Automatic Gain Control /AGC/ channel automatically compensates for gain changes in the azimuth error channel due to time varying optical sight degrading effects. This system is useful in remote television monitors, automatic navigation systems, and surveying and mapping instrumentation

Mrus, G.

Zukowsky, W.

English

NASA Technical Reports Server

ecember 1966 
•ksd
Brief 66-10596 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Optical Automatic Gain Channel 
Sway Detector 
lao Swciy Filter
Sway and AGC 
'•%.••.* Beamsplitter 
AGC Filter	
.,Re-imaging Lens 
AGC Detector Mirror	 Light Returning 
From  Target Prisms 
Beamsplitter Beamsphtter	
Synchro 
________ Acquisition 
Acquisition 
Detector 
Inertial Acquisition Filter  Synchro Detector Filter 
Inertial Detector Filter 
The problem: 
Theodolites sighting over land have their per-
formance degraded due to optical gain changes in 
the atmosphere and to shimmer. This effect has been 
compensated for in previous instruments by manually 
adjusting gain pots in the electronic error channel. 
The solution: 
An automatic gain control (AGC) channel that 
automatically compensates for gain changes in the 
azimuth error channel due to time varying optical 
sight degrading effects. 
How it's done: 
The complete device is an automatic azimuth laying 
theodolite capable of determining simultaneously, but 
separately, the azimuth orientation of two porro 
prisms mounted within an inertial guidance system
some 1,000 feet distant. The instrument performs 
this function automatically while tracking the swaying 
target. 
By placing a trihedral with the two target prisms, 
so that an autocollimated signal from it could, be 
sensed in the theodolite by appropriately located 
detectors, it is possible to provide a signal for sensing 
deleterious signal effects in the optical path. Porro 
prisms are azimuth orientation sensitive to on-coming 
collimated light. Trihedrals can return this light upon 
itself. The optical signal from a porro prism not at null 
thus contains azimuth information attenuated by at-
mospheric effects. 
The light from the trihedral, however, always 
returning on itself, can provide, under the best viewing 
conditions, a reference base from which to sense deg-
radation of the optical signal.
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government not any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19660000594 2020-03-11T17:45:16+00:00Z
The optimum optical signal from the trihedral is 
used to set the bias voltage in the AGC circuit. This 
bias voltage is then introduced into the azimuth error 
signal computer amplifier and automatically sets the 
gain of this channel so that signal output is solely a 
function of azimuth error—independent of atmos-
pheric transmission variables. Consequently, at any 
subsequent moment, degrading effects in the optical 
path will cause appropriate gain increases in the error 
channels. 
The trihedral supplying the AGC signal is dichroi-
cally coated to return energy to the theodolite in the 
1.8 to 2.6 micron range. This optical signal is thus 
discrete from the azimuth error signals which occupy 
the 0.7 to 1.8 micron range. By placing appropriately 
coated filters within the theodolite, the return beam is 
differentiated into various optical channels within the 
instrument. 
The figure illustrates how this may be accom-
plished. Besides transmitting light to the error de-
tectors, the clear beamsplitter reflects light return-
ing to the autocollimator from the vehicle prisms to 
the second beamsplitter located directly above the 
clear beamsplitter, the bandwidth transmitted by the 
second beamsplitter being approximately 1.8 to 2.6 
microns. After reflection by the mirror, light is re-
imaged by the lens onto the third beamsplitter that
transmits light to the sway detector and filter and 
reflects light to the AGC detector and filter. Both 
filters transmit only the 1.8 to 2.6 micron bandwidth. 
Since the filters absorb the 0.7 to 1.8 bandwidth, the 
sway and AGC channels are not affected by the iner-
tial and synchro prism error and acquisition channels. 
Notes: 
I. This system should be useful in remote television 
monitors, automatic navigation systems, and sur-
veying and mapping instrumentation. 
2. Inquiries concerning this invention may be di-
rected to:
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B66-10596 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546. 
Source: George Mrus and Walter Zukowsky
of Perkin-Elmer Corporation 
under contract to
Marshall Space Flight Center 
(M-FS- 1550) 
Brief 66-10596	 Category 02


Optical automatic gain channel

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