AMBIGUOUS RETURNS

1-6 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