Who Goes There: Friend or Foe? - Federation of American Scientists

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.- 40 I Who Goes There: Friend or Foe? Box 3-A-The Electromagnetic Spectrum Chapter 3 discusses in general terms-and the next two chapters discuss in much more specific detail--some oft he identification and communication techniques important for avoiding friendly fire. Many of these techniques depend on detecting some form of electromagnetic radiation. The figure 3-A represents the electromagnetic spectrum. Visible light is the most familiar part of the spectrum but radiation at higher and lower wavelengths is exactly the same physical phenomenon, just not detectable by the human eye. Radiation of different wavelengths interacts with matter in different ways, which has two important consequences: first, the wavelength determines how easily the radiation passes through the atmosphere, including obscurants, such as fog, smoke, rain, or dust; second, the wavelength affects how the radiation is generated and detected. In general, radiation is little disturbed by particles smaller than the radiation’s wavelength, thus longer wavelengths tend to pass more easily through airborne particulate like smoke or fog. Infrared radiation--with a longer wavelength than visible radiation--can be used to see through clouds of obscurants that appear impenetrable to the human eye and even longer wavelength radar waves can pass through rain that would stop infrared radiation. at a characteristic frequency not used by friendly forces, or the sound of a tank that is different from that of friendly tanks. The observer could be active and transmit energy to the target. Most commonly this is in the form of radio signals of some sort. Even if the target is passive, the shooter can still bounce radar signals off it—this is standard radar. Thus an active shooter can detect the radar return of a passive target and get some information about the Figure 3-5—Examples of Approaches to IFF L SOURCE: DoD and OTA. TARGET Passive Active Visual ID of Beacons: weapons type, e.g., Budd-lights, painted insignia DA RPA-lights Question- Radar and-answer systems target that might allow it to be classified as friend or foe. Alternately, the target might actively transmit energy in response to the shooter’s transmission. In such a system, the shooter sends a special signal to the target. If the target wishes to be identified, and is equipped with the proper transponders, then it sends back a signal signifying that it is a friend. This is the principle of ‘‘cooperative’ IFF systems, such as the MARK XII. These are often called ‘‘question-andanswer” systems. Finally, each branch of the tree shown in the figure can be cooperative or noncooperative. Painting special insignia on a vehicle for others to see is an example of a passive/passive cooperative system. Normal radar is an active/passive noncooperative system, but if the target adds special radar reflectors to enhance radar echoes, it becomes an active/passive cooperative system. Just because a target gives an active response to an active observer does not imply necessarily that the target is cooperative; it might be tricked into responding. Military electronics experts are always trying to find ways to cause an enemy to transmit energy revealing his position, intentions, or capability.
Chapter 3—Avoiding Fratricide: Genera Considerations 141 Longer wavelength radiation may be able easily to penetrate the atmosphere but another problem arises as longer wavelength radiation is used: the ultimate resolution of any imagining device is limited by the ratio of the wavelength and the size of the imaging optics. Radars using radiation with waves several centimeters or meters long may easily penetrate long distances, but to achieve any resolution requires proportionally large antennas. Thus, long wavelength radiation may be useful for communication and detection but is not much use for deriving an image. Several parts of the spectrum are of importance to the problem of avoiding fratricide. Communication usually uses UHF and VHF radio bands. The radar bands shown are used to detect objects and perhaps identify them, The infrared bands are important because ground forces use them for seeing at night and under conditions of limited visibility. With increasing wavelength, infrared blends into the millimeter wave bands that will be used by the proposed question-and-answer identification system for ground combat vehicles. The Electromagnetic Spectrum Frequency (Hz) 1072 1 o?~ 1 ().8 1 o’~ 1014 1 o’? 1 o’~ 1 Oa 1 OF 10’ lo~ 1 Wavelength (m) 10 ‘ 3 10 “ 109 10’ 105 0.4 0.9 1.1 8 10 12 I Visible Iight ‘ I I I { Sensitive range of IR lmaglng/targeting night vision goggles viewer 103 0.1 10 103 1 o’ 1 o~ Nominal frequency ruu~cpprpy -CDbm range (GHz) Li.3~r’d O-J03 Wave lengths (microns)
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