Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2000-07-17
2003-04-29
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S206100, C250S234000, C359S225100
Reexamination Certificate
active
06555803
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of imaging a two-dimensional field of regard. More particularly, the present invention relates to imaging of the full-Earth disk by a spacecraft that scans the field of view of an imager across the full Earth disk.
2. Background Information
One of the most common uses for artificial planetary satellites is to produce images of the planet's surface. Many Earth-orbiting satellites capture images of Earth for purposes ranging from military intelligence to weather forecasting. Orbital imaging for weather forecasting and for scientific purposes demands images of vast areas of the Earth at once.
It is common to image a planet (e.g., Earth) from space using an imaging electro-optical sensor constructed from a telescope that collects radiation from a remote source and brings it to focus on one or more focal plane arrays (FPA's) with each FPA containing many detector elements. A scanning sensor moves the image of the scene over one or more FPA'S, each FPA usually having many detector elements perpendicular to the direction of the scan motion. Each element converts the radiation from an instantaneous field of view (IFOV) in the scene into an electronic signal. The image of the scene in the spectral band of the sensor is reconstructed from these electronic signals.
The angular field of view (FOV) of the telescope multiplied by its effective focal length (EFL) equals the dimensions of the telescope's focal plane, which contains the FPA. For application in which an imaging sensor must cover a two-dimensional field of regard (FOR) that exceeds the telescope's FOV, a plane scan mirror may be located in front of the imaging sensor's telescope to scan the FOV across the FOR. For example, the Earth subtends a circle approximately 17.4° in diameter from geosynchronous altitude. An instrument that is capable of imaging the full-Earth disk must have a field of regard (FOR) that not only includes this full-Earth disk, but also allows it to view deep space to measure the background signal in each channel. Most multispectral instruments that image the Earth from this altitude use a large, reflective telescope with a field-of-view (FOV) that is much smaller than the required FOR. A two-dimensional raster scanning procedure is required to cover the FOR, and is usually implemented with a plane mirror in front of the telescope's aperture.
A number of geosynchronous weather satellites, including the EUMETSAT and GOES-1 through GOES-7 satellites, are “spinners” that rotate about the north-south axis. The imager on each of these spinning satellites has a telescope that is aligned with the north/south axis of the spacecraft. A plane mirror with a single rotational axis, perpendicular to the spin axis, reflects the optical axis of the telescope towards the Earth. The spacecraft's rotation scans the line of sight (LOS) in the east/west direction. To form a two-dimensional map of the Earth, the plane mirror is only required to step in the north/south direction. The main disadvantage of a spinning satellite is that it only allows the imager to view the Earth's surface for less than 5% of its total duty cycle.
Beginning with GOES 8, the geostationary weather satellites operated by the United States (developed for NOAA by LORAL with instruments from ITT) have been three-axis stabilized. In this configuration, the imager continuously points toward the Earth, permitting it to operate at a high duty cycle and to be far more flexible than a spinning satellite in imaging arbitrary areas of the Earth's surface. These prior art GOES imagers routinely produce 3000 km by 5000 km images of the contiguous United States (CONUS) and 1000 km by 1000 km images of severe storms. Scanning is performed by a plane scan mirror mounted on a two-axis gimbal. Rotation of this mirror about the inner gimbal axis scans the LOS in the east/west direction. Between scan lines, incremental rotations about the outer gimbal axis move the LOS from north to south. When scanning the Equator, the GOES imager projects its detector arrays onto the Earth's surface in the optimal manner, with the cross-scan axis of each detector array (its long axis) projected in the north-to-south direction. When scanning north of the Equator, the projection of this axis is tilted in the northeast-to-southwest direction; when scanning south of the Equator, the projection is tilted northwest-to-southeast. The tilt angle varies from zero at the Equator to 8.7° at the North and South Poles. This phenomenon, known as image rotation, is an intrinsic problem in a two-axis scanning system that uses a single scan mirror in object space. See J. J. Shea, “Image correction via lunar limb knife-edge OTF's”,
Proc. SPIE, Earth Observing Systems III
, vol. 3439, Jul. 19-21, 1998, pp 165-186.
Referring to
FIG. 1
, the geometric configuration of the GOES 8 & 9 imager is illustrated. The GOES convention for spacecraft coordinates is portrayed by a set of orthogonal coordinates 10 wherein +x=east, +y=south, and +z=nadir. For simplicity of illustration, the GOES telescope is represented as a single lens
12
and crossed axes
14
represent the image as presented at the focal plane array. Note that the lens
12
inverts the image of the axes
14
. The telescope's optical axis
16
points due east along the x-axis. The scan mirror
18
is an optical flat with an elliptical cross section and has a reflective surface (not visible from the viewpoint of
FIG. 1
) that directs light from the Earth (shown in phantom) into the telescope
12
. The scan mirror
18
is mounted on a first axle
20
that provides an inner axis of rotation with respect to which the inner gimbal angle (iga) is measured. The inner axis of rotation is coincident with the short dimension of the ellipse and perpendicular to the normal vector of the mirrored surface. The first axle
20
permits the scan mirror
18
to rotate about the inner axis of rotation with respect to a yoke
22
. The yoke
22
has a second axle
24
that is perpendicular to the first axle
20
. The second axle
24
lies along the extension of the optical axis
16
of the telescope
12
, along the x-axis, and allows the yoke
22
to rotate about this outer axis of rotation that is fixed with respect to the telescope
12
, and with respect to which the outer gimbal angle (oga) is measured. The orientation of the first axle
20
always remains perpendicular to the x-axis, but rotates in the y-z plane when the yoke
22
is pivoted about the second axle
24
.
Referring to
FIG. 2
, projections
42
,
44
,
46
,
52
,
54
,
56
of the crossed axes in the focal plane
14
onto the Earth's surface
30
are illustrated. The line with the arrowhead
14
′ is parallel to the z-axis and corresponds to the along-scan direction of the array. The line with the circle
14
″ is parallel to the y-axis and corresponds to the cross-scan direction of the array.
The intersection of the crossed axes is projected onto the equator
32
when the position of the yoke
22
on the outer axle
24
aligns the inner axle parallel to the y-axis. This angle can be defined as the home position of the outer axle, at oga=0. When the outer axle
22
is fixed at this position and the scan mirror
18
is rotated about its inner axle
20
, the projection
42
,
44
,
46
of the focal plane
14
is scanned along the equator
32
. The y-axis of the focal plane remains perpendicular to the direction of the scan and the z-axis of the focal plane is projected along the direction of scan.
When the yoke
22
is rotated about the outer axle
24
in the +oga direction, the crossed axes in the focal plane
14
are projected
52
,
54
,
56
into the Northern Hemisphere. The array's projection
52
,
54
,
56
rotates clockwise, as viewed from space. When the oga remains fixed and the scan mirror
18
is rotated about the inner axle
20
, the z-axis of the focal p
Allen Stephone B.
Glass Christopher W.
Roberts Abokhair & Mardula LLC
Swales Aerospace
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