PROPAGATION TIME / RESOLUTION

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

Rules of Thumb
In one usec 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 usec

1 yd = 0.006 usec

1 ft = 0.002 usec

1 NM = 12.35 usec

1 km = 6.7 usec

 

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 usec
1 micro sec (us) 1000 ft . 1 km 3.3 usec
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 inter-pulse 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 R = Range and Pms = Period (i.e. PRI or T) 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.

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