U.S. patent application number 09/756395 was filed with the patent office on 2002-09-05 for method and sensor for capturing rate and position and stabilization of a satellite using at least one focal plane.
This patent application is currently assigned to The Boeing Company. Invention is credited to Davis, John E..
Application Number | 20020121574 09/756395 |
Document ID | / |
Family ID | 25043276 |
Filed Date | 2002-09-05 |
United States Patent
Application |
20020121574 |
Kind Code |
A1 |
Davis, John E. |
September 5, 2002 |
METHOD AND SENSOR FOR CAPTURING RATE AND POSITION AND STABILIZATION
OF A SATELLITE USING AT LEAST ONE FOCAL PLANE
Abstract
An optical sensor includes dual fields of view including a
panoramic field of view spanning 360.degree. in azimuth angle in a
direction perpendicular to an axis of the sensor, and a
limb-looking field of view non-perpendicular to the axis for
viewing the limb of the earth. Both fields of view are imaged onto
annular regions of one of more focal plane arrays comprising pixels
arranged in a rectangular array of rows and columns. The sensor is
used in a method for capturing rate and direction of rotation of a
satellite about its axes and for detecting orientation of the
satellite about two of its axes. Rate and direction are determined
by finding the center of the earth relative to axes of the focal
plane array based on the image of the earth limb from the panoramic
field of view, and comparing the earth center location at a series
of sequential times. The rate and position information are used for
stabilizing an initially tumbling satellite after tip-off. Once the
satellite is stabilized, its orientation is detected by finding the
earth center location on the focal plane array based on the earth
limb image from the limb-looking optics of the sensor. In one
embodiment of the invention, both fields of view are imaged onto
concentric inner and outer ring-shaped regions of the same focal
plane array.
Inventors: |
Davis, John E.; (Claremont,
CA) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA
101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Assignee: |
The Boeing Company
Seattle
WA
|
Family ID: |
25043276 |
Appl. No.: |
09/756395 |
Filed: |
January 8, 2001 |
Current U.S.
Class: |
244/171 |
Current CPC
Class: |
G02B 13/06 20130101;
B64G 1/244 20190501; B64G 1/365 20130101 |
Class at
Publication: |
244/171 |
International
Class: |
B64G 001/36 |
Claims
What is claimed is:
1. A method for determining direction and rate of rotation of a
satellite about first, second, and third mutually orthogonal body
axes fixed relative to the satellite and for determining
orientation of the satellite relative to earth, comprising:
mounting a dual field-of-view optical sensor on the satellite with
an optical axis of the sensor in a predetermined orientation with
respect to the body axes, the sensor having at least one focal
plane array comprising a plurality of pixels arranged as a grid;
directing radiant energy from a first field of view of the sensor
onto a first annular region of the at least one focal plane array,
the first field of view spanning 360.degree. in azimuth angle about
the optical axis and a range of elevation angles including
90.degree. in elevation angle relative to the optical axis;
directing radiant energy from a second field of view of the sensor
onto a second annular region of the at least one focal plane array,
the second field of view spanning 360.degree. in azimuth angle
about the optical axis and a range of elevation angles
non-perpendicular to the optical axis; determining a relative
location of a subtense of a limb of the earth on the first annular
region of the at least one focal plane array at each of a plurality
of successive times, and determining direction and rate of rotation
of the satellite about the body axes based on changes in the
relative location of the earth limb subtense on the first annular
region; and determining a relative location of the earth limb
subtense on the second annular region of the at least one focal
plane array, and determining orientation angles between the body
axes and a nadir vector to the earth based on the relative location
of the earth limb subtense on the second annular region.
2. The method of claim 1, further comprising: substantially
reducing tumbling of the satellite by applying corrective thrusts
to the satellite based on the rates and direction of rotation
determined from the first field of view of the sensor; and placing
and stabilizing the satellite in a predetermined orientation
relative to the nadir vector by applying stabilizing thrusts to the
satellite based on the orientation angles determined from the
second field of view of the sensor.
3. The method of claim 1, wherein the first and second fields of
view are directed respectively onto concentric first and second
annular regions of the same focal plane array.
4. The method of claim 3, wherein the first annular region
comprises an inner ring and the second annular region comprises an
outer ring lying radially outwardly of the inner ring.
5. The method of claim 4, wherein the pixels of the focal plane
array are arranged in a plurality of parallel rows and a plurality
of parallel columns perpendicular to the rows, the relative
location of the earth limb subtense being determined on each of the
inner and outer rings by detecting transition pixels in each row
and each column at which a transition between earth and space
falls.
6. The method of claim 1, wherein sensor electronics connected to
the sensor capture signals from the at least one focal plane array
by sampling the signals periodically to create a series of
sequential frames, and wherein the location of the subtense of the
earth limb on each of the annular regions is sampled at least three
times for each frame.
7. The method of claim 1, further comprising directing radiant
energy from at least one of the fields of view onto more than one
focal plane array.
8. The method of claim 7, wherein a separate set of electronics is
provided for each focal plane array to enable multiple signal paths
for enhanced reliability.
9. The method of claim 1, wherein the at least one focal plane
array is operable to detect radiant energy in a wavelength band
chosen so as to provide both day and night limb contrast and such
that limb height changes caused by season and diurnal changes are
minimized.
10. The method of claim 9, wherein the wavelength band lies within
a range from about 3.5 .mu.m to about 4.5 .mu.m.
11. The method of claim 1, wherein sensor electronics connected to
the sensor capture signals from the at least one focal plane array
by sampling the signals periodically at a defined sampling rate to
create a series of sequential frames, and wherein the sampling rate
is at least about 10 frames per second.
12. The method of claim 1, wherein rotational orientations of the
satellite about two of the body axes are determined from the second
annular region of the at least one focal plane array, and further
comprising determining a rotational orientation of the satellite
about the third body axis by directional radio reception.
13. The method of claim 1, wherein rotational orientations of the
satellite about two of the body axes are determined from the second
annular region of the at least one focal plane array, and further
comprising determining a rotational orientation of the satellite
about the third body axis by directional magnetic fields.
14. The method of claim 1, wherein rotational orientations of the
satellite about two of the body axes are determined from the second
annular region of the at least one focal plane array, and further
comprising determining a rotational orientation of the satellite
about the third body axis by directional inertial rotation.
15. A dual field-of-view optical sensor comprising: at least one
focal plane array comprising a plurality of pixels arranged in a
grid; panoramic optics for capturing radiant energy from an annular
panoramic field of view spanning 360.degree. in azimuth angle about
an optical axis of the sensor and covering a range of elevation
angles including 90.degree. in elevation angle relative to the
optical axis, the panoramic optics re-directing and focusing the
radiant energy from the panoramic field of view onto a first
annular region of the at least one focal plane array; and
limb-looking optics for capturing radiant energy from an annular
field of view spanning 360.degree. in azimuth angle about the
optical axis and covering a range of elevation angles
non-perpendicular to the optical axis such that at least a major
circumferential portion of a limb of the earth is within the field
of view of the limb-looking optics when the optical axis of the
sensor points toward a centroid of the earth, the limb-looking
optics re-directing and focusing the radiant energy onto a second
annular region of the at least one focal plane array.
16. The sensor of claim 15, wherein the panoramic optics include a
convex mirror of generally annular form.
17. The sensor of claim 16, wherein the convex mirror re-directs
the radiant energy from the panoramic field of view along a
direction generally parallel to the optical axis and away from the
at least one focal plane array, and wherein the panoramic optics
further includes a concave mirror that receives the radiant energy
from the convex mirror and re-directs the radiant energy back
generally toward the at least one focal plane array.
18. The sensor of claim 17, wherein the panoramic optics include a
curved meniscus lens that receives the radiant energy from the
concave mirror at a central portion of the meniscus lens, and a
final lens that receives the radiant energy from the central
portion of the meniscus lens and focuses the radiant energy on the
first annular region of the at least one focal plane array.
19. The sensor of claim 18, wherein the first and second annular
regions respectively comprise concentric inner and outer rings
defined on the same focal plane array, the limb-looking optics
re-directing the radiant energy from the field of view of the
limb-looking optics onto an outer portion of the curved meniscus
lens lying radially outward of the central portion thereof, and the
outer portion of the meniscus lens serving as a final optic for
focusing the radiant energy onto the second annular region of the
focal plane array.
20. The sensor of claim 19, wherein the limb-looking optics
comprise a plurality of lenses.
21. The sensor of claim 16, wherein the convex mirror re-directs
radiant energy from the panoramic field of view generally inwardly
and toward the at least one focal plane array, and wherein the
panoramic optics further comprise lenses for focusing the radiant
energy from the convex mirror onto the at least one focal plane
array.
22. The sensor of claim 21, wherein the panoramic optics comprise a
first lens having a central portion that receives radiant energy
from the convex mirror, and a second lens that receives radiant
energy from the central portion of the first lens and focuses the
radiant energy onto the first annular region of the at least one
focal plane array.
23. The sensor of claim 22, wherein an outer portion of the first
lens receives radiant energy from the limb-looking optics and
serves as a final lens that focuses the radiant energy from the
field of view of the limb-looking optics onto the second annular
region of the at least one focal plane array.
24. The sensor of claim 23, wherein the first and second annular
regions are defined on the same focal plane array and respectively
comprise inner and outer concentric rings.
25. The sensor of claim 23, wherein the limb-looking optics
comprise a plurality of lenses.
26. The sensor of claim 16, wherein the panoramic optics further
comprise a pair of lenses in series that receive radiant energy
from the convex mirror, and a curved meniscus lens having a central
portion that receives radiant energy from the pair of lenses and
focuses the radiant energy onto the first annular region of the at
least one focal plane array.
27. The sensor of claim 26, wherein the limb-looking optics
comprise lenses for directing radiant energy from the field of view
of the limb-looking optics onto an outer portion of the curved
meniscus lens, the outer portion of the curved meniscus lens
serving as a final lens for focusing the radiant energy onto the
second annular region of the at least one focal plane array.
28. The sensor of claim 27, wherein the pair of lenses comprise two
meniscus lenses with a concave surface of one of the meniscus
lenses facing a concave surface of the other meniscus lens.
29. A method for determining direction and rate of rotation of a
satellite about first, second, and third mutually orthogonal body
axes fixed relative to the satellite, comprising: mounting an
optical sensor on the satellite with an optical axis of the sensor
in a predetermined orientation with respect to the body axes, the
sensor having at least one focal plane array comprising a plurality
of pixels arranged as a grid; directing radiant energy from a
panoramic field of view of the sensor onto an annular region of the
at least one focal plane array, the panoramic field of view
spanning 360.degree. in azimuth angle about the optical axis and a
range of elevation angles including 90.degree. in elevation angle
relative to the optical axis; and determining a relative location
of a subtense of a limb of the earth on the annular region of the
at least one focal plane array at each of a plurality of successive
times, and determining direction and rate of rotation of the
satellite about the body axes based on changes in the relative
location of the earth limb subtense on the annular region.
30. The method of claim 29, further comprising: substantially
reducing tumbling of the satellite by applying corrective thrusts
to the satellite based on the rates and direction of rotation
determined from the panoramic field of view of the sensor.
31. The method of claim 29, wherein the pixels of the focal plane
array are arranged in a plurality of parallel rows and a plurality
of parallel columns perpendicular to the rows, the relative
location of the earth limb subtense being determined on the focal
plane array by detecting transition pixels in each row and each
column at which a transition between earth and space falls.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an optical sensor for
viewing earth from an orbit thereabout and determining positional
information of the sensor relative to earth by observing the limb
of the earth. The invention further relates to a method for
determining the rate and direction of rotation of an initially
tumbling satellite about all three axes of the satellite, for
eliminating the tumbling motion and stabilizing the satellite in a
desired orientation, and for sensing the orientation of the
stabilized satellite about two of the three axes, all using a
single optical sensor.
BACKGROUND OF THE INVENTION
[0002] Optical sensors for satellites have been developed for
viewing earth in order to derive position information of the
satellite relative to earth. In such schemes, it is known to direct
light from a field of view of the sensor onto a focal plane array,
such as a charge coupled device (CCD), comprising a grid of pixels.
The field of view and the optics of the sensor are typically
designed such that at least part, and more typically all, of the
circumference of the earth's limb (i.e., the transition region
between the earth and space) can be imaged onto the focal plane
when the sensor is pointed in a suitable direction relative to the
earth. The relative location of the image of the earth limb on the
focal plane is determined by finding the pixels at which a large
gradient in intensity of the incident light energy, indicating a
transition between earth and space, is located. Using an
appropriate algorithm, it is possible to determine the rotational
orientation of the sensor, and hence of the satellite, about two
orthogonal axes based on the locations of the transition pixels of
the focal plane array. See, for example, U.S. Pat. No.
6,026,337.
[0003] A number of patents for various types of optical sensors
have been acquired by the assignee of the present application,
including U.S. Pat. Nos. 5,502,309, 5,534,697, 5,627,675, and
5,841,589, the entire disclosures of which are hereby incorporated
herein by reference. The sensors described in all of the
aforementioned patents have a single field of view for looking at
the limb of the earth. On satellites using a limb-looking optical
sensor as described above, the optical sensor is generally used for
deriving position information about two axes after the satellite
has been stabilized following tip-off from the launch vehicle. An
entirely different system is used for bringing the initially
tumbling satellite into a controlled condition and stabilizing the
satellite in that condition. Typically, inertial measurement units
(IMUs), i.e., gyroscopes, are used for detecting the rotation rate
of the tumbling satellite about all three axes, and this rate
information is used by the satellite's control system to stop the
tumbling motion. Once the satellite is no longer tumbling, it is
then manipulated to place it in the desired orientation. Still
other sensors are typically used to aid in this process, since it
is possible for the satellite to be brought to a stabilized
condition in an orientation in which the limb-looking sensor is
looking away from earth so that the earth is not in its field of
view. For instance, a sun sensor and/or sensors for viewing stars
or other celestial bodies may be used to aid in maneuvering the
satellite toward the desired orientation, at least until the earth
comes into the field of view of the limb-looking sensor. It is
apparent that this approach requires a considerable number of
sensing devices.
SUMMARY OF THE INVENTION
[0004] The present invention provides an optical sensor and a
method for determining the rate and direction of rotation of an
initially tumbling satellite about all three axes of the satellite,
for eliminating the tumbling motion and stabilizing the satellite
in a desired orientation, and for sensing the orientation of the
stabilized satellite about two of the three axes, all using a
single optical sensor. The invention thus allows the usually
required IMUs and sun or star sensors to be eliminated, thereby
providing substantial savings in weight, cost, and complexity.
[0005] To these ends, an optical sensor in accordance with a
preferred embodiment of the invention comprises at least one focal
plane array comprising a plurality of pixels arranged in a grid,
and both panoramic optics and limb-looking optics each of which
maps its field of view onto a separate region of the at least one
focal plane array. The panoramic optics capture radiant energy from
an annular panoramic field of view spanning 360.degree. in azimuth
angle about an optical axis of the sensor and covering a range of
elevation angles including 90.degree. in elevation angle relative
to the optical axis. The panoramic optics re-direct and focus the
radiant energy from the panoramic field of view onto a first
annular region of the at least one focal plane. The limb-looking
optics capture radiant energy from an annular field of view
spanning 360.degree. in azimuth angle about the optical axis and
covering a range of elevation angles non-perpendicular to the
optical axis such that at least a major circumferential portion of
a limb of the earth is within the field of view of the limb-looking
optics when the optical axis of the sensor points toward a centroid
of the earth. The limb-looking optics re-direct and focus the
radiant energy onto a second annular region of the at least one
focal plane.
[0006] A particularly simple sensor construction is made possible
by directing the radiant energy from both fields of view onto the
same focal plane array. Preferably, the panoramic field of view
that looks generally perpendicular to the optical axis is imaged
onto an inner ring-shaped region of the focal plane array, and the
field of view that looks non-perpendicular to the optical axis is
imaged onto an outer ring-shaped region radially outward of the
inner ring-shaped region. Alternatively, where redundancy is
desired for enhanced reliability, either or both of the fields of
view can be optically split and imaged onto more than one focal
plane array. Redundant electronics can be provided for the various
focal plane arrays if desired.
[0007] Various optical arrangements for the sensor can be used. In
one embodiment, the panoramic optics include a convex mirror of
generally annular form that re-directs the radiant energy from the
panoramic field of view along a direction generally parallel to the
optical axis and away from the focal plane array, and a concave
mirror that receives the radiant energy from the convex mirror and
re-directs the radiant energy back generally toward the focal plane
array. A curved meniscus lens receives the radiant energy from the
concave mirror at a central portion of the meniscus lens, and a
final lens receives the radiant energy from the central portion of
the meniscus lens and focuses the radiant energy on the first
annular region of the focal plane array. The limb-looking optics
re-direct the radiant energy from the field of view of the
limb-looking optics onto an outer portion of the curved meniscus
lens lying radially outward of the central portion thereof, and the
outer portion of the meniscus lens serves as a final optic for
focusing the radiant energy onto the second annular region of the
focal plane array. Preferably, the limb-looking optics comprise a
plurality of lenses.
[0008] In another embodiment of the sensor, the convex mirror
re-directs radiant energy from the panoramic field of view
generally inwardly and toward the focal plane array, and the
panoramic optics include a first lens having a central portion that
receives radiant energy from the convex mirror, and a second lens
that receives radiant energy from the central portion of the first
lens and focuses the radiant energy onto the first annular region
of the focal plane array. This embodiment thus eliminates the
concave mirror of the previously described embodiment.
[0009] In still another embodiment of the sensor, the panoramic
optics include a convex mirror that re-directs radiant energy from
the panoramic field of view generally inwardly and toward the focal
plane array, a pair of lenses in series that receive radiant energy
from the convex mirror, and a curved meniscus lens having a central
portion that receives radiant energy from the pair of lenses and
focuses the radiant energy onto the first annular region of the
focal plane array. The limb-looking optics comprise lenses for
directing radiant energy from the field of view of the limb-looking
optics onto an outer portion of the curved meniscus lens, the outer
portion of the curved meniscus lens serving as a final lens for
focusing the radiant energy onto the second annular region of the
focal plane array. The pair of lenses preferably comprise two
meniscus lenses with a concave surface of one of the meniscus
lenses facing a concave surface of the other meniscus lens.
[0010] The invention also provides a method for determining the
direction and rate of rotation of a satellite about first, second,
and third mutually orthogonal body axes fixed relative to the
satellite and for determining orientation of the satellite relative
to earth. In accordance with this aspect of the invention, a dual
field-of-view optical sensor is mounted on the satellite with an
optical axis of the sensor in a predetermined orientation with
respect to the body axes of the satellite. Radiant energy is
directed onto a first annular region of a focal plane array from a
first field of view of the sensor spanning 360.degree. in azimuth
angle about the optical axis and a range of elevation angles
including 90.degree. in elevation angle relative to the optical
axis. Radiant energy is directed onto a second annular region of
the same or a different focal plane array from a second field of
view of the sensor spanning 360.degree. in azimuth angle about the
optical axis and a range of elevation angles non-perpendicular to
the optical axis. Direction and rate of rotation of the satellite
about the body axes are determined based on changes in the relative
location of the earth limb subtense on the first annular region of
the focal plane array over time. The sensor electronics
periodically take readings from the focal plane array(s), and the
location of the earth limb on the focal plane array(s) is compared
at successive times to derive the direction and rate information.
Orientation angles between the body axes of the satellite and a
nadir vector to the earth are determined based on the relative
location of the earth limb subtense on the second annular region of
the focal plane array(s). In the general case, the earth limb
location on the second annular region of the focal plane array can
be used for deriving rotational orientation about two axes. The
rotational orientation of the satellite about the third body axis
can be determined by directional radio reception (e.g., from sister
satellites in a constellation), by directional magnetic fields, or
by directional inertial rotation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The above and other objects, features, and advantages of the
invention will become more apparent from the following description
of certain preferred embodiments thereof, when taken in conjunction
with the accompanying drawings in which:
[0012] FIG. 1 is a diagrammatic view depicting a satellite in orbit
about earth and having an optical sensor for viewing the earth in
accordance with the present invention;
[0013] FIG. 2 is a schematic view of a focal plane array of the
sensor, illustrating two annular regions onto which the two fields
of view of the sensor are imaged;
[0014] FIG. 3 is a greatly enlarged view of a small region of the
focal plane array, showing a subtense of the earth limb imaged onto
the region;
[0015] FIG. 4 is an optical schematic of a sensor in accordance
with a first embodiment of the invention;
[0016] FIG. 5 is an optical schematic of a sensor in accordance
with a second embodiment of the invention;
[0017] FIG. 6 is an optical schematic of a sensor in accordance
with a third embodiment of the invention; and
[0018] FIG. 7 is a diagrammatic depiction of an attitude control
system in accordance with the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The present invention now will be described more fully
hereinafter with reference to the accompanying drawings, in which
preferred embodiments of the invention are shown. This invention
may, however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein; rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. Like numbers refer to like
elements throughout.
[0020] FIG. 1 diagrammatically depicts a satellite S in orbit about
the earth. A sensor 10 in accordance with the present invention is
mounted on the satellite. The sensor is mounted such that when the
satellite is stabilized in a desired orientation relative to earth,
the sensor points in a direction enabling it to see the earth. The
sensor 10 is unique in that it has two fields of view that both
span 360.degree. in azimuth angle about the optical axis of the
sensor. Thus, a first field of view is a panoramic view that looks
essentially perpendicular to the optical axis of the sensor (in
FIG. 1, the optical axis points at the centroid of the earth) and
covers a range of elevation angles about the perpendicular
direction. For example, the elevation angles relative to the
optical axis can cover a range of about 90.A-inverted.3.degree.. As
explained further below, the panoramic field of view is used for
capturing the rate and direction of rotational motion of the
satellite about its axes.
[0021] A second field of view of the sensor looks non-perpendicular
to the optical axis of the sensor for seeing the horizon or limb of
the earth. As an example, the second field of view can span
elevation angles relative to the optical axis of about
28.A-inverted.8.degree.. As described below, the second field of
view is used once the satellite has been stabilized and oriented
generally in its desired attitude, for detecting orientation of the
satellite relative to the nadir vector to the centroid of the
earth.
[0022] As explained in further detail below, the radiant energy
from the two fields of view of the sensor 10 are imaged, using
suitable optics, onto annular regions of one or more focal plane
arrays. Each focal plane array comprises a plurality of pixels each
of which is sensitive to radiant energy striking the pixel so as to
emit an electrical signal as a function of the intensity of the
radiant energy. The pixels are arranged in a rectangular or square
grid that preferably defines a planar surface for receiving the
incident radiant energy. The focal plane array(s) can be sensitive
to a single wavelength of light while being substantially
insensitive to light of other wavelengths, or the focal plane
array(s) can be multispectral such that a plurality of different
wavelengths are detected. The focal plane array can be cooled in
any suitable manner, if cooling is required.
[0023] FIGS. 2 and 3 schematically depict a focal plane array 12 in
accordance with a preferred embodiment of the invention. As shown
in FIG. 2, the focal plane array 12 defines an inner annular region
14 and an outer annular region 16 that lies radially outward of and
is concentric with the inner annular region 14. In accordance with
the invention, radiant energy from the panoramic field of view of
the sensor 10 is imaged onto the inner annular region 14. Radiant
energy from the other field of view of the sensor is imaged onto
the outer annular region 16 of the focal plane array. The center of
both regions 14 and 16 is represented by an origin of orthogonal x
and y axes in FIG. 2.
[0024] As shown in FIG. 3, which represents a small region of the
surface of the focal plane array 12, the focal plane array has
pixels 18 arranged in rows and columns. Each row extends parallel
to the x-axis and each column extends parallel to the y-axis. The
sensor generally will be mounted on the satellite in a fixed and
known orientation relative thereto, and hence the x- and - and
y-axes of the focal plane array 12 will be in a fixed and known
orientation relative to defined body axes of the satellite.
Accordingly, the location of an image of earth on the focal plane
array 12 can be geometrically related to the orientation of the
satellite's body axes relative to earth.
[0025] More particularly, FIG. 2 shows an image of the limb 20 of
the earth on the outer annular region 16 of the focal plane array,
and FIG. 3 represents a region R at which the limb is located. For
a given row of pixels 18, there will be at most two crossings of
the earth limb. Each crossing is characterized by a change in
incident energy intensity from a relatively higher intensity
emitted by the earth to a relatively lower intensity emitted by
space. The width of the earth limb on a given row of pixels can be
found by finding the two transition pixels at the opposite edges of
the limb at which the transition from high to low, or low to high,
intensity occurs. The locations of these transition pixels are
saved in memory for each row, in terms of values of the x
coordinate at the known y coordinate to which the row corresponds.
A similar process is undertaken for each column of pixels in order
to find and save the transition y coordinates for each column. By
averaging the transition x values for all rows and averaging the
transition y values for all columns, a single value of x and a
single value of y are obtained corresponding to the coordinates of
the center C of the earth. The offset between the center of the
earth and the center of the focal plane array is related to the
orientation of the sensor relative to earth, and thus can be used
as a parameter for guiding the satellite into a desired orientation
in two axes.
[0026] Even if the center of the earth is not within the view of
the sensor such that there is only one crossing of the earth limb
for some rows and/or columns, suitable algorithms can be used for
determining the location of the earth center. For example, with a
knowledge of the locations of the earth limb on a plurality of
adjacent rows or columns, the shape of the arc of the earth limb
can be determined and an orthogonal to the arc can be struck. By
striking orthogonals to the arc from two different directions, the
intersection of the two orthogonals can be found, which represents
the earth center. If desired, orthogonals can be struck from more
than two directions, and variation in the location of the earth
center calculated from various pairs of orthogonals can give an
indication of the level of confidence in the solution as well as
the uncertainty in the calculation.
[0027] If desired, the accuracy of the determination of the earth
limb location on the focal plane array can be improved by taking
into consideration not just a single pixel representing the
transition location, but instead also including data from the eight
pixels immediately surrounding the pixel of interest. Thus, data
from a total of nine pixels would be used for deriving a location
of the limb crossing. Oversampling and digitizing of the pixel
intensity data can be used to interpolate and thereby find the limb
location to within about one-tenth of a pixel dimension.
[0028] Based on the foregoing, it will be recognized that a method
has been disclosed for obtaining orientation of a satellite about
two of its three axes. One of these two axes is tangential to the
orbit of the satellite and the other is perpendicular to the first
axis and to the nadir vector that points toward the earth centroid.
For example, the sensor 10 can be mounted relative to the satellite
such that the earth limb image on the focal plane array can be used
for sensing rotational positions of the satellite about its pitch
and roll axes. In this case, the sensor cannot provide orientation
about the third or yaw axis because the circular limb of the earth
appears the same regardless of the rotation about the yaw axis.
[0029] To get the orientation of the satellite about the third
axis, various methods can be used. One method is to use a sensor to
view another celestial body such as the sun, moon, or stars. Sun
sensors are relatively low in cost but do not provide a high degree
of accuracy. Star sensors are relatively more expensive and require
a star catalog for precise pointing and tracking. Another method
for getting the third axis orientation is to use directional radio
reception of signals from other satellites in orbit about the
earth. For instance, the satellite on which the sensor 10 is
mounted may be one of a constellation of satellites in the same
orbit, each transmitting radio-frequency signals, such as global
positioning system (GPS) satellites in circular orbit. The
satellite's position about the yaw axis can be steered based on the
radio signal from one or more such other satellites. Other methods
that can be used for getting the third axis orientation include
using directional magnetic field detection, and using directional
inertial rotation (e.g., gyroscopes).
[0030] Thus far, a method has been described for guiding a
satellite in order to maintain it in a desired orientation relative
to the earth. Although the view of earth imaged onto the outer
annular region 16 of the focal plane array 12 need not be such that
the earth center lies on the focal plane array, in general it is
contemplated that the sensor would be directed in such a manner
that the optical axis of the sensor would point toward the center
of the earth, i.e., along the nadir vector, when the satellite is
in its desired orientation. Thus, the outer annular region 16 would
be used primarily for maintaining the satellite in a stabilized
orientation wherein the center of the earth would be essentially
centered on the focal plane array. However, it will be appreciated
that when the satellite is initially deployed from a launch
vehicle, it will generally be tumbling with rotation components
about all three axes. The problem then becomes how to bring the
satellite under control and maneuver it into the desired
orientation.
[0031] To facilitate this process, the sensor 10 in accordance with
the invention includes the second field of view comprising a
360.degree. panoramic view perpendicular to the axis A of the
sensor. This panoramic field of view is imaged onto the inner
annular region 14 of the focal plane array. The location of the
earth center relative to the axes of the focal plane array at a
given instant in time is determined based on the instantaneous
image of the earth limb on the inner annular region 14 in a manner
similar to that described above. In particular, it is preferable to
strike orthogonals to the arc of the earth limb from two or more
different directions and find the earth center based on the
intersection of the orthogonals, since in the general case only a
fraction of the earth may be within the panoramic field of
view.
[0032] In order to determine the direction and rate of rotation of
the satellite about its axes, the location of the center of the
earth imaged onto the inner annular region of the focal plane array
relative to the axes of the focal plane array is tracked over a
series of sequential times. Knowing where the edge of the earth
limb is moving relative to the focal plane axes enables the
direction of rotation of the satellite about each axis to be
determined; knowing the rate of movement of the earth limb enables
the rate of rotation about each axis to be determined. The
direction and rate information can be used by the satellite's
guidance and navigation system to reduce the tumbling of the
satellite and bring the satellite into a controlled orientation,
which generally will be an orientation in which the limb-looking
field of view of the sensor would have the earth limb in view such
that the sensor can be used for continuing guidance of the
satellite to maintain the desired orientation as previously
described.
[0033] More particularly, with respect to the process of stopping
the tumbling of the satellite using the rate capture portion of the
sensor, the direction of the earth in roll from the satellite is
determined from the concave or convex curvature of the earth limb
imaged onto the inner ring 14 of the focal plane array. The
direction of the earth in yaw or pitch from the satellite is
determined from the changes in the limb between sequential frames
from the focal plane array. For example, an increasing view of the
limb indicates that the satellite is tumbling in the axis in which
the highest part of the earth limb is showing. The stabilizing jets
on the satellite are activated to slow that motion. If the image is
rotating such that the curve of the limb is growing on one side and
retreating on the other side of the ring 14, that indicates that
the satellite is tumbling on the axis of the satellite pointing
toward the limb. The jets are activated to slow that motion.
[0034] The first task of the satellite control system is to remove
the rate of rotation in all three axes. Once that is accomplished,
the control system then uses the jets to slowly move the satellite
until the optical axis of the sensor points at the center of the
earth. This is accomplished by detecting the bend of the earth limb
on the inner ring 14 of the sensor and commanding the satellite to
turn the image away from the limb. This will bring the image into
the outer ring 16 of the sensor's focal plane array. When the earth
limb starts showing on the outer ring 16, the jets are commanded to
slow the rotation; during this process, the rates of rotation are
determined by the motion of the limb on the outer ring 16. It
should be noted that the rate of the indicated motion is different
between the inner and outer rings, and the attitude control
electronics must account for this rate difference.
[0035] Having described the method of determining direction, rate,
and orientation of the satellite using a single optical sensor,
attention is now turned to a description of several preferred
embodiments of sensors in accordance with the invention. FIG. 4
depicts an optical schematic of a first preferred embodiment of a
sensor 10. The sensor 10 has a set of panoramic optics 30 for
capturing radiant energy from a 360.degree. panoramic field of view
looking perpendicularly outward from the optical axis A of the
sensor and spanning a range of elevation angles as indicated by the
ray traces showing the elevation angle limits. The sensor also has
a set of limb-looking optics 40 for capturing radiant energy from a
360.degree. field of view about the optical axis A looking outward
in a direction non-perpendicular to the axis A. Both sets of optics
include elements for re-directing and focusing the captured radiant
energy onto a focal plane array 12 mounted in the sensor.
[0036] More particularly, the panoramic optics 30 include a
360.degree. convex mirror 32 that re-directs the radiant energy
from the panoramic field of view in a direction parallel to the
optical axis A and away from the focal plane array 12. The
re-directed radiant energy from the convex mirror 32 is then
re-directed by a 360.degree. concave mirror 34 in a radially inward
direction toward the focal plane array 12. The mirrors 32 and 34
preferably comprise a Mersenne optic pair. The energy from the
mirror pair 32, 34 passes through a strongly curved meniscus lens
50. More specifically, the energy from the mirror pair passes
through a central portion 36 of the meniscus lens 50, which central
portion functions as a part of the panoramic optics 30. A final
component of the panoramic optics is a plano-convex lens 38 that
receives the energy from the central portion 36 of the meniscus
lens and focuses the energy onto an inner annular region of the
surface of the focal plane array 12. Of course, it will be
understood that any or all of the various lenses in the panoramic
optics could be replaced by suitably configured reflective
surfaces.
[0037] The limb-looking optics 40 include a pair of back-to-back
concave-convex lenses 42 and 44, followed by an intermediate
bi-convex lens 46, which transmit radiant energy from the
non-perpendicular field of view to an outer annular portion 48 of
the meniscus lens 50 that surrounds the central portion 36. The
outer portion 48 of the meniscus lens serves as a final lens that
focuses the energy onto an outer annular region of the focal plane
array 12. The lenses 42, 44, 46 could be replaced in whole or in
part by a plurality of reflective surfaces of suitable design.
[0038] FIG. 5 depicts a second embodiment of a sensor 10' in
accordance with the invention. In contrast to the sensor 10 of FIG.
4 in which a Mersenne pair is used for bending the radiant energy
from the panoramic scene toward the focal plane array, the
panoramic optics of the sensor 10' employ a single hyperbolic
convex mirror 33 for this purpose. The energy from the mirror 33
passes through a central portion 36' of a nearly plano-convex lens
50', and then through a final conic lens 38' that focuses the
energy on the inner annular region of the focal plane array 12. The
limb-looking optics of the sensor 10' comprise three lenses 42',
44', 46' in series, followed by the nearly plano-convex lens 50'
whose outer portion 48' serves as a final lens focusing the energy
on the outer annular region of the focal plane array 12.
[0039] FIG. 6 shows a third embodiment of a sensor 10" in
accordance with the invention. The panoramic optics of the sensor
10" include a single convex mirror 33' for bending the radiant
energy from the panoramic scene toward the focal plane array 12.
The sensor 10" differs from the previous two embodiments in that
the final optic for the panoramic optics is not located between the
focal plane and the final focusing element for the limb-looking
optics as is the case with the previous embodiments. Thus, the
energy from the convex mirror 33' passes through a back-to-back
pair of meniscus lenses 35 and 37 and finally through a central
portion 39 of a strongly curved meniscus lens 50" that focuses the
energy on the inner annular region of the focal plane array 12. A
stop, not visible in FIG. 6, is located between the two meniscus
lenses 35, 37. The first surface of the second meniscus lens 37 is
conic. The limb-looking optics of the sensor 10" comprise only
three lenses 42", 44", and the outer portion 48" of the lens
50".
[0040] In all of the described and illustrated sensor embodiments,
the limb-looking optics are inverse telephoto types in which there
is a negative power group followed by a positive power group. The
inverse telephoto design is particularly suitable for wide fields
of view. The meniscus lens near the focal plane serves to flatten
the field and correct astigmatism.
[0041] It will be recognized by those skilled in the art that a
negative lens could be used in place of the mirror or pair of
mirrors of the panoramic optics for any of the sensors. The
mirrors, however, have the advantage of producing substantially
less pupil foreshortening than a negative lens; on the other hand,
the mirrors tend to introduce astigmatism that must be corrected by
the focusing lenses. From the foregoing, it should be apparent that
the particular types and arrangements of optical elements chosen
for accomplishing the objectives of the dual field-of-view sensor
can be varied in many different ways, and all of these variations
are intended to be encompassed by the present invention and the
appended claims.
[0042] In the sensor in accordance with the invention, although a
single focal plane array is shown, it is also possible to employ an
optical splitter (not shown) for splitting radiant energy from
either or both of the two fields of view and directing the radiant
energy onto two or more focal plane arrays. This provides
redundancy for enhanced reliability. Each focal plane array can
have its own set of electronics. It is also possible to direct the
radiant energy from the panoramic field of view onto one focal
plane array and to direct the radiant energy from the limb-looking
optics onto a different focal plane array. Thus, the invention as
defined in the appended claims is not limited to the single focal
plane arrangements illustrated and described herein.
[0043] The sensor and method of the present invention can be
employed for satellites in various orbits including circular or
elliptical orbits, from low altitude up to geosynchronous
altitude.
[0044] FIG. 7 diagrammatically depicts an attitude control system
in accordance with the invention. The system includes attitude
control electronics 60 for controlling the stabilizing jets.
Coupled with the attitude control electronics are computational
electronics 70 for making the necessary calculations of rate and
position based on the signals from each of the rings 14, 16 of the
focal plane array (FPA) 12. The signals from the FPA 12 are
communicated via drive electronics 80 to the computational
electronics. The electronics connected to the sensor capture
signals from the focal plane array by sampling the signals
periodically to create a series of sequential frames.
Advantageously, the electronics are operable to create at least
about 10 frames per second, and preferably the location of the
subtense of the earth limb on each of the annular regions is
sampled at least three times for each frame. Below the 10 frame per
second rate, it is anticipated that it would be difficult to
maintain stabilization of the satellite in the desired
orientation.
[0045] Many modifications and other embodiments of the invention
will come to mind to one skilled in the art to which this invention
pertains having the benefit of the teachings presented in the
foregoing descriptions and the associated drawings. Therefore, it
is to be understood that the invention is not to be limited to the
specific embodiments disclosed and that modifications and other
embodiments are intended to be included within the scope of the
appended claims. Although specific terms are employed herein, they
are used in a generic and descriptive sense only and not for
purposes of limitation.
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