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IAC-09-B1.2.3

GEO-AFRICA:

A DEDICATED AFRICAN SPACE OBSERVATORY

Author

Mr Cyrille Tourneur EADS Astrium, Toulouse, France, cyril.tourneur@astrium.eads.net

Co-author Mr Dominique Pawlak

EADS Astrium, Toulouse, France, dominique.pawlak@astrium.eads.net Mr Xavier Sembely

EADS Astrium, Toulouse, France, xavier.sembely@astrium.eads.net

ABSTRACT

GEO-Africa is the name of a new space mission, presently under discussion and consolidation in a group associating major African space remote sensing and applications actors, international organisations and a European team led by EADS Astrium. GEO-Africa is a geostationary space imagery infrastructure aimed at being one of the contributions for answering to the level & quality of medium resolution EO data that the main applications (management of environmental resources such as water management, land use-land change, soil erosion-desertification, forest monitoring, risk management, agriculture, etc…) require at the African Continent scale. GEO-Africa is analysed as one of the bricks of the system of systems that Africa needs for a better management of its natural resources and environment and a particular attention is given to the synergy and complementarity with existing-forthcoming equipments and programs (space and ground infrastructures, capacity building) national and international. GEO-Africa features innovative, yet technologically available features such as permanent observation and fast access, continental coverage with up to 13 hours of observation per day over Africa. This paper provides a first overview of the overall GEO-Africa concept and performance.

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FULL TEXT

GEO AFRICA SENSING CONCEPT The GEO Africa mission is based on an optical observation satellite placed in the geostationary orbit.

The remote sensing concept allows production of 300 km x 300 km elementary scenes in Visible & Near InfraRed (VNIR) and Short Wavelength InfraRed (SWIR) spectral domains.

Figure 1: Africa mapping with 300 x 300 km scenes.

Less than 420 scenes are needed to map political Africa, including mainland and islands.

The corresponding nadir looking Ground Sampling Distances (GSD) are respectively 25m for VNIR bands and 75m for SWIR band.

The selected imaging principle is based on the ‘step and stare’ concept. It consists of two consecutive phases: The ‘step’ phase during which the entire satellite body is pointed toward the target scene and the ‘stare’ phase, consisting of the actual image collection with detector matrix.

Stare phaseStep phase

Telescope

Arraydetector

Imaged scene

Scene N

Scene N+1

Satellitebody

motion

Figure 2: Principle of the ‘step and stare’ concept.

The super spectral instrumental concept offers VNIR imaging capacities in 10 narrow bands and a large panchromatic band, complemented by a SWIR channel. Wavelengths are selected to provide atmospheric correction capabilities and enable thematic applications based on super spectral imagery.

Domain Central

wavelength (nm) Width (nm)

Visible 443 40 Visible 490 20 Visible 550 20 Visible 640 20 Visible 672 20 Visible 702 20 Visible 722 20 NIR 782 20 NIR 865 40 NIR 910 40 VNIR (PAN) 650 400 SWIR 1600 100 Table 1: GEO Africa spectral bands. The number of

bands to be collected can be selected for each image acquisition

THE OBSERVATION DOMAIN

Optical observation from the geo stationary orbit takes place in a extended domain with regard to the familiar Low Earth Orbit (LEO) concepts. This concerns mainly the relative sun to satellite observation angles which range differs from the LEO based satellites performing image collection at fixed local time. GEO Africa daily timeline

GEO Africa daily operations timeline is driven by the satellite concept which provides the capability to perform image collection up to 13 hours per day.

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Observationperiod

00:00

12:00

06:00 18:00

Rest and maintenanceperiod

Sun direction

Figure 3: Satellite daily timeline: Thanks to the satellite design concept, image collection can begin from 5:30 (satellite local time) up to 18:30 (satellite local time)

Regarding the incoming optical flux, the radiance levels available over Africa for Geo stationary observation allow an image acquisition duration of 11 to 12h throughout the year as shown on figure 4. This duration is consistent with the satellite’s capability

Figure 4: Various radiance levels over Africa for

different hours and seasons: Observation can begin from 5:00 GMT on Madagascar & Somalia till 17:00 GMT over westernmost areas.

Angular domain In terms of observation angles, two angles are considered to compare GEO Africa observation domain with the classical LEO missions: • The Sun Zenithal Angle (SZA) which is

the angle of the incoming solar flux with the local nadir. LEO missions such as SPOT5 are operated with SZA extending up to 60°

• The phase angle which is the direct angle between the observation line of sight and the sun illumination angle. For SPOT5, the maximum phase angle is up to 75°

Figure 5: Observation angles definition

According to these definitions, 2 observation domains can be considered for GEO Africa: • The LEO ‘like’ domain for which the

above defined angles do not exceed those of the classical LEO missions. GEO Africa can perform up to 10 hours of continuous observation over Africa within this domain.

• The ‘extended’ GEO domain for which the phase angle can extend up to 95° for an SZA below 60°. This domain allows continuous observation from 5:00 GMT up to 17:00 GMT but requires advanced Bidirectional Reflectance Distribution Function (BRDF) inversion models for image exploitation.

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Instant accessibility vs time for various phase angles

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LEO Domain

Figure 6: Instantaneous accessibility over Africa (average over one year) for different hours and phase angles with an SZA below 60°. The classical ‘LEO domain’ corresponds to phase angles below 75°.

Aerosol specifics

Aerosols represent a specific concern for visible observation over Africa, typically for sub-Sahel areas, since they affect image exploitation due to attenuation, diffusion and reflection effects.

Figure 7: Example of aerosol attenuation: Resulting

optical transmission due to aerosol optical depth at 11:00 GMT at Spring equinox

The aerosol coverage therefore results in a 20% average reduction of the instantaneous accessibility over Africa and the ‘observation window’ for highly affected areas can be reduced to a couple of hours per day. Consequently, aerosol coverage has been taken into consideration for GEO Africa observation windows and operational tasking concept.

THE SYSTEM CONCEPT Image downlink concept With regard to the regulatory aspects, few bands of frequencies are available for GEO Africa to allow image data downlink since many satellites currently operate close to foreseen GEO Africa position on the geostationary arc. In accordance with ITU regulation, an opportunity in the L-Band domain is available for GEO Africa provided that the EIRP density does not exceed -52 dBW/Hz. This constraint comes from Meteosat8 (10°E) so as to avoid interferences at ground station level. This can be easily fulfilled with a data rate of 26 Mbps for an occupied bandwidth of 36 MHz.

This frequency is also perfectly adapted to GEO Africa timeline with regard to the usual scintillation concerns in L-Band : According to ITU-R P.531-9, ionospheric scintillation impacts inside the tropical belt become significant after 19:00 local time; i.e. well after the completion of GEO Africa observation period.

Based on the available data rate, the image downlink concept selected for GEO Africa is the following:

• Continuous & permanent image downlink during the observation period

• Fixed downlink antenna parallel to the observation line of sight

• Semi global aperture of 12°

Figure 8: Downlink coverage example for 5 different

observation directions (1 nadir + 4 extremes). The large downlink aperture combined with the ground segment architecture allows image data receiving for any target location in Africa.

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Elementary timeline

According to the selected downlink concept and the step and stare imaging principle, GEO Africa elementary timeline lasts less than 8 minutes. This duration accounts for the acquisition of a 300 x 300 km scene in all spectral bands.

Satellite manœuvre & tranquilization

< 80 s

Visible & SWIR images acquisition

< 400 s

Images compression & downlink

< 380 s

Elementary image cycle (max case) < 480 s

Figure 9: Breakdown of GEO Africa elementary

timeline. Image data downlink is performed in real time and no additional latency has to be considered

System architecture

The system architecture takes benefit of the downlink and operations concept. It is based on: • A central mission and control centre

(MCC) in charge of satellite house keeping and programming. The mission establishment, including requests collection and work plans elaboration, is performed in a fully automated manner

• Regional Image Centres (RIC) dedicated to image data reception, processing and dissemination to regional end users. Each RIC also collects local end users requests and transmits them to the MCC.

Central Mission &

Control Centre

Regionalimage centre

Regionalimage centre

Regionalend user

Regionalend user

Regionalend user

Regionalend user

Telecommands

Housekeeping Telemetry

Image telemetry

End products

Image telemetryEnd

products

Endproducts

Figure 10: GEO Africa system architecture

The flexible concept of operation

The GEO Africa concept of operations (CONOPS) significantly differs from the classical operations principles of LEO missions: The CONOPS takes advantage of the satellite geostationary position, allowing permanent Radio Frequency contact with the ground segment, and the near real time availability of meteorological data from METEOSAT, refreshed every 15 minutes. The CONOPS is based on a 2 phases daily cycle: • The preparation phase which is

completed at night and during which all user requests are compiled to establish and optimize a preliminary collection plan for the coming day. The automatic elaboration of this preliminary collection plan is based on requests priority rules and meteorological forecasts.

• The ‘live’ operation phase extending over the whole observation period of the current day. During this phase, the collection plan is automatically updated every 30 minutes, considering the current meteorological & aerosol situation and the emergency requests which may be raised during the day.

Figure 11: GEO Africa tasking concept. The CONOPS

take benefit of the satellite geostationary position to allow dynamic update of the current work plan every 30 minutes.

Three categories of requests can be considered: User defined requests, divided into routine and emergency requests, and automatic meteo/aerosol opportunity requests for highly cloud/aerosol covered areas. The latter type of requests are automatically generated for areas with high cloud or aerosol coverage statistics when an observation opportunity is detected thanks to the latest METEOSAT live update.

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User defined parameters typically include the observation area, the spectral bands and the expected date of collection or the deadline. Optional parameters such as the expected local time for acquisition and the repeat frequency can also be defined. Routine requests incorporated in the preliminary collection plan are selected with automatic ranking algorithms based on criteria such as request deadline, meteo success rate with regards to forecasts, preferred local time, etc The emergency requests which are immediately taken into account during the live operation phase are selected according to criteria to be further refined by the operators based on the system governance. Figure 12 provides an illustration of GEO Africa collection plan for a typical day, including the preliminary collection plan established during the night and the actual plan completed at the end of the day.

Figure 12: Typical day illustration.

Up: Preliminary collection plan elaborated during the preparation phase (routine requests only).

Bottom: Actual plan at the end of the day, including routine requests (blue), emergency requests (red) and meteo/aerosol (green) automatic requests generated during the day.

GEO Africa performance Sensing and capacities

GEO Africa essential performance are summarized below. GSD Nadir 25 mSwath 300 x 300 kmSpectral bands 11 bands + 1 PANSignal to Noise Ratio 110Modulation Transfer Function > 10%Capacity 90 images/day

Revisit/Refresh rate30 minutes non tasked

8 minutes tasked Table 2: GEO Africa main performance

• The image quality performance (Modulation Transfer Function and Signal to Noise Ration) are similar to those available for classical LEO mission so as to allow reuse of existing Earth observation applications.

• The system capacity, in terms of images per day, combined with the large time consistent swath of 300 x 300 km enables global mapping of Africa in 4 to 5 days.

• The time resolution (revisit) is the ultimate breakthrough of the system since it provides 8 minutes revisit in tasked mode and 30 minutes in non-tasked mode (emergency requests) which correspond to the elementary programming period during the live operation phase

• The GSD variation over is according to figure 13. This demonstrates the high interest of geostationary observation concept for Africa.

Figure 13: GSD variation with respect to nadir

resolution.

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Beating the clouds

While GEO Africa is a permanent complement to existing or planned LEO systems, GEO Africa has a valuable advantage which is the ability to take every clear sky opportunity to perform image acquisition. This ability provides significant benefits for Africa remote sensing in the visible spectrum, particularly inside the inter-tropical belt.

8:00 10:00 12:00 14:00 16:00 18:008:00 10:00 12:00 14:00 16:00 18:00

LEO passLEO passOpportunity

�30’

for GEO AfricaOpportunity

�30’

for GEO Africa

Figure 14: Advantage of the geo stationary observation

concept: GEO Africa can take any clear sky opportunity of 30 minutes whereas a LEO satellite passes at a fixed local time and therefore depends on the sky conditions at the time of pass.

Figure 15: Totally cloud free statistics over Africa for

the year 2006 (%of time). The figure clearly illustrates the important cloud coverage statistics inside the inter-tropical belt and a worst case in the lake Victoria area.

These advantages have been quantified in collaboration with METEO FRANCE for 3 typical African areas of 300 x 300 km:

• 2 areas with high cloud coverage statistics, respectively over Lake Victoria (worst statistics) and Malabo island area in the Gulf of Guinea

• 1 area with high cloud coverage variability located in the vicinity of Cape Town

Malabo areaHigh cloud

coverage statistics

Lake Victoria areaHigh cloud

coverage statistics

Cape Town areaHigh cloud

coverage variability

Figure 16: Areas selected for ‘beating the clouds’

advantage evaluation.

The following comparative statistics have been computed over the last 3 years on the selected areas:

• Clear sky statistics during the observation period

• Image acquisition success rate with respect to cloud coverage

These statistics have been calculated for a morning pass LEO satellite with a 5 days revisit and for a 30 minutes observation opportunity during the whole daylight period (GEO Africa).

Comparative

statistics

Lake

Victoria

Malabo

island

Cape

Town

Clear sky statistics LEO satellite 22% 12% 47%

GEO Africa 29% 19% 48%

Success rate LEO satellite 5% 3% 9%

GEO Africa 57% 46% 72%

Table 3: Cloud coverage impacts comparative statistics for GEO Africa and a LEO satellite with a 5 days revisit.

These results show that the cloud coverage impacts are reduced by a factor close to 16 for the highly affected equatorial areas and 8 for high variability areas.

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CONCLUSION

Based on a new observation concept, GEO Africa extends the classical Optical Earth Observation domain regarding: • The revisit/time resolution and tasking

flexibility since GEO Africa grants permanent daylight access over Africa

• Super spectral applications thanks to 10 narrow bands

• The swath dimension which allows regional applications over very large time consistent areas of 300 x 300 km and rapid African continent mapping.

• Selectable local time and possibility to perform observations of the same area at different times of day, enabling a new range of applications to be deployed

• Ability to surpass the problem of cloud obstructions, especially over highly cloud covered African areas for which visible imagery was up to now rarely available.

GEO Africa therefore enables the reuse of traditional remote sensing applications, inherited from the standard LEO observation missions, but also provides the opportunity to develop new thematic applications in this extended domain.