was important that the chart angle and elevation of an airplane determined
by the listening equipment were quickly available at the field glasses and
the searchlight. Therefore electrical transport was the best solution. The
development of such a transport system at the Measurements Building was
started in 1929.
The value of chart angle and elevation were in the equipment available each as the rotation of a shaft. As servo systems did not exist in the early thirties the electrical transmission of a shaft position required heavy cables, which were impractical in use. So the Measurements Building developed a novel design called "Step system".
On the transmitter side it consisted of an electro-mechanical arrangement. The mechanism separated forward and backward movement of the shaft and translated each into separate trains of plus and minus impulses respectively. The two trains were transmitted with separate wires the number of impulses being proportional to the angle of rotation.
The receiver converted the received train of impulses into a proportional forward or backward pointer position. The pointer moved on a round dial over 360 degrees (or 6400 "artillery" milliradians) in 640 steps. By keeping the rotation mechanism under mechanical pressure of a wound spring the current impulses for the pointer transport in each lead could be reduced to 25 milliamps.
A tracking pointer adjustable by hand, covering with the moving pointer, served to transport the indicated angle value with a transmitter as previously described to a different location. The maximum transmission speed amounted to 150 impulses (=steps) per second.