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Most this range can never be achieved due to low power of the transmitter. Thus always remains enough time for a measurement of the difference frequency. This corresponds to a range resolution of 0. This example shows impressively the advantage of the FMCW radar: A pulse radar must measure these 4 ns delay difference, resulting in a considerable technical complexity.
A difference in frequency of 1 kHz, however, is much easier to measure because it is in audio range. As with any radar in the FMCW radar, besides the allocated bandwidth, the antenna beamwidth determines the angular resolution in detecting objects. There are several possible modulation patterns which can be used for different measurement purposes:. In a linear sawtooth frequency changing see Figure 1 a delay will shift the echo signal in time i.
This results in a frequency difference between the actual frequency and the delayed echo signal, which is a measure of the distance of the reflecting object. An occurring Doppler frequency would now move the frequency of the entire echo signal either up moving towards the radar or down moving away from the radar. In this form of modulation, the receiver has no way to separate the two frequencies. Thus, the Doppler frequency will occur only as a measurement error in the distance calculation.
In the choice of an optimum frequency sweep can be considered a priori, that the expected Doppler frequencies are as small as the resolution or at least, that the measurement error is as small as possible. This will be the case for example in maritime navigation radar: Boats move in the coastal area at a limited speed, with respect to each other perhaps with a maximum of 10 meters per second. In this frequency band of these radar sets X-Band mostly , the expected maximum Doppler frequency is Hz If the radar signal processing uses a resolution in the kilohertz range per meter, this Doppler frequency is negligible.
The measurement error caused by the Doppler frequency can be greater than the distance to be measured. The target signs would then theoretically appear in a negative distance, i. In a triangular-shaped frequency changing, a distance measurement can be performed on both the rising and on the falling edge.
In Figure 3, an echo signal is shifted due to the running time compared to the transmission signal to the right. Without a Doppler frequency, the amount of the frequency difference during the rising edge is equal to the measurement during the falling edge.
A Doppler frequency shifts the echo signal in height green graph in the figure 3. This opens up the possibility of making an accurate distance determination, despite the frequency shift caused by the Doppler frequency, which then consists of the arithmetic average of the two parts of measurements at different edges of the triangular pattern. At the same time the accurate Doppler frequency can be determined from two measurements. The difference between the two difference frequencies is twice the Doppler frequency.
Since the two differential frequencies, however, are not simultaneously available, this comparison requires digital signal processing, with intermediate storage of the measured results. The Doppler frequency-adjusted frequency for the distance determination and the Doppler frequency of a moving target is calculated by:. However, this method has the disadvantage that, if appear a plurality of reflective objects, the measured Doppler frequencies cannot be uniquely associated with a target.
The assignment of the wrong Doppler frequency to a destination in the wrong distance can lead to ghost targets. In figure 4 a graphical solution is shown. The intersection of the two lines is the position of the target 1. When a second object [……] 2 with a second Doppler frequency appears Just then both pairs of linear slopes give a total of four intersections, two of which are the ghost targets. The position of ghost targets also depends on the steepness of the modulation pattern.
The transceiver is simply switched back and forth with a rectangular control voltage between two transmission frequencies. There are two principal ways to process the output signals of the transceiver. The first possibility is to measure the duration of the frequency change. A signal appears at the output of the transceiver whose envelope is a pulse having a given pulse width as a measure for the distance. However, this measurement is a pure waste of time like the measurement of pulse radar and is therefore either inaccurate or technologically very complex.
A second possibility is to compare the phase angle of the echo signals of the two frequencies. During the pulse top of the rectangular pulse, the radar operates at the first frequency, and during the interpulse period the radar operates at the second frequency. During these times in the millisecond range, the radar will work as for CW radar method.
The output of the down mixer see block diagram , a DC voltage appears as a measure of the phase difference between the reception signal and its transmission signal. The phase difference between the echo signals of different transmission frequencies technically: the voltage difference at the output of the mixer is a measure of the distance. However, because of the periodicity of the sine wave, this method has only a very limited unambiguous measurement distance that is even this range; there the phase difference between the both echo signals is smaller than the half-wavelength.
A frequency difference of 20 MHz between two transmission frequencies results in an unambiguous measuring range of 15 meters. Multiple targets at close range cannot be separated, since only one phase angle can be measured at the output of the mixer stage. Several targets overlap to only a single output voltage at which dominates the strongest target. If both analysis methods in time and in phase are applied simultaneously, then the time-dependent distance determination can used to as a rough evaluation.
The detailed results of the phase analysis can then be multiplied until the result is close enough to the distance from the measurement of time. The bad unambiguous maximum range of the measurement of phase difference is thus avoided. In general, the same advantages and disadvantages of a stepped frequency modulation as the method with a square-wave modulation apply.
However, the FMCW radar is now working with several successive frequencies. In each of these individual frequencies, a phase angle of the echo signal is measured. The unambiguous measurement range widens considerably, however, since now the phase relationships between several frequencies must be repeated to create ambiguities. This method will be very interesting if resonances for individual component frequencies can be observed at the irregularities of the reflecting object.
This measurement method is then a field of interferometry. An FMCW radar consists essentially of the transceiver and a control unit with a microprocessor. The transceiver is a compact module, and usually includes the patch antenna implemented as separate transmit and receive antenna.
The high frequency is generated by a voltage controlled oscillator which directly feeds the transmitting antenna, or its power is additionally amplified. A part of the high frequency is coupled out and fed to a mixer which down converts the received and amplified echo signal in the baseband.
The control board contains a microprocessor that controls the transceiver, converts the echo signals in a digital format as well usually via USB cable ensures the connection to a personal computer or laptop. Using a digital to analog converter, the control voltage is provided to the frequency control. As noted in the previous section, a radar is essentially a ranging or distance measuring device. It consists fundamentally of a transmitter, a receiver, an antenna, and an electronics system to process and record the data.
The transmitter generates successive short bursts or pulses of microwave A at regular intervals which are focused by the antenna into a beam B. The radar beam illuminates the surface obliquely at a right angle to the motion of the platform. The antenna receives a portion of the transmitted energy reflected or backscattered from various objects within the illuminated beam C. By measuring the time delay between the transmission of a pulse and the reception of the backscattered "echo" from different targets, their distance from the radar and thus their location can be determined.
As the sensor platform moves forward, recording and processing of the backscattered signals builds up a two-dimensional image of the surface. While we have characterized electromagnetic radiation in the visible and infrared portions of the spectrum primarily by wavelength, microwave portions of the spectrum are often referenced according to both wavelength and frequency.
The microwave region of the spectrum is quite large, relative to the visible and infrared, and there are several wavelength ranges or bands commonly used which given code letters during World War II, and remain to this day. Here are two radar images of the same agricultural fields, each image having been collected using a different radar band.
The one on the top was acquired by a C-band radar and the one below was acquired by an L-band radar. You can clearly see that there are significant differences between the way the various fields and crops appear in each of the two images.
This is due to the different ways in which the radar energy interacts with the fields and crops depending on the radar wavelength. We will learn more about this in later sections. When discussing microwave energy, the polarization of the radiation is also important. Polarization refers to the orientation of the electric field recall the definition of electromagnetic radiation from Chapter 1.
Most radars are designed to transmit microwave radiation either horizontally polarized H or vertically polarized V. Similarly, the antenna receives either the horizontally or vertically polarized backscattered energy, and some radars can receive both.
These two polarization states are designated by the letters H for horizontal, and V, for vertical. Thus, there can be four combinations of both transmit and receive polarizations as follows:. The first two polarization combinations are referred to as like-polarized because the transmit and receive polarizations are the same.
The last two combinations are referred to as cross-polarized because the transmit and receive polarizations are opposite of one another. These C-band images of agricultural fields demonstrate the variations in radar response due to changes in polarization. The upper left image is the result of displaying each of the three different polarizations together, one through each of the primary colours red, green, and blue. Similar to variations in wavelength, depending on the transmit and receive polarizations, the radiation will interact with and be backscattered differently from the surface.
Both wavelength and polarization affect how a radar "sees" the surface. Therefore, radar imagery collected using different polarization and wavelength combinations may provide different and complementary information about the targets on the surface. Typical output products e. For RADARSAT imagery, this must include the squaring of the digital values and the application of a lookup table which can have range dependent values.
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