An ordinary aircraft altimeter uses a type of Aneroid Barometer
which measures the changes in air pressure at different heights.
However, changes in air pressure due to the weather can adversely
affect the altimeter's readings. It is far more favorable to use a
radar (or radio) altimeter for enhanced accuracy when the bomb
reaches Ground Zero.
While Frequency Modulated-Continuous Wave (FM CW) is more
complicated, the accuracy of it far surpasses any other type of
altimeter. Like simple pulse systems, signals are emitted from a
radar aerial (the bomb), bounced off the ground and received back
at the bomb's altimeter. This pulse system applies to the more
advanced altimeter system, only the signal is continuous and
centered around a high frequency such as 4200 MHz. This signal is
arranged to steadily increase at 200 MHz per interval before
dropping back to its original frequency.
As the descent of the bomb begins, the altimeter transmitter will
send out a pulse starting at 4200 MHz. By the time that pulse has
returned, the altimeter transmitter will be emitting a higher
frequency. The difference depends on how long the pulse has taken
to do the return journey. When these two frequencies are mixed
electronically, a new frequency (the difference between the two)
emerges. The value of this new frequency is measured by the
built-in microchips. This value is directly proportional to the
distance travelled by the original pulse, so it can be used to give
the actual height.
In practice, a typical FM CW radar today would sweep 120 times per
second. Its range would be up to 10,000 feet (3000 m) over land and
20,000 feet (6000 m) over sea, since sound reflections from water
surfaces are clearer.
The accuracy of these altimeters is within 5 feet (1.5 m) for the
higher ranges. Being that the ideal airburst for the atomic bomb is
usually set for 1,980 feet, this error factor is not of enormous
concern.
The high cost of these radar-type altimeters has prevented their
use in commercial applications, but the decreasing cost of
electronic components should make them competitive with barometric
types before too long.
