RADAR Electromagnetic Energy - multiple echos
how should be treat this? for a simple pulsed radar,if we treat the coming radiation (burst of pulse) as waves then there will be multiple waves coming from the RADAR..then how can it receive all the waves and treat it as echo for the tx burst?
can someone please clarify in this regard?
Hi pavannanduri - this is a LOT bigger question than can be answered simply in a forum ;) For starters, go peek at: http://en.wikipedia.org/wiki/Radar
The short answer is some RADARs use pulses, some use continuous waves - and others use a combination of both. In addition, there are many possible modulation formats for both pulsed and CW RADAR, depending upon their intended application, power, bandwidth constraints, antenna design, range, jamming immunity, resolution etc etc etc...
In the case of the simple pulsed RADAR you describe, the pulse repetition frequency (PRF) directly affects the maximum range that can be unambiguously observed - i.e. the round trip time of the previous RADAR pulse (and the return echo) must be < 1/PRF. These sort of systems have generally been made obsolete by better signal processing/modulation techniques that offer vastly improved spectrum efficiency and target resolution.
Hello,
I very much appreciate your answer. i think i can not explain my question in a clear manner. let me put in again. let us say i am not using CW Radar and i am using pulsed RADAR, which has got a PRF of 5000 pulses/sec which will give me an unambiguous range of 30 km. for this example let us say my radar works at 3 GHz. now my radar will radiate 3 GHz frequency wave bursts at the rate of 5000. hope this time i am clear.
now for this scenario assume that for the first burst (which is also a wave for a finite pulse duration τ), now will this radiate from antenna EM energy which can be assumed as a short burst of EM energy or EM waves for certain finite τ duration? ifbvas the later is the case we should be receiving echos from all the EM waves with in the short pulse duration? so in that case which should be treated as an echo?
My question basically relates with radiated EM energy rather than the RADAR as such
The narrow repetitive pulse is a constant frequency carrier. Baseband pulse of broad spectrums such as lightning or echos of other EM energy are rejected due to narrow channel rejection filters centered on a 3 GHz carrier may be on order of > 140dB after 3 superhet conversions.. When pulsed at your example of 5KHz with a duration such as 1μS. It creates a spectrum of 5KHz sidebands up to 1MHz and then repeating above that at 1/τ duration levels. Choices of coherent and incoherent detection help improve resolution of the target and rejection of echos of other EM noise sources.
The "range" (distance to target) is the relative group delay of the center carrier frequency or time delay of the detected pulse is compared with pulse sent using time interval count. The carrier is enhanced to reject all other EM pulses of other carrier frequencies and baseband pulses using several stages of frequency conversion (IF filters) Significant loss is expected with inverse square law in each direction so adaptive loss/gain to prevent front end saturation, distortion yet improve sensitivity with time delay after each pulse, enhances resolution.
Each echo carrier pulse received is expected to be weaker with distance but after compensation;
1. The log of amplitude may indicate size of target after delay compensation in both directions
2. Frequency offset of received pulse may indicate relative speed AND/OR vibration of target.
3. Noise from outside channel will be rejected as defined by filter response
4. Noise inside the channel due to thermal noise may be limiting factor for range with low power.
5. Noise from broad spectral pulsed modulation that scatters IN BAND will be incoherent with PRF so coherent carrier frequency detection will improve on jammer rejection for static targets
6. Coherent pulse response in time domain not simply from averaging but from quasi peak maximum likelihood statistical time coherence of moving targets to cause coherent reflects are accepted
7. Further coherent rejection from random simple modulated carrier bursts or ones design to emulate radar reflects will look like "bogey targets"
8. Bogey target rejection can be designed using ultra stable carrier with unpredictable modulation within each pulse but is is predictable by radar and is coherently detected to reject bogey noise.
9. Time averaging along with signature coherence computations where SNR is improved by √N for N samples averaged.
e.g. In one second (1 sec) image detection SNR can be improved √(5000) = 70 times = 18dB after averaging.
10. Significant noise broad spectrum noise from solar storms , aurora borealis and other plasma dynamic arcing of apparatus near radar station can reduce sensitivity of RADAR.
Such events use tunnelling effects where electrons escape energy levels thru a conductance path of negative resistance and low ESR which accelerates flow of ions , reduces rise times and increases bandwidth of spectral emissions.
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Consider police radar where the beam is fixed direction and pulsing.
Here they are more interested in Doppler freq. or velocity of car so the frequency shift after mixing several stages with successively smaller BW to raise the SNR. In the offset frequency for a car speed can be calibrated with a simple tuning fork, where the freq of the tuning fork represents a speed of a car which produces a an offset freq. equivalent to the tx carrier.
Consider Airport radar. here the tx signal is pulse at 1KHz with a 1uS burst of a certain GHz carrier. After processing in both frequency and Fourier transformed time domain, the group delay of the carrier envelopes which are received are compared with the Tx time for range and velocity is the amount of carrier shift in freq. while angle is relative to the antenna direction.