You must convert the aircraft ground speed to feet per second to determine the exposure interval or frequency between frames. The conversion factor for

Figure 4-22: -Multi-finger divider.

converting airspeed (in knots) to feet is 1.7 (kts x 1.7 = ft/sec). For example, when the airspeed is 125 knots, the ground speed, in feet, is 212.5 feet per second (125 x 1.7 = 212.5).

Head winds and tail winds must also be considered. When head winds are present, you should subtract the head wind from the airspeed. Tail winds should be added to the airspeed. For example, when the aircraft is flying north at 125 knots and the wind is blowing from the north at 10 mph, then the corrected airspeed is 115 knots (125 - 10= 115). When the air craft is flying south, then the corrected airspeed is 135 knots (125 + 10 = 135). The corrected airspeed must be used to find the ground speed, in feet.

For the time interval between exposures, the following formula should be used:

Where

T = Time in seconds

D = Ground-gained forward

S = Ground speed in feet per second

NOTE: When the interval between exposures can be accomplished in full seconds only, the tenths of a second should be dropped. By doing so, slightly more than the required 60-percent forward overlap is provided. This can be advantageous.