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The International Archives of the Photogramme try, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008
ALISEO main characteristics
Detector
Silicon CCD 2D-array (1024 x 1024
elements)
Spectral range
400- 1000 nm
Spectral resolution
Better than 5 nm @650 nm
Swath
10 Km
FOV
1.15°
Spatial resolution
10m
Digitalization
12 bit
Expected SNR
200 @ 650 nm
Size
60 cm (along flight direction) x 30 cm
(along nadir axis) x 30 cm
Weight
Less than 20 Kg
Power
Less than 25 W
Table 2. ALISEO main characteristics
2.ALISEO INSTRUMENT CONCEPT
ALISEO acquires the image as modulated by a pattern of
autocorrelation functions of the energy coming from the
observed scene. The complete interferogram of any target
locations is observed introducing relative source-observer
motion, which allows each pixel to be observed while going
across the entire pattern of ElectroMagnetic (EM)
autocorrelation function.
The interferometer optical layout is shown in the figure below.
Figure 1. Interferometer Optical layout
Light is collected from the objective L, then the two interfering
rays are generated by the beam-splitter BS and travel the
triangular ray path in opposite directions by means of two
folding mirrors Ml and M2. The two rays are then focused onto
the CCD by the camera lens P. It is easy to demonstrate that the
BS is the fundamental component, which provides the basic
phase-delay between the two coherent rays. Nonetheless, it can
be shown that phase-delay is heavily affected by the overall
instrument geometry: the BS to Ml and M2 distances and the
orientation of the two folding mirrors. Figure 2 shows a picture
of the airbone ptototype of ALISEO.
This Sagnac interferometer produces a fixed (stationary) pattern
of interference fringes of equal thickness (Fizeau fringes). The
Optical Path Difference (OPD) between the recombining beams
linearly changes with the angle (slope) of the entering ray onto
the instrument optical axis.
Figure 2: Picture of the inner part of the imaging interferometer
developed at our Institute
Due to the absence of entrance slit the device acquires the
image of the target superimposed to a fixed pattern of
interference fringes. Figure 3 displays a single-frame
acquisition of the landscape around Florence (Italy). Due to the
low coherence of solar radiation, the fringes visibility (and their
number) is limited to a small central region of the collected
image. The dark central fringe corresponds to the null optical
path difference between the two interfering rays.
Due to the relative sensor-object motion each target’s location
crosses the entire interference pattern, hence the corresponding
pixel is observed under different phase delays when the scene is
framed repeatedly in time (Barducci, 2001; Barducci, 2002). In
a different wording, the collected image sequence forms a 3-
dim array of data (image stack). This data-cube is first
processed in order to extract the complete interferogram of
every target’s point, and then it is cosine inverse transformed to
yield a hyperspectral data-cube. Stack geometry is related to
important characteristics of sensor and acquired data. As an
instance, image sequence and target-sensor relative velocity
govern the actual optical path difference between consecutive
sample of a retrieved interferogram.