Distributed Satellite Systems for Earth Observation and Surveillance




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Distributed Satellite Systems for Earth Observation and Surveillance

Rainer Sandau

German Aerospace Center

Rutherfordstr. 2, 12489 Berlin

Germany

rainer.sandau@dlr.de

A bstract

The recent trend of miniaturization accompanied with increased or even novel high performance features allows to implement distributed space systems with different basic characteristics. Distributed space systems with satellites carrying different payloads can for instance replace larger spacecraft providing also the potential of realizing performances and missions unachievable using the monolithic approach. After an introduction to the different concepts of distributed space systems like inspection and docking systems, formation flying, satellite constellations and spacecraft swarms, the paper concentrates on constellations and formations. Their widespread possibilities are presented by means of example missions.

1.0 Introduction

The trend to smaller satellites was and is supported by the improvements in diverse fields of technology as for instance optics, mechanics and materials, electronics, signal processing, communication and navigation. Mass, volume and power consumption of the spacecrafts and the instruments followed the trend to miniaturization allowing at the same time significant increases of performance. These trends can be observed for passive optical space borne systems as well as for active micro wave systems, e. g. SAR systems. They all benefit from the technology improvements.

The manifold application areas of space borne systems for Earth observation and surveillance cover very large ranges in terms of ground resolution, spectral resolution and time resolution. Some applications need to be implemented using distributed satellite systems like constellations or formations. Distributed satellite systems provide a number of advantages like:

  • Increase of time resolution and daily coverage depending on the number of satellites within a constellation

  • Substitution of complex satellite systems (or complementing them) using different (distributed) instruments on small satellite platforms making use of data fusion

  • Easy replacement of a satellite within a constellation or formation due to the relative low costs of a single satellite

  • Soft degradation of the system performance caused by the malfunction of one satellite

Thinking about distributed space systems, we may distinguish between different systems based on the distance between the satellites and the requirements concerning the control of their distance [1]. With this approach we get the following categories:

  • Constellations

  • Formations

  • Swarms

  • Inspection and docking systems

Figure 1 shows local systems with separations between the spacecrafts of a few meters, regional separations of typically a few ten meters to several hundreds of kilometres, and global systems with separations of more than thousand kilometres. Inspection and docking involves two objects in space in close vicinity. This is obvious for an inspector micro- or nano satellite orbiting for instance the International Space Station (ISS). Another example is ESA’s Automated Transfer Vehicle (ATV) docking the ISS. This poses very high demands on the control accuracy. At a given separation of for instance ten meters, the control accuracy should be at least a factor of ten better resulting in one meter. The control is based on sensors which again need to provide a tenfold better accuracy, i.e. 10 cm in this case.



Figure 1 Requirements for distributed space systems, qualitatively

Formation flying of satellites is typically associated with a small number of spacecraft flying in a concerted way at regional intersatellite separations. The mission objectives determine the requirements on the control accuracy. A Science mission using interferometry may have high control demands. A formation of two satellites with different instruments can have relaxed control requirements. Chapter 3 provides examples for different applications.

To achieve global coverage on Earth with high time resolution requires a satellite constellation. Chapter 2 provides examples for different applications.

While docking, formation flying and constellations are well established implementations of distributed space systems, swarms of spacecraft consisting of several ten or thousands of satellites have not been deployed yet. Swarms of satellites can characterize for instance the local, regional or global Earth environment making in situ measurements of the atmosphere or the radiation conditions.

This paper concentrates on constellations and formations. The concept of formation flying of satellites is frequently confused with that of satellite constellations. In the following chapters we will distinguish between both concepts using the definitions of NASA GSFC:



  • A constellation is composed of two or more spacecraft in similar orbits with no active control by either to maintain a relative position.

  • Formation flying involves the use of an active control scheme to maintain a relative position.

For a better understanding, the terms relative position and active control scheme are to be understood as:

  • Active control scheme: the scheme can be executed in orbit or on-board, but it can also be implemented on the ground. It can be executed in real-time but it can also be implemented post-time.

  • Relative position: The measurement of relative position can be done directly or indirectly. Direct measurement is usually applied to precisely tight formation flight while the indirect one is often used in a coarsely loose formation flight.

In addition to the facts mentioned above, we can state the comparison between constellation and formation flights of satellites as follows:

  • For constellations, the relative position and velocity between the spacecrafts are not controlled, except to orbital station keeping points predefined at mission designs. But for the formation flights, the relative position and velocity between the spacecrafts are controlled and possibly at certain parts also the relative attitudes.

  • For constellations, there is no plane defined for the inter spacecraft positions other than the orbital planes. For the formations flight, a plane is defined for the inter spacecraft positions with an arbitrary orientation in space and with respect to a possible local orbital frame.

2.0 Constellations

2.1 Disaster Monitoring Constellation DMC-1

Small satellites provide a unique opportunity for affordable constellations. In this respect, small satellites can do things that are not practical with large satellites.



Figure 2: (a) One day coverage provided by four spcecraft at 600 km with 30° off-pointing capability compared with (b) single satellite capability [2]

DMC may serve as the example for a constellation of five small satellites (DMC 2008). DMC has a GSD of 32 m and a swath width of 600 km (Landsat: GSD = 30 m, Swath width = 185 km). It provides a daily coverage of the Earth (figure 2). The five satellites

  • AlSat-1

  • BILSAT-1

  • NigeriaSat-1

  • UK-DMC-1

  • Beijing-1

from five countries have been launched with three COSMOS launchers into the same orbit. There are more constellations in sight, for instance DMC-2, a follow up of DMC-1 with improved performances based on new technologies, and RapidEye which was launched in August 2008 (see figure 3). The facts discussed in chapters 2.1 – 2.3 may lead to the conclusion: small satellites have the potential to change the economics of space and to increase the tempo of space exploitation.

F
igure 3: SSTL high resolution mission products [2]


    1. RapidEye

RapidEye is a commercial multispectral Earth observation mission of RapidEye AG of Brandenburg, Germany, that includes a constellation of five minisatellites. The mission will provide high-resolution multispectral imagery along with an operational GIS (Geographic Information System) service on a commercial basis. The objectives are to provide a range of Earth-observation products and services to a global user community. The RapidEye sensor images five optical bands in the 400-850nm range and provides 6.5m pixel size at nadir. It provides products for the following applications:

  • Agricultural producers (farmers): Crop monitoring and mapping, yield prediction;

  • Agricultural insurance: Provision of regularly updated field maps to help insurers assess insurance contracts and claims by providing quick and reliable information about damaged areas;

  • Cartography - satellite based maps (scale 1:25,000), ortho photos, DEM (Digital Elevation Model) generation

  • Other markets - disaster assessment, 3-D visualization

  • Service spectrum at completion mission: Guaranteed daily revisit, global coverage, product delivery to the customer within 24 hours, possibility of dedicated programming, capability of direct transmission and imagery transfer within hours, global digital database of "orthomaps" of 1:25,000 scale and DEMs of 20 m x 20 m resolution. The service permits also the merging of multi-temporal imagery with information from other sources.



Figure 4: The RapidEye constellation consists of five satellites

The five RapidEye earth observation satellites have been launched on a single Russian Dnepr rocket from the Baikonur Cosmodrome in Kazakhstan in August of 2008. They are deployed in orbits at an altitude of 630 km. The satellites are placed equally spaced in a single sun-synchronous orbit to ensure consistent imaging conditions and a short revisit time. The satellites follow each other in their orbital plane at about 19 minute intervals (figure 4). The constellation approach in a single orbital plane permits a cumulative swath to be built up (the spacecraft view adjacent regions of the ground, with image capture times separated by only a few minutes). A revisit time of one day can be obtained anywhere in the world (±70º latitude) with body pointing techniques. The average coverage repeat period over mid-attitude regions (e.g., Europe and North America) is 5.5 days at nadir. The RapidEye system can access any area on Earth within one day and cover the entire agricultural areas of North America and Europe within five days. The swath width is 80 km and the maximum scene length per orb it is 1500 km.

Table 1 gives some more information of the RapidEye project (see also [3]).

Table 1: RapidEye, some project data

Number of satellites

5

Launch

August 2008

Launching site

Baikonur, Kazakhstan

Rocket type

DNEPR-1

Orbit

sun-synchronous, 630 km

Mission duration

7 years

Satellite mass

150 kg

Satellite size

1 m x 1 m x 1 m

Camera

5 spectral bands (VIS and NIR)

Geometric resolution

6.5 m/Pixel

Scene length/orbit

1500 km

Satellite command center

RapidEye AG, Brandenburg

Data receiving station

Svalbard, Norway



2.3 SAR-Lupe

SAR-Lupe is a satellite constellation for synthetic aperture reconnaissance. It consists of up to five spacecraft in three different polar orbit planes, at an altitude of 500 km (figure 5). The spacecraft feature a mass of 770 kg and an average power of 250 W. For an increased integration time of the radar image and an enhanced resolution, the three-axis stabilised satellite performs a slewing manoeuvre during imaging.



Figure 5: SAR-Lupe Constellation

The SAR-Lupe satellite concept is characterised by the following features:

  • Large conventional parabolic SAR antenna, which is rigidly mounted to the satellite structure to achieve a simple mechanical design

  • SAR antenna dual used for imaging and data transmission

  • Attitude Control System, which performs high accuracy manoeuvres of the entire satellite for pointing during imaging

  • Integrated and modular design for satellite bus and SAR payload sensor

  • Few and simple mechanisms yield reliability and cost efficiency

  • All satellite control commands are produced in the ground station, which simplifies the OBDH architecture

  • Command data relaying through Inter-Satellite Link yields reduced system response time.

The ACS features 3 operational modes and an accuracy of σ ≤ 0.05°. Its actuators comprise reaction wheels and magnetic coils; its sensor suite consists of a star tracker assembly with σ ≤ 0.01° accuracy, a magnetometer and sets of sun sensors and gyroscopes. The OCS compensates the altitude decrease due to the residual air drag and relies on a mono-propellant hydrazine system. Telemetry is done within the X-band, and tele-control and housekeeping data are exchanged via S-band radio waves. The power subsystem features

  • 2.4 m2 solar panel of GaAs solar cells

  • 550 W @ EOL and perpendicular to radiation of light

  • 2 x 50 Ah battery capacity NiH2 batteries

  • 2900 W / 100 A peak power / current

From this concept OHB’s standard medium-sized platform with agile and precise pointing capabilities is derived. [4]



Figure 6: SAR-Lupe Satellite

3.0 FORMATIONS

The recent trend of miniaturization and insertion of novel high performance, including micro and nano, technologies is about to enable space missions which rely on co-operating satellites. Not only can a system of coordinated platforms replace a larger spacecraft, but it also has the capability of realizing performance and missions unachievable using a monolithic approach.

This chapter is intended to highlight current activities, plans, and visions for future Earth observation missions building on distributed systems which synergistically use payloads on board different satellites rather than multiplying payloads for coverage enhancement (constellations). Such an approach poses challenges from the points of view of system design (centralized or de-centralized approaches), bus design (performance, inter-link, autonomy, etc), payload design (synchronization, data processing, etc), and formation flying guidance, navigation, and control.

3.1 NASA’s A-Train

The A-Train satellite formation consists of six satellites flying in close proximity. These satellites will for the first time ever combine a full suite of instruments for observing clauds and aerosols from passive radiometers to active lidar and radar sounders. The first one, Aqua, was launched in 2002. The formation was finalized in 2008 after AURA, CloudSat, CALIPSO, PRABOL and OCO have been added. The satellites will cross the equator within a few minutes of one another at around 1:30 p.m. local time. By combining the different sets of observations by means of data fusion methods, scientists will be able to gain a better understanding of important parameters related to climate change.

The A-Train formation will allow for coordinated measurements.  Data from several different satellites can be used together to obtain comprehensive information about atmospheric components or processes. Combining the information from several sources gives a more complete answer to many questions than would be possible from any satellite taken by itself. [5]



Figure 7: A-Train with equator crossing times



Figure 8: A-Train: inter-satellite time distance (Image Credit NASA [6])

Figure 7 depicts the satellites that make up the Afternoon Constellation (not to scale) – “The A-Train”. Listed under each satellite’s name is its equator crossing time. Note that though Aura crosses the equator eight minutes behind Aqua, in terms of local time, because it is along a different orbit track, it actually lags Aqua by fifteen minutes. Note also that CALIPSO trails CloudSat by only 15 seconds to allow for synergy between Aqua, CloudSat, and CALIPSO (see figure 8 showing the relative time distances between the satellites).





The A-Train Satellites



Aqua is designed to acquire precise atmospheric and oceanic measurements to provide a greater understanding of their role in the Earth's climate and its variations. The satellite's instruments provide regional to global land cover, land cover change, and atmospheric constituents.











Aura's mission is designed to observe the atmosphere to answer the following three high-priority environmental questions: Is the Earth's ozone layer recovering? Is air quality getting worse? How is the Earth's climate changing? Aura's new objective over previous atmospheric research missions is also to probe the Earth's troposphere.









CloudSAT, a cooperative mission with Canada, will use advanced radar to "slice" through clouds to see their vertical structure, providing a completely new observational capability from space. CloudSAT will look at the structure, composition, and effects of clouds and will be one of the first satellites to study clouds on a global basis.










CALIPSO will provide key measurements of aerosol and cloud properties needed to improve climate predictions. CALIPSO will fly a 3-channel LIDAR with a suite of passive instruments in formation with Aqua to obtain coincident observations of radiative fluxes and atmospheric conditions. CloudSAT will also fly in formation with CALIPSO to provide a comprehensive characterization of the structure and composition of clouds and their effects on climate under all weather conditions.









PARASOL (Polarization and Anisotropy of Réflectances for Atmospheric Sciences coupled with Observations from a Lidar) is a microsatellite project.of CNES, France. Its main purpose is to improve the characterization of the clouds and aerosols microphysical and radiative properties, needed to understand and model the radiative impact of clouds and aerosols.










The OCO provides space-based observations of atmospheric carbon dioxide (CO2), the principal anthropogenic driver of climate change. This mission uses mature technologies to address NASA's carbon cycle measurement requirement. OCO generates the knowledge needed to improve projections of future atmospheric CO2.

The A-Train formation will help answer these important questions.

  • What are the aerosol types and how do observations match global emission and transport models?

  • How do aerosols contribute to the Earth Radiation Budget (ERB)/climate forcing?

  • How does cloud layering affect the Earth Radiation Budget?

  • What is the vertical distribution of cloud water/ice in cloud systems?

  • What is the role of Polar Stratospheric Clouds in ozone loss and denitrification of the Arctic vortex?

It will be tough to get these satellites to work harmoniously together, because of the great variety of instruments and resolutions. And the formation will need to be precisely aligned, which means a coordinated manoeuvring of the different satellites. [5]

    1. TOPOLEV – a distributed RADAR mission of ESA

Mission Objectives

The Topographic Levelling Mission (TOPOLEV) is aimed at mapping surface topography and its temporal change to very high accuracy (1 m in elevation). There is high need of precise elevation data for a wide range of scientific and operational applications. The focus of the mission is on low and moderate relief terrain, including water surfaces.

Number of Spacecraft

The proposed minimum constellation consists of two satellites in close formation flight, to obtain bistatic radar measurements. On the Master satellite an active SAR operates; on the Slave a SAR receiver. Optionally, more than one Slave can be added, as proposed for classical Cartwheel configurations. This increases the percentage of good baselines over an orbit, but would only be useful if the operation time of the Master SAR is increased, requiring increased resources for power, data storage and downlink.

Mission Analysis and Orbit

The mission applies single-pass across-track interferometry for accurate measurement of topographic elevation, in a bistatic InSAR configuration with two satellites in close formation (see figure 9). The SAR operates in stripmap mode, in Transmit and Receive mode (T/R) on the Master satellite, and in Receive-only mode on the Slave satellite. For meeting the requirements in elevation accuracy and horizontal spacing, phase stability is of key importance. In addition, comparatively high chirp bandwidth is needed. Because of the higher surface scattering contribution, VV is the preferred polarisation.

To achieve a height accuracy of  1.0 m, the distance between the two antennas needs to be known at 1 2 mm accuracy (depending on the baseline length and attitude), if no ground control points are available. In addition, the absolute satellite positions need to be known at sub-metre accuracy. Two orbit phases (sun-synchronous, dawn/dusk orbits) are considered:

  • A Global Levelling phase (GL-Phase): 91 day repeat orbit (1394 tracks) enabling access to global land areas at seasonal intervals. This orbit corresponds to the ICESat orbit, thus enabling validation and improvement of the absolute height accuracy of the TOPOLEV DEM product over stable surfaces by means of ICESat data. This mode is also very interesting for mapping seasonal or annual changes of surface height, e.g. for glaciers, river plains, wetlands, inter-tidal zones, etc.

  • A Hydrology and geophysical Process studies phase (HP-Phase): 15 day repeat orbit to study specific geophysical and hydrological phenomena with higher temporal accuracy (these are mainly processes related to water and ice). This orbit provides only partial coverage of the global land surfaces. A sampling strategy needs to be developed to obtain relevant statistics for various climate zones and regions.



Figure 9: TOPOLEV mission configuration

Table 2: Proposed orbit specifications

Orbit Parameters

Orbit Phase GL

Orbit Phase HP

Altitude

600 km

628 km

Inclination (deg.)

97.8 sun-synchronous

97.9 sun-synchronous

Repeat Cycle

91 days

15 days

Coverage

Global access

Subset of land surfaces





Figure 10: Sketch of TOPOLEV Cartwheel orbit configuration with two satellites

The proposed formation flight configuration is the “classical” interferometric Cartwheel, as shown in Figure 10 for a formation of two satellites. The orbits have small eccentricities deviating slightly from the circular reference orbit. The relative position of the satellites is determined by the movement on an elliptically shaped wheel along the reference orbit. The interferometric baselines vary over the orbit. With two satellites, as proposed as baseline configuration for TOPOLEV, the baseline can be optimized for latitude bands, but not over the whole orbit. This is easily compatible with the objectives and duty cycle of the mission.

General

For high absolute geolocation accuracy of the InSAR product the absolute position of each satellite should be known to an accuracy of  1m. For interferometric retrieval of very precise topographic data, the relative position between the two satellites needs to be known to 1 2 mm accuracy. Recent studies for GRACE indicate that inter-satellite distance can be measured at an accuracy of 1 mm using carrier-phase differential GPS (and in future also the Galileo system). Such an approach is used for TanDEM-X orbit and interferometric baseline determination. In addition, the relative phase between the two independent oscillators on the Master and Slave satellite needs to be known for the interferometric retrieval, for which an inter-satellite link may be required. [7]

3.3 SWARM

The ESA Swarm mission under the Living Planet Programme consists of three identical spacecraft orbiting in near polar orbits with altitudes varying between 400 km to 550 km (see figures 11 and 12). This constellation is to map the magnetic field of the Earth with unprecedented spatial and temporal accuracy. For this purpose, each spacecraft will be equipped with a vector field magnetometer and three star trackers co-mounted in an optical bench, which will ensure 100 % data coverage over the orbit with arcsecond accuracy. This accuracy of the magnetometry package is essential for fulfilling the mission objectives.



Figure 11: Swarm constellation of three satellites

The Swarm mission was selected as the 5th mission in ESA's Earth Explorer Programme in 2004. The mission will provide the best ever survey of the geomagnetic field and its temporal evolution that will lead to new insights into the Earth system by improving our understanding of the Earth's interior and its effect on Geospace, the vast region around the Earth where electrodynamic processes are influenced by the Earth's magnetic field. Scheduled for launch in 2010, the mission will comprise a constellation of three satellites, with two spacecraft flying side-by- side at lower altitude (450 km initial altitude), thereby measuring the East-West gradient of the magnetic field, and the third one flying at higher altitude (530 km). High-precision and high-resolution measurements of the strength, direction and variation of the magnetic field, complemented by precise navigation, accelerometer and electric field measurements, will provide the necessary observations that are required to separate and model the various sources of the geomagnetic field. This results in a unique "view" inside the Earth from space to study the composition and processes of its interior. It also allows analysing the Sun's influence within the Earth system. In addition practical applications in many different areas, such as space weather, radiation hazards, navigation and resource management, will benefit from the Swarm concept. [8]



Figure 12: Swarm satellite will be about 8 m long and have a weight of 300-400 kg

The research objectives of Swarm mission are:

  • Related to the Earth’s Interior:

  • Map the core flow

  • Determine core dynamics

  • Investigate jerks: their time-space structure and recurrence

  • Understand core-mantle coupling and its implication for Earth rotation

  • Perform 3D imaging of mantle conductivity

  • Determine remanent and induced magnetisation of the lithosphere

  • Related to the Earth’s environment:

  • Determine the position and development of the radiation belts and their near-Earth effects

  • Investigate the time-space structure of the magnetospheric and ionospheric current systems on all time scales

  • Monitor the solar wind energy input into the upper atmosphere and sense its effect on the thermospheric density

  • Sound the electron density of the ionosphere/plasmasphere and relate it to magnetic activity

The scientific payload consists of the following instruments:

  • Vector Field Magnetometer (VFM), which is co-mounted together with a stellar compass for determining the components of the magnetic field very accurately

  • Absolute Scalar Magnetometer (ASM), which is used primarily for calibrating absolutely the vector field magnetometer.

  • Electrical Field Instrument (EFI)

  • Accelerometer (ACC)

3.4 GRACE

The GRACE mission (figure 13) was selected as the second mission under the NASA Earth System Science Pathfinder (ESSP) Program in May 1997. Launched in March of 2002, the GRACE mission will accurately mapped variations in the Earth's gravity field over its 5-year lifetime. The GRACE mission will have two identical spacecrafts flying about 220 km apart in a polar orbit 500 km above the Earth. [9]



Figure 13: GRACE – Orbiting twins [10]

Mission Synopsis

The gravity field of the Earth is variable in both space and time, and is an integral constraint on the mean and time variable mass distribution in the Earth. The science data from GRACE mission are to estimate global models for the mean and time variable Earth gravity field approximately every 30 days for the 5 year lifetime of the mission. The science data from GRACE mission consists of the inter-satellite range change measurements, and the accelerometer, GPS and attitude measurements from each satellite (figure 14).

The gravity variations that GRACE studied: changes due to surface and deep currents in the ocean; runoff and ground water storage on land masses; exchanges between ice sheets or glaciers and the oceans; and variations of mass within the Earth. Another goal of the mission is to create a better profile of the Earth's atmosphere. The results from GRACE will bring a huge contribution to the goals of NASA's Earth Science Enterprise, Earth Observation System (EOS) and global climate change studies.

GRACE is a joint partnership between the National Aeronautics and Space Administration (NASA) in the United States and Deutsche Forschungsanstalt für Luft und Raumfahrt (DLR) in Germany.



Figure 14: The GRACE flight configuration [9]



Figure 15: GRACE – Mission concept

4.0 References

  1. Gill, E., Together in Space, Potentials and Challenges of Distributed Space Systems. Inaugural speech, TU Delft, Faculty of Aerospace Engineering, September 17, 2008

  2. http://www.dlr.de/iaa.symp/Portaldata/49/Resources/dokumente/archiv5/0301_daSilvaCuriel.pdf

  3. http://www.dlr.de/rd/desktopdefault.aspx/tabid-2440/3586_read-5336/

  4. http://www.dlr.de/iaa.symp/Portaldata/49/Resources/dokumente/archiv5/0304_Koebel.pdf

  5. http://csc.gallaudet.edu/soarhigh/A-TrainExplain.html

  6. http://www.nasa.gov/images/content/112931main_a-train.jpg

  7. Sephton, T., Rott, H., Wishart, A., Grafmueller, B., Hall, D., Pasternak, F., Strauch, K., Formation Flying for EO small satellite missions. Proc. ‘The 4S Symposium - Small Satellites Systems and Services’, 26–30 May 2008, Rhodes, Greece, 10 p.

  8. Merayo, J. M. G., Jørgensen, J. L., Friis-Christensen, E., Brauer, P., Primdahl, F., Jørgensen, P. S., Allin, T. H., Denver, T., The Swarm Magnetometry Package. In: Sandau, Rainer, Röser, Hans-Peter, Valenzuela, Arnoldo (Eds.): Digest of the 6th International Symposium of the International Academy of Astronautics, Berlin, April 23 - 26, 2007, 103-106

  9. http://www.csr.utexas.edu/grace/overview.html

  10. http://www-app2.gfz-potsdam.de/pb1/op/grace/index_GRACE.html



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