“Iridium NEXT Partnership in Earth Observation: Hosting Radio Occultation Sensors on
                   Commercial Communications Satellite Constellations"
                                                        Don Thoma

Iridium Satellite LLC plans to launch NEXT, the next generation satellite constellation of 66 satellites plus spares, starting
in 2013. Iridium proposes to include secondary sensor payloads that will measure parameters in unprecedented detail that
are crucial to societal needs, for example, the monitoring and modelling of global climate change. The GEO Secretariat
recognizes the potential value of the project to the GEOSS 10-Year Implementation Plan and proposes to support the
feasibility study into the use of the constellation for Earth observation including radio occultation.

Iridium NEXT
Iridium Satellite LLC currently owns and operates a constellation of 66 satellites in low Earth orbit (780 km) arranged in 6
planes of 11 providing global voice and data communications to approximately 220,000 subscribers. The rapid growth of
subscribers and the financial success of Iridium makes necessary the replenishment and enhancement of this
constellation. Beginning in 2013 through to 2016, the full constellation will be replaced, plus 12 spares, to ensure
continuous operation beyond 2030.

Opportunity for Earth Observation
It is proposed that Earth Observation Sensor (EOS) payloads be hosted on the NEXT constellation. This represents a
unique opportunity to achieve many of the priorities of the 68 member states and agencies of the Group on Earth
Observations and will provide a backbone for planning over 25 years. Flying EO sensors in a constellation format with
real-time access to the sensor data can provide unprecedented geospatial and temporal coverage. By sharing the space
infrastructure with a commercial satellite venture, the EO community will be capable of gaining an access to a orbiting
platform at a fraction of the cost of conventional missions. Without intending to exclude any feasible payload, a study by
ESA recommends a focus directed towards the oceans, ocean-atmosphere interactions and the coastal zones.

Proposed Approach
The sensor selection has been constrained to <50 kg; 30 x 40 x 70 cm; < 50 W average power & 200 W peak and <
1Mbps data from each sensor. About 24 currently available sensors meet the engineering specification but the following
are considered to make best use of the constellation (although any other suggestions are welcome and will be
     Sensor                Quantity     Measured Parameter(s)
     Altimeter                24        Mean sea level; altitude over land; vegetation thickness
     GPS Occultation          12        Atmospheric water vapour content
     Optical Imager            6        Broad band radiometer; ocean colour & land imaging
     Radiometer               24        Earth radiation budget; energy source for the climate system

Radio Occultation Mission
The opportunity to fly between 12 to 66 GPS Occultation Sensors on NEXT provides an unprecedented capability for
profiling atmospheric temperature & humidity. Such a configuration can provide up to 1000 soundings per day, with
300km x 100m resolution. A radio occultation data set from NEXT can support a dynamic 3D global view of the
temperature and water vapor profile with accurate input to climate models, long term variation monitoring, good data over
the poles and oceans and improvement in forecasting reliability including extreme events

Societal Benefits
The proposed payloads make a cohesive group of missions to give long term continuity of measurements valid for time
scales ranging from now-casting, to daily, weekly, seasonal forecasting to monitoring environmental and climate change
and providing invaluable data for long climate change models and the resulting consequences. The data sets can be
applied to all themes outlined in the GEO 10 Year Implementation Plan, the NRC forward look and European aims for EO.
However, full involvement of scientists who will evaluate and define the missions is just beginning and we need to bring
the engineers and scientists together nationally and internationally.

Plan of Action
The GEO Secretariat has been very supportive of the effort since Jan 2007 and the project will be tabled for consideration
at the GEO Summit in Cape Town (11/07). GEO, NASA, NOAA, France and the UK have set up working groups to assess
the opportunity. Iridium will be finalizing plans and specifications for the NEXT system in 2008 in preparation for
contracting for the deployment of NEXT. It is essential that the radio occultation community assess this opportunity and
voice its support to their appropriate Governments to ensure this unique opportunity is not missed.

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