repetition. For example, an analytical stereoplotter 
must calculate and change photo-carriage positions in 
no more than 0.05 second, or else the operator will 
notice a delay between the time he moves a hand- 
wheel or footwheel and the time the floating mark is 
seen to move in the stereo model. Automatic con- 
touring requires even less delay in controlling photo- 
carriages—often as little as 0.005 second—if smooth 
performance is to be maintained. 
Service programs are not dependent on execution 
time, except in a gross sense. During the execution of 
these programs, the operator is not actively using the 
instrument. An example of a typical service program 
is the calculation of a rigorous least-squares exterior 
orientation of a stereo pair of photographs. This pro- 
gram may require more than a minute for comple- 
tion. 
In performing both kinds of programs, substantial 
arithmetic computations and logical operations are 
required. Arithmetic computations are needed to 
process the necessary photogrammetric equations, 
often with precisions of 6 to 9 decimal digits. Logical 
operations are needed for interpreting control panel 
commands, editing data, detecting limits, and control- 
ling equipment. The real-time operations place an 
exceptionally heavy computational burden on the 
computer; one study of real-time computational re- 
quirements for an automated analytical plotter con- 
cluded that 30,000 additions, subtractions, and 
logical operations plus 6,000 multiplications and 
divisions would be required every second during real- 
time operation. Essentially all these operations must 
be carried out with a precision on the order of 30 
bits. 
CONTROL COMPUTERS 
Control computers are used to directly control 
equipment other than that used solely to read or 
record computer data. A real-time computer is re- 
quired to receive, process, and return data either 
within a few seconds of a specific request or at a 
regular rate of several times per second. Photogram- 
metric applications may require rates of hundreds of 
times per second. 
Photogrammetric real-time control systems have 
demanding computational requirements. The neces- 
sary calculations are complex, the repetition rates are 
high, and the outputs must be controlled with high 
accuracy. Therefore, the design of such a control 
system is a difficult task requiring a careful balance 
between performance and expense for a cost-effective 
solution. In the past, these considerations have re- 
sulted in the use of a special-purpose computer: one 
designed with special internal organization. Presently, 
22 
even the very demanding requirements of automation 
in photogrammetry can be met with a parallel-arith- 
metic computer of general-purpose internal organiza- 
tion. Careful cost considerations in design are still 
essential: the control equipment can cost up to one- 
half of the total cost of the system, and the interface 
components between computer and instrument can 
cost more than the computer itself. 
Special input-output facilities must be provided 
for photogrammetric control-computers; circuits are 
needed to detect handwheel and footwheel positions 
and to detect and control leadscrew positions, instru- 
ment control-panel switches, and analog voltages. 
Conventional input-output facilities have been re- 
stricted largely to a paper-tape reader, tape punch, 
and possibly a typewriter. 
The characteristics of photogrammetric-instrument 
control computers used by Bendix are summarized in 
Table I, which indicates the rather modest memory 
sizes of these computers. As a result, the software 
(defined here as the special program library written to 
assist the user in preparing his operating programs) 
has been limited in availability and utility. Software 
for small computers generally has been limited, some- 
times requiring extra-expense memory options or 
input-output facilities. 
Table I shows the trend toward general-purpose 
internal organization. This change is brought about 
by the increasing speed and decreasing cost of digital- 
computer basic components, and by the increased 
convenience in programming. Software availability 
contributes to the programming convenience. It 
appears that computers with general-purpose internal 
organization will predominate in future photogram- 
metric control applications. Of course, efficient and 
economical communication between the computer, 
the instrument, and the operator still must be spe- 
cially designed for the individual application. This is 
an appreciable task, especially for the more complex 
instruments. 
It is interesting to compare photogrammetric re- 
quirements with those of other real-time computer 
applications. For real-time industrial process control, 
such as control of chemical plants, substantially fewer 
computations normally are required, and these at 
lower rates. The number of inputs and outputs is 
generally higher, however, and many input-output 
devices may be located remotely. An individual input 
or output rate more than once per second is rare. For 
real-time inquiry systems, such as airline reservation 
systems, few regular computations are required, most 
computation being in response to random inquiries. 
The time to respond to an inquiry can generally be 
one second or more. In some cases, few arithmetic 
computations are required. The number of inquiry 
FORREST, WHITESIDE, AND HORNBUCKLE