Medium Earth Orbit Architecture for an Integrated
Environmental Satellite System
Andrew J. Gerber, Richard L.
Baron, David M. Tralli, Michael Crison, Shyam Bajpai, and
Gerald Dittberner
Introduction
Operational Earth observing satellites generally
reside in low Earth orbits (LEO) at less than 1,000 km
altitude and in geostationary Earth orbits (GEO) at
~35,800 km altitude. LEO enables high spatial resolution
from greater proximity to the Earth surface, however with
low temporal resolution due to the revisit time between
observations of a given point on the surface.
Comparatively speaking, GEO provides lower spatial
resolution, but higher temporal resolution by remaining
fixed in the equatorial plane over a given point. The
National Oceanic and Atmospheric Administration (NOAA)
uses both observation venues for their environmental
satellites. Medium Earth orbit (MEO), at altitudes between
1,000 and 35,800 km, may be able to capture the preferred
attributes of both LEO and MEO, thus providing the
capability for higher spatial and temporal resolution
environmental observations. The results of an ongoing MEO
system architecture study provide insight into
cost-effective operational benefits.
The Current Configuration
The NOAA operational environmental satellite system
is comprised of geostationary satellites for short-range
warning, and polar-orbiting satellites for longer-term
forecasting. The Geostationary Operational Environmental
Satellite (GOES) system maintains a continuous data stream
from two satellites, one located at 75 W longitude and the
other at 135 W longitude, in support of National Weather
Service (NWS) and other users requirements, transmitting
environmental data, and visible and infrared images
covering the regions of the world from approximately 20 W
longitude to 165 E longitude. Imagers on each satellite
have the additional capability to focus on narrow regions
of the globe, such as to obtain coverage of a hurricane.
The Polar Operational Environmental Satellite (POES)
system provides daily global coverage, with morning and
afternoon low earth orbits that deliver data, including
cloud cover, storm location, temperature, and atmospheric
heat balance for improved weather forecasting.
Additionally, the Defense Meteorological Satellite Program
(DMSP), run by the Air Force Space and Mission Systems
Center, is used for monitoring meteorological,
oceanographic, and solar-terrestrial physics environments.
The future National Polar-orbiting Operational
Environmental Satellite System (NPOESS) and its managing
Integrated Program Office (IPO) were established in 1994
to converge existing multi-agency polar-orbiting
satellites (POES and DMSP) into an integrated national
program that will be used to monitor global environmental
conditions, collect and disseminate data related to
weather, atmosphere, oceans, land, and the near-space
environment. First NPOESS launch readiness is scheduled
for 2009, with launches for a fully operational system
planned through about 2016.
An Alternate View
As part of an ongoing system architecture study,
the NOAA National Environmental Satellite, Data, and
Information Service (NESDIS) Office of Systems Development
(OSD) and the Jet Propulsion Laboratory (JPL) are
providing input to the NOAA development process for an
environmental observing system that potentially would
follow the GOES-R series, scheduled for launch readiness
in 2012. A long-term goal of the relationship between
NESDIS and JPL is investigating the merits of combining
the capabilities of LEO and GEO satellite systems into a
consolidated MEO constellation and system.
The study suggests that a MEO satellite
constellation may afford the greatest potential for
providing NOAA with the most cost-effective path to the
high spatial, temporal, and spectral resolution
environmental data it needs to achieve its 21st century
strategic plan. The plan recognizes the increasing
linkages between the environment, the economy, and public
safety with an implementation goal of transitioning from
individual polar and geostationary observational programs,
to an integrated system that meets current and future
observational, processing and communications requirements.
The continued optimization of the MEO architecture
includes consideration of the data collected by other
Earth observing systems and platforms, both nationally and
internationally, and recognizes the increased widespread
interest in implementing an integrated global earth
observation strategy.
Furthermore, the architecture study is driving the
need to understand more comprehensively, the attendant
science requirements from the broader ocean and atmosphere
communities, towards providing a complete real-time global
weather monitoring system and long-term climate and
environmental observations. These requirements are being
used to define instrument suite options for a potential
MEO demonstration mission in the 2012-2013 timeframe. The
study process also identifies new sensor needs that would
drive technology planning, investment and development.
Demonstration Mission
The rationale for a MEO demonstration mission is to
validate that basic imaging, temperature sounding, and
wind and liquid/solid water profile measurement
requirements can be met from 10,400 km MEO altitude, and
provide a communications backbone for a variety of NASA,
NOAA, and NPOESS IPO missions. The overarching purpose of
such a mission would be to demonstrate a sustainable,
extremely capable system that is superior to current
implementations and affords significantly reduced
long-term costs compared to other implementations that
could deliver similar data products and capabilities.
Recognizing that observational requirements dictate
temporal resolution, spectral coverage, and resolution,
spatial resolution and radiometric performance, an
objective of the study is to provide definitive studies
and trades as input to roadmaps for MEO and other
satellite constellation options for NESDIS/OSD.
Equatorial-polar (EP), Walker and ICO (i.e.,
Intermediate Circular Orbit, as for communications
satellites) constellation configuration tradeoff studies
were performed and evaluated with respect to imaging and
sounding coverage, constellation coverage and
inter-satellite and downlink communications complexity.
The study recommends an EP configuration (Figure 1).
Global imaging robustness in terms of each
constellation’s 24-hr average coverage percent under a
single satellite failure condition was examined (Figures 2
and 3). The concept for an integrated environmental
satellite system entails developing a constellation of
four equatorial and four polar MEO satellites for the year
2020 timeframe that can make global observations of the
weather, atmosphere, oceans, land, and the near-space
environment, and acquire real-time images of any place on
Earth at any time of day or night.
An evolutionary MEO road map to a post-GOES-R
observational capability must support continually
improving weather predictions, plus climate and
environmental assessments and forecasts, with near
real-time data availability, total global coverage, 0.5 km
at 0.5 µm instantaneous geometric field of view or better
spatial resolution, improved spectral coverage (including
microwave for cloudy weather conditions), and globally
consistent, long-term accurate and stable data collection.
Based on architecture studies performed to-date, a MEO
system could be developed to serve three functions: (1) to
demonstrate MEO-observing capabilities with GOES-like
instrumentation (Figures 4 and 5); (2) to provide an
operational communications system; and (3) to provide a
risk retirement test bed for evaluating new instruments
developed to exploit MEO opportunities and provide NOAA a
cost-effective, risk-reduction path for developing
environmental observation instruments for future orbital
and ground systems architectures.
The MEO demonstration plan calls for launching 4
satellites in the equatorial plane of the constellation
augmenting the capabilities of GOES-R, other
meteorological satellites and NPOESS, while continuing the
usage of NPOESS for the polar regions. Furthermore, it
provides a platform for validating and demonstrating new
instruments to substantially improve the environmental
data collection and weather prediction capabilities of
NPOESS and GOES-R, such as an advanced optical and
infrared imaging and sounding and a high-performance
microwave radiometer/sounder with spatial resolution of
the order of 1 km and 50 km, respectively. The first pair
launch would be about 2013, after the first launches of
GOES-R. The focus would be on an operational duration of
five to eight years. Emphasis would be on reducing
operating costs (compared to existing observational
systems), and achieving satisfactory long-term
reliability. The MEO mission also would demonstrate
satellite-to-satellite and satellite-to-Earth
communication capabilities.
Imager and Sounder
Numerical weather prediction and forecasting models
are moving towards improved spatial resolution. An imager
provides high spatial resolution for improved feature
definition, and a sounder provides high spectral
resolution for good radiometry, temperature and water
vapor profiling, and trace gas amounts. Imaging and
sounding both are needed together. One instrument option
for MEO is based on combining the capabilities of two NASA
Earth Observation System instruments—the Moderate
Resolution Imaging Spectroradiometer (MODIS) with those of
the Atmospheric Infrared Sounder (AIRS). The MODIS
instrument provides high radiometric sensitivity in 36
spectral bands ranging in wavelength from 0.37 µm to 14.4
µm. The AIRS instrument, with spectral coverage from 3.7
to 15.4 µm, is the first high-spectral-resolution
infrared sounder developed by NASA in support of
operational weather forecasting by NOAA. Design studies
are in progress for potential development of such an
integrated instrument with capabilities that exceed those
on GOES-R.
Microwave Radiometer
As an extension of a GEO synthetic thinned aperture
radiometer (GeoSTAR) microwave sounder study by JPL, an
initial assessment for aperture synthesis in MEO was
performed. The objective is to add microwave sounding
capabilities to future systems such as GOES-R, both to
complement infrared sounding systems (such as the
Hyperspectral Environmental Suite—HES) and to provide
all-weather standalone microwave soundings. Aperture
synthesis is an approach that does not suffer from the
limitations of typically real aperture systems, namely
diffraction effects in the effective aperture formed by
the reflector, thereby limiting spatial resolution;
rather, it uses an array of receivers to form an
equivalent aperture with on-board signal processing to
measure the phase properties of the radiometric field.
For a stand alone capability, GeoSTAR can be
deployed on a separate platform, but to complement an IR
sounder it is important to have the same field of regard,
preferably on the same platform. At MEO, although it would
be possible to implement a two-dimensional (i.e., star
array) system such as GeoSTAR, there are aliasing issues
due to the large solid angle subtended by the Earth as
seen from MEO. Instead, one-dimensional approaches are
being considered—where along-track coverage is obtained
through orbital motion. Many details remain to be worked
out and evaluated before definitive recommendations can be
made. Nonetheless, early indications are that such a
microwave radiometer system is also a viable option for
MEO.
Communications
Broader bandwidth communications are needed as a
result of higher resolution global coverage and temporal
availability, with an approach that reduces the resource
load on satellites in order to support the primary
function of data collection. Three options for MEO
communications architectures were evaluated.
Option 1 consists of a distributed high-speed
ground network with commercial rebroadcast, with upgraded
SafetyNet sites and ground network, and with medium data
rate rebroadcast. Option 1 is namely a modified NPOESS
SafetyNet with commercial rebroadcast, using Ka-band
downlinks from the individual satellites using
mechanically steered parabolic antennas. This option calls
for much higher data rates and extensive network upgrades.
Commercial satellites would be used for data rebroadcast
(e.g., AWIPS-like broadcast). The rebroadcast data rate is
limited, albeit about 25 times higher than the current
GOES rebroadcast network, but orders of magnitude lower
than the instrument data collection rate. Option 2
comprises optical satellite crosslinks to a downlink
satellite with a Ka-band data downlink to a single ground
node and a client-server data redistribution network
(e.g., no network of ground stations). Onboard processing
for routing data satellite-to-satellite would be needed,
increasing satellite resources. The satellites would
exchange their observational data and satellite health and
safety data with each other and with ground-based
receiving stations in real-time. The data rates for ground
redistribution are limited, but each user gets data as
requested. Option 3 consists of optical satellite
crosslink with hybrid data redistribution, using medium
data rate rebroadcast and a client-server ground network.
Requirements
MEO mission requirements are derived from
requirements in the GOES-R Preliminary Requirements
Document. The demonstration constellation of four
equatorial satellites, for example, would make
observations over the entire range from 60 degrees north
to 60 degrees south of the equator, and include
interleaving hemispheric (full disc), synoptic (regional
CONUS) and mesoscale (rapid-scan) imaging. Full disc
imagery data would be taken every 15 minutes, and CONUS
imagery data taken every 5 minutes—an equivalent GOES-R
“full disc” as seen by the MEO constellation being a
composite image from sensors on the multiple satellites.
For severe weather activity, updated satellite imagery
data covering areas at least 1000 km square area would be
taken every 30 sec. The four-satellite constellation would
be expected to have an operational availability of at
least 98%.
Conclusion
MEO architecture affords NOAA the potential to
provide high spatial, temporal and spectral resolution
environmental data comparable to or exceeding that of
NPOESS and GOES-R. The architecture study is ongoing, as
the opportunities for visible/IR imaging and sounding and
microwave radiometry, and a global communications backbone
are considered. A draft Program Plan has been developed,
and a Program Implementation Plan that would help NOAA
transition into an integrated environmental system that
meets current and future observational, processing and
communications requirements is in progress.
Acknowledgements
This work was performed by JPL as a member of the
NOAA advanced architecture team under a contract to NOAA
NESDIS. JPL is a Federally Funded Research and Development
Center managed by the California Institute of Technology
under contract to NASA. We thank Tom Pagano and Bjorn
Lambrigtsen for analyses and discussions related to AIRS
and GeoSTAR, respectively, and David Oh for the
communications architecture component.
About the Authors
Andrew J. Gerber, Richard L. Baron and David M.
Tralli; National Space Technology Applications (NSTA)
Office, Jet Propulsion Laboratory, California Institute of
Technology. Focusing on R&D and applications needs of
national security and economic significance, NSTA offers
organizations opportunities to apply the unique
scientific, technological and engineering capabilities
resident at JPL to help meet their strategic visions.
Michael Crison, Shyam Bajpai and Gerald Dittberner;
Requirements, Planning and Systems Integration Program/
Office of Systems Development (OSD)/ NESDIS/NOAA. OSD
manages the NOAA operational geostationary and
polar-orbiting environmental satellites programs and is
responsible for defining user requirements and developing
designs of future satellite systems to meet those
requirements.
For further information, contact [email protected].
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