Radar Basics
With everything we’re already learned about the electromagnetic spectrum, this is an opportune time to talk about radar, something we rely on in our everyday lives. When you access a weather app like Storm Radar on your cell phone, you can actually view a picture of your local area, informed by radar. Weather forecasters use radar to look at meteorological information to predict the weather. We all rely on it to tell us when to plan on staying home because a storm is coming, and what day we can plan a picnic because it will be warm and sunny.
What is radar? The word RADAR is an abbreviation for RAdio Detection And Ranging. Let’s take each word one at a time. Radio we talked about last week. Radio means electromagnetic waves in the radio frequencies. Detection is exactly what it sounds like – detecting if something is there. The word range has multiple meanings. The one that’s most useful here is “an interval of values which something can take on”, like the range of a function. Ranging means how far away something is from another thing, in this case, the radar sensor itself.
Radar uses pulses of electromagnetic energy to detect the presence of an object and its distance away from you. Airplanes and airports use radar to monitor the airspace around them. Remember, the airspace can get pretty crowded with so many planes in the air, especially around busy airports! Each flight follows a flight plan, an agreed-upon route from its departure point to its destination. The pilots in the planes, and the air traffic controllers on the ground, rely on their radars to keep the planes on course according to their flight plans. They also use radar to bypass hazardous weather conditions and avoid unexpected objects (including other planes!) in their path. You can get an idea of how the radar is translated into human-usable information by looking at a flight-tracking app like FlightAware on your phone.
An airport has a powerful radar system which both sends out and collects electromagnetic emanations. Its primary surveillance radar transmits and receives EM pulses through its rotating antenna, sensing planes and other objects in the airspace around the airport. The secondary surveillance radar antenna is attached to the top of the primary radar antenna. It transmits and receives area aircraft data for barometric altitude, identification code, and emergency conditions. As the FAA tells us (https://www.faa.gov/air_traffic/technology/asr-11/), “military, commercial, and some general aviation aircraft have transponders that automatically respond to a signal from the secondary radar by reporting an identification code and altitude. The air traffic control center uses this system data to verify the location of aircraft within a 60-mile radius of the radar site. The secondary radar also provides rapid identification of aircraft in distress. The secondary radar operates in the range of 1030 to 1090 MHz. Transmitting power ranges from 160 to 1500 watts.”
So how does the primary radar work? Let’s make the problem really easy to start with, and assume that both the radar and the object are stationary. The radar sends out an electromagnetic pulse and measures how long it takes for the pulse to be echoed back after hitting the object. Since radar signals travel at the speed of light c, this elapsed time represents the amount of time it took the pulse to travel d meters to the target, and d meters back to the radar. Therefore, the speed of light times the elapsed time is equal to 2 times the distance to the object. Let’s look at an example. Suppose it takes 1 millisecond for a radar pulse to return to the transmitting radar. Then the distance d to the object is
d = ½ * c meters/second * 1 millisecond
= ½ * 3 * 108 * 10-3 = 1.5 * 105
= 100,000 meters
= 100 kilometers.
So now we see how radar can be used to determine how far apart two stationary objects are. That’s all very well and good, but a flying plane isn’t stationary. Airports and pilots must find radar useful even when the planes are moving! Can radar tell the relative speed and direction between a stationary object (the airport) and an airborne plane? How about two planes in flight?
Of course, the answer to these questions is yes. Radar came into its own during World War II. Huge investments in radar were made by both sides, and they paid off handsomely for the Allies. One U.S. physicist, Lee DuBridge, stated that “the [atomic] bomb may have ended the war, but radar won the war.” Radar would not have been the gamechanger it was if it couldn’t answer simple questions like how far away the enemy planes were, what direction they were traveling in, and how fast they were going.
Unfortunately, we’re going to need a little more math and physics to answer how radar does these things, so we’ll defer that discussion to next week . At that point, we’ll talk about the Doppler effect, an essential feature of a special kind of radar, Doppler radar, which can find a moving object’s speed.