Continuous-wave radar
Encyclopedia
Continuous-wave radar is a type of radar
system where a known stable frequency continuous wave
radio
energy is transmitted and then received from any reflecting objects.Continuous wave (CW) radar uses Doppler
, which renders the radar immune to interference from large stationary objects and slow moving clutter
.
CW radar systems are used at both ends of the range spectrum.
There are two different antenna configurations used with continuous wave radar.
The main advantage of CW radar is that energy is not pulsed so these are much simpler to manufacture and operate. They have no minimum or maximum range, although the broadcast power level imposes a practical limit on range. Continuous wave radar maximize total power on a target because the transmitter is broadcasting continuously.
The military uses continuous-wave radar to guide semi-active radar homing
(SARH) air-to-air missile
s, such as the U.S.
AIM-7 Sparrow
and standard missile
. The launch aircraft illuminates the target with a CW radar signal, and the missile homes in on the reflected radar waves. Since the missile is moving at high velocities relative to the aircraft, there is a strong Doppler shift. Most modern air combat radars, even pulse Doppler
sets, have a CW function for missile guidance purposes.
Maximum distance in a continuous wave radar is determined by the overall bandwidth and transmitter power. This bandwidth is determined by two factors.
Doubling transmit power increases distance performance by about 20%. Reducing the total FM transmit noise by half has the same effect.
Frequency domain
receivers used for continuous wave Doppler radar receivers are very different from conventional radar receivers. The receiver consists of a bank of filters, usually more than 100. The number of filters determines the maximum distance performance.
Doubling the number of receiver filters increases distance performance by about 20%. Maximum distance performance is achieved when receiver filter size is equal to the maximum FM noise riding on the transmit signal. Reducing receiver filter size below average amount of FM transmit noise will not improve range performance.
A CW radar is said to be matched when the receiver filter size matches the RMS bandwidth of the FM noise on the transmit signal.
when objects are moving. There is no way to evaluate distance. This type of radar is typically used with competition sports, like golf, tennis, baseball, and NASCAR.
The transmitter is the strongest source of RF energy in the vicinity of the receiver unless there is a large physical barrier along the line of sight, such as a mountain. There must be a strong Doppler shift between the receiver and the reflector, otherwise the bleed-through signal reaching the receive antenna from the transmit antenna will overwhelm the reflected signal arriving at the receiver.
Continuous wave radar with no FM modulation only detects moving targets, as stationary targets (along the line of sight) will not cause a Doppler shift. Reflected signals from stationary and slow-moving objects are masked by the transmit signal, which overwhelms reflections from slow-moving objects during normal operation.
Distance measurement is implemented using a technique known as frequency modulated continuous-wave radar. In this system the signal is not a continuous fixed frequency, but varies up and down over a fixed period of time. Range is determined by evaluating the frequency-spread of the received signal.
FM modulation is used with CW radar altimeters used with aircraft, CW early warning radar, and proximity sensors. Doppler shift is not always required for detection when FM modulation is used.
The transmitter frequency can slew up and down as follows.
FM modulation on the receive signal increases with distance. This smears out, or blurs, the Doppler signal. CW radar range detection involves FM demodulation to sharpen the Doppler signal by reducing the blurring like auto-focus on a camera.
The transmit frequency is used to down-convert the receive signal to baseband
, and the amount of frequency shift between the transmit signal and the reflected signal increases with time delay (distance). FM Modulation index riding on the receive signal corresponds with range. Small frequency-spread is produced by nearby reflections. A larger frequency-spread corresponds with more time delay and a longer range. The amount of demodulation required to sharpen up the signal corresponds with distance.
Range demodulation is usually limited to 1/6 wavelength of the transmit modulation. Instrumented range for 100Hz FM modulation would be 250km. The radar is not guaranteed to report the correct distance for reflections from distances beyond the instrumented range.
.
Feed-through null
is typically required to eliminate bleed-through between the transmitter and receiver to increase sensitivity in practical systems. This is typically used with continuous wave angle tracking (CWAT) radar receivers that are interoperable with surface to air missile systems.
Interrupted continuous wave can be used to eliminate bleed-through between the transmit and receive antenna. This kind of system typically takes one sample between each pair of transmit pulses, and the sample rate is typically 30kHz or more. This technique is used with the least inexpensive kinds of radar, such as those used for traffic monitoring and sports.
. The transmitter is fairly expensive, while the receiver is fairly inexpensive and disposable.
This is typically used with semi-active radar homing
including most surface to air missile systems. The transmit radar is typically located near the missile launcher. The receiver is located in the missile.
The transmit antenna illuminates the target in much the same way as a search light. The transmit antenna also issues an omnidirectional
sample.
The receiver uses two antennas - one antenna aimed at the target and one antenna aimed at the transmit antenna. The receive antenna that is aimed at the transmit antenna is used to develop the feed-through null
, which allows the target receiver to operate reliably in or near the main beam of the antenna.
Because of their simplicity, CW radar are inexpensive to manufacture relatively free from failure, and cheap to maintain.
CW radars, such as Trackman and Personal Pitcher, are inexpensive and are being used in sports to track ball and equipment parameters.
More sophisticated CW radar systems can reliably achieve accurate detections exceeding 100km distance while providing missile illumination.
Cheap unmodulated radar systems are only reliable when used with one object in a sterile environment free from vegetation, aircraft, birds, weather phenomenon, and other nearby vehicles.
With 20dB antenna side-lobes
, a truck or tree with 1,000 square feet of reflecting surface behind the antenna can produce a signal as strong as a car with 10 square feet of reflecting in front of a small antenna that lacks directionality.
This is a typical problem with radar speed guns
used by law enforcement officers, NASCAR events, and sports, like baseball, golf, and tennis. Interference from a second radar, automobile ignition, other moving objects, moving fan blades on the intended target, and other radio frequency sources will corrupt measurements. These systems are limited by wavelength, which is 0.3 meter at Ku band
, so the beam spread exceeds 45 degrees if the antenna is smaller than 12 inches (0.3 meter). Significant antenna side-lobes extend in all directions unless the antenna is larger than the vehicle on which the radar is mounted.
side-lobe suppression and FM range modulation are required for reliable operation. There is no way to know the direction of the arriving signal without side-lobe suppression, which requires two or more antenna, each with its own individual receiver. There is no way to know distance without FM range modulation.
Speed, direction, and distance are all required to pick out an individual object.
This limitations is well known among radar engineers.
Law enforcement agencies include hand held laser in the mix of tools needed for law enforcement to confirm reliable speed and position of an individual vehicle in traffic after radar detects excessive speed.
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
system where a known stable frequency continuous wave
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
energy is transmitted and then received from any reflecting objects.Continuous wave (CW) radar uses Doppler
Doppler
-Doppler effect and its applications:* Doppler effect* Doppler beaming* Doppler broadening* Doppler cooling** Doppler cooling limit* Doppler echocardiography** Doppler ultrasound, also called Doppler sonography** Transcranial doppler* Doppler fetal monitor...
, which renders the radar immune to interference from large stationary objects and slow moving clutter
Clutter
Clutter may refer to any of the following:*Excessive physical disorder** Clutter , a confusing or disorderly state or collection, and possible symptom of compulsive hoarding** A type of light pollution...
.
CW radar systems are used at both ends of the range spectrum.
- Inexpensive radio-altimeters, proximity sensors and sport accessories that operate from a few dozen feet to several kilometers
- Costly early warning CW angle track (CWAT) radar operating beyond 100km for use with surface to air missile systems
Operation
There are two types of continuous wave radar.- Unmodulated continuous wave
- Modulated continuous wave
There are two different antenna configurations used with continuous wave radar.
- Monostatic radarMonostatic radarMonostatic radar is the term given to a radar in which the transmitter and receiver are collocated. This is the conventional configuration for a radar, but the term is used to distinguish it from a bistatic radar or multistatic radar....
- Bistatic radarBistatic radarBistatic radar is the name given to a radar system which comprises a transmitter and receiver which are separated by a distance that is comparable to the expected target distance. Conversely, a radar in which the transmitter and receiver are collocated is called a monostatic radar...
The main advantage of CW radar is that energy is not pulsed so these are much simpler to manufacture and operate. They have no minimum or maximum range, although the broadcast power level imposes a practical limit on range. Continuous wave radar maximize total power on a target because the transmitter is broadcasting continuously.
The military uses continuous-wave radar to guide semi-active radar homing
Semi-active radar homing
Semi-active radar homing, or SARH, is a common type of missile guidance system, perhaps the most common type for longer-range air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is only a passive detector of a radar signal – provided by an external ...
(SARH) air-to-air missile
Air-to-air missile
An air-to-air missile is a missile fired from an aircraft for the purpose of destroying another aircraft. AAMs are typically powered by one or more rocket motors, usually solid fuelled but sometimes liquid fuelled...
s, such as the U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
AIM-7 Sparrow
AIM-7 Sparrow
The AIM-7 Sparrow is an American, medium-range semi-active radar homing air-to-air missile operated by the United States Air Force, United States Navy and United States Marine Corps, as well as various allied air forces and navies. Sparrow and its derivatives were the West's principal beyond visual...
and standard missile
Standard missile
Standard Missile can refer to a family of several different American missiles:* RIM-66 Standard , a medium range surface-to-air missile, the successor of the RIM-24 Tartar missile...
. The launch aircraft illuminates the target with a CW radar signal, and the missile homes in on the reflected radar waves. Since the missile is moving at high velocities relative to the aircraft, there is a strong Doppler shift. Most modern air combat radars, even pulse Doppler
Pulse-doppler radar
Pulse-Doppler is a 4D radar system capable of detecting both target 3D location as well as measuring radial velocity . It uses the Doppler effect to avoid overloading computers and operators as well as to reduce power consumption...
sets, have a CW function for missile guidance purposes.
Maximum distance in a continuous wave radar is determined by the overall bandwidth and transmitter power. This bandwidth is determined by two factors.
- Transmit energy density (watts per Hertz)
- Receiver filter size (bandwidth divided by the total number of filters)
Doubling transmit power increases distance performance by about 20%. Reducing the total FM transmit noise by half has the same effect.
Frequency domain
Frequency domain
In electronics, control systems engineering, and statistics, frequency domain is a term used to describe the domain for analysis of mathematical functions or signals with respect to frequency, rather than time....
receivers used for continuous wave Doppler radar receivers are very different from conventional radar receivers. The receiver consists of a bank of filters, usually more than 100. The number of filters determines the maximum distance performance.
Doubling the number of receiver filters increases distance performance by about 20%. Maximum distance performance is achieved when receiver filter size is equal to the maximum FM noise riding on the transmit signal. Reducing receiver filter size below average amount of FM transmit noise will not improve range performance.
A CW radar is said to be matched when the receiver filter size matches the RMS bandwidth of the FM noise on the transmit signal.
Unmodulated Continuous Wave
This kind of radar can cost less than $100. Return frequencies are shifted away from the transmitted frequency based on the Doppler effectDoppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
when objects are moving. There is no way to evaluate distance. This type of radar is typically used with competition sports, like golf, tennis, baseball, and NASCAR.
The transmitter is the strongest source of RF energy in the vicinity of the receiver unless there is a large physical barrier along the line of sight, such as a mountain. There must be a strong Doppler shift between the receiver and the reflector, otherwise the bleed-through signal reaching the receive antenna from the transmit antenna will overwhelm the reflected signal arriving at the receiver.
Continuous wave radar with no FM modulation only detects moving targets, as stationary targets (along the line of sight) will not cause a Doppler shift. Reflected signals from stationary and slow-moving objects are masked by the transmit signal, which overwhelms reflections from slow-moving objects during normal operation.
Modulated Continuous Wave
Frequency-modulated (FM) continuous wave radar are capable of determining distance. This increases reliability by providing distance measurement along with speed measurement, which is essential when there is more than one source of reflection arriving at the radar antenna.Distance measurement is implemented using a technique known as frequency modulated continuous-wave radar. In this system the signal is not a continuous fixed frequency, but varies up and down over a fixed period of time. Range is determined by evaluating the frequency-spread of the received signal.
FM modulation is used with CW radar altimeters used with aircraft, CW early warning radar, and proximity sensors. Doppler shift is not always required for detection when FM modulation is used.
The transmitter frequency can slew up and down as follows.
- Sine waveSine waveThe sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...
, like air raid siren - Sawtooth waveSawtooth waveThe sawtooth wave is a kind of non-sinusoidal waveform. It is named a sawtooth based on its resemblance to the teeth on the blade of a saw....
, like the chirp from a bird - Triangle waveTriangle waveA triangle wave is a non-sinusoidal waveform named for its triangular shape.Like a square wave, the triangle wave contains only odd harmonics...
, like police siren in the US - Square waveSquare waveA square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels...
, like police siren in the UK
FM modulation on the receive signal increases with distance. This smears out, or blurs, the Doppler signal. CW radar range detection involves FM demodulation to sharpen the Doppler signal by reducing the blurring like auto-focus on a camera.
The transmit frequency is used to down-convert the receive signal to baseband
Baseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
, and the amount of frequency shift between the transmit signal and the reflected signal increases with time delay (distance). FM Modulation index riding on the receive signal corresponds with range. Small frequency-spread is produced by nearby reflections. A larger frequency-spread corresponds with more time delay and a longer range. The amount of demodulation required to sharpen up the signal corresponds with distance.
Range demodulation is usually limited to 1/6 wavelength of the transmit modulation. Instrumented range for 100Hz FM modulation would be 250km. The radar is not guaranteed to report the correct distance for reflections from distances beyond the instrumented range.
Monostatic Radar
The radar receive antenna is located nearby the radar transmit antenna in monostatic radarMonostatic radar
Monostatic radar is the term given to a radar in which the transmitter and receiver are collocated. This is the conventional configuration for a radar, but the term is used to distinguish it from a bistatic radar or multistatic radar....
.
Feed-through null
Feed-through null
Feed-through null follows the duplexer and is commonly used with continuous-wave radar to improve performance.-Definition:The transmitter is constantly issuing high power RF with continuous wave radar...
is typically required to eliminate bleed-through between the transmitter and receiver to increase sensitivity in practical systems. This is typically used with continuous wave angle tracking (CWAT) radar receivers that are interoperable with surface to air missile systems.
Interrupted continuous wave can be used to eliminate bleed-through between the transmit and receive antenna. This kind of system typically takes one sample between each pair of transmit pulses, and the sample rate is typically 30kHz or more. This technique is used with the least inexpensive kinds of radar, such as those used for traffic monitoring and sports.
Bistatic Radar
The radar receive antenna is located far from the radar transmit antenna in bistatic radarBistatic radar
Bistatic radar is the name given to a radar system which comprises a transmitter and receiver which are separated by a distance that is comparable to the expected target distance. Conversely, a radar in which the transmitter and receiver are collocated is called a monostatic radar...
. The transmitter is fairly expensive, while the receiver is fairly inexpensive and disposable.
This is typically used with semi-active radar homing
Semi-active radar homing
Semi-active radar homing, or SARH, is a common type of missile guidance system, perhaps the most common type for longer-range air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is only a passive detector of a radar signal – provided by an external ...
including most surface to air missile systems. The transmit radar is typically located near the missile launcher. The receiver is located in the missile.
The transmit antenna illuminates the target in much the same way as a search light. The transmit antenna also issues an omnidirectional
Omnidirectional antenna
In radio communication, an omnidirectional antenna is an antenna which radiates radio wave power uniformly in all directions in one plane, with the radiated power decreasing with elevation angle above or below the plane, dropping to zero on the antenna's axis. This radiation pattern is often...
sample.
The receiver uses two antennas - one antenna aimed at the target and one antenna aimed at the transmit antenna. The receive antenna that is aimed at the transmit antenna is used to develop the feed-through null
Feed-through null
Feed-through null follows the duplexer and is commonly used with continuous-wave radar to improve performance.-Definition:The transmitter is constantly issuing high power RF with continuous wave radar...
, which allows the target receiver to operate reliably in or near the main beam of the antenna.
Advantage
Continuous wave radar are cheap enough for the average home budget. Some are small enough to carry in your pocket.Because of their simplicity, CW radar are inexpensive to manufacture relatively free from failure, and cheap to maintain.
CW radars, such as Trackman and Personal Pitcher, are inexpensive and are being used in sports to track ball and equipment parameters.
More sophisticated CW radar systems can reliably achieve accurate detections exceeding 100km distance while providing missile illumination.
Limitations
Unmodulated continuous wave radar cannot measure distance, and the beam is usually broad with side-lobes that extend to the side and behind the radar antenna . There is no way to determine which object corresponds with which speed measurement when there is more than one moving object near the receiver. Moving objects include birds flying near objects in front of the antenna. Reflections from small objects directly in front of the receiver can be overwhelmed by reflections entering antenna side-lobes from large object located to the side, above, or behind the radar, such as trees with wind blowing through the leaves, tall grass, sea surface, freight trains, busses, trucks, and aircraft.Cheap unmodulated radar systems are only reliable when used with one object in a sterile environment free from vegetation, aircraft, birds, weather phenomenon, and other nearby vehicles.
With 20dB antenna side-lobes
Side lobe
In antenna engineering, side lobes or sidelobes are the lobes of the far field radiation pattern that are not the main lobe....
, a truck or tree with 1,000 square feet of reflecting surface behind the antenna can produce a signal as strong as a car with 10 square feet of reflecting in front of a small antenna that lacks directionality.
This is a typical problem with radar speed guns
Radar gun
A radar speed gun is a small doppler radar unit used to measure the speed of moving objects, including vehicles, pitched baseballs, runners and other moving objects. Radar speed guns may be hand-held, vehicle-mounted or static...
used by law enforcement officers, NASCAR events, and sports, like baseball, golf, and tennis. Interference from a second radar, automobile ignition, other moving objects, moving fan blades on the intended target, and other radio frequency sources will corrupt measurements. These systems are limited by wavelength, which is 0.3 meter at Ku band
Ku band
The Kμ band is a portion of the electromagnetic spectrum in the microwave range of frequencies. This symbol refers to —in other words, the band directly below the K-band...
, so the beam spread exceeds 45 degrees if the antenna is smaller than 12 inches (0.3 meter). Significant antenna side-lobes extend in all directions unless the antenna is larger than the vehicle on which the radar is mounted.
side-lobe suppression and FM range modulation are required for reliable operation. There is no way to know the direction of the arriving signal without side-lobe suppression, which requires two or more antenna, each with its own individual receiver. There is no way to know distance without FM range modulation.
Speed, direction, and distance are all required to pick out an individual object.
This limitations is well known among radar engineers.
Law enforcement agencies include hand held laser in the mix of tools needed for law enforcement to confirm reliable speed and position of an individual vehicle in traffic after radar detects excessive speed.