A method of detecting the presence, position, and direction of motion of distant objects (such as ships and aircraft) by means of their ability to reflect a beam of electromagnetic radiation of centimetric wavelengths. It is also used for navigation and guidance. It consists of a transmitter producing radio-frequency radiation, often pulsed, which is fed to a movable aerial from which it is transmitted as a beam. If the beam is interrupted by a solid object, a part of the energy of the radiation is reflected back to the aerial. Signals received by the aerial are passed to the receiver, where they are amplified and detected. An echo from a reflection of a solid object is indicated by a sudden rise in the detector output. The time taken for a pulse to reach the object and be reflected back (t) enables the distance away (d) of the target to be calculated from the equation
d = ct/2
, where c is the speed of light. In some systems the speed of the object can be measured using the Doppler effect. The output of the detector is usually displayed on a cathode-ray tube in a variety of different formats (see illustration). 
Types of cathode-ray tube radar display
- dispersion
- dispersion forces
- dispersive power
- displacement
- distortion
- distributive law
- diurnal
- divergence
- divergence theorem
- divergent series
- diverging lens or mirror
- dl-from
- DLA
- domain
- donor
- doping
- Doppler effect
- dose
- dosimeter
- dot product
- double refraction
- doublet
- drain
- drift-tube accelerator
- dry cell