Keywords: Sensor alignment, Collinearity condition, ADEOS/OCTS
Abstract: The alignments of ADEOS/OCTS were measured before the launch. But it was
found out that the values of the alignments were changed after the launch, which resulted in the
serious amount of geometric distortion. The alignments were corrected on the basis of
photogrammetry using OCTS imagery and GCPs.
1. Introduction
The ADEOS(Advanced Earth Observing Satellite) was launched in August 1996 by NASDA
(National Space Development Agency of Japan). The OCTS(Ocean Color and Temperature
Scanner) on board the ADEOS spacecraft is an optical mechanical scanner with 12 channels
and the spatial resolution of 680 m at nadir. It observed the Earth surface only in daytime. The
geometric accuracy of the OCTS at initial check out was about 10 km on the ground. It was too
terrible for some applications which needed high geometric accuracy like mosaicking, image
composite, etc. The NASDA and some organization examined the reasons for such a terrible
accuracy and ascertained three kinds of factors; 1) the bug of software for calculating satellite
position, 2) the inadequacy of the algorithm for satellite attitude, 3) the change of alignments
after the launch. In those factors, the bug for satellite position was fitted easily. The algorithm
for satellite attitude was installed on the onboard computer. The raw attitude data were
processed by the algorithm and transmitted to the ground. So the attitude data could be no
longer corrected. But the only raw attitude data for first orbit each day in GMT were
downlinked for housekeeping. Such raw data were processed with the new algorithm adopted
for ADEOS-2/GLI (Global Imager). The subject of this paper is the correction of the
alignments utilizing OCTS imagery on the basis of photogrammetry.
2. Scan geometry of OCTS
Figure 1 shows the optical pass from the focal plane to the ground. The orbital coordinates are
introduced which is right handed where the flight and zenith direction correspond to x and z
axis respectively. The OCTS has the tilt function by inclining the scanning mirror around
y-axis (NASDA,1996). The vector
a from the center of the focal plane to the scanning mirror
is expressed as (1,0,0). If the tilt angle
(
t) is equal to zero, the normal vector of mirror
0will be
( cos(45
°
+
e
), 0, sin(45
°
+
e
) ), where
e
is the miss-inclination of mirror and equal to 14
milli-radians. When the scanning mirror is tilted by
t
and the scan angle is
q
, the normal
vector of the mirror
1
is expressed as follows.
If the alignments of the scanning mirror (
a1,
b1,
l1) are considered, the normal vector of the
mirror is expressed as
=Q
1•
1.
The view vector reflected from the mirror is expressed as
1=
-2(
,
)•
.
If the alignments of the sensor itself to the platform
(
a2,
b2,
g2) are considered, the view vector will
be re-written as
=Q
2•
1.
In these equations, both Q
1 and Q
2 are the rotation matrices around
three axes. The view vector is expressed by the components as follows.
Figure 1 Scan geometry of OCTS (q=0)
3. Alignments to be determined
The effect of each alignment on the view vector is examined. The view vector without any
alignment is expressed as
1=(1-2h
x2,
-2h
xh
y,-2h
xh
z.
If each alignment exist individually, the view vector is expressed as follows.
The differences of view vectors are expressed in Figure 2 by the error vectors on the image
where each alignment has 0.001 radian independently. The calculation was performed on the
equatorial region. Both the formula and the errors on the image proved that the
a1.and
a2 have
the same effect on the image. Eventually the parameters (
b1,
g1) and
(
a2,
b2,
g2) are selected
as the alignments to be determined and the value of
a1 is fixed to the re-launch value in this
work.
Figure 2 Errors on image raised by alignment error
