RADAR
What is Radar?
Definition
Probably the most famous acronym in all of wireless communications, radar stands for RAdio Detecting And Ranging, which is a real big hint as to what it does. It uses radio waves to detect things. More specifically, today's advanced radars can measure four distinct characteristics about an object. Radar can detect if an object is present, how far away it is, where it is and, most impressively, how fast it is moving. (Anyone who has ever gotten a speeding ticket can attest to that.)
Radar is used in many places, including on the ground (called ground-based), on a ship (called shipboard), in the air (called airborne), and out in space (called spaced-based).
The Role of Frequency
Like every other wireless application in the United States, the FCC has allocated specific frequency bands for radar. Some of the allocated radar frequency bands are shown in Table 6-2.
| Radar Band | Frequency Range(s) | Some Uses |
|---|---|---|
| UHF | 220–225 MHz | Early warning, satellite surveillance. |
| VHF | 420–450 MHz | Early warning, satellite surveillance. |
| L-band | 960–1215 MHz | Air traffic control. |
| S-band | 2.3–2.5 GHz 2.7–-3.0 GHz | Shipboard military, early warning. |
| C-band | 5.25–5.925 GHz | Altimeters, weather. |
| X-band | 8.5–10.55 GHz | Airborne fighter, weather, police. |
| Ku-band | 13.4–14 GHz 15.7–17.7 GHz | Airborne fighter, police. |
There are four main factors which dictate what frequency is selected for a particular application. All four, which are highlighted in Table 6-3, must be taken into account when making the frequency decision. Notice in Table 6-3 that several of the words are surrounded by quotes, which is because each of these is a relative measure. For instance, a "large" radar system for airborne applications will probably be considered a "small" radar system for ground-based applications.
| Factors | Implication |
|---|---|
| As the frequency goes up, the atmospheric absorption (attenuation) goes up. | If the radar needs to detect something "far" away, a "low" frequency is required. |
| As the frequency goes up, the size of the system's components gets smaller. | If space is limited (as on an airplane), the frequency must be "high." |
| For a given frequency, as the antenna size gets bigger, the beamwidth gets smaller. | If radar accuracy is important, a "large" antenna is needed. |
| As the output power goes up, the system size and weight goes up. | If "high" power is required, a "large" space is needed to house the radar. |
Table 6-3 contains a word you may not have seen before called beamwidth. The beamwidth of a radiated RF signal describes how wide the antenna pattern is from an antenna which is not omnidirectional. (See Figure 3-6 for a review of antenna patterns.) As you recall, an antenna which is not omnidirectional radiates RF energy in only one specific direction. Of course, RF energy can be radiated in a wide pattern or a narrow pattern (see Figure 6-5). Antenna beamwidths are measured in degrees (of a circle). With respect to the antenna patterns in Figure 6-5, the antenna on the left has a narrow beamwidth (perhaps 10 degrees out of 360) while the antenna on the right has a wide beamwidth (perhaps 45 degrees out of 360).
Figure 6-5. A comparison of antenna beamwidths.
It should be intuitive that the narrower the beamwidth, the more accurately a radar's antenna can locate an object. As with all aspects of the RF world, there is a price to be paid for this accuracy. As mentioned in Table 6-3, a narrow beamwidth (i.e., more accuracy) requires a large antenna, which is heavy and difficult to move.
How Radar Works
The Role of Reflection
Radar works off the simple principal, covered back in Chapter 2, that when an RF signal encounters a solid object, at least some of the RF energy is reflected. The key concept to this RF "reflection" is something called radar cross section. Every solid object has a radar cross section. Radar cross section can be visualized by viewing the two-dimensional silhouette of any three-dimensional object. To help you envision this, imagine looking at a football straight on at the pointy end. Even though the football is oblong and comes to a point, when viewed from its end the radar cross section of a football is a perfect circle. It's like looking at a football which has been cut in half.
The fundamental principal of radar is this: the greater the area of the cross section, the more RF energy which gets reflected, and the more RF energy which gets reflected, the easier it is to find the object. Remember, it isn't the size of the object which matters, but the area of its radar cross section (silhouette). In that respect, a football is easier for radar to find than a fishing rod.
Did You Know?
The B-2 bomber in the United States Air Force is almost invisible to radar. It accomplishes this feat in two ways. First, the entire outside of the plane is covered with an RF absorbing material. Almost all of the RF energy (radar from an enemy) that hits the plane gets absorbed (converted to heat) rather than reflected. Second, the unique design of the plane gives it an incredibly small radar cross section when viewed from the front. It's almost like looking at a pencil, which also has a small radar cross section.
How Radar Determines Distance
To determine an object's distance—and consequently its presence—the radar simply invokes the easiest equation you ever learned back in Algebra: distance equals rate times time (D = R × T). The key to using this equation is that the radar system already knows the rate (at which the RF signal is traveling). RF waves travel at the speed of light, which just happens to be 186,000 miles per second. To put it in perspective, light can race around the equator of the Earth almost eight times in one second. (And you thought Ferraris were fast.)
In determining distance, the radar's transmitter sends out a signal and the super-fast electronics within the radar counts the time until the reflected signal reaches the receiver. Dividing this count by two (because the RF energy has to travel there and back), gives the time required for the signal to reach the object. This time, multiplied by the speed of light, determines the distance to the object. Radar systems which only determine distance are known as pulsed radars, because it "pulses" the transmitter off and on. During its brief "on" period, the transmitter sends out RF energy at a particular frequency. During its off period, the receiver "listens" for the reflected signal. Since the reflected signal is at the same frequency as the original (transmitted) signal, the transmitter must be off while the receiver listens for the reflected signal. (Otherwise, all the receiver hears is the transmitted signal.)
Did You Know?
Back in the 17th century, Sir Isaac Newton—the genius that discovered gravity—actually tried to measure the speed of light. He did so by standing on top of a mountain with a lantern and he had a colleague stand on another mountain, not far away, with another lantern. Newton's intention was to uncover his lantern and, upon seeing the light, have his partner uncover his lantern. All Newton needed to do was to count the time between uncovering his own lantern and seeing the other light, divide by two and he had the speed of light, or so he thought. Needless to say, this did not work very well, given the infinitesimal time it took for the light to make the trip. However, rumor has it that it did cause Newton to ponder his dilemma by sitting under an apple tree.
How Radar Determines Direction
Radar determines an object's location by moving the antenna in a process called scanning. Scanning involves pointing the antenna in a single direction, transmitting a signal in that direction, and waiting for the reflected signal. If a reflected signal is received, the radar knows which way the antenna is pointing and therefore it knows in what direction the object is located.
After a short period of time, the antenna moves a small amount and repeats the process. The radar repeats this three step process (move antenna, transmit a signal, wait for a reflection) until it has covered the entire area of interest and then points the antenna back to the first position and starts the process all over again.
This is where beamwidth size comes in. Smaller beamwidths require more of these "antenna moves" in the scanning process but allow the radar to pinpoint the object more exactly. Obviously there is a trade-off. Narrower beamwidths take longer to find the object, but once it's found its location is known more precisely. When airborne radar is used in combat, a wide beamwidth is initially used to scan, just to know if there is something out there. Once an object is detected, a narrower beamwidth is used to pinpoint the actual location of the target.
Think about all of the different trade-offs which need to be made in designing a radar system. If you are the pilot of a fighter aircraft and you are hunting down enemy aircraft, you want a radar which can produce a very narrow beamwidth so that you can locate the target precisely before you fire your missile. A narrow beamwidth requires one of two things: either a big antenna or a high frequency of transmission. A big antenna is an excellent choice if it does not keep the aircraft from taking off in the first place. Obviously, there is a limitation. On the other hand, a high frequency is a good choice to produce a narrow beamwidth, except for one thing: high frequencies suffer from extreme atmospheric attenuation, which means that the radar can only locate the closest of targets. If I were flying a fighter aircraft, I'd want to see all of the targets (near and far). Today's radars are a combination of constant technological innovation and performance tradeoffs.
Did You Know?
One of the drawbacks to conventional radar is that it requires the antenna to physically move to scan the area of interest. Not only does it require expensive motors to move the (heavy) antenna, but it is impossible for the antenna to move from one position to another, non-adjacent position instantaneously. As a result of this limitation, a new technology has been developed called electronically scanned arrays, in which the antenna pattern in the radar moves without anything physically moving. To understand the details of this would truly require a degree in engineering, so you will just have to take my word for it.
How Radar Determines Velocity
Back in the 1800s, a clever Austrian physicist named Christian Doppler made an amazing discovery. He observed that the frequency of sound waves emanating from a moving object changed as the object moved by. And as things turned out, it isn't just true for sound waves but all waves including RF. In fact, this observed "frequency shift" (called a Doppler shift) is proportional to the velocity of the moving object. This is the fundamental principal underlying what is known as Doppler radar. Doppler radar is used to determine an object's velocity. It is Doppler radar which is responsible for all the "false" readings the police use to give out speeding tickets.
Doppler radar is somewhat different from conventional radar in that the transmitter is always on. This type of radar is known as continuous wave or CW radar. The transmitter must stay on continuously because, unlike conventional radar which counts the time between transmission and reception, Doppler radar is looking for a change in frequency. Since this change in frequency may not last long, the transmitter must stay on continuously.
You may be wondering why the transmitter doesn't interfere with the receiver if it is always on? It would, if the receiver were listening for the same frequency which the transmitter is transmitting, but it isn't. It is looking for a signal which is frequency shifted from the transmitted signal. In fact, the receiver filters out any reflected signals at the transmitter's frequency. Reflected signals at the transmitter's frequency are, by definition, not moving (there is no frequency shift). Doppler radar does not care about stationary objects.
Perhaps you are thinking that in certain circumstances it would be advantageous to combine the capabilities of conventional radar (distance and location) with the capability of Doppler radar (velocity). In fact, today's most sophisticated radar systems, called pulsed Doppler radar, do just that. It not only counts the time lapse of the received signal (to determine distance), but it also looks for frequency shifts (to determine velocity).
Did You Know?
If you have ever heard a train whistle's tone change as it passed by you, you have experienced a Doppler shift. What you heard, as the train came toward you, was the frequency of the train's whistle "shifted" by the velocity of the train. Since the train was coming toward you, the frequency was shifted up. As the train passed by you, the tone dropped, shifted downward by the velocity of the train going away from you.
Different Radar Systems
Pulsed Radar Systems
As mentioned before, pulsed radar systems are used to calculate distance and, as a consequence, presence. Two common systems using pulsed radar are automatic door openers (found at many supermarkets) and automatic toilet flushers (found in many airports). The systems' operations are simple to understand. In both cases the system continuously transmits a radar pulse and waits for a response. The systems expect to receive a response at one of two time intervals. In the case when no person is present, the signal reflects off of the ground (for the door opener) or off the lavatory door (for the flusher), which result in a "long" time delay. With this long time delay, the system knows to do nothing. When a person is present, the signal in both cases reflects off of the person, which is closer to the system, resulting in a shorter time interval. When the system senses this shorter time interval, it knows to do its thing (open the door or flush the toilet).
A new application for pulsed radar is called near object detection system or NODS. NODS is nothing more than an inexpensive pulsed radar on the rear bumper of an automobile. When the car is put in reverse, the radar turns on and begins pulsing and timing the response. While backing up, if the car gets too close to an object (the time delay gets too short), it signals a warning—which should make parallel parking easier.
Did you ever wonder how commercial airline pilots know what altitude the airplane is at? They know because they use a clever little device called an altimeter. You have probably heard the word before but did not know what it was or how it worked. Well, now you do. An altimeter is nothing more than pulsed radar in an airplane, pointing down at the ground. It sends out a signal, waits for a response, and converts the time delay into a distance (see Figure 6-6), which is the airplane's altitude.
Figure 6-6. The signal path of a radar altimeter.
Now for the fun part. If an airplane is cruising at 33,000 feet, how long does it take for the round trip of the altimeter's signal? That is 66,000 feet (the length of the signal's round trip) divided by 984 million feet per second (the speed of light), or about 67 millionths of a second. (I guess that's what they mean by how time flies.)
Another useful pulsed radar system is weather radar. It works simply by detecting the RF energy reflected off of raindrops. No rain, no reflection; a little rain, a little reflection; a lot of rain, a lot of reflection. These differences in reflected RF power show up as different colors on the weather maps shown on the evening news. And, of course, taking several of these (radar) weather readings periodically results in the "moving cloud" display which captures everyone's attention during hurricane season.
Doppler Radar Systems
The most common use of Doppler radar is the radar gun used by your local law enforcement to punish you for running late. The radar gun simply detects the change in frequency from the signal reflected off the automobile and converts it to a speed in miles per hour. (I understand that the next generation of radar guns will be able to convert a car's speed measurement directly into a debit in a checking account.)
A more interesting use of Doppler radar, used in fighter aircraft, is called fire control radar. (It is called fire control radar because it "controls" the "firing" of the aircraft's missiles.) In today's modern aircraft, pulsed Doppler radar is used to determine both location and velocity.
In the nose of every fighter aircraft is a pulsed Doppler radar. In air combat, fighter pilots only care about situations (called threats) in which an enemy aircraft is coming toward them. The pulsed Doppler radar uses a wide beamwidth in the pulsed mode to scan (remember scanning?) the sky for any threats. Once it detects a threat, the radar locks onto the target with a narrow beam. The receiver then analyzes the return signal for one of three conditions: no change in frequency, a lower frequency, or a higher frequency.
When the receiver senses no change in frequency, it is implied that there is no relative difference in the velocities of the two airplanes, which means the enemy plane is traveling away from the pilot's plane at exactly the same speed. Interpretation: the enemy pilot is prudent.
When the receiver senses a drop in frequency, it means that the enemy plane is traveling away at a greater velocity and thereby increasing the distance from the pilot's plane. Interpretation: the enemy pilot is chicken. Finally, if the receiver senses an increase in frequency, it means that the enemy aircraft is heading right for the pilot, and the greater the frequency shift, the faster the enemy is closing in. Interpretation: the enemy pilot is crazy.
Did You Know?
Police radar jammers—which may be illegal—work by transmitting an RF signal at a constantly varying frequency within the frequency band of the radar gun. The radar gun is expecting to receive a single frequency (which reflects the speed of the car). Instead, it receives a whole bunch of different frequencies which drives the display crazy. Oh well.
The police aren't the only ones having fun with Doppler radar. Weather forecasters are also taking advantage of it. Believe it or not, Doppler radar can be made to measure the velocity of wind. How it does this depends on the RF energy's ability to reflect off moving air differently than static air. With this capability, Doppler radar is used to detect things such as wind shear, which is a strong and sudden change in the wind's direction near the ground (usually near airports).
The last radar I will mention, only briefly, is a relatively new system called collision avoidance. Collision avoidance is similar to NODS, only for the front of an automobile. It has the ability to detect distance and velocity of objects in front of the car. With both of these pieces of information, an onboard computer determines whether the car in front is too close for the speed it is traveling. If the car is too close, the system either gently applies the brakes or berates the driver incessantly about their overly aggressive driving behavior.
The reason that you may not have seen collision avoidance systems on automobiles yet (circa 1999) is that car manufacturers in the U.S., Europe and Asia cannot agree on which frequency band to use. Stay tuned.