Continuous-wave radar
The main advantage of CW radar is that energy is not pulsed so these are much simpler to manufacture and operate.
Continuous-wave radar maximize total power on a target because the transmitter is broadcasting continuously.
The launch aircraft illuminates the target with a CW radar signal, and the missile homes in on the reflected radio 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.
Reducing receiver filter size below average amount of FM transmit noise will not improve range performance.
This type of radar is typically used with competition sports, like golf, tennis, baseball, NASCAR racing, and some smart-home appliances including light-bulbs and motion sensors.
Sawtooth modulation is the most used in FM-CW radars where range is desired for objects that lack rotating parts.
Modulation can be turned off on alternate scans to identify velocity using unmodulated carrier frequency shift.
The time delay is thus a measure of the range; a small frequency spread is produced by nearby reflections, a larger frequency spread corresponds with more time delay and a longer range.
The beat signals are passed through an analog-to-digital converter, and digital processing is performed on the result.
As explained in the literature, FM-CW ranging for a linear ramp waveform is given in the following set of equations:[7] Then,
The amount of spectrum spreading caused by modulation riding on the receive signal is proportional to the distance to the reflecting object.
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.
This is typically used with semi-active radar homing including most surface-to-air missile systems.
The bistatic FM-CW receiver and transmitter pair may also take the form of an over-the-air deramping (OTAD) system.
Significant leakage will come from nearby environmental reflections even if antenna components are perfect.
This is impractical for bistatic systems because of the cost and complexity associated with coordinating time with nanosecond precision in two different locations.
The design constraint that drives this requirement is the dynamic range limitation of practical receiver components that include band pass filters that take time to settle out.
The phase shift and attenuation are set using feedback obtained from the receiver to cancel most of the leakage.
Leakage reduction of 120 dB requires 14 recover bandwidth time constants between when the transmitter is turned off and receiver sampling begins.
The interruption concept is widely used, especially in long-range radar applications where the receiver sensitivity is very important.
Because of simplicity, CW radar are inexpensive to manufacture, relatively free from failure, cheap to maintain, and fully automated.
More sophisticated CW radar systems can reliably achieve accurate detections exceeding 100 km distance while providing missile illumination.
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.
Small radar systems that lack range modulation are only reliable when used with one object in a sterile environment free from vegetation, aircraft, birds, weather phenomenon, and other nearby vehicles.
This is a typical problem with radar speed guns used by law enforcement officers, NASCAR events, and sports, like baseball, golf, and tennis.
There is no way to know the direction of the arriving signal without side-lobe suppression, which requires two or more antennae, each with its own individual receiver.
