I never knew anything like this existed. Maybe it isn't a problem with the number of signals we have now, but are there any areas where no signals are present for a passive radar to use?

If I understand this all correctly, the passive radar works by picking up radio or other signals that just happen to be in an area, then it can use the reflection of those signals off of nearby plane or ships or whatever to figure out their location.

Is there anywhere in the world where there just aren't any signals available?

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