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# Electronic Warfare and Radar Systems Engineering Handbook - Propagation Time / Resolution -

PROPAGATION TIME /

RESOLUTION

1. ROUND TRIP RANGE: with t = time to reach target

Rules of Thumb

 In one μsec round trip time, a wave travels to and from an object at a distance of: The time it takes to travel to and from an object at a distance of: ≈ 150 m≈ 164 yd≈ 500 ft≈ 0.08 NM≈ 0.15 km ≈ 1 m ≈ 0.0067 μsec≈ 1 yd ≈ 0.006 μsec≈ 1 ft ≈ 0.002 μsec≈ 1 NM ≈ 12.35 μsec≈ 1 km ≈ 6.7 μsec

2. ONE WAY RANGE: R = ct with t = time to reach target

 Time Distance Traveled Distance Time it Takes 1 milli sec (ms) 165 NM 1 NM 6.18 μsec 1 micro sec (μs) 1000 ft 1 km 3.3 μsec 1 nano sec (ns) 1 ft 1 ft 1 nsec

3. UNAMBIGUOUS RANGE

(DISTANCE BETWEEN PULSES):

Normally a radar measures "distance" to the target by measuring time from the last transmitted pulse. If the interpulse period (T) is long enough that isn't a problem as shown in "A" to the right. When the period is shortened, the time to the last previous pulse is shorter than the actual time it took, giving a false (ambiguous) shorter range (figure "B").

Rules of Thumb

RNM ≈ 81Pms

RKm ≈ 150Pms

Where Pms is PRI in milliseconds

4. RANGE RESOLUTION

Rules of Thumb

500 ft per microsecond of pulse width

500 MHz IF bandwidth provides 1 ft of resolution.

5. BEST CASE PERFORMANCE:

The atmosphere limits the accuracy to 0.1 ft

The natural limit for resolution is one RF cycle.