Continuous Wave Radar
Created | Updated Feb 9, 2007
The History of Radar | Radar History: Isle of Wight Radar During The Second World War | Radar: The Basic Principle
Radar Technology: Main Components | Radar Technology: Side Lobe Suppression | Radar Technology: Airborne Collision Avoidance
Radar Technology: Antennas | Radar Technology: Antenna Beam Shapes | Radar Technology: Monopulse Antennas | Radar Technology: Phased Array Antennas | Radar Technology: Continuous Wave Radar | Theoretical Basics: The Radar Equation
Theoretical Basics: Ambiguous Measurements | Theoretical Basics: Signals and Range Resolution
Theoretical Basics: Ambiguity And The Influence of PRFs | Theoretical Basics: Signal Processing | Civilian Radars: Police Radar | Civilian Radars: Automotive Radar | Civilian Radars: Primary and Secondary Radar
Civilian Radars: Synthetic Aperture Radar (SAR) | Military Applications: Overview | Military Radars: Over The Horizon (OTH) Radar
How a Bat's Sensor Works | Low Probability of Intercept (LPI) Radar | Electronic Combat: Overview | Electronic Combat in Wildlife
Radar Countermeasures: Range Gate Pull-Off | Radar Countermeasures: Inverse Gain Jamming | Advanced Electronic Countermeasures
In contrast to pulsed radars, a continuous wave radar (CW radar) has a transmitter that operates all the time. This has the consequence that echoes also arrive all the time. CW radars can be thought of as the degenerate case of either
- High PRF radars with an infinitely high pulse repetition frequency (PRF), or
- Low PRF radars with infinite pulse duration and no waiting time.
This description is only true for pure CW radars that transmit without ever changing their frequency. There are methods available to derive range readings from a CW radar too, but these will be kept aside for now.
Having the transmitter switched on all the time brings about the necessity to isolate the receiver from the transmitter signal to prevent it from being blinded. Therefore a filter is placed at the receiver's input, and many CW radars use a set of two antennae, one for transmission and another for reception.
So what is a CW radar good for?
CW radars are far less complex than pulsed systems because they don't have all the clocking circuitry and high frequency switching devices of low-, medium- or high-PRF radars. Their transmitter hardware is less expensive because there is no need for high power devices capable of handling signals in the megawatt range for the duration of a microsecond. The magnetron taken out of your microwave oven could do the job nicely.
There are applications in which range information doesn't matter, but unambiguous velocity measurement without blind speeds is required.
Sometimes a CW radar works in collaboration with other radars which can determine range but may need some cueing in order to find a target in the first place.
CW Radar Applications
Continous wave radars are used in the following applications:
Some Police Radars are built as CW radars because it's only the velocity that determines whether or not a speeding ticket is to be issued.
Some diseases of blood vessels or the heart can be examined by Doppler profiling. The device doesn't need to determine where a blood vessel is because this information can be looked up in anatomy books. Blood can flow freely within a vessel when no obstacles are present. If there are objects in the way then the blood is forced to flow faster (just like the water of a river is faster when it has to pass rapids). A more scientific explanation can be found in Bernoulli's law of fluid dynamics: the velocity of a fluid is inversely proportional to the aperture area that it flows through.
Cases of thrombosis or arteriosclerosis entail the presence of obstacles in a blood vessel, and their location is determined by recording Doppler profiles.
|============blood vessel================ --> -------> ----> -------------> ------> ---------------> -------> -------------> ------> -------> ----> /========\ --> / obstacle \ ==(A)===================(B)============= (length of arrow indicates speed)|
|Doppler profiles: Speed Speed : : : :|| : :|| :\ :||| :||| :|||| :|||| :|||| :||||| :|||| :|||||| :||||| +---------> +---------> at (A) at (B)|
A Doppler profile plots the amount of blood versus the speed that it travels at. A healthy vessel shows a smooth curve with a low maximum value. The presence of an obstacle leads to a narrow curve with a pronounced peak and a significantly higher average speed.
This is essentially the same method as the one described above. This time it is the movement of water vapour that indicates air speed. Lacking range measurement capabilities, a pure CW radar cannot produce a plot containing wind speed against height, therefore wind profiling usually employs FMCW which will be explained further down.
CW radars are also used for target illumination. This is a straightforward application: the radar's beam is kept on target by slaving it to a target tracking radar and the reflection from the target is used by an anti-air missile to home in on it.
CW radars are somewhat hard to detect. Therefore they are classified as low probability of intercept radars.
CW radars lend themselves well as detectors for low-flying aircraft which try to overcome an enemy's air defence by 'ground-hugging'. A pulsed radar has difficulties in discriminating between ground clutter and low-flying aircraft. A CW radar can close this gap because it is blind to slow-moving ground clutter and can pinpoint the direction where something is going on. This information is relayed to a co-located pulsed radar for closer examination and further treatment.
Range Measuring CW Radars: FMCW
So far, only pure CW radars operating on one and the same frequency have been described. The CW signal can be expressed as a series of 'simple' pulses that have been glued together. There is no variation in this signal and thus, range-measuring capability has gone lost._____________________________________________ _____________________________________________ _____________________________________________ __OOO____OOO_____OOOOOOOOOOOOOOOOOOOOOOOOO... _____________________________________________ simple pulses glued together: CW signal Admittedly, some variation is necessary, and this can be done... by gluing together chirps rather than 'simple' pulses. ____O____O______________O_______O_______O____ ___O______O____________O_O_____O_O_____O_O___ __O________O__________O___O___O___O___O___O__ _O__________O________O_____O_O_____O_O_____O_ O____________O______O_______O_______O_______O chirped pulses glued together: FMCW
This is a continuous wave signal but it has frequency variations on it. Therefore it is called 'Frequency Modulated CW', or FMCW in short. Just as in a pulsed radar, ranging is feasible by comparing the transmitted signal and the echo which is basically a time-delayed replica. Of course, the length of each 'up-sweep' or 'down-sweep' must be chosen carefully to avoid misinterpretations regarding which part of the signal had caused which part of the echo - there is a need to take care of range ambiguities.
History: Overview | Isle of Wight Radar During WWII
Technology: Basic Principle | Main Components | Signal Processing | Antennae | Side Lobe Suppression | Phased Array Antennae | Antenna Beam Shapes | Monopulse Antennae | Continuous Wave Radar
Theoretical Basics: The Radar Equation | Ambiguous Measurements | Signals and Range Resolution | Ambiguity and PRFs
Civilian Applications: Police Radar | Automotive Radar | Primary and Secondary Radar | Airborne Collision Avoidance | Synthetic Aperture Radar
Military Applications: Overview | Over The Horizon | Low Probability of Intercept | How a Bat's Sensor Works
Electronic Combat: Overview | Electronic Combat in Wildlife | Range Gate Pull-Off | Inverse Gain Jamming | Advanced ECM | How Stealth Works | Stealth Aircraft