Frequency Modulated Continuous Wave (FMCW) Radars Have Advantages Over Pulse Doppler Radars For Ground Surveillance:Ground Surveillance Radars can build a virtual wall around facilities or on a border. They provide operators and agents more response time to access, prioritize and apprehend intruders. They provide wide area surveillance and tracking over a large, 360 degree area, directing responders even after an intrusion has occurred. But, all GSR technologies are not the same. There are two primary GSR technologies - Pulsed Doppler radar technology and Frequency Modulated Continuous Wave (FMCW) radar technology. Most Pulsed Doppler radars are derivatives of legacy military battlefield radar being applied for wide area surveillance, while a new generation of FMCW radar technology was developed for wide area surveillance, site security and force protection. It was specifically developed to detect and track walking personnel. ICx Radars use FMCW radar technology. Frequency Modulated Continuous Wave (FMCW) Radars:FMCW radars operate on the imaging principle; that is, they break up the background into small segments, or resolution cells, and then measure changes in the signal return from each cell to detect small targets, such as walking people. Typical resolutions for long range FMCW radars are less than 1 meter in range and less then 1 degree in azimuth. The smaller the cell the easier it is to detect and track a target. FMCW operation is independent of the speed or direction of travel of the target, only its size with respect to the resolution cell in which it is located. Modern FMCW radars can detect people moving at near zero speed and walking in any direction with respect to the radar. Pulse Doppler (PD) Radars:Pulse Doppler Radars operate on the Doppler principle, which states that all moving objects will exhibit a frequency shift from the transmitted signal to the received signal, which is proportional to the speed of the target in the direction of the radar. If a target is walking directly toward the radar at 3MPH, the radar will detect a frequency difference in the received signal and declare that a 3 MPH target has been detected. If the target is walking at 45 degree angle to the radar, the Doppler signal will be 3 MPH times the cosine of the angle, or about 2.1 MPH. However, background clutter like trees and bushes also have some apparent speed when the wind blows. In order not to have a large number of false alarms, that low speed signal return from the clutter must be filtered out. A virtual velocity threshold (blind speed) is created below which targets will not be reliably detected. That means that some slowly moving targets could be filtered out along with the clutter. It also means that higher speed targets moving across the radar beam may be filtered out because speed only generates a Doppler signal proportional to the incoming or outgoing speed, which is called radial speed (approaching or receding in the beam).

Implications of Using Doppler as the Detection Technique:A fundamental deficiency exists such that wide area surveillance systems using Pulse Doppler radars have large areas where slow targets will not be detected. In fact, if an intruder walks at a speed somewhat below the velocity threshold (defined as the "blind speed") of the radar, it doesnt matter in what direction the intrusion takes place, the intruder will likely not be detected at all the intruder can simply walk through the perimeter or across the border and the radar will not detect the target. Alternatively, an intruder can walk between two radars spaced along a border and will be moving across the beams, or tangentially to each radar, and therefore, can walk at a higher speed than the velocity threshold, and still not be detected. This deficiency gives the intruders a major advantage. Those familiar with border operations know that intruders learn to avoid areas where they are apprehended regularly. Thus, holes in coverage inherent to Pulsed Doppler radars will be found and exploited, nullifying the very purpose of the radars. Changing the spacing or offsetting radars in latitude will somewhat change the shape of the nondetect zones, but will not eliminate the deficiency. In summary, PD radars have an inherent flaw when used in ground surveillance applications. There is a conflicting trade off between minimizing clutter returns and the minimum detection speed of the target. Most PD radars will never detect at speeds less than 1.5 miles per hour (a distinct probability with walkers carrying 50 pounds or more of contraband). The FMCW Advantage - Summary:The STS-12000 radar has the advantage of being designed specifically for perimeter and border surveillance using the most optimum technology for this mission: frequency modulated continuous wave (FMCW). The benefits of FMCW over other technologies such as pulse doppler (PD) are numerous: FMCW is less complex, safer and lower cost than PD FMCW gives low false alarm rates Proven in Government testing - The only radars to pass stringent U.S. Air Force false alarm testLess likely to alarm with wind blown objects --- grass and leaves, rain o One FMCW installation has 31 radars netted together using only one operator FMCW sees a higher percentage of valid targets o Wont miss slower targets or tangential ones no holes in coverage no one penetrates Smaller beamwidth for better pointing of cameras

Frequency-Modulated Continuous-Wave Radar:-

transmitted signal received echo signalFigure 1: Ranging with an FMCW system

CW radars have the disadvantage that they cannot measure distance, because it lacks the timing mark necessary to allow the system to time accurately the transmit and receive cycle and convert the measured round-trip-time into range. In order to correct for this problem, phase or frequency shifting methods can be used. In the frequency shifting method, a signal that constantly changes in frequency around a fixed reference is used to detect stationary objects and to measure the rage. In such a Frequency-Modulated Continuous Wave radars (FMCW), the frequency is generally changed in a linear fashion, so that there is an up-and-down or a sawtoothlike alternation in frequency. If the frequency is continually changed with time, the frequency of the echo signal will differ from that transmitted and the difference f will be proportional to round trip time t and so the range R of the target too. When a reflection is received, the frequencies can be examined, and by comparing the received echo with the actual step of transmitted frequency, you can do a range calculation similar to using pulses:c0 |t | c0 |f | c0 = speed of light = 3108 m/s t = measured time-difference [s] R = distance where: altimeter to (1) terrain [m] df/dt = transmitters frequency shift per unit time

R= 2

= 2 (df/dt)

Accordingly, measuring the difference between the transmitted and received frequencies gives the range to the stationary target. It is generally not easy to make a broadcaster that can send out random frequencies cleanly, so instead these frequency-modulated continuous-wave radar, use a smoothly varying ramp of frequencies up and down. If the frequency modification is linearly over a wide area, so within this region by a frequency comparison f, the distance can be determined on a simple way. Since that only the absolute value of the difference can be measured, the results with increasing frequency modification equal to a decreasing frequency change at a static scenario. Sawtooth modulation forms are preferred for imaging radar; triangular shaped modulation is used more for non-imaging radars. Characteristic feature of an FMCW radar is:

the distance measurement is done by comparing the actual frequency of the received signal to a given reference (usually direct the transmitted signal): the duration of the transmitted signal is much larger than the time required for measuring the installed maximum range of the radar

By suitable choice of frequency deviation per time unit can be varied the radar resolution, and by choice of the duration of the time of the frequency shift the maximum range can be varied. For example, a radar with a linear frequency increase over 1 ms duration can measure a time-limited maximum range of nearly 150 km. If the maximum frequency deviation is 65 MHz, then stay about 433 Hz per meter for the filter for analysis. Of course, the amount of frequency modulation must be significantly greater than the expected Doppler shift or the results will be affected. The simplest way to modulate the wave is to linearly increase the frequency. In other words, the transmitted frequency will change at a constant rate.

Figure 2: Strip-line patch antenna of maritime FMCW- navigation radar operating in X-Band

As a result of the proceedings (simultaneous transmission and receiving), a ferrite circulator shall make the separation of transmit and receive path, when using a single antenna. But using of separate transmitting and receiving antennas is much cheaper in today's common used patch antennas in strip-line technology. On a common substrate transmitting and receiving antenna are mounted directly above each other as an antenna array. The direction of the linear polarization is rotated against each other by 180 degrees. An additional shielding plate reduced a direct "cross talk" (i.e. a direct coupling of both antennae) often. Since the measurement is performed to as a frequency difference between transmit and receive signal, the signal that arises from this direct coupling is suppressed due to the same frequency.

Imaging FMCW Radar This radar method is used in so-called Broadband Radar as a navigation radar for maritime applications. Here, the frequency sweep after reaching the maximum measuring distance is, however, stopped. The transmitted signal looks more like the signal from a pulse radar using intra pulse modulation therefore. This break, however, has no direct effect on the maximum measuring distance, in contrast to the pulse radar. However, it is necessary to read the very many measured data from a memory buffer, and to transmit this data without loss through a narrow-band line to the radar scope. Because of its principle of operation frequency comparison of the received echo signal with the transmitted signal, which is available over the whole range sweep it remains an FMCW radar. The transmitter is switched off for a few milliseconds only, as more data are simply not needed. An imaging radar carries out a distance measurement for each point or pixel on the monitor. The radars range resolution depends more on the size of a pixel on this screen therefore, and depends on the capacity of signal processing to provide the data in the required speed. With the given as an example of frequency shift of 65 MHz per millisecond, the radar obtains good values of range resolution. You need a high-resolution screen with the required number of pixels. If it is possible to make a frequency comparison during a clock of the length of 15 nanoseconds, the imaging FMCW radar can achieve a range resolution of a slightly more than 2 meters. Non-Imaging FMCW Radar

Figure 3: Analog indicator of a radar altimeter

The measurement result of this FMCW radar is shown as a numerical value on a moving coil meter or digitized as alpha-numeric symbols on a screen. It can only be a single dominant object to be measured, but of this with a much high degree of accuracy down to the centimeter range. The most common form of FMCW radar is the radar altimeter used on aircraft to determine height above the ground, especially during the landing procedure of aircraft. A possible Doppler frequency fD is displayed on the moving coil meter as a measuring error. The gradient of the slope can be chosen that the influence of the Doppler frequency is very small in contrast to the measured frequency difference. An analysis of the Doppler frequency is possible by using a triangular shaped modulation and a separate frequency comparison during

the rising and falling side of the triangle shaped modulation. For a reflective object with a positive (moving towards the radar) radial velocity the entire received signal will be moved by the Doppler frequency to higher frequencies. Compared to a fixed reflector, the frequency difference between transmit and receive signals on the rising edge of the triangle is reduced by the Doppler frequency and increased on the falling edge by the Doppler frequency. The difference between the two difference frequencies is therefore twice the Doppler frequency. Since both of difference frequencies are not available simultaneously, therefore this comparison, however, requires a digital signal processing.