84 SURVEY NAVIGATION, CORTEN 
1. the fact that the systems are self-contained without references to outside sources 
gives rise to a serious propagation of errors, and 
2. the fact that the systems are complicated servo loops gives rise to unwanted oscilla- 
tions. 
The first source of errors (build-up of errors) must be met by designing and con- 
structing component parts with incredibly high accuracy, e.g. gyros, accelerometers 1), 
servo meters, computers and integrators. In addition, the propagation of errors is greatly 
reduced by slaving the inertial system to another reference such as Doppler radar, 
astrocompass, ete. 
The second source of errors — the oscillations of the pendulous servo systems - is 
greatly reduced by slowing the oscillations down to a 84 minutes oscillation period. An 
1/R 
oscillating system having this oscillation period 7 = 84 min = 27x |/ is equivalent to 
a physical pendulum of an earth’s radius (R) length. Such an earth pendulum or Schu- 
ler pendulum will be influenced by horizontal accelerations in such a way that it oscillates 
with a radius = R, consequently its attitude will follow the earth’s curvature, consequent- 
ly it will “remain horizontal” when it oscillates due to accellerational disturbances. The 
fact that cumulative errors can also be restricted, is an important additional advantage 
of Schuler-tuned servo systems. 
Instruments available. 
A complete inertial navigational system may consist of an inertial platform (con- 
taining gyros, accelerometers, gimbals, preamplifiers, heaters, synchros and torques), 
electronic unit, navigational computer and display or output. Such a system can provide 
outputs of heading, present position, speed, acceleration, in other words complete attitude 
and behavior on earth or in space. 
These systems are extremely compli- 
cated. For photogrammetry, Aeroflex 
designed an inertial system, containing 
only what is needed for a photogram- 
metric vertical reference in nominally 
straight and level, "low-speed survey 
flights. Figures 24 and 25. The func- 
tioning of this PVRU (photogramme- 
tric vertical reference unit) is de- 
scribed in a paper by T. Trott. It is 
under flight test and is expected to 
command a truly vertical camera axis 
Lp £2...9c If 
this system should come up to these 
expectations, it would provide an im- 
with mean errors m, 
portant contribution to photogramme- 
tric operations. 
1) For instance, the errors of accelero- 
  
meters for precision systems should 
not exceed 1 part in 100.000 of accel- 
eration response. Fig.24. Aeroflex inertial platform PVRU.