EO TECHNOLOGICAL STANDPOINT
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Contents |
History
The first true Earth observation satellites saw light for the first time thirty years ago. From the distance, i.e. from Earth orbits, one can observe aspects of Earth not observable from ground and many informations regarding the surface, the atmosphere, the oceans, the subsurface, etc. have been obtained thanks to satellites. Imagery is not by far the only means for studying the Earth but it represents certainly one of the most efficient and most spectacular techniques when imaging the surface; we will in the following analysis concentrate on this application, for the civilian domain. With satellites it is possible to
- examine very frequently any given zone;
- examine any part of the world without authorisation;
- observe in one shot very wide surface.
At different scales the details seen vary. Also with the
wavelength analysed by the instrument, the information
contained in the “image” changes.
Many satellites are now in orbit to observe the Earth
and several systems have been implemented on private
bases. Many applications have been developed and the
“market” should continue developing. [1]
Since the availability of Russian analogue KVR-1000
space imagery in the early 90s, the limit
of geometric resolution of non-classified earth
observation imagery shifted from the then best resolution
of 10 m—captured by the SPOT satellites
to the domain of about 2m per pixel. Similar to
the heritage of the Russian data, such information
was formerly only available for strictly classified
military reconnaissance programmes of the
former military super-powers. Since the successful
launch and operations of Space Imaging IKONOS II satellite (after an initial launch failure) in 1999,
this “metric” or even “submetric” resolution is now
available on a commercial basis. This domain of
geometric resolution is often referred to as “very
high resolution” (VHR) data. Responding to the
challenge from former Russia, US companies, utilising
technologies developed initially for military
tasks, took the lead in optical metric systems. Meanwhile
European states and other countries of the
world keep up with providing similar systems and
exploring other domains of the electromagnetic spectrum,
such as radar imaging. Besides space agencies
and entities from the traditionally space faring
nations, also satellite operators from “countries in
transition” and even from “developing countries”
enter the arena. International organisations such as
Committee on Earth Observation Satellites (CEOS)
try to keep up to date with the plans of their
members, but do not list purely commercial or
some dual-use enterprizes.
Though, most of the operators or distributors of
these earth imaging systems are private enterprises,
still the vast majority of customers for the very
high resolution data are governmental entities, especially
military and security agencies . To satisfy
the increasing need of US intelligence customers
on VHR space data on one hand and to reduce the
costs of operations, the US Presidential directive of
April 25, 2003 further eased previous restrictions
on the commercial collection of space imagery and
demanded US federal agencies to purchase satellite
information from commercial companies. The
National Geospatial Intelligence Agency (NGA, former
NIMA) awarded a first NextView contract to
DigitalGlobe, Longmont, Co, USA, for the delivery
of very high resolution space imagery to the
US armed forces. Intentionally, the contract would
allow DigitalGlobe to finance its next generation
satellite system, called WorldView. At the time of
writing, a call for a second NextView
contract was finished and has been awarded to Orbimage.
With the delay in the deployment of the
future US military reconnaissance satellites (called
Future Imagery Architecture, or FIA) and the global
military engagement of US forces, data purchase
agreements such as NextView will stimulate the
delivery of VHR data for quite a couple of years.
Besides such governmental data purchase agreement,
the rest of the governmental and private customer
market is believed too small to operate such systems
profitable. [2]
Past evolution of satellite imagery
NOAA-A/Landsat 1 satellite was launched in orbit
in 1972. It has been the true forerunner with its
resolution in the 100-m range. It has paved the way
for better and better satellites. SPOT family with five
satellites launched today, the latest one on the 4th of
April 2002 has been greatly developing the commercial
market for space imagery of the Earth. The geometrical
resolution has continuously improved, for instance,
from 100m for the first NOAA-A satellite to
10m for NOAA-K or from 10m for SPOT 1 launched
in 1986 to 2.5m for the recently launched SPOT 5.
Other satellites have been launched over the past 2
or 3 years with performances down to 0.7 m resolution
but with less wide field of view than SPOT or Landsat.
With the improvement in the resolution generally
the size of the satellite has a tendency to decrease in
weight. One major reason for this is the evolution in
detectors and in the performance of telescope.
Note: The geometric resolution is improved by two
and the field of view is decreased by two when the
altitude of the orbit is lowered by two. Most civilian
satellites operate at altitudes between 400 and 1000 km
and this involves a compromise between image performance
(field of view and geometric resolution) and orbit
perturbation due to drag forces in high atmosphere,
which affects the lifetime of the satellite in orbit.
Together with the improvement in resolution and
size of images, comes the increase in data volume to be
transmitted from satellite to the ground stations. Improvement
in the data compression algorithm, however
limits the growth in data quantity to be transmitted.
Also it is worthwhile mentioning that the size of the
on-board memory increased over time tremendously
and with the progress in memory chips, the volume
and power consumption of these on-board memory
have greatly decreased.
The Earth stations necessary to receive the data
transmitted by the satellites have also decreased a lot
inspite of data rate increase, because of better antennas
and better transmitters on board satellites and better
receivers in the ground stations. [1]
Status : the application of remote sensing/imagery
The military demand for observation of territories has always existed, e.g. through look out posts and balloons. With the origination of flights, airplanes became relevant for this purpose. Of course when the technology has become available, this is being attempted with satellites. In the civilian domain the need has existed for less long but have proven to be also important. Among the applications are
- Cartography at every scales in 2D and 3D: The demand for good maps arises from many different needs, e.g., the regional development, the town and country planning with roads and railtracks, parks and forests, water supply and other installations.
- Land cover inventories: For agriculture, forest management, buildings, observation via satellites offers unique possibilities.
- Closed waters, open seas, humid zones surveillance: this area offers certainly one of the biggest challenge of the century, as keeping the waters healthy could well become one of our priorities.
- Installation of new collective equipment: For instance, the installation of new telecommunication and television relays requires extensive study of topography, terrain occupation and population density.
- Agriculture aids and management: Images of cultures could give informations on expected yields (crop evaluation) or for the farmer it can give informations on demands for fertilisers, for watering or for chemical treatments.
- Disaster management.
- Analysis of changes, etc.
The alternatives to satellite are the observations directly
on the ground or aerial photography. In fact, they
complement each other and not compete. The smaller
the distance the better the resolution, but the smaller
the swath the more difficult it is to make frequent observations
of remote areas.
Some applications require extremely high geometric
resolution but many are dedicated to wide coverage
or frequent observations.
Airplanes are perfectly adapted to extremely highresolution
image of surfaces not too wide and in areas
that are not too remote. Satellites are perfect when
wide coverage is required if high resolution (down to
0.5 m) is sufficient and of course if the zone is not
easily accessible. [1]
The European situation
The situation for European earth observation satellite systems is only partially comparable to those in the US: though also used by military and security forces, the requirements of European military forces would not have so far justified the deployment of commercial operations. European military interests have been not as global as the US ones and European military budgets are significantly smaller than those of its Atlantic ally.
Future trends and expected technological evolution
Earth observation is one of the application in which satellites have gained a market niche
and also became indispensable to public services. But the applications are slow to develop
and are not able to cover the costs of development and renewal of satellites and ground
segments.
When we analyse the past evolution of the remote sensing activity, we observe a continuous
improvement in performances and precisely in the geometric resolution of the satellites.
From the first satellites with 100 m accuracy to now 1 m or even better, the satellites have
made tremendous progresses.
Also on another side, the general move towards « faster, better, cheaper » has affected this
domain and we now currently see operational satellites with sizes in the mini range.
Geometric resolution
Improvements in geometric resolution and swath for new satellites are related to the decreasing in the size of individual transistors composing integrated circuits.
Satellite size
The development of small launchers, sometimes based or derived from reconverted missiles, has allowed or pushed the development of smaller spacecraft.
Access time
One of the characteristics of Earth observation satellites systems is the access time, i.e. the time needed to make available to a user an image he requested.
A possible scenario for the long term
Certainly small satellites are there for ever, or at least for long, but a number of problems are to be solved if we want to transform technical experiments, or lucky opportunities, in real and sustained business. From what has been said in the preceding paragraphs, one can derive a scenario for the future of Earth observation satellite. [1]
NASA's future Earth observation plans
Improving life here on planet Earth is foremost in NASA ’s vision, and the larger purpose of NASA’s Earth science activities within its Science Mission Directorate (SMD). The research strategy is supported by information obtained from a variety of space vantage points and complemented by airborne and in situ observational data. [3]
References
[1] D.Hernandez
"Possible scenario for future mission in Earth observation".
[2] G.Schreier, S.Dech
"High resolution Earth observation satellites and services in the next decade - A european perspective".
[3] S.P.Neeck, G.E.Paules, J.D.McCuistion Ramesh
"NASA's future Earth observation plans".
