Development of star intensity measuring system which minimizes effects of outside interferenc

Davis, E. S.

English

NASA Technical Reports Server

REPLY TO 
ATTN OF: 
FROM : 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
WASI~INGTON. D.C. 20546 
April 5, 1971 
~~~/~cientific & Technical Information Division 
Attention: Miss Winnie M. Morgan 
~P/~ffice of Assistant General 
Counsel for Patent Matters 
SUBJECT: Announcement of NASA-Owned 
U.S. Patents in STAR 
In accordance with the procedures contained in the Code GP 
to Code US1 memorandum on this subject, dated June 8, 1970, 
the attached NASA-owned U.S. patent is being forwarded for 
abstracting and announcement in NASA STAR. 
The following information is provided: 
U.S. Patent No. 
Corporate Source 
Supplementary 
Corporate Source 
: California Institute of Technology 
: Jet Propulsion Laboratory 
NASA Patent Case No.: XNP-06510 
Please note that this patent covers an invention made by an 
employee of a NASA contractor. Pursuant to Section 305(a) of 
the National Aeronautics and Space Act, the name of the 
Administrator of NASA appears on the first page of the patent; 
however, the name of the actual inventor (author) appears at 
the heading of Column No. 1 of the Specification, following the 
words 'I. . . with respect to an invention of. . . ." 
~ayfe Parker 
Enclosure : 
Copy of Patent 
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k. 17, 1968 JAMES E. WEBB 3,417,247 
ADMINISTRATOR O F  T H E  NATIONAL AERONAUTICS 
A N D  SPACE ADMlNlSTRATlON 
RADIANT ENERGY INTENSITY MEASUREMENT SYSTEM 
Filed June 29, 1966 4 Sheets-Sheet 2 
W I D T H  O F  T H E  IMAGE DISSECTOR S L I T  
4 / 
F I R S T  H A R M O N I C  
I R E F E R E N C E  S I G R A L  AT b 
"c 
P R E A M P  O U T P U T  AT e 
l-..-- OUTPUT OF 
S Q U A R I N G  C I R C U I T  03 
I 
FIG. 2 
INVENTOR. 
BY 
A T T O R N E Y S  
Dec. 17, 1968 JAMES E. WEBB 3,417,247 
ADMINISTRATOR OF T H E  NATIONAL AERONAUTICS 
A N D  SPACE ADMINISTRATION 
RADIANT ENERGY INTENSITY MEASUREMENT SYSTEM 
Filed June 29, 1966 4 Sheets-Sheet 3 
WIDTH OF THE IMAGE DISSECTOR S L I T  
-- 
I 
SWEEP SIGNAL AT a 
FIRST HARMONIC 1 I 'w / i- REFERENCE SIGNAL AT b 
t 
SQUARIWG CIRCUIT 13 
FIG. 3 
INVENTOR. 
EDGAR S. D A V I S  
Dec. 17, 1968 JAMES E. WEBB 3,417,247 
ADMINISTRATOR OF T H E  NATIONAL AERONAUTICS 
A N D  SPACE ADMINISTRATION 
RADIANT ENERGY INTENSITY MEASUREMENT SYSTEM 
Filed June 29, 1966 4 Sheets-Sheet 4 
FIG. 4 ( a )  
bi-SYAIGMRONOUSLY 
DEMODULATED %IG#AB. 
FIG. 4 ( b )  
ARBITRARY 
U N I T S  
I 
F IG.  5 
INVENTOR. 
E D G A R  S. D A V I S  
BY 
3,4 17,247 United States Patent O6ce , .... ,  ,, 
1 2 
same locations as those for which waveshapes are shown 
3,417,247 in FIGURE 2, however this time the star which is being 
RADIANT ZPPJEFPGU INTENSTY MEASmEMENT scanned is off the sensor optical axis; SYSmM FIGURE 4 illustrates the DC average value of the 
J a t a a i i  : 5 respective demodulator o ~ t p u t  signals as a function of 
vention of Edgar S ,  Davis, La Canada, Calif. star location in the scanned field of view which are 
Filed June 29, 1966, Ser. No. 562,445 observed at the outputs of low pass filters 18 and 28; and 
6 Claims. (el. 250-203) FIGURE 5 is the waveshape of a typical star intensity 
signal obtained at the output of the apparatus. 
The invention described herein was made in the per- 10 Referring now to FIGURE 1, there may be seen a 
formance of work under a NASA contract and is sub- block diagram of an embodiment of the invention. Thic: 
ject to the provisions of Section 305 of thc National includes an image dissector camera lube 10 which i s  
Aeronautics and Space Act of 1958, Public Law 85-568 used to scan the sky only along a single axis. To achieve 
(72 Stat. 435; 42 USC 2457). thus, a slit aperature is employed instead of the usual 
This invention relates to  a radiant energy intensity 15 opening found in television type inaage dissector camera 
measurement system, and more particularly to improve- tubes. In order to sweep the field of view containi~~p: 
ments therein. the desired star across the slit aperture, the triangular 
In attitude control systems as well as in navigational sweep generator 12 is employed, which applies i t s  output 
systems which operate by locating and tracking a par- to the image dissector. The output of the image dissector 
ticular star, one )of the pieces of information used for 20 is applied to the preamplifier 14. The output ef the pre- 
these operations is the intensity of illumination from amplifier is applied to a first demodulator 16. The output 
the particular star which is being scanned. It is known that from the triangular sweep generator is applied lo a squar- 
other celestial light sources in the field of scan can cause ing circuit 13. The output from the squaring circuit 13 is 
errors in the reading of the particular star intensity. These applied as a second input to the first demodnPator 13. 
errors are called noise. The presently favored "peak de- 25 The output from the first demodulator is applied to a 
tecting" systems used for measuring the intensity of illu- low pass filter 18. The low pass filter output is applied to 
mination from a star are not very effective at eliminat- an output amplifier 20, the o u t p ~ ~ t  signal of which corra- 
ing noise. prises the star position error signal. As thus far described, 
An object of this invention is to provide a star intensity the apparatus is known and has been used for f i x  purpose 
signal measuring system which minimizes the effects of 30 indicated. 
noise. In accordance with this invention, output from the 
Another object of the present invention is to provide triangular sweep generator 12 is used to generate a first 
a unique star light intensity measuring system which harmonic reference signal. This comprises a square wave 
has a lower background sensitivity and a lower stray signal which is at twice the frequency of the triangular 
light sensitivity than previous systems. 35 sweep generator. Thus, since normally the triangular 
Still another object of the present invention is the pro- sweep generator signal is a t  a 1000 Hz, the first harmonic 
vision of a novel and unique star light intensity measur- reference signal has a frequency of 2000 Hz. This i s  
ing system. achieved by applying the output of the triangular sweep 
These and other objects of the present invention are generator 12 to a full wave rectifier 22, that produces as 
achieved by adding circuitry to that already provided 40 its output a triangular waveshape at twice the frequency 
upon the guided vehicle for determining the roll error of of the triangular sweep generator 12. The output of the 
the vehicle. The roll error detecting circuitry normally full wave rectifier 22 is applied to a square wave generator 
includes an image dissector camera tube wherein the 24, whose output comprises the first harlnonic reference 
optical image of the field viewed by the tube, including the signals of the proper phase which have been described. 
star of concern, is deflected past a slit aperture, using a 45 The output of the preamplifier 14 is applied to a sec- 
triangular sweep. The output signal of the image dissec- ond demodulator 26 whose other input is the output of 
tor tube is amplified, then applied to a demodulator where the squaring circuit 24. This second demodulator 26 may 
it is demodulated using the triangular sweep signal as be called the bisynchronous demodulator. Its output is 
the second input, then filtered and applied to an output applied to a low pass filter 28, the output of which is 
amplifier. This invention utilizes the amplified output 50 applied to an absolute value amplifier 30, The output of 
signal and applies it to a demodulator having as its second low pass filter 18 is also applied to an absolute value 
input a first harmonic reference signal which is derived amplifier 32. The amplifiers 30 and 32 provide unity 
from the triangular sweep signal. The output of the gain and effectively operate as isolatio~i amplifiers. Their 
demodulator is then filtered through a low pass filter, outputs are applied to a summing network comprising the 
and then added to the filtered output from the poll error 55 two resistors 34, 36 which have one of their ends con- 
signal channel. The sum signal represents an average in- nected to the respective amplifiers 30, 32 and the ,other 
tensity of the star regardless of its location within the ends joined together. The output of the surnmii~g network 
area scanned by the aperture. is applied to an inverter amplifier 38. The outpat of the 
The novel features that are considered characteristic inverter amplifier 38 comprises the average intensity of 
of this invention are set forth with particularity in the GO the star as it is scanned across the area scanned by the 
appended claims. The invention itself both as to its or- slit aperture of the image dissector. 
ganization and method of operation, as well as additional Considering now the four waveshapes shown in FIG- 
objects and advantages thereof, will best be understood URE 2, the first waveshape 48 is a representation of the 
from the following description when read in connection triangular sweep generator output at point a in FIGURE 
with the accompanying drawings, in which: 65 1. The width of the image dissector slit is represented by 
FIGURE 1 is a block schematic diagram of an embodi- the disance between the two spaced lines along the 
ment of the invention; ordinate. The ordinate designated V, is the amplitude of 
FIGURE 2 illustrates waveshapes which occur at  vari- the voltage of the triangular sweep waveshape 480 which 
ous locations throughout the operation of the system; varies with time in the manner shown. When the sweep 
FIGURE 3 shows typical waveshapes obtained at the 70 voltage is zero the star, if on axis, is positioned right in f i e  
3,417,247 
3 4 
center of tile slit and thus provides its maximum light application in other areas where this type of an operation 
output. The variations of the output signal of the image is required. 
dissector, as the electron image is being swept across the What is claimed is: 
aperture is represented by the waveshape 42, which oc- 1. In apparatus wherein a source of light in a field is 
curs at point c in FIGURE 1, which is at the output of the 5 viewed by an image dissector camera wherein the elec- 
preamplifier 14. 41: mill be seen that as the optical image of tron image of the field of view is deflected past a slit 
t k  star being scanned comes into the aperture there is a aperture onto a target by a triangular sweep signal, means 
sharp increase in the output of the preamplifier which sub- for producing a signal representative of the average in- 
stantially maintains its level across the width of the aper- tensity of said source of illumination regardless of its 
tllre and when the image reaches the other side of the position within the said scanned field of view, said means 
aperture there is a sharp drop in the output signal of the comprising first and second demodulators, means for 
preamplifier. ap~lying the output from said image dissector camera to 
The first harmonic reference signal 44 which occurs at said first and second demodulators, means for generating 
point b of FIGURE I, which is the output of the squaring a first harmonic reference signal responsive to said tri- 
circuit 24, occurs at twice the frequency of the triangular 1 j angular sweep signal, means for applying said triangular 
sweep sigi~al and is eEectively shifted 90" relative thereto. sweep signal to said first demodulator, means for apply- 
$his signal, as can be seen from the waveshapes, is in ing said first harmonic reference signal to said second 
phase with the preamplifier output at a. The output of demodulator, and summing network means for adding the 
squaring circuit 13 is represented by the waveshape 45. outputs of said first and second demodulators to pro- 
FIGURE 3 shons the same waveshapes as are shown 20 vide a signal representative of the average intensity of 
in FIGURE 2, for a star which is off of null by an angle the source of illumination regardless of its position within 
corresponding to ?A of the peak sweep. The respective said scanned field of view. 
waveshapcs arc given the reference numerals LBO', 44', 42' 2. In apparatus as recited in claim 1 wherein said 
and 45'. The indication that the star is off of null is shown means for generating a first harmonic reference signal 
by the shrft in the position of the two lines representing 25 generates a signal at twice the frequency of said triangular 
the width of the image dissector slit. These are shifted sweep signal having a rectanguIar waveshape and phase 
50 that these two lines are no longer symmetrical about shifted 90" from said triangular sweep signal. 
the axis of the a'sscissa. While the sweep signal and 3. Apparatus as recited in claim 1 wherein there is 
f irst harmonic lckrcnce signal remain the same, the included a first and second low pass filter, means coupling 
preamplifier ouput is shifted relative to these tho  signals. 30 said first low pass filter between said first demodulator 
Since a phase seasitive demodulator 14 is employed, and said summing network means, and means for con- 
the det:iodillation of the signal 42' using the signal 48' necting said second low pass filter between said second 
produces at the output of the low pass filter 18 a P)C demodulator and said summing network means. 
signal . ~ v ~ i c h  is p~oportional to the magnitude and 4. Apparat~~s as recited in claim 3 wherein said ap- 
polarity of the deviation of the star from the optical axis 33 paratus includes a preamplifier having an input connected 
of the sensor in one axis. to the output of said image dissector camera and its out- 
FIGURE 4 shows the respective waveshapes 45 and 48 put connected to one input to said first and second de- 
which represent the signals seen at  points d and e in modulators, and there are first and second absolute value 
FIGURE 1 which are located at the outputs of the re- amplifiers respectively connected between the output of 
spectivc low pass filters 28 and 18. The waveshape 46 40 the first low pass filter and said summing network means 
shows ihc variation of the voltage output at the low pass and between the output of said second low pass filter and 
filter with the ~osiiion 8, of the star in the field of view. said summing network means. 
If ( 9 ,  represents the amplitude of the peak sweep, then 5. Apparatus as recited in claim I wherein said means 
li,/op i s  the normalized star position. The waveshape 48 for generating a first harmonic reference signal responsive 
shows the variation in output voltage at point d with the 45 to said triangular sweep signals includes full wave rectifier 
position of the star in the field of view. The abscissa is means having an input to which said triangular sweep 
0,/9,, or the ilormalized star position. signals are applied, square wave generator means having 
FIGURE 5 shows a waveshape 58 which represents the an input to which an output from said full wave rectifier 
sum of the absolute values of waveshapes 46 and 48. This means is applied, and means for applying the output of 
waveshape 50 is representative of the output signal or star 50 said square wave generator means to said second de- 
intensity signal over the field of view. This signal is modulator. 
exactly equal to the average value of the signal out of 4. Apparatus for generating a signal representative of 
the preampiificr, independent of the position of the star the average intensity of a source of illumination com- 
within the field of view. Even more important, the noise prising an image dissector camera having a slit aperture, 
bandwith of this signal is determined by the simple low 55 said image dissector camera scanning a field of view in- 
pass filter, Since the noise has a zero average, it does not clnding said source of illumination, triangular sweep gen- 
modify the intensity voltage. Accordingly, a given star erator means for generating a triangular sweep signal at 
prodcrces the same average voltage output at the output a predetermined frequency, means for applying the out- 
of the amplifier 38, independent of the background bright- put of said triangular sweep generator to said image dis- 
ncss aro~tnd the star. As a result, the present invention 60 sector camera for passing an image of said field of view 
overcomes the major fault of the conventional peak detec- including said source of illumination repetitively by said 
tor apparatus for nleasuring star intensity. The peak slit aperture, means for deriving an output signal from 
detector detects the peak of not only the star in the field said image dissector camera having an amplitude repre- 
of view but also of the noise, thus producing an incorrect sentative of the illumination of the image which passes 
signal indication. 65 through said slit aperture, means for generating a first 
There has been accordingly described and shown herein harmonic reference signal responsive to said triangular 
a novel and useful arrangement for obtaining a signal sweep signal generator output, said first harmonic refer- 
representative of the average intensity of a light source ence signal having a frequency which is twice that of said 
which is scanned by an image dissector or other mod~la-  first harmonic sweep generator frequency and having a 
tor-detector arrangement camera having a silt aperture, 70 substantially rectangular wave shape, a first and second 
regardless of the position of the light source within the demodulator each having first and second inputs and an 
field of view scanned. It  will be appreciated that despite output, means for applying the output signal from said 
the fact that the description herein has lbeen made with image dissector camera to the respective first inputs of 
respect Lo guided vehicles using stars for guidance, this said first and second demodulators, means for applying 
nrrangenlend is not to be restricted thereto but can find 75 otttput from said triangular sweep generator to said first 
demodulator second input, means for applying output References Cited 
from said first harmonic reference generator to said sec- 
ond demodulator second input, first and second low pass UNITED STATES PATENTS 
filter means, means for applying the output of said first 3~043,907 7/1962 W. L. Martin -------- 17R-6.8 
demodulator to the first low pass filter means, means for 5 3,043 ,go7 7/1962 Martin --------- ----- 178-6.8 
ao~lvine the o u t ~ u t  of the second demodulator to the 371343022 5/1964 et al. -------- 250-203 
A * .  
second low pass filter means, means for adding the out- JAMES W. LAWRENCE, Primary Examiner. 
puts of said first and second low pass filter means to pro- R. F. HOSSFELD, ~~~i~~~~~ ~ ~ ~ i ~ ~ ~ .  
vide an output signal whose value is representative of the 
average value of the intensity of said source of illumina- U.S. C1. X.R. 
tion regardless of its location within the aperture slit. 250-206, 234 


Radiant energy intensity measurement system Patent

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