Images produced by pinhole cameras using film sensitive to atomic oxygen provide information on the ratio of spacecraft orbital velocity to the most probable thermal speed of oxygen atoms, provided the spacecraft orientation is maintained stable relative to the orbital direction. Alternatively, as it is described, information on the spacecraft attitude relative to the orbital velocity can be obtained, provided that corrections are properly made for thermal spreading and a co-rotating atmosphere. The LDEF orientation, uncorrected for a co-rotating atmosphere, was determined to be yawed 8.0 minus/plus 0.4 deg from its nominal attitude, with an estimated minus/plus 0.35 deg oscillation in yaw. The integrated effect of inclined orbit and co-rotating atmosphere produces an apparent oscillation in the observed yaw direction, suggesting that the LDEF attitude measurement will indicate even better stability when corrected for a co-rotating atmosphere. The measured thermal spreading is consistent with major exposure occurring during high solar activity, which occurred late during the LDEF mission

Gregory, John C.

Peters, Palmer N.

English

Images produced by pinhole cameras using film sensitive to atomic oxygen provide information on the ratio of spacecraft orbital velocity to the most probable thermal speed of oxygen atoms, provided the spacecraft orientation is maintained stable relative to the orbital direction. Alternatively, information on the spacecraft attitude relative to the orbital velocity can be obtained, provided that corrections are properly made for thermal spreading and a corotating atmosphere. The Long Duration Exposure Facility (LDEF) orientation, uncorrected for a corotating atmosphere, was determined to be yawed 8.0 +/- 0.4 degrees from its nominal attitude, with an estimated +/- 0.35 degree oscillation in yaw. The integrated effect of inclined orbit and corotating atmosphere produces an apparent oscillation in the observed yaw direction, suggesting that the LDEF attitude measurement will indicate even better stability when corrected for a corotating atmosphere. The measured thermal spreading is consistent with major exposure occurring during high solar activity, which occurred late during the LDEF mission

NASA Technical Reports Server

N92-23283
PINHOLE CAMERAS AS SENSORS
FOR ATOMIC OXYGEN IN ORBIT; APPLICATION TO
ATTITUDE DETERMINATION OF THE LDEF
Palmer N. Peters
Space Science Laboratory, NASA/MSFC
Huntsville, Alabama 35812
Phone: 205/544-7728, Fax: 205/544-7754
John C. Gregory
The University of Alabama in Huntsville*
Huntsville, Alabama 35899
Phone: 205/895-6028, Fax: 205/895-6349
SUMMARY
Images produced by pinhole cameras using film sensitive to
atomic oxygen provide information on the ratio of spacecraft
orbital velocity to the most probable thermal speed of oxygen
atoms, provided the spacecraft orientation is maintained stable
relative to the orbital direction. Alternatively, as described
here, information on the spacecraft attitude relative to the
orbital velocity can be obtained, provided that corrections are
properly made for thermal spreading and a co-rotating atmosphere.
The LDEF orientation, uncorrected for a co-rotating atmosphere,
was determined to be yawed 8.0 ° + 0.4 ° from its nominal attitude,
with an estimated + 0.35 ° osciTlation in yaw. The integrated
effect of inclined orbit and co-rotating atmosphere produces an
apparent oscillation in the observed yaw direction, suggesting
that the LDEF attitude measurement will indicate even better
stability when corrected for a co-rotating atmosphere. The
measured thermal spreading is consistent with major exposure
occurring during high solar activity, which occurred late during
the LDEF mission.
INTRODUCTION
A requirement to study the LDEF attitude was identified and
a pinhole camera was developed for this purpose as part of
Experiment A0114 (refs. 1-3) . The atomic oxygen sensitive
pinhole camera uses the fact that oxygen atoms dominate the
atmosphere in low-Earth orbits, and formation of a nearly
*Work supported in part by a grant from UAH Research Institute
and NASA grant NAGW-812 and contract NAS8-36645.
61
https://ntrs.nasa.gov/search.jsp?R=19920014040 2020-03-17T11:49:38+00:00Z
collimated beam of oxygen atoms passing through a pinhole in a
satellite front surface occurs as a result of the orbital
velocity being greater than the most probable Maxwell-Boltzmann
speed of the oxygen atoms. Thus, the range of incidence angles
of atoms to satellite surfaces is very limited, as shown by the
angular distribution curves for two different temperatures in
fig. 1 and described in greater detail elsewhere (ref. 4). The
same maximum oxygen atom intensity was used for both temperatures
to illustrate how the intensity spreads into the wings for higher
temperatures. A thin film of material (silver in this case),
which is sensitive to atomic oxygen, then forms an image of the
impact spot.
The temperature of the thermosphere depends upon solar
activity; the 700 K temperature in fig. 1 is characteristic of a
solar minimum and the 1500 K is closer to a solar maximum. LDEF
altitude was high during the solar minimum of September 1986
(initially deployed at 480 km in April 1984) where oxygen density
was lower and had decayed by the time solar maximum was reached
in June 1989 (recovery occurred at 310 km in January 1990). Most
of the exposure in the pinhole camera occurred close to solar
maximum. When the altitude was lower, the oxygen density was
greater, and the angular distribution for atom incidence was
widest. As will be described later, a well-defined spot was
measured on the pinhole camera's silver sensor surface. Although
overall darkening from overexposure (scattered atoms within the
camera) was observed, this spot has been interpreted as being
from the direct incidence beam and was used to determine the
orientation of the LDEF relative to the orbital velocity.
MEASUREMENTS
The pinhole camera consisted of a 0.3 mm thick stainless steel
hemisphere 3.25 cm (1.28 in.) radius, polished on the concave
surface and coated with vacuum-evaporated silver. Silver was
used because it discolors from formation of oxide (ref. 5). The
pinhole had a conical shape with an included angle much wider
than the maximum atom incidence angle and terminated as knife
edges at a pinhole diameter of 0.5 mm (0.020 in.). The pinhole
was positioned at the center of the silvered hemisphere. As
shown in fig. 2, the exposure at any point on the hemisphere will
depend upon the solid angle subtended by the pinhole from that
point and the point's angular displacement from the orbital
direction, i.e., the atom fluence as a function of angle from the
velocity vector as shown in fig. I. For orientations within i0 °
of the orbital direction, the solid angle subtended by the
pinhole is constant within 2%; the predominant effects of pinhole
size and thus solid angle are to reduce the overall fluence, or
exposure, and increase resolution by reducing pinhole size.
62
Thus, the spot produced behind the pinhole should be centered
with the LDEF's velocity vector and the spot's intensity should
correspond to the distribution shown in fig. i. Any variation in
the attitude of the LDEF's velocity vector relative to the
atmosphere would cause the spot to wander, producing a
nonspherical, larger than normal, spot compared to that produced
by thermal spreading of the beam.
Two techniques were used to determine the spot center and
its shape: the first technique involved measurements taken
directly from an enlarged photograph of the hemisphere taken on-
axis with a 120 mm format camera and a 80 mm macro lens, and the
second technique involved digitizing a 512 x 512 pixel CCD video
camera image of the hemisphere and processing it to obtain both
the spot and hemisphere centers and the spot geometry. Both
techniques gave similar results.
DISCUSSION
Assuming that misalignment of the pinhole camera relative to
the LDEF frame was negligible (machined surfaces and robust
structures offer assurance of this), an LDEF orbiting with
nominal attitude should have produced a spot centered on the
hemisphere and uniformly round• The actual spot, as shown in
fig. 3, was off-center, as would be produced by 8° + 0.4 ° clock-
wise yaw viewed from the space end. The spot was elliptical
(major axis 14 8 ° and minor axis 14 1 °• . , as subtended from the
pinhole), with the major axis in the satellite yaw direction. It
is noted that a yaw of 8 ° should have narrowed the spot in the
yaw direction, not widened it as observed; thus, an oscillation
in atom incidence along the yaw direction is the likely cause.
This originally led us to conclude that the LDEF oscillated in
the yaw direction (i.e., about its long axis), but it has been
brought to our attention (Bourrassa, private communication, 1990)
that a co-rotating Earth's atmosphere interacting with an in-
clined orbit produces an oscillation in the angle of incidence of
oxygen atoms at the surface. We have verified that the oscilla-
tion occurs in the yaw direction, as observed, but the maximum
range should be about + 1.5 ° , not the estimated + 0.35 ° obtained
from the ellipticity measured on the spot. While the center of
the spot is rather well defined and is believed to be the average
orientation for the LDEF, oscillations, thermal spreading, and
other influences on exposure, such as multiple scattering must be
separated• Some considerations are
I. The exposure of the silver was an integrated effect
which occurred over 5 3/4 years, over a wide range in oxygen atom
temperature, and with an excess background from
multiply-scattered atoms• However, most of the oxygen exposure
was received during the last six months of the flight.
63
2. We have not been able to depth profile the exposed
silver film, particularly across the spot. Although a nearly
circularbull'seyepattern suggests a profile similar to those in
fig. i, we have not yet devised a satisfactory technique for
measuring optically opaque profiles.
3. Without a depth-composition profile it is not possible
to fit the oxygen exposure to a known temperature distribution
and there is some uncertainty as to the exact limits of the spot
diameter (i.e., where the spot ends and the background takes
over); however, it appears that rings on the spot represent equal
thicknesses of oxide and provide the measured ellipticity. The
minor axis of the spot could represent temperatures as high as
1500 K if assigned FWHM in fig. i.
4. An oscillating structure and the apparent oscillation
caused by an inclined orbit and rotating atmosphere do not yield
the same angular flux distribution in a pinhole camera. An
oscillating structure sweeps rapidly through the zero displace-
ment and pauses at the extreme angular displacement; The opposite
is true for the rotating atmosphere effect. Thus, a mechanical
oscillation has a larger integral effect on spot diameter for the
same number of degrees of oscillation. We are calculating these
profiles with atmospheric oscillations included. Further study
is needed to accurately determine the LDEF's range of oscilla-
tion.
Analysis by x-ray diffraction of the black powder flaking
from much of the camera interior confirmed that it was Ag20. For
reasons yet unknown, the primary exposed spot was more stable
than the rest of the background exposed surface; this assisted
our investigation.
AKNOWLEDGEMENTS
The authors are grateful for continued assistance from David
Carter, William Kinard, and Jim Jones of the NASA Langley
Research Center; to Howard Foulke of the General Electric Company
for explanations of their satellite stabilization studies; to
William Witherow for digital image measurements; and to Charles
Sisk for computer assistance.
64
REFERENCES
I ,
•
.
.
•
Clark, L. G., Kinard, W. H., Carter, D. J., and Jones, J.
L., Jr., Eds., "The Long Duration Exposure Facility (LDEF):
Mission 1 Experiments," NASA SP-473, Scientific and
Technical Information Branch, NASA, Washington, D.C. (1984).
Siegel, S. H. and Das, A., "Passive Stabilization of the
LDEF," Final Report on contract NASI-13440, GE Document No.
74SD4264, November 1974, General Electric Company,
Astrospace Division, Philadelphia, PA.
Siegel, S. H. and Vishwanath, N. S., "Analysis of the
Passive Stabilization of the LDEF, " GE Document No.
78SD4218, August 1977, General Electric Company, Astrospace
Division, Philadelphia, PA.
Peters, P. N., Sisk, R. C., and Gregory, J. C., "Velocity
Distributions of Oxygen Atoms Incident on Spacecraft
Surfaces," J. Spacecraft and Rockets, 25(II , 53-58 (1988) .
Thomas, R. J. and Baker, D. J., "Silver Film Atomic Oxygen
Sensors," Can. J. Phys., 50, 1676 (1972).
65
I-i@ @
CAP-THETA
1500 K, s- = 6.25
700 K, s_ = 9.13
i
r
J
3_
Fig. i. Intensity of oxygen atoms versus incidence angle,
cap-theta, in degrees from the orbital ram direction for two
equilibrium temperatures of the atoms.
ORBITAL D IR/KYF ION
-_ _ YAW
Fig. 2. Schematic of pinhole camera with off-centered spot due
to yaw of the LDEF and showing thermal spreading about the spot
center due to the effect shown in Fig. i.
66
¸SPACE
Fig. 3. Photograph of exposed silver hemisphere from pinhole
camera; overall dark flaking area is interpreted as overexposure
from multi-scattered atoms, and the spot, which is more stable,
is believed to be from direct incidence.
OR/_Ih'.AL PAGE
BLACK AND WHITE PHOTOGRAPH
67



Pinhole cameras as sensors for atomic oxygen in orbit: Application to attitude determination of the LDEF

PINHOLE CAMERAS AS SENSORS
FOR ATOMIC OXYGEN IN ORBIT; APPLICATION TO
ATTITUDE DETERMINATION OF THE LDEF
Palmer N. Peters
Space Science Laboratory, NASA/MSFC
Huntsville, Alabama 35812
Phone: 205/544-7728, Fax: 205/544-7754
John C. Gregory
The University of Alabama in Huntsville*
Huntsville, Alabama 35899
Phone: 205/895-6028, Fax: 205/895-6349
i
SUMMARY
images produced by pinhole cameras using film sensitive to
atomic oxygen provide information on the ratio of spacecraft
orbital velocity to the most probable thermal speed of oxygen
atoms, provided the spacecraft orientation is maintained stable
relative to the orbital direction. Alternatively, as described
here, information on the spacecraft attitude relative to the
orbital velocity can be obtained, provided that corrections are
properly made for thermal spreading and a co-rotating atmosphere.
The LDEF orientation, uncorrected for a co-rotating atmosphere,
was determined to be yawed 8.0 ° + 0.4 ° from its nominal attitude,
with an estimated + 0.35 ° osciYlation in yaw. The integrated
effect of inclined orbit and co-rotating atmosphere produces an
apparent oscillation in the observed yaw direction, suggesting
that the LDEF attitude measurement will indicate even better
stability when corrected for a co-rotating atmosphere. The
measured thermal spreading is consistent with major exposure
occurring during high solar activity, which occurred late during
the LDEF mission.
INTRODUCTION
A requirement to study the LDEF attitude was identified and
a pinhole camera was developed for this purpose as part of
Experiment A0!I4 (refs. 1-3) . The atomic oxygen sensitive
pinhole camera uses the fact that oxygen atoms dominate the
atmosphere in low-Earth orbits, and formation of a nearly
*Work supported in part by a grant from UAH Research Institute
and NASA grant NAGW-812 and contract NAS8-36645.
https://ntrs.nasa.gov/search.jsp?R=19920001860 2020-03-17T15:11:35+00:00Z
collimated beam of oxygen atoms passing through a pinhole in a
satellite front surface occurs as a result of the orbital
velocity being greater than the most probable Maxwell-Boltzmann
speed of the oxygen atoms. Thus, the range of incidence angles
of atoms to satellite surfaces is very limited, as shown by the
angular distribution curves for two different temperatures in
fig. 1 and described in greater detail elsewhere (ref. 4). The
same maximum oxygen atom intensity was used for both temperatures
to illustrate how the intensity spreads into the wings for higher
temperatures. A thin film of material (silver in this case),
which is sensitive to atomic oxygen, then forms an image of the
impact spot.
The temperature of the thermosphere depends upon solar
activity; the 700 K temperature in fig. I. is characteristic of a
solar minimum and the 1500 K is closer to a solar maximum. LDEF
altitude was high during the solar minimum of September 1986
(initially deployed at 480 km in April 1984) where oxygen density
was lower and had decayed by the time solar maximum was reached
in June 1989 (recovery occurred at 310 km in January 1990). Most
of the exposure in the pinhole camera, occurred close to solar
maximum when the altitude was lower, the oxygen density was
greater, and the angular distribution for atom incidence was
widest. As will be described later, a well-defined spot was
measured on the pinhole camera's silver sensor surface. Although
overall darkening from overexposure (scattered atoms within the
camera) was observed, this spot has been interpreted as being
from the direct incidence beam and was used to determine the
orientation of the LDEF relative to the orbital velocity.
MEASUREMENTS
The pinhole camera consisted of a 0.3 mm thick stainless steel
hemisphere 3.25 cm (1.28 in.) \radius, polished on the concave
surface and coated with vacuum-evaporated silver. Silver was
used because it discolors from formation of oxide (ref. 5). The
pinhole had a conical shape with an included angle much wider
than the maximum atom incidence angle and terminated as knife
edges at a pinhole diameter of 0.5 mm (0.020 in.). The pinhole
was positioned at the center of the silvered hemisphere. As
shown in fig. 2, the exposure at any point on the hemisphere will
depend upon the solid angle subtended by the pinhole from that
point and the point's angular displacement from the orbital
direction, i.e., the atom fluence as a function of angle from the
velocity vector as shown in fig. I. For orientations within 10 °
of the orbital direction, the solid angle subtended by the
pinhole is constant within 2%; the predominant effects of pinhole
size and thus solid angle are to reduce the overall fluence, or
exposure, and increase resolution by reducing pinhole size.
Thus, the spot produced behind the pinhole should be centered
with the LDEF's velocity vector and the spot's intensity should
correspond to the distribution shown in fig. i. Any variation in
the attitude of the LDEF's velocity vector relative to the
atmosphere would cause the spot to wander, producing a
nonspherical, larger than normal, spot compared to that produced
by thermal spreading of the beam.
Two techniques were used to determine the spot center and
its shape: the first technique involved measurements taken
directly from an enlarged photograph of the hemisphere taken on-
axis with a 120 mm format camera and a 80 mm macro lens, and the
second technique involved digitizing a 512 x 512 pixel CCD video
camera image of the hemisphere and processing it to obtain both
the spot and hemisphere centers and the spot geometry. Both
techniques gave similar results.
DISCUSSION
Assuming that misalignment of the pinhole camera relative to
the LDEF frame was negligible (machined surfaces and robust
structures offer assurance of this), an LDEF orbiting with
nominal attitude should have produced a spot centered on the
hemisphere and uniformly round. The actual spot, as shown in
fig. 3, was off-center, as would be produced by 8° + 0.4 ° clock-
wise yaw viewed from the space end. The spot was elliptical
(major axis 14 8° and minor axis 14 1 °. . . , as subtended from the
pinhole), with the major axis in the satellite yaw direction. It
is noted that a yaw of 8 ° should have narrowed the spot in the
yaw direction, not widened it as observed; thus, an oscillation
in atom incidence along the yaw direction is the likely cause.
This originally led us to conclude that the LDEF oscillated in
the yaw direction (i.e., about its long axis), but it has been
brought to our attention (Bourrassa, private communication, 1990)
that a co-rotating Earth's atmosphere interacting with an in-
clined orbit produces an oscillation in the angle of incidence of
oxygen atoms at the surface. We have verified that the oscilla-
tion occurs in the yaw direction, as observed, but the maximum
range should be about + 1.5 °, not the estimated + 0.35 ° obtained
from the ellipticity measured on the spot. Whil--e the center of
the spot is rather well defined and is believed to be the average
orientation for the LDEF, oscillations, thermal spreading, and
other influences on exposure, such as multiple scattering must be
separated. Some considerations are:
I. The exposure of the silver was an integrated effect
which occurred over 5 3/4 years, over a wide range in oxygen atom
temperature, and with an excess background from
multiply-scattered atoms. However, most of the oxygen exposure
was received during the last six months of the flight.
2. We have not been able to depth profile the exposed
silver film, particularly across the spot. Although a nearly
circular bul!eye pattern suggests a profile similar to those in
fig. !, we have not yet devised a satisfactory technique for
measuring optically opaque profiles.
3. Without a depth-composition profile it is not possible
to fit the oxygen exposure to a known temperature distribution
and there is some uncertainty as to the exact limits of the spot
diameter (i.e., where the spot ends and the background takes
over); however, it appears that rings on the spot represent equal
thicknesses of oxide and provide the measured ellipticity. The
minor axis of the spot could represent temperatures as high as
1500 K if assigned FWHM in fig. i.
4. An oscillating structure and the apparent oscillation
caused by an inclined orbit and rotating atmosphere do not yield
the same angular flux distribution in a pinhole camera. An
oscillating structure sweeps rapidly through the zero displace-
ment and pauses at the extreme angular displacement; The opposite
is true for the rotating atmosphere effect. Thus, a mechanical
oscillation has a larger integral effect on spot diameter for the
same number of degrees of oscillation. We are calculating these
profiles with atmospheric oscillations included. Further study
is needed to accurately determine the LDEF's range of oscilla-
tion.
Analysis by x-ray diffraction of the black powder flaking
from much of the camera interior confirmed that it was Ag20. For
reasons yet unknown, the primary exposed spot was more stable
than the rest of the background exposed surface; this assisted
our investigation.
AKNOWLEDGEMENTS
The authors are grateful for continued assistance from David
Carter, William Kinard, and Jim Jones of the NASA Langley
Research Center; to Howard Foulke of the General Electric Company
for explanations of their satellite stabilization studies; to
William Witherow for digital image measurements; and to Charles
Sisk for computer assistance.
REFERENCES
I •
•
.
.
,
Clark, L. G., Kinard, W. H., Carter, D. J., and Jones, J.
L., Jr., Eds., "The Long Duration Exposure Facility (LDEF):
Mission 1 Experiments," NASA SP-473, Scientific and
Technical information Branch, NASA, Washington, D.C. (1984).
.Siegel, S. H. and Das, A., "Passive Stabilization of the
LDEF," Final Report on contract NASI-i3440, GE Document No.
74SD4264, November 1974, General Electric Company,
Astrospace Division, Philadelphia, PA.
Siegel, S. H. and Vishwanath, N. S., "Analysis of the
Passive Stabilization of the LDEF, " GE Document No.
78SD42!8, August 1977, General Electric Company, Astrospace
Division, Philadelphia, PA.
Peters, P. N., Sisk, R. C., and Gregory, J. C., "Velocity
Distributions of Oxygen Atoms Incident on Spacecraft
Surfaces," J. $_macecraf_/]d Rockets, 25(!), 53-58 (1988).
Thomas, R. J. and Baker, D. J., "Silver Film Atomic Oxygen
Sensors," Can. J. Phys., 50, 1676 (1972).
FIGURE CAPTIONS
Fig. i. Intensity of oxygen atoms versus incidence angle,
cap-theta, in degrees from the orbital ram direction for two
equilibrium temperatures of the atoms.
Fig. 2. Schematic of pinhole camera with off-centered spot due
to yaw of the LDEF and showing thermal spreading about the spot
center due to the effect shown in Fig. i.
Fig. 3 Photograph of exposed silver hemisphere from pinhole
camera'; overall dark flaking area is interpreted as overexposure
from multi-scattered atoms, and the spot, which is more stable,
is believed to be from direct incidence.
6
|1
700 K, s_ : 9.13
_c L
ORBITAL D IREL-TI(I_
____/ _-____, _=
i
ORIGINAL, PAGE IS
OF POOR QUALITY
!:
.i
_J_
ORIGINAL PAQE 18
OF POOR QUALITY


Pinhole cameras as sensors for atomic oxygen in orbit; application to attitude determination of the LDEF

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