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Shop Shop. Fish Finder Selector. Johnson Outdoors Family. This translates to better situational awareness in low-visibility conditions, and helps identify birds and floating obstacles for clues to where gamefish are located. You can also track weather and vessels with amazing definition. Figure 1: Input and output signals of a pulse compression stage, the received signal in noise is hardly noticeable, so the pulse compression results in a clear echo signal.
Pulse compression is a method for improving the range resolution of pulse radar. This method is also known as intra-pulse modulation modulation on pulse, MOP because the transmitted pulse got a time-dependent modulation internally.
In publications the inaccurate term Chirp—Radar is often taken which only describes a part of the possible modulation methods. Pulse compression combines the energetic advantages of very long pulses with the advantages of very short pulses. The range resolution of a simple pulse-modulated radar depends on the pulse duration.
Two reflective objects located within the spatial extent of the pulse are only displayed as one target. To improve the range resolution for a relatively long transmission pulse duration, the transmission pulse is modulated internally.
Now a frequency comparison can be made in the received echo, for example, which makes it possible to localize the reflecting object within the pulse. The Pulse Compression Ratio PCR is the ratio of the time length of the uncompressed transmitted pulse to the length of the compressed pulse. The noise is always broadband and the noise pulses have a statistical distribution. The frequency-synchronous part of the noise i. Therefore, the non-frequency synchronous part of the input noise is reduced by the filters.
This way an output signal is still obtained even if the input signal has long since been lost in the noise and would thus be lost for simple demodulation. Compared to the non-modulated pulse, an additional gain is thus obtained, the pulse compression gain or pulse compression factor, which is approximately equal to the Pulse Compression Ratio PCR.
For a linear i. For further calculations, the time-bandwidth product is introduced, the derivation of which results from the ratio of the different range resolutions :. The range resolution of a pulse modulated radar is therefore a multiple by a factor of the Pulse Compression Rate PCR of the range resolution of an intra-pulse modulated radar:. With the help of pulse compression, a relatively long transmission pulse with comparatively low peak power can achieve a better, longer range than the basic radar equation would suggest.
This is because pulse compression can still detect echo signals that have already disappeared in the noise before pulse compression. When a linearly swept or chirp signal is run through a linear delay vs frequency network, as in Fig.
This piling up does not result in a perfectly rectangular pulse, but instead the signal assumes the shape of the pulse shown in Fig.
A mathematician at this point might correctly point out that chirp radar signal processing is nothing but a means of taking the Fourier transform of the rectangular energy spectrum of the transmitted signal. If the sidelobes of this waveform are eliminated, a very good approximation to a conventional rectangular pulse results.
A linear delay vs frequency network is a reciprocal device. This signal is then transmitted at microwave frequency at the required high-power level. In the process, a significant improvement in range, resolution, and peak-power requirement is obtained.
The chirp ratio can be as high as several hundred although the minimum chirp ratio meeting system requirements is always chosen, since the wide receiver bandwidths needed add greatly to system cost and complexity. A chirped radar is compared to a conventional radar in Fig. Here the chirp ratio is about five. There are a number of ways of generating the swept signal. If the same network is used for both chirping and dechirping, any system non-linearities or distortions cancel, giving a cleaner signal than would otherwise be possible.
This is called a matched-filter technique, a tremendously significant radar tool. It is possible also to actively generate a linearly swept frequency without using a delay network.
This method is simpler but requires very careful control of system linearity and sweep rate. There are likewise a number of dechirp, or pulse compression, methods. Certain ultrasonic aluminum delay lines, as well as special quartz delay lines, can directly produce the required delay vs frequency characteristic.
A delay line that delays various frequencies different lengths of time is called a dispersive line. A second method uses a bridged-T network. By carefully "stacking" the right number of bridged-T's, with properly chosen delay widths and center frequencies, a linear delay vs frequency can be very closely approximated.
The complexity of this method is offset by the wide bandwidth and adjustability attainable. There are some more subtle but equally significant advantages of chirp. Note that a chirp system has to have a much wider receiver bandwidth than a conventional pulsed radar of equal peak power. Also note that the energy transmitted is distributed over a much wider range of frequencies.
This makes the radar relatively immune to jamming. There is another significant advantage of chirp. The delay network will only pile up, or compress, one particular swept frequency whose slope and bandwidth exactly match the network. Random signals fed into the delay network will not pile up and will come out of the network with the same amplitude they had when they went in assuming a lossless network.
However, the real signal will pile up by the chirp ratio and increase in amplitude by the same factor. This means that the signal-to-noise ratio of the radar echo returns is considerably improved going through the delay network.
Costas Code waveform but this adds to the complexity of processing. However, in this case, each sub-chirp could be transmitted opportunistically when its sub-band is free. Practically an existing pulse radar such as the ASR-E could use the stepped chirp waveform and can thus take the advantages of the improved range resolution only by software changing.
In practice, however, this would be disadvantageous by the time budget of the radar.
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