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RANGE Radar measurement of range, or distance, is made possible because of the properties of radiated electromagnetic energy. This energy normally travels through space in a straight line, at a constant speed, and will vary only slightly because of atmospheric and weather conditions. The effects atmosphere and weather have on this energy will be discussed later in this chapter; however, for this discussion on determining range, these effects will be temporarily ignored. Electromagnetic energy travels through air at approximately the speed of light, which is 186,000 STATUTE MILES per second. The Navy uses NAUTICAL MILES to calculate distances; 186,000 statute miles is approximately 162,000 nautical miles. While the distance of the statute mile is approximately 5,280 feet, the distance for a nautical mile is approximately 6,080 feet. Radar timing is usually expressed in microseconds. To relate radar timing to distances traveled by radar energy, you should know that radiated energy from a radar set travels at approximately 984 feet per microsecond. With the knowledge that a nautical mile is approximately 6,080 feet, we can figure the approximate time required for radar energy to travel one nautical mile using the following calculation:
The same answer can be obtained using yards instead of feet. In the following calculation, the 6,080 foot approximation of a nautical mile is converted to 2,027 yards and energy speed is changed from 984 feet to 328 yards per microsecond:
A pulse-type radar set transmits a short burst of electromagnetic energy. Target range is determined by measuring elapsed time while the pulse travels to and returns from the target. Because two-way travel is involved, a total time of 12.36 (6.18 x 2) microseconds per nautical mile will elapse between the start of the pulse from the antenna and its return to the antenna from a target. This 12.36 microsecond time interval is sometimes referred to as a RADAR MILE, RADAR NAUTICAL MILE, or NAUTICAL RADAR MILE. The range in nautical miles to an object can be found by measuring the elapsed time during a round trip of a radar pulse and dividing this quantity by 12.36. In equation form, this is:
For example, if the elapsed time for an echo is 62 microseconds, then the distance is 5 miles, as shown in the following calculation:
NOTE: Unless otherwise stated all distances will be expressed as nautical miles throughout this module. Minimum Range Recall from NEETS, Module 11, Microwave Principles, that the DUPLEXER alternately switches the antenna between the transmitter and receiver so that only one antenna need be used. This switching is necessary because the high-power pulses of the transmitter would destroy the receiver if energy were allowed to enter the receiver. As you probably already realize, timing of this switching action is critical to the operation of the radar system. What you may not realize is that the minimum range ability of the radar system is also affected by this timing. The two most important times in this action are PULSE WIDTH and RECOVERY TIME. This timing action must be such that during the transmitted pulse (pulse width), only the transmitter can be connected to the antenna. Immediately after the pulse is transmitted, the antenna must be reconnected to the receiver. The leading edge of the transmitted pulse causes the duplexer to align the antenna to the transmitter. This action is essentially instantaneous. At the end of the transmitted pulse, the trailing edge of the pulse causes the duplexer to line up the antenna with the receiver; however, this action is not instantaneous. A small amount of time elapses at this point that is referred to as recovery time. Therefore, the total time in which the receiver is unable to receive the reflected pulse is equal to the pulse width plus the recovery time. Note that any reflected pulses from close targets returning before the receiver is connected to the antenna will be undetected. The minimum range, in yards, at which a target can be detected is determined using the following formula (pulse width and recovery time are expressed in microseconds or fractions of microseconds):
For example, minimum range for a radar system with a pulse width of 25 microseconds and a recovery time of 0.1 microseconds is figured as follows:
Most modern radar systems are designed with such small recovery times that this figure can often be ignored when figuring minimum range. Maximum Range The maximum range of a pulse radar system depends upon CARRIER FREQUENCY, PEAK POWER of the transmitted pulse, PULSE-REPETITION FREQUENCY (prf) or PULSE REPETITION RATE (prr), and RECEIVER SENSITIVITY with prf as the primary limiting factor. The peak power of the pulse determines what maximum range the pulse can travel to a target and still return a usable echo. A usable echo is the smallest signal detectable by a receiver system that can be processed and presented on an indicator. The frequency of the rf energy in the pulse radiated by a radar is referred to as the CARRIER FREQUENCY of the radar system. The carrier frequency is often a limiting factor in the maximum range capability of a radar system because radio frequency energy above 3,000 megahertz is rapidly attenuated by the atmosphere. This decreases the usable range of radio-frequency energy. Therefore, as the carrier frequency is increased, the transmitted power must also be increased to cover the same range. Long-range coverage is more easily achieved at lower frequencies because atmospheric conditions have less effect on low-frequency energy. Radar systems radiate each pulse at the carrier frequency during transmit time, wait for returning echoes during listening or rest time, and then radiate a second pulse, as shown in figure 1-3. The number of pulses radiated in one second is called the pulse-repetition frequency (prf), or the pulse-repetition rate (prr). The time between the beginning of one pulse and the start of the next pulse is called PULSE-REPETITION TIME (prt) and is equal to the reciprocal of prf as follows:
Figure 1-3. - Radar pulse relationships.
AMBIGUOUS RETURNS. - The radar timing system must be reset to zero each time a pulse is radiated. This is to ensure that the range detected is measured from time zero each time. The prt of the radar becomes important in maximum range determination because target return times that exceed the prt of the radar system appear at incorrect locations (ranges) on the radar screen. Returns that appear at these incorrect ranges are referred to as AMBIGUOUS RETURNS or SECOND-SWEEP ECHOES. Figure 1-4 illustrates a radar system with a 1 millisecond prt. The pulses are shown at the top, and examples of two transmitted pulses hitting targets and returning are shown at the bottom. In the case of target A, the pulse travels round trip in 0.5 millisecond, which equates to a target range of 82,000 yards. Since 0.5 millisecond is less than 1 millisecond, displaying a correct range is no problem. However, target B is 196,800 yards distant from the radar system. In this case, total pulse travel time is 1.2 milliseconds and exceeds the prt limitation of 1 millisecond for this radar. While the first transmitted pulse is traveling to and returning from target B, a second pulse is transmitted and the radar system is reset to 0 again. The first pulse from target B continues its journey back to the radar system, but arrives during the timing period for the second pulse. This results in an inaccurate reading. In this case, the first return pulse from target B arrives 0.2 millisecond into the second timing period. This results in a range of 32,800 yards instead of the actual 196,800 yards. You should see from this example that pulse returns in excess of the prt of the radar system result in ambiguous ranges while pulse returns within the prt limits result in normal (unambiguous) ranges. The maximum unambiguous range for a given radar system can be determined by the following formula:
Figure 1-4. - Maximum unambiguous range.
Q.3 What is the speed of electromagnetic energy traveling through air? |
 
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