Midcourse navigation and guidance simulator - sextant sighting performance in simulated environmen

Lampkin, B. A.

Randle, R. J.

English

NASA Technical Reports Server

Sextant Sighting Performance in the Ames Miiico- 
Xmigation and Guidance Simulator + L  
Robert J. Rsndle ' I .  
/ 
Biotechnology Mvis ion 
and 
Bedford A. fnmnk.ln 
FLtll-Scale and Systems Research Division 
National Aeronautics and Space Administratis 
Ames Research Center, Moffett Field, California 
The Ames Research Center is studying the role of the crew in the 
pavigation, guidance, and control for  the midcourse phase of manned 
8pace missions. 
navigation and guidance sirnulator has been constructed at Ames. 
In pursuit of these investigations, a lunar midcourse 
This device presents, for one thing, a controlled visual task to 
the human operator in a relatively realist ic manner. Our principal 
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purpose in discussing it here is t o  l e t  you how that it exists and t o  
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let  you form some idea of its capabilities as a research device f o r  
visual problems. 
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To achieve this purpose we will briefly describe t h e ,  
major features of the simulator and the conduct of an exploratory study 
of sextant sighting performance in the simulated task environment. 
The visual scene is a 25O portion of the sky, including a simulated 
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moon which is translatable in accordance with long-period vehicle-moon 
relative motions for a typical trajectory. 
Paper presented to the National Research Council (Conm&ttee on 
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vision) Washington, D. C. 4ril  23-24, 1964. - 
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https://ntrs.nasa.gov/search.jsp?R=19650019776 2020-03-24T05:06:17+00:00Z
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i Figure 1 shows the moving cab munted on an air bearing. !he cab 
is driven by an on-board cold gas system. 
portion of a 105-inch sphere which allows rotational motion 
in pitch and r o ~ .  and 
has been succeesf'ully used to stabilize the cab both nmnually and auto- 
!The air-bearing support is a 
to S O o  
in yaw. The on-bod  cold gas'control system 
matically during Optical Sighting tasks. 
cycle operation. 
It has also provided 1-t 
pigure 2 depicts the celestial visual  scene that has been simulated 
in  the midcourse simulator. The direct optical planetarium approach t o  I 
visual scene generation has been utilized. 
about k, feet from the viewing point, is composed of 64 stars which are 
contained in the 25O segment of the sky surrounding the m n .  
The visual scene, located 
This se-t 
. results from the choice of a specific trajectory which is taken as being 
tygical  of several possible lunar trajectories. 
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I The simulated stars are simply 0.005-inch diameter holes i n  the end 
I of a tube lighted by a grain-of-wheat lamp. They have a suBtended engle ~ - 
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of about 2 arc seconds and maintain their  relative positions withdn- arc' 
seconds for as long as 8 hours. 
planetarium is subject t o  optical pard lax  due t o  the f in i te  distance 
between the l lght source and the viewer. 
I Unfortunately, the direct optical 
To minimize the errors in 
angUla;r m e s s U r e ~ t S  due to pardlax, f O U r  Of the Star -6 We= C O l l i -  
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mated by 11~8118 of &inch parabolfc mirrors with GIIIEIU light sources at 
the focus to represent the stars 
device. The images produced in $he parabolic mirrors, when viewed from 
the simulator cab, appear t o  be at infinity and have the s e  direction 
when viewed m e r e  within t h e  viewing limits of the parabolic mirrors. . 
To extend the viewing area these c o l l i ~ ~ ~ t i ? d  st;a;rS are being fitted with 
F'igure 3 is a picture of the collimating 
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=-inch parabolic mirrors. 
extend the magnitude of rotational oscillations which may be employed in 
Use of the larger mirror Kill of cowse 
research on the effects of motion on sighting accuracy. 
This very brief description of the simulation faci l i ty  is taken 
from an unpublished paper by Donald Smith of Ames Research Center. 
the research with this faci l i ty  concerns ntahlythe idqtifica- 
tion and verification of' performance capabilities of the human operator 
as an active participant in the positive fbdng and guidance of the 
vehicle during translunar o r  midcourse flight. This use of the simulator 
rests upon the conviction that man can be a usef'ul adjunct toyor replace- 
ment for, fully automatic primary systems. His capabilities in  this 
task environment thus must be fully explored and delineated (1). One 
of the important tasks which w i l l  concern the astronaut is that of obtain- 
ing navigation information. For the mment we are investigating manual 
sextant sighting performance as a possible minimum manual system for - 
gathering navigation data. Later this task will be integrated with the 
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larger task of inputting the data to  the on-board computer and ultimately . - 
with the broader task context of vehicle alinement, sighting, cornputstion, 
and velocity correction. Our immediate larger goal is to  simulate up t o  
an 8-hour segment of the midcourse phase of the translunar trajectory. - -  
I would like n6w t o  discuss briefly this first- task elexent - sextant 
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sighting -,which has been our ini ial concern. ;" 
There have been many m a n d  vigation schemes proposed in the - 51. 
literature fo r  finding one's way,about in the solar system (2r3r5 ,7r8 ,~) .  
'Ilhese are generally of two kinrls: 
(1) Extensione of ConventionaI. celestial navigation techniques for 
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(2) *licit guidance through the determination of the elements 
of the orbi t  and their departure f r o m  or agreement with a reference 
orbit which has been predetermined t o  intersect the coordinstes of the 
desired target point in space. 
Both of these methods involve considerable mathematical computation, 
but, each, i n  mst cases, depends first upon the seemingly simple task 
of ~ a s u r i n g  an angle with an optical sighting device. In marine and 
air navigation the accuracy of the celestial fix depends largely upon 
the measurement of the altitude of the body of interest above the natural 
sea horizon or the bubble horizon. 
of englee may be of interest. 
In navigation i n  space, several kind8 
Some of these are: 
(1) The angle between the line of sight of a lunar or terrestrial 
or planetary -k snd a star. I 
(2) 
(3) 
'Ihe-sngular extension of any of the planets, noon, o r  sun. 
The angle between a star and the linib of a planet, moon, or - 
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SM. 
(4) 
horizon. 
The angle between the  earth or moon and the vehicle centered 
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Since the marine sextant may be rotated through any angle t o  m e ~ ~ u r e  I 
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the angle between any two points of interest, this is the instrument we 
have been using. 
of its dependence upon a gravita io& field. 
!&e bubble sextant is disqUif ied,  of course, becauee 
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Specifically, we have had early access t o  the Navy Mark II Mod 0 
I - -  hand-held sextant and have used it mainly because of its availability. 
This sextant has a 3 power scope and a loo field of view. 
we have come upon a &ern sextant with interchangeable telescopes and 
t h i s  will be used in future studies. 
M o r e  recently 
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It probably appears an audacious b i t  of romance t o  bring forward 
- this time-honored device for evaluation against the precision require- 
ments of space navigation. However, as an entrance point to the study 
of the fundmentd sighting task its employment is inescapable and appears 
thus far t o  be quite fruitArlo This is particularly true where it ie 
desired t o  estimate relative accuracies under various sighting conditiane 
end the inherent sextant errors bias onlythe determination of the trus 
angle. 'Ihe variance of the operator's sighted angles about their  nmn 
is the criterion measure for a given sighting session, and the change 
in th is  score with 'the conditions of the study is the experimental 
variable of interest. 
Pigure 4 is a photograph of the marue sextant we have been using. 
Its design, as for all sextants, is based upon the optical principle 
that the angle between the first and last directions of a ray of light 
that has undergone two  reflections in the same plane is twice the angle 
that the two reflecting surfaces make with each other. Since the index 
arm is mechanically linked to the index mirror, the position of the arm 
on the l i m b  indicates the angle between the two  mirrors. The l i m b  thus 
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must be calibrated so that 0.3' of a r c  reads 1' of arc. Orig.fnnly, 
the limb was a sixth of a circle, whence the name sextant; however, on 
modern sextants it is usuallymore than a sixth. 
sources of error associated with t h i s  sextant. Some are due t o  mirror 
There are several 
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i - telescope misalinement, others dw t o  eccentricity i n  the index 
arm, errors of graduation, and lack of parallelism between mirror and 
6h& glasses (4). 
Obviouely, these are not highly accurate instruments i n  terms of 
error tolerances for space navigation. Position fixing on the open sea 
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does not haw? stringent accuracy requirements nor does air navigation, 
necessarily, mere cr i t ical  corridors and terminal points are approached . 
using other, mre precise, techniques. 
Office considers any sextant used fo r  marine navigation purposes to be 
In fact the German Hydrographic 
free from errors for a l l  practical use if the error goes up to  twenty n 
seconds" of arc. 
guidance enqloy an accuracy model of at most10 seconds of arc in sight-  
Homer, our theoreticians in space navigation and $ 
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ing perf'ormance i n  their  analyses of navigation requirements (6,9,10). 
The literature is not at all replete with controlled studies of ' 
sextant sighting accuracy. 
was used, measured performance i n  sighting on actual celestial obJects 
One study, in which a modified modern sextant 
i n  the night sky (12). 
m n  crater were the targets. The sextant was fitted w i t h  a 6 x 9 tele- 
Two stars, a star and a planet, and a star azd a 
scope and it was munted on a modified telescope munt. The micmmter 
vernier permitted angular interpolation t o  3 arc seconds. - 
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For the measurements of angle between two stars and a planet and a 
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star,the standard deviation was of the order of 10 seconds of arc and 
for the star-crater pair it was of the order of 26 seconds of arc. , 
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In  our case, we used a hand-held sextant and a sextant gimbal mounted I 
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t o  the cab. Our telescope was three power with a loo field of 
view. 
interpolation to 3 seconds of arci. Our interest WSB not, however, in 
The vernier &IS readable t o  6 seconds of arc with a dubious 
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I - the ascertainment of absolute sight ing performance. We were trying to  
determine whether there was a q f e r e n c e  in performance between the band- 
held and the gimbaled sextant and also whether oscillatory motion affected 
performance to any great extent. 
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Accordingly, an arbitrary limit cycle f'unction was programmed on 
an analog computer t o  drive the cab using the cold gas je t  system. 
The l i m i t  cycle was restricted t o  the yaw axis, spurious nntions in 
the other axes being damped t o  relatively smaU amplitudes. 
of rate were used: 
A s ta t ic  condition wa8 also included. The oscillations were contained 
within a So band about the l ine bisecting the stars of interest. T w  
Three levels ~ 
1/2O per second, lo per second, and 1-1/2' per second. 
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of our collimated s t a r s ,  oriented in  a near vertical position, were used 
as targets. The task was t o  measure the angle between these stare by 
superpositioning one s t a r  over the other in the sextant f ie ld  of view. 
For sd jec t s ,  we emgloyed three Air Force navigator instructors 
at nearby k t h e r  A i r  Force Base, an advanced A i r  Force navigation school. 
Four professionals engaged i n  related studies a t  Ames Research Center 
were also included. 
Since these investigations are still in  process, it would be pre- - .- 
mature t o  present firm data at this time; however, S O ~  info- state- 
merits regardin@; apparent trends are in order. 
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Training is an important variable, a t  least  for the task env i ropn t  
we are providing. 
week or mre was necessary t o  bring the subjects down t o  asyqtote. 
W e  found that using two  sighting sessions per day, a 
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Twenty-four sightings were taken i n  each session. 
navigators, as well as for the prpfessionals. 
This was true for the 
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For the navigators, trans- 
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- fe r  of training cannot be evaluaded because of their  lack of recent 
intensive use of their  bubble sextants and the f'mdammtal differences 
between that taak and the present one. One of the major differences i a  
due t o  the use w i t h  e r n  bdble  sextants of manual or  automstic inted 
grating devices which allow fo r  the Civeraging of a conthuous sighting 
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over a minute or two of t i m e .  me marine sextant is a single shot 
device and is so used in sea navigatioa. 
regressed considerably in  their  learning af ter  the intervention of a 
weekend. However, the losses were quickly recovered after some additional - + 
sessions. It is  mandatory that the extent of retraining required after 
We noticed that our subjects 
varying periods of nonpractice be determined if a s t r ic t ly  mantad 
sighting scheme is t o  be seriously considered as an alternate xwde for 
space navigation. 
The cab mtion  does not appear t o  have a systematic effect on 
performance at  the relatively large rates that we used. Homer, the - 
effects of less discernible rates in both the l i m i t  cycle and the long -. 
period relative motions of the vehicle and bodies in the solar system 
have yet t o  be est,imated. 
Indications are that the ginbaled sextant has a slight edge, i n  
terms of accuracy, over the hand-held. 
ginibsled sextant, particularly after having had some experience with 
both types. However, this is not sufficient t o  qualify the g m e d  
and disqualify the hand-held, the difference amounting t o  some 3 
The subjects preferred the 
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seconds of arc in standard deviation. This is not very much when it 
is lmown that the standard deviations f o r  a l l  s a j e c t s  ranged from 
5 seconds t o  46 seconds with a mean of 22 seconds of arc. 
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The three power telescope ma not be the best for mst accurate 7 
performance. We tried the modem' sextant w i t h  a 6 X 30 telescope and 
though there was considerable random mtion in the t w o  star targets due 
t o  hand tremor, performance was better. 
based on only  a few subjects, we are going t o  investigate the contrlbu- 
Since this determination was 
tion t o  performance of various telescope magnifications using the newe=: 
sextant . 
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In the process of defining the details of the task it beconres 
apparent that the visual processes involved require separate treat- 
ment for their  investigation. Accordingly, plans are being made t o  
assess matters such as the relation of visual acuity to  perfornnmce; 
the effects of fatigue; the effects of reticle geometry such as cross 
hairs versus concentric circles versus gun-sight type displays; the . .  
variance of performance with varying * geometries of celestial targets; 
, the effects of varying contrasts in  the field of view; and the effect8 
of training, lack of practice, and retraining on all of these. These 
are general statements because they result from an initial judgment 
of what is in Sighting performance as identified in the 
simulated task environment. The opportunity t o  make these judgments . 
i s  provided byt4e midcourse.navigation and guiaance simulator. 
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References I 
1. Christensen, J. M. Psychological aspects of extended manned space 
flight.  Behavioral Sciences Laboratory, Air Force Systems Commend, 
‘WPAFB, Ohio, Tech. Doc. Rep. AMRL-TDR 63-81, Sept. 1963. 
2. &viland, R. Po, & Hause, C. M. Celestial navigation in space,. ‘ 
advances in the astronautical sciences. Vol. 13, Proc. Ninth 
Annual &et- Am. Astronautical Society, Jan. 15-17, 1963, 
Los Angeles, C a l i f .  
3. Havill, D. C. An emergency midcourse navigation procedure for a 
space vehicle returning f r o m  the moon. NASA Tll D-1765, 1963. 
4. U, Conmnnnder J. C., Utegaard, L t .  Commander T. F., & Riordan, G. 
Duttons navigation and p i l o t i q ,  U. S. Naval Insti tute,  Annapoliie, 
Maryland, Fourth Printing, May 1961. 
5. Lillestrand, R. L. , & Carroll, J. E. Horizon based satellite 
I.E.E.E. transactions on aerospace and 
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navigation systems. 
navigational electronics, vol. ANE-10, no. 3, e p t .  1963. 
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6 .  McLean, J. D., Schmidt, S. F., &McGee, L. A. Optimal filtering, 
and l inear prediction applied t o  a midcourse navigation s y s t e m  
for the circumlunar mission. NASA TI? D-1208, 1962. - 
7. Moskowitz, S., k Weinschel, P. Instrumentation for  space navigation. 
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I.E.E.E. transactions on aerospace and navigational electronics, 
vol. AIJE-10, no. 3, sept. $963. 
8. Space navigation handbook. Navpers 92988. 
9. Smith, G. L. Secondary errors and off-design conditions i n  optimal 
estimation of space vehicle trajectories. NASA TN D-2129, lg64. 
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10. Smith, G o  Le, & Harper, E. V. Midcourse guidance using radar 
tracking and on-board observation data. NASA Tm 11-2238, 1964. 
11. White, J. S., & Wingrove, R.  A survey of guidance and navigation 
problem6 for the msnned lunar mission. 
vol. 9, no. 2, 1962. 
J. Inst. Navigation, 
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12. Yachter, M., I% Goetz, R. A. Astronomical angular measurements 
via an astro sextat .  Arma Division, American Bosch Arnrr Corp., 
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Sextant Sighting Performance in the Ames Midcourse Navigation and Guidance Simulator

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