Question 3-92O1

For a RADAR operating on a long-range setting like 48 miles, two key parameters, pulse width (PW) and pulse repetition rate (PRF), are selected to optimize performance: 1. **Pulse Repetition Rate (PRF):** The PRF determines the maximum unambiguous range. To detect targets at 48 miles, the radar pulse must travel to the target and return before the next pulse is transmitted. * Maximum unambiguous range (R_max) = c / (2 * PRF), where c is the speed of light. * If R_max must be at least 48 miles (approximately 77,248 meters), then PRF <= c / (2 * R_max) = (3 x 10^8 m/s) / (2 * 77,248 m) ≈ 1941 pps. * Therefore, PRF options of 2,000 pps or 2,500 pps are too high for a 48-mile range setting, as they would lead to range ambiguity where echoes from distant targets are mistaken for closer ones. A PRF of 500 pps ensures an unambiguous range much greater than 48 miles (R_max = 3x10^8 / (2*500) = 300,000 meters = 186 miles). 2. **Pulse Width (PW):** Pulse width affects the amount of energy transmitted per pulse and the range resolution. * For *longer ranges* (like 48 miles), a *longer pulse width* is generally preferred. A longer pulse contains more energy, which improves the signal-to-noise ratio of the return echo, making it easier to detect distant targets that have undergone significant signal attenuation. * A very short pulse width like 0.05 µs is typically used for short-range, high-resolution applications. For 48 miles, 1.0 µs or 2.5 µs provide more energy. Considering these factors: * Options A, B, and C use PRFs (2,000 or 2,500 pps) that are too high for a 48-mile unambiguous range. * Option D, with a 1.0 µs pulse width, provides sufficient energy for long-range detection, and a 500 pps PRF ensures no range ambiguity for a 48-mile setting. The final answer is $\boxed{D}$