A series of reference systems of various accuracies is available for use with 
such a mounting system? . This paper will be confined to a discussion of the 
most precise such system, the PVRU. 
The problem of the vertical in moving aircraft is today in about the same 
state in which the problem of Longitude was in the 18th Century. At that time, 
there existed a theoretically correct method for obtaining Longitude, however, 
a sufficiently accurate clock independent of ship motions did not become avail- 
able until Harrison built his chronometer in 1736. Today, a theoretically cor- 
rect method for establishing true vertical exists. It was first described by 
Prof. Maximilian Schuler? in 1923. Sufficiently accurate instruments to re- 
alize this method did not become available until quite recently. This method 
forms the theoretical basis for the Aeroflex PVRU. 
It must be understood that no system involving pendulums or other grav- 
itation sensitive devices can alone serve to indicate the vertical if Einstein's 
theorem of the equivalence of gravitational and acceleration mass be true. A 
pendulum will align itself with the resultant of the combination of gravitational 
acceleration with the other accelerations acting on its pivot. Some indication 
of the problem is given by considering that an acceleration of 0.00087 g will 
produce the allowed error of 3 minutes of arc. This acceleration is produced 
when the aircraft speed changes at the rate of 1 knot per minute of time. In an 
aircraft traveling at 600 knots, a rate of turn of 0. 14 degrees per minute will 
produce this error. Even in uniform "straight and level" flight the Coriolis 
Acceleration at Latitude 45° would produce this error at an aircraft speed of 
150 knots. It is apparent that this level of accuracy is quite difficult to attain 
with any of the common means of indicating the vertical. 
-2 
dd FE RER