26-02-2013, 12:51 PM
Range & Doppler Accuracy
Range & Doppler.pptx (Size: 3.07 MB / Downloads: 26)
Doppler Shift:
A frequency shift in electromagnetic waves due to the motion of scatters toward or away from the observer
Analogy: The Doppler shift for sound waves is the change in frequency one detects as race cars or airplanes approach and then recede from a stationary observer
Doppler Radar:
A radar that can determine the frequency shift through measurement of the phase change
that occurs in electromagnetic waves during a series of pulses
Stationary Sound Source
Sound waves are produced at a constant frequency f0, and the wavefronts propagate symmetrically away from the source at a constant speed v, which is the speed of sound in the medium. The distance between wavefronts is the wavelength. All observers will hear the same frequency, which will be equal to the actual frequency of the source.
How can we relate Doppler Effect to a RADAR ?
Doppler Radar:
A radar that can determine the frequency shift through measurement of the phase change that occurs in electromagnetic waves during a series of pulses
If both the source and the receiver of the sound remain stationary, the receiver will hear
the same frequency sound produced by the source. This is because the receiver is
receiving the same number of waves per second that the
source is producing.
Doppler Dilemma
Now we can see , that the radars ability to measure an ambiguous range and unambiguous Doppler frequency (i.e. the aims radial speed) depends on the transmitter’s carrier frequency only.
In the absence of noise ,the peak of the signal term would give a time delay corresponding to the targets Range .
However this noise term will cause this peak to move around .In practical the noise term is not zero at correct time delay, the peak is displaced.
As a result the time delay of the peak will take a range of values and this noise can be related with the Gaussian noise with a mean value occurring a true delay .
Good range accuracy is not dependent on short pulse lengths, but needs large bandwidth :good Doppler accuracy needs long pulses.