NASHUA, N.H. — A weather radar watching a thunderstorm and a military radar tracking an aircraft both use radio waves to gather information. That does not necessarily mean the two systems need to work the same way.

Radar sends radio-frequency energy toward an area and receives energy that returns after interacting with an object. The time it takes a signal to travel out and return can help determine distance, while the direction of the beam provides information about where the object is located.

What a radar needs to detect can vary considerably. One system might watch a large area for aircraft, while another might distinguish precipitation from other objects in the atmosphere. And a military radar may track targets while separating them from reflections caused by terrain or the ocean.

Engineers design radar around what the system needs to find and what information it needs to provide.

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The job comes first

Before choosing an antenna or determining how a radar will process a return, engineers have to define what the system needs to do. Air traffic control provides a straightforward example. The Federal Aviation Administration's (FAA) Airport Surveillance Radar, or ASR-11, uses primary radar to determine the location of aircraft and other objects around airports. The system also uses secondary surveillance radar, which communicates with aircraft transponders to receive information such as identification and altitude.

Weather radar has another job. National Weather Service systems send pulses into the atmosphere and analyze the energy that returns from precipitation and other objects, allowing forecasters to examine the location and characteristics of storms.

The amount of information a radar needs can also change with its mission. A system that provides general surveillance may need to determine that an object is present and establish its approximate position. Tracking applications may require the radar to update that position repeatedly as the target moves.

Those requirements influence the frequency, antenna, transmitted signal and processing used in the radar.

Frequency affects what radar engineers can build

Radar systems do not all operate at the same frequency. Engineers can choose from different portions of the radio-frequency spectrum depending on what they need the system to do. Frequency and wavelength are directly related. A lower frequency has a longer wavelength, while a higher frequency has a shorter one. That choice affects several parts of radar design, including the antenna and the way radio waves propagate.

The FAA's ASR-11 shows how different frequencies can even appear within the same overall surveillance system. Its primary radar operates in S-band between 2.7 and 2.9 GHz, while its secondary surveillance radar operates in L-band between 1.03 and 1.09 GHz.

Wavelength also affects the physical size of antenna components. Shorter wavelengths can allow engineers to build smaller antenna elements, an important consideration when the radar has to fit aboard an aircraft or another platform with limited space.

Higher frequencies can support narrow beams with smaller antennas, but signals at those frequencies can behave differently as they travel through the atmosphere. MIT Lincoln Laboratory, for example, has studied the challenges of extending radar and communications signals over longer distances at millimeter-wave frequencies.

Some radars need to measure movement

Knowing that something is present and determining its distance may provide only part of the information an operator needs.

Doppler radar adds motion information. When an object moves relative to the radar, that motion changes the frequency of the returned signal. The radar can measure that change to determine movement toward or away from it.

The National Weather Service uses Doppler information to examine the movement of precipitation and winds within storms rather than simply showing where precipitation exists.

Radar engineers can also use Doppler techniques to help separate moving targets from background returns. A radar searching over land, for example, may receive strong reflections from the ground along with a much smaller return from the object it needs to detect. Techniques such as moving-target indication and pulse-Doppler processing can help distinguish moving objects from that clutter. The radar's processing then has to sort through the returns and determine which information is useful for its particular mission.

The antenna determines where the radar looks

Radar also needs a way to direct radio energy toward the area it wants to observe. Some systems physically move an antenna. The FAA's ASR-11 uses a continuously rotating antenna to scan the area surrounding an airport. National Weather Service radar also rotates while changing antenna elevation to examine different portions of the atmosphere.

How the radar scans affects how often it can look at a particular area. A system covering a wide region has to distribute its time across that coverage, while a tracking radar may need to revisit a particular target more frequently.

Other systems steer radar beams electronically.

An active electronically scanned array, or AESA, uses many antenna elements rather than depending entirely on mechanical movement to point the radar beam. By controlling the signals sent through those elements, the system can steer the beam in different directions. MIT Lincoln Laboratory's airborne AESA testbed, for example, can direct radio waves across a range of angles without rotating its antenna.

Range is not the only design goal

It might seem logical to make every radar see as far as possible. Range, however, is only one part of radar performance. Engineers may also need to consider how small a target the radar can detect, how precisely it can determine location and how quickly it can revisit an area.

Available power, antenna size and processing hardware add more constraints. Increasing performance in one area can require additional hardware or resources elsewhere in the system.

The platform matters as well. A large radar installed on the ground does not face the same size and weight limits as one carried by an aircraft or uncrewed vehicle. An airborne radar has to share limited space and electrical power with the other equipment aboard the aircraft. Engineers also have to account for the weight of the antenna, electronics and supporting hardware.

Radar transmits radio energy and analyzes what comes back. What engineers want to find, how far away it is, how quickly it moves and where the radar has to operate determine the design between those two steps.