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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
3. REQUIRED COMPONENTS FOR OPERATION OF
A SMALL SATELLITE FOR EARTH OBSERVATION
3.1 Launch
For satellite launches several competitive possibilities exist.
They can be launched by a separate mission or in piggyback.
Recently retrievable launch vehicles, such as the German
Phoenix are under development. Traditionally, launchers have
been used in the Russian Federation, in India, from ESA, and in
the United States. A launch cost estimate is about 10 000 $ per
| kg of mass. A retrievable launcher would cut this cost to
about half.
3.2 Choice of Orbit
The choice of the orbit is crucial for the performance of an
earth observation satellite.
Geostationary orbits at 36 000 km are preferred for
communication satellites. Earth observation satellites prefer
sunsynchronous polar orbits at orbital heights between 400 and
1000 km. If the observations are aimed at specific areas of the
globe, then lower inclinations combined with elliptical orbits
(150 to 500 km) can be chosen. Such orbits do not provide ideal
illumination conditions, however. The choice of the orbit
determines repeatability of sensing.
3.3 Attitude and Orbit Controls
The orbit can best be monitored by an on-board GPS satellite
receiver. The positions of the satellite are downlinked and
monitored in the ground facilities. Orbit corrections are possible
by hydrocyne propulsion systems with the need to carry this
fuel on board.
The attitude of the satellite is warranted first by 3 axes gyros.
For higher precisions a sun sensor is required. After monitoring
the attitude on the ground the propulsion systems can carry out
the attitude corrections for pointing of sensors. For smaller
corrections the sensor my be reoriented.
3.4 Sensors
For earth observations sensors are required, which sense
reflections of the earth within the range of the electromagnetic
spectrum. Optical and thermal (passive) sensors require
sensitive elements, which need to be read out and transmitted to
earth. Onboard processing by microprocessors is possible.
Active sensors (such as radar) require antennas for transmission
of electromagnetic pulses and for reception of the backscattered
reflections from the ground. Obviously passive sensors require
less mass.
The sensor limitations for use in small satellites have been
discussed in detail in the paper *High resolution mapping with
small satellites" by Rainer Sandau, presented to the ISPRS
2004 Congress (1). They refer mainly to optical and thermal
sensors. They are:
- Spatial resolution by the optical system, which is
governed by the diffraction limitation;
- sensitivity of the detector elements requiring a minimum
exposure time of about 1 msec;
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- image motion, due to the forward motion of the satellite
movement in the order of 7.4 km/sec or 7.4 m/msec.
If higher resolutions than 7 m are desired, then image motion
compensation must be applied. This is possible by time delay
integration sensors (TDI), in which instead of a single array of
detector elements across the platform motion a number of
arrays in direction of the platform motion are utilized, over
which the detector signals are averaged before readout. High
resolution sensors with ground resolutions better than 7 m
therefore not only need smaller resolution elements, e.g. by
staggered arrays, but also means of image motion
compensation, wither by TDI arrays or by rotation of the
satellite sensor during the exposure time.
3.5 Power
For the requirements of on-board sensing, processing and the
reception and transmission of sensing and auxiliary data, as
well as for the control options of the satellite power systems
must be provided. The principal source are batteries (NiCd,
NiH or Li-Ion). For longer duration missions they need to be
charged by solar energy, which has to be collected by solar
panels.
3.6 Data Readout
The charges received at the sensor elements of the arrays need
to be transmitted at appropriate readout rates to the ground
stations during a ground station contact time of about 10
minutes. At a rate of 100 Mb/s up to 60 GB of data can be
transmitted. If higher data rates are required data compression
must be utilized for transmission.
3.7 Ground Station Processing
The received signals at a ground antenna must be stored and
processed at the ground station facility.
4. CONCLUSIONS
Simple imaging of the ground requires a linear array sensor
perpendicular to satellite motion. Images of such a sensor
system require geocoding. This is possible using the transmitted
GPS and attitude orbital data within accuracy limitations.
Ground control points can increase the accuracy of geocoding.
However, the images cannot be orthorectified onto a
cartographic projection unless a digital elevation model of the
terrain is known from other sources, and used in the ground
processing chain.
To be independent of the knowledge of the digital elevation
model stereo sensing can be utilized, using at least two sensor
systems operating with a forward and a backward inclination
along the orbital path against the nadir. Rainer Sandau
describes the possible stereo sensing configurations in his
paper, which have been utilized in satellite systems (1).
As the swath widths for small satellite sensors are usually small
due to the use of narrow angle optical systems optimal
base/height ratios of 1:1 permitting to acquire digital elevation
models with sufficient accuracy are difficult to achieve.
