Editing Continuous-wave radar (section)

== Configurations ==
[[File:Bsp2 CW-Radar.EN.png|thumbnail|upright=1.5|Block diagram of a simple continuous-wave radar module: Many manufacturers offer such [[transceiver]] modules and rename them as "Doppler radar sensors"]]
There are two different antenna configurations used with continuous-wave radar: ''[[monostatic radar]]'', and ''[[bistatic radar]]''.

===Monostatic===
The radar receive antenna is located nearby the radar transmit antenna in [[monostatic radar]].

[[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 30&nbsp;kHz or more. This technique is used with the least expensive kinds of radar, such as those used for traffic monitoring and sports.

FM-CW radars can be built with one antenna using either a circulator, or circular polarization.

===Bistatic===
The radar receive antenna is located far from the radar transmit antenna in [[bistatic radar]]. 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 antenna|omnidirectional]] sample.

The receiver uses two antennas{{spaced ndash}}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.

The bistatic FM-CW receiver and transmitter pair may also take the form of an over-the-air deramping (OTAD) system. An OTAD transmitter broadcasts an FM-CW signal on two different frequency channels; one for synchronisation of the receiver with the transmitter, the other for illuminating the measurement scene. Using directive antennas, the OTAD receiver collects both signals simultaneously and mixes the synchronisation signal with the downconverted echo signal from the measurement scene in a process known as over-the-air deramping. The frequency of deramped signal is proportional to the bistatic range to the target less the baseline distance between the OTAD transmitter and the OTAD receiver.<ref name="reflect">M. Ash ''et al.'', A New Multistatic FMCW Radar Architecture By Over-The-Air Deramping, [https://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=7182747&reload=true&newsearch=true&queryText=over-the-air%20deramping IEEE Sensors Journal], No. 99, 2015.</ref>

Most modern systems FM-CW radars use one transmitter antenna and multiple receiver antennas. Because the transmitter is on continuously at effectively the same frequency as the receiver, special care must be exercised to avoid overloading the receiver stages.

===Monopulse===
{{Main|Monopulse radar}}

Monopulse antennas produce angular measurements without pulses or other modulation. This technique is used in [[semi-active radar homing]].

===Leakage===
The transmit signal will leak into the receiver on practical systems. Significant leakage will come from nearby environmental reflections even if antenna components are perfect. As much as 120&nbsp;dB of leakage rejection is required to achieve acceptable performance.

Three approaches can be used to produce a practical system that will function correctly.
* Null
* Filter
* Interruption

Null and filter approaches must be used with bistatic radar, like [[semi-active radar homing]], for practical reasons because side-lobes from the illumination radar will illuminate the environment in addition to the main-lobe illumination on the target. Similar constraints apply to ground-based CW radar. This adds cost.

Interruption applies to cheap hand held mono-static radar systems (police radar and sporting goods). 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.

====Null====
The null approach takes two signals:
* A sample of the transmit signal leaking into the receiver
* A sample of the actual transmit signal

The actual transmit signal is rotated 180 degrees, attenuated, and fed into the receiver. The phase shift and attenuation are set using feedback obtained from the receiver to cancel most of the leakage. Typical improvement is on the order of 30&nbsp;dB to 70&nbsp;dB.

====Filter====
The filter approach relies on using a very narrow band reject filter that will eliminate low velocity signals from nearby reflectors. The band reject area spans 10 mile per hour to 100 mile per hour depending upon the anticipated environment. Typical improvement is on the order of 30&nbsp;dB to 70&nbsp;dB.

====Interruption, FMICW====
While interrupted carrier systems are not considered to be CW systems, performance characteristics are sufficiently similar to group interrupted CW systems with pure CW radar because the pulse rate is high enough that range measurements cannot be done without frequency modulation (FM).

This technique turns the transmitter off for a period before receiver sampling begins. Receiver interference declines by about 8.7&nbsp;dB per time constant. Leakage reduction of 120&nbsp;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. It is commonly known as "frequency modulated interrupted continuous wave", or FMICW.