ul 2004
may be
iry disk
iportant
iX
d À the
n d the
by the
lution.
agth of
r to the
iximum
ects. F
| £52
is
e target
cts the
te AI.
Frm is
> linear
of the
L.
jitter 1s
hat the
andom
(4)
thumb,
system
ystems
t high
pment.
led by
ng the
ımera
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B1. Istanbul 2004
(Ap - detector area, Toy, — transmission of the optics, t, —
integration time, F —f-number, Ry — detector responsivity, L —
radiation flux) with tin « tq.
Once the detector is selected, Ap and Ry are given. L is also
given as well as F and Topties When the optics is selected or
designed taking into account the technological or mission
constraints. AA is fixed in most cases, so that the only real
variable part is the integration time ti. For a satellite in LEO,
the satellite ground track velocity is about 7 km/s. In other
words, the dwell time is 1 ms for a ground sample distance GSD
of 7 m. For high resolution imagers with GSD of about 1 m, tin
« ]/7 ms is too short for a sufficient good signal and SNR.
uu m)! Qiu (0m) = 1/10 (6)
dwe
Even more severe is the influence of the pixel field of view
(IFOV).
IFOV (1m)/ IFOV (10m) «1/100 (7)
Taking both aspects into account, redusing the GSD by a factor
of 107" causes a time related and geometry related decrease of
energy at the detector of about 10°.
There are two possibilities to overcome this obstacle:
- use TDI technology with N stages in order to increase the
signal N-fold and improve the SNR by the factor of JN
(this technology is used e. g. in the IKONOS and QuickBird
missions)
- use the so-called slow-down mode in order to decrease the
ground track velocity of the line projection on the surface.
with respect to the satellite velocity in order to obtain the
necessary dwell time tq.
18 + te
= 14
o
S 12
Ew detector size x > d
9 detector limited
= 8
zo
2 4 detector < d
> optics limited
Eu
0 b
0 2 4 6 8 10
f-number
Airy disk parameter d as a function of the
f-number F (470.55 um)
Figure 7
3.3 Mass, Volume, Power Consumption
3.3.1 Microelectronics: Since the launch of Landsat-l in
1972, the progress in microelctronics enabled more
sophisticated instrument designs. The developments for the
MESUR Network Mission may serve as an example, how much
microelectronics technology may influence the overall mission
design. The MESUR (Mars Environmental Survey) Network
Mission concept consisted of up to 16 small spacecraft (that
time planned to be launched in 2001). As often in
extraterrestrial missions, there was a pressure to miniaturization
by need. Reference mission. was the MESUR Pathfinder
Mission, one of the first missions under NASA's Discovery
program of smaller, low-cost missions to be launched 1997.
In [3] the benefits have been assessed which may occur when
the electronics technology used in the MESUR Pathfinder
mission is replaced by advanced microelectronics technology.
“ The MESUR Network study team found out that advanced
microelectronics packaging technologies could be applied to the
implementation of subsystem functions for
- the Attitude and Information Management System AIMS
- the Radio Frequency Subsystem RF
- the Power and Pyro Subsystem PP.
As a result, a factor of three or better reduction in mass, volume,
and power consumption were projected relative to the MESUR
Pathfinder baseline (see table 8).
The key to realize these reductions lies in the utilization of
industry-based advanced microelectronics packaging
technologies, including:
- multichip module (MCM) technology
- three-dimensional MCM stacking
- Die stacking for memory.
Pathfinder | Network Net Fractional
Reduction | Reduction
Mass 47 kg 11 kg 36 kg 4.3 x
Volume 46 dm’ 6.5 dm? 39.5 dm? 7.x
Power 74 W 26 W 49 W 29x
Table 8 Projected total reduction in mass, volume, and
power consumption for MESUR Network in
comparison to MESUR Pathfinder
The leverage of these reductions to the spacecraft is obvious.
The advanced microelectronics packaging technologies have
been widely used for instance in a joint NASA/DLR study for
the ROSETTA lander carrying among other cameras a stereo
camera with 10 mm GSD [4] and in a joint DLR/NASA three-
line stereo camera concept for planetary exploration [5]. The
effects have been remarkable. The latter concept resulted for
instance in very small stereo camera for a GSD of 20 m and a
swath width of 250 km from an orbit altitude of 250 km, and
with a weight of 2 kg and a power consumption of 12.5 Watts
including a 1 Gbit mass memory.
3.3.2 Detector: Pixel size influence - For mapping purposes
the pixel size of the detector is projected via the focal length to
the ground pixel size to be obtained, the smaller the detector
elements x the shorter the focal length f (see figure 9). As an
example, the stat-of-the-art CCD pixel size of 7um results in a
focal length of f = 4.2 m. Of course with smaller detector sizes
less energy is integrated. If the sensitivity of the pixel element is
not sufficient to obtain the necessary SNR, TDI needs to be
applied or a so called slow-down mode allows to enlarge the
dwell time to the sufficient extent (should not be used in stereo
imaging).
Impact of staggered configurations - Volumes and mass of an
optics depends significantly on the focal length and the
aperture, but also on the image field size determined by the
