Ring laser angle encoder with a scanning photometer autocollimator and an isolation axis, provides continuous digital readout. It measures the angular difference in inertial attitudes of target /any phenomena generating or reflecting a light beam/ two at a time relative to target one at a time

Coccoli, J. D.

Lawson, J. R.

Mc Garty, T. P.

Nickles, J. E.

English

NASA Technical Reports Server

April 1969	 Brief 69-10115 
Ah	 CORRECTED COPY 
Ring Laser Angle Encoder 
A new angle encoder concept is presented, which 
would provide continuous digital readout with great 
precision. It has characteristics particularly applicable 
to space navigation, space- and ground-based survey-
ing, inertial attitude determination and alignment, 
antenna or telescope attitude determination, and 
rotary table precision-angle determination, with im-
provements of at least an order of magnitude over 
present techniques. A unique feature is that this en-
coder measures the angular difference in inertial atti-
tudes Of target two at time t 2 relative to target one at
node-angular spacing for the equivalent circular ring 
laser would be of the order of an arc second. Since 
electromagnetic radiation in empty inertial space is 
described entirely by Maxwell's equations and boun-
dary conditions, if loss at the boundaries is sufficiently 
small, the reference wave remains practically station-
ary in inertial space when the ring is rotated about its 
axis. Each antinode of the ring laser reference wave 
is detected by a photosensor and level-detection cir-
cuitry and is labelled a beat. Beat count including time 
interpolation gives rotation-angle digital resolution 
from 0.1 to 0.001 second. 
In practice, nonreciprocal effects result in motion 
of the reference wave around the optical path. When 
this motion is proportional to optical cavity rotation 
rate, the beat counting technique remains accurate. 
This motion is intrinsic to noncircular geometries. 
Also, Doppler shifts of the laser optical dispersion 
with cavity rotation produce this type of reference 
motion. Dispersion drifts occur with changes in en-
vironment temperature and electronic components. 
The time constants are long compared to the opera-
tional cycle, contributing negligible angle measure-
ment errors. 
There are nonreciprocal effects independent of rota-
tion rate, e.g., Langmuir flow of the laser gas. Then, 
the beat counting technique must be supplemented by 
subtraction of the bias motion. Accuracy is not af-
fected, since the nonreciprocal properties also remain 
sufficiently constant during the operational cycle. 
Even without nonreciprocity the scale factor relat-
ing beat count to rotation angle depends on rotation 
rate. The standing wave, considered resolved into two 
oppositely-directed traveling waves, experiences back-
scatter with Doppler shift at mirror imperfections. 
This mechanism couples the traveling wave modes, 
(continued overleaf) 
. time t1 . "Target" means any phenomena which
generate or reflect a detectable light beam. 
The encoder combines a ring laser, a scanning pho-
tometer autocollimator, and an isolation axis with a 
rate-servo drive. The ring laser is rotated at an ap-
proximately constant inertial rate above an inertially 
fixed axis. The photometer is rigidly fixed to the ring 
laser body and its optical axis scans in the plane of 
rotation. 
An operational cycle consists of rotating the assem-
bly through 360°. Initially, the plane of the ring laser 
is oriented to coincide with the angle measurement 
plane formed by the lines of sight to the two targets. 
When the first target center is detected, two counts of 
the ring laser beat are initiated. The first count termi-
nates at detection of the center of the second target. 
The second count terminates after redetection of the 
first target after 360° rotation. The ratio of these two 
counts constitutes a measurement of the angle between 
the two targets. This operational cycle provides an 
intrinsic calibration when the first target is inertially 
stationary or moves in a sufficiently defined manner, 
eliminating the effect of principle error sources. 
A single-mode ring laser establishes a standing op-
tical reference wave around a closed path. Typical 
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resulting in a hyperbolic functional relation between 
the beat rate and the rotation rate, with a dead zone 
below a certain rotation rate. Therefore, beat-count 
technique accuracy deteriorates with increasing varia-
tion of rotation rate about the average rate. Proper use 
of a ring laser with this nonlinearity dictates minimiz-
ing variations about the calibrated rotation rate by 
sensing ring laser beat-count rate deviation and cor-
recting the rotation rate. Contributions to the measure-
ment error can be made as low as 0.01 arc second. 
Beat counting and time interpolation logic define the 
attitude of the photometer relative to the laser ref-
erence wave at the coincidences of the optical axis 
with the two target beams. 
Each target beam is transformed by the photometer 
optics to an image at the photometer focal plane. The 
optical axis is defined by a slit aperture at the focal 
plane. The target image position at the aperture is 
determined by combination of the photodetector and 
the scanner estimation logic. This photometer optical 
axis is related to the ring laser plane by the optics 
mechanical assembly.
The optics mechanical assembly, which transforms 
the target beams to images at the aperture and relates 
the image to the laser reference wave, is a critical part 
of the design. Limitations due to the optical transfor 
mation and slit precision and the mechanical rigidity 
and drift characteristics are analyzed and discussed. 
Note: 
Documentation is available from: 
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Price $3.00 
Reference: TSP69-101 15 
Patent status: 
Title to this invention has been waived under the 
provisions of the National Aeronautics and Space Act 
[42 U.S.C. 2457 (f)], to the Massachusetts Institute 
of Technology, Cambridge, Massachusetts. 
Source: J. D. Coccoli and J. R. Lawson of
Massachusetts Institute of Technology 
under contract to

Manned Spacecraft Center
(MSC-13099)
. 
. 
. 
Brief 69-10115	 Category 01


Ring laser angle encoder

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