Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
1997-06-18
2001-04-10
Garber, Wendy R. (Department: 2612)
Television
Camera, system and detail
Solid-state image sensor
C348S145000, C348S202000, C358S483000, C358S494000, C382S280000
Reexamination Certificate
active
06215522
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of acquiring a satellite observation image of the earth by “push-broom” scanning using a strip or a matrix of detectors of the charge coupled device (CCD) type moving relative to the observed zone.
BACKGROUND OF THE INVENTION
The principle of “push-broom” type scanning is shown in
FIG. 1
for the case of a detector strip
1
.
As the satellite carrying the strip
1
moves, the strip observes successive lines L
1
, L
2
, . . . , Ln extending perpendicularly to its displacement direction (arrow D). At any instant, an instrumentation optical system
2
forms an image of a line of the scene on a line of detectors, the strip
1
being located in the focal plane of the optical system
2
extending perpendicularly to the velocity vector of the satellite. The scene passes in front of each detector which integrates the light flux during exposure time and transforms it into a proportional electrical charge.
FIG. 2
shows a conventional system for processing images taken in this way.
In outline, the processing system comprises a unit
3
for processing and amplifying the output from the detectors of the strip
1
, an analog-to-digital encoder
4
receiving the output signal from the unit
3
, means
5
for transmitting the digital images taken in this way from the satellite to the ground, and a unit
6
on the ground for reconstituting images.
The unit
3
comprises, in particular, a shift register into which the information integrated and stored in each detector of the strip
1
in the form of charge is transferred at the end of the exposure time. This register then transfers charge electronically, converting it into a succession of voltages proportional to points of light flux as received and integrated.
The unit
6
on the ground reconstitutes images in particular by implementing deconvolution processing to compensate for instrument defects, and also, where appropriate, interpolation processing for reconstituting certain pixels of the image.
Usually, the acquisition time between two successive lines, or “sampling time”, is such that the point on the ground situated vertically below the satellite is displaced through a distance equal to the dimensions of an individual detector as projected onto the ground.
With reference to the focal plane of the instrument, this gives rise to line and column sampling frequencies that are the same and equal to the reciprocal of the size of an individual detector in the focal plane.
However, this approach does not take account of the instrument modulation transfer function (MTF).
It is recalled that the modulation transfer function of an optical system is a function in frequency space representing the suitability of the system for transmitting various frequencies. It is characteristic of the reproduction of contrast in the scene by the system.
For push-broom type acquisition, the modulation transfer function depends mainly on the optics of the system, on the smearing effect (reduction in contrast due to motion), and on the detectors.
Observation systems are characterized by the cutoff frequency fc above which the MTF is negligible. A cutoff frequency can be associated with each of the effects contributing to the overall modulation transfer function (optical system, smearing, and integration on the photosensitive zone), the overall cutoff frequency being the smallest of the three above values.
The modulation transfer function associated with integration in the photosensitive zone cancels at a frequency equal to the reciprocal of the size of an individual detector which constitutes a first approximation to the corresponding cutoff frequency.
It is the integration effect on the photosensitive zone which generally determines the overall cutoff frequency.
For conventional acquisition where the sampling frequency fe is equal to the cutoff frequency fc, the Shannon condition (fe≧2.fc) is not satisfied and this gives rise to a high degree of spectrum aliasing which introduces artifacts and makes any attempt at deconvolution or at interpolation difficult.
SUMMARY OF THE INVENTION
An object of the invention is to propose a processing method which makes it possible:
to limit spectrum aliasing, in order to make satisfactory restoration and/or interpolation of the image on the ground possible; and
to minimize redundancy in the information transmitted to the ground so as to make best use of the data transmission capacities of the instruments.
A processing method has already been proposed in patent application FR 2 678 460 enabling images to be recorded and retransmitted with line and column sampling pitches that are half the size of a conventional scanning instrument, but with identical telescope and individual detectors.
In that method, which is illustrated in
FIG. 3
, oversampling is performed by:
dividing the integration time and the sampling time by two, the column sampling frequency thus becoming equal to twice the cutoff frequency; and
for lines, superposing measurements taken by two strips B
1
and B
2
that are offset relative to each other by a distance equal to half the size of an individual detector.
Two strips can be superposed by means of a line optical divider or by using the motion of the satellite by shifting in the field of the second strip by an integer number of sampling steps in the advance direction and by half a step in the lengthwise direction of the strip. The offset of the two sampling grids may be other than half a pixel, providing it is appropriately taken into account during processing, however, the optimal value of the offset is half the line and column size of an individual detector.
FIG. 4
shows a matrix of pixels obtained by such processing. Pixels X
B1
are those acquired with the strip B
1
, while pixels X
B2
are those acquired with the strip B
2
.
Each of the two strips B
1
and B
2
therefore generates a rectangular sampling grid having a pitch along the lines equal to the size of an individual detector and a pitch down the columns equal to half of said value, the two grids being offset by a distance equal to half an individual detector. As shown in
FIG. 4
, a square sampling grid is indeed reconstituted by interleaving, with the line pitch coinciding with the column pitch and being equal to half the size of an individual detector.
A method is thus made available that satisfies the Shannon sampling condition at the cost of quadrupling the information rate.
FIG. 5
is a representation in the Fourier plane of a realistic modulation transfer function (MTF) for a push-broom type observation instrument, used in application of the acquisition method described in FR 2 678 460. This MTF is expressed as the product of a transfer function mtfl of normalized line frequencies fx and a transfer function mtfc of column frequencies fy, these two functions being given by the following equations for the SPOT 5 instrument:
mtfl
=
ⅇ
(
-
3.431
fx
)
sin
(
2
π
fx
)
2
π
fx
mtfc
=
ⅇ
(
-
3.03
fy
)
sin
(
2
π
fy
)
sin
(
π
fy
)
2
π
2
fy
2
In this representation, the modulation transfer function is normalized to 1 at the origin and the frequencies fx and fy are normalized relative to twice the conventional sampling frequency, and the various curves represent level curves with a step size of 0.01 up to the value 0.1.
It can be seen in this representation that the modulation transfer functions are small over a large portion of the spectrum of frequency origin.
The functions mtfl and mtfc cancel for fx=fy=0.5, and the Shannon condition is therefore satisfied.
All of the significant information is essentially contained in the top triangle I
s
, whose vertices correspond to the origin in the Fourier plane and to the points (0, 0.5) and (0.5, 0).
The MTF curve of level 0.03 is tangential to this zone, and below that level the signal is assumed to be buried in noise. Beyond the curve, i.e. outside the above-specified
Favard Jean-Claude
Latry Christophe
Pauc Gilbert
Rouge Bernard
Centre National d'Etudes Spatiales
Garber Wendy R.
Pollock, Vande Sande & Amernick, R.L.L.P.
Vu Ngoc-Yen
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