can be detected when a helicopter is moving slow near terrain and weather. Helicopters appears like a rapidly pulsing noise emitter except in a clear environment free from clutter. An audible signal is produced for passive identification of the type of airborne object. Microwave Doppler frequency shift produced by reflector motion falls into the audible sound range for human beings (), which is used for target classification in addition to the kinds of conventional radar display used for that purpose, like A-scope, B-scope, C-scope, and RHI indicator. The human ear may be able to tell the difference better than electronic equipment. A special mode is required because the Doppler velocity feedback information must be unlinked from radial movement so that the system can transition from scan to track with no lock. Similar techniques are required to develop track information for jamming signals and interference that cannot satisfy the lock criterion. Multi-mode Pulse-Doppler radar must be multi-mode to handle aircraft turning and crossing trajectory. Once in track mode, pulse-Doppler radar must include a way to modify Doppler filtering for the volume of space surrounding a track when radial velocity falls below the minimum detection velocity. Doppler filter adjustment must be linked with a radar track function to automatically adjust Doppler rejection speed within the volume of space surrounding the track. Tracking will cease without this feature because the target signal will otherwise be rejected by the Doppler filter when radial velocity approaches zero because there is no change in frequency. Multi-mode operation may also include continuous wave illumination for semi-active radar homing. See also Radar signal characteristics (fundamentals of the radar signal) Doppler radar (non-pulsed; used for navigation systems) Weather radar (pulsed with Doppler processing) Continuous-wave radar (non-pulsed, pure Doppler processing) Fm-cw radar (non-pulsed, swept frequency, range and Doppler processing) Aliasing - the reason for ambiguous velocity estimates Doppler sonography - velocity measurements in medical ultrasound. Based on the same principle External links Doppler radar presentation, which highlights the advantages of using the autocorrelation technique Pulse-Doppler radar handouts from Introduction to Principles and Applications of Radar course at University of Iowa Modern Radar Systems by Hamish Meikle () Advanced Radar Techniques and Systems edited by Gaspare Galati () References Bibliography Radar Radar signal processing Measuring instruments Microwave technology Navigational equipment Air traffic control Science and technology during World War II Targeting (warfare)