Buzz
Operation Specialist 2nd Class Gilbert Lundgren manages radar systems in the combat information center aboard the USS Carney.
Photo courtesy Department of DefenseWhat exactly is radar? This technology is ubiquitous yet often unseen. Radar, short for radio detection and ranging, is utilized by air traffic controllers to monitor aircraft both on the ground and in the sky. Law enforcement employs radar to measure the speed of vehicles on the road.
NASA leverages radar for mapping Earth and other celestial bodies, monitoring satellites and space debris, and assisting in tasks such as docking and navigation. The military relies on radar to identify hostile aircraft using bursts of short radio waves.
Meteorologists rely on Doppler weather radar to monitor weather phenomena such as storms, hurricanes, and tornadoes, thanks to its ability to function effectively across various weather conditions. For instance, Doppler radar is capable of identifying precipitation.
Radar technology is even present in everyday scenarios, such as automatic store doors that operate using radar sensors. It's clear that radar is a highly versatile and valuable technology. To grasp how radar functions, it's essential to explore its diverse applications.
Radar System Basics
Radar is typically employed to achieve one of three primary objectives:
- Detect objects: Often, the object in question is in motion, such as an airplane tracked by air traffic controllers. However, radar can also locate stationary objects underground. In certain instances, radar can even provide specifics, such as the type of aircraft detected.
- Measure an object's speed: This is the principle behind police radar guns used for speed enforcement.
- Create maps: Space shuttles and satellites utilize synthetic aperture radar to produce intricate topographic maps of planetary and lunar surfaces.
These three tasks can be achieved using two familiar concepts from daily life — echo and the Doppler effect, also known as Doppler shift. These principles are easily understood in the context of sound, as your ears perceive echoes and the Doppler effect regularly. Radar systems apply these same principles using radio waves.
In this article, we’ll delve into the inner workings of radar. We’ll start with the concept of sound, as it’s something you’re likely already familiar with.
Echo
MytourEchoes are a common experience. When you shout into a well or a canyon, the sound bounces back after a brief delay. This happens because some of the sound waves from your shout reflect off a surface, such as the water at the bottom of the well or the far wall of the canyon, and return to your ears. Essentially, echoes are reflected sound waves. The time delay between your shout and the returning echo depends on the distance between you and the surface causing the reflection.
When you yell into a well, the sound travels downward and bounces off the water surface at the bottom. By measuring the time it takes for the echo to return and knowing the speed of sound, you can accurately determine the well's depth.
Doppler Radar
Doppler effect: A person behind the car perceives a lower pitch than the driver because the car is moving away. Conversely, someone in front of the car hears a higher pitch than the driver as the car approaches.
MytourTo grasp the Doppler effect, imagine a car moving toward you at 60 mph in a parking lot with its horn blaring. As the car approaches, you hear the horn at a certain pitch. However, once the car passes you, the pitch suddenly drops. The horn's sound remains constant, but the change you perceive is due to the Doppler effect.
Here’s the explanation. The speed of sound through the air in the parking lot is constant. For simplicity, let’s assume it’s 600 mph (the actual speed depends on air pressure, temperature, and humidity). If the car is stationary, exactly 1 mile away, and honks its horn for one minute, the sound waves will travel toward you at 600 mph. You’ll experience a six-second delay (as the sound covers 1 mile at 600 mph) followed by exactly one minute of sound.
Now, imagine the car is approaching you at 60 mph. Starting from a mile away, it honks its horn for exactly one minute. You’ll still experience a six-second delay, but the sound will only last for 54 seconds. This occurs because the car reaches your location after one minute, and the final sound arrives instantly.
Doppler Frequency Shift
From the driver’s perspective, the horn blares for a full minute. However, because the car is moving, the sound is compressed into 54 seconds from your viewpoint. The same number of sound waves are squeezed into a shorter time frame, increasing their frequency and making the horn’s tone sound higher. As the car passes and moves away, the process reverses, stretching the sound waves and lowering the tone.
You can combine echo and the Doppler effect in this way: Imagine sending a loud sound toward a car moving toward you. Some sound waves bounce off the car as an echo. Since the car is approaching, the sound waves compress, causing the echo to have a higher pitch than the original sound. By measuring the pitch of the echo, you can calculate the car’s speed.
While discussing sound and motion, let’s explore sonic booms. Suppose the car moves toward you at the speed of sound—around 700 mph—while honking its horn. The sound waves from the horn can’t travel faster than the speed of sound, so both the car and the sound approach you at 700 mph. The sound waves "stack up," and you hear nothing until the car arrives. At that exact moment, all the sound reaches you at once, creating a loud sonic boom.
Understanding Radio Waves and Radar Technology
Left: Antennas at the Goldstone Deep Space Communications Complex, part of NASA's Deep Space Network, facilitate radio communications for interplanetary spacecraft. Right: A guided missile destroyer is equipped with surface search radar and air search radar mounted on its foremast.
Photo courtesy NASA (left), Department of Defense (right)We’ve learned that sound echoes can measure distance and that the Doppler effect of an echo can determine speed. This principle is the foundation of "sound radar," which is essentially what sonar is. Submarines and ships frequently use sonar. While similar principles could apply to sound in air, there are challenges:
- Sound has limited range, typically no more than a mile.
- Since most people can hear sound, a "sound radar" would likely cause noise disturbances (this issue can be mitigated by using ultrasound instead of audible sound).
- Sound echoes are often faint, making them difficult to detect.
Radar overcomes these limitations by using radio waves instead of sound. Radio waves travel long distances, are undetectable to humans, and can be easily identified even at low intensities.
Air-Based Radar
Consider a standard radar system designed to detect aircraft in flight. The radar activates its transmitter, emitting a brief, high-power burst of high-frequency radio waves, typically lasting a microsecond. After transmitting, the radar switches to its receiver mode, listening for echoes. It calculates the time taken for the echo to return and analyzes the Doppler effect to gather additional data.
Radio waves move at the speed of light, approximately 1,000 feet per microsecond. With a precise high-speed clock, the radar can accurately determine the airplane's distance. Advanced signal processing tools also allow the radar to measure the Doppler effect with high precision, enabling it to calculate the airplane's speed.
Ground-Based Radar
Ground-based radar faces more interference from other radar signals compared to air-based systems. When a police radar emits a pulse, it reflects off various objects like fences, bridges, mountains, and buildings. To eliminate this clutter and enhance accuracy, radar systems filter out non-Doppler-shifted signals. Police radars focus solely on Doppler-shifted signals, and since the radar beam is narrow, it typically targets only one vehicle at a time.
Some law enforcement agencies now employ a laser-based method to measure vehicle speed. Known as lidar, this technique utilizes brief bursts of light rather than short pulses of radio waves. For more details on lidar technology, refer to How Radar Detectors Work.
CW Radar vs. Pulse Radar
While pulse radar has been our focus, there’s also CW radar, or continuous wave radar. Unlike pulse radar, CW radar emits uninterrupted electromagnetic waves. Both systems rely on radar antennas and electronic components, but they differ in energy consumption. CW radar operates continuously, consuming energy without interruption.
