936 
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.