Biomass (Biomass monitoring mission for Carbon Assessment)
In May 2013, ESA's Earth Observation Program Board selected the Biomass mission as its 7th Earth Explorer mission following the review of three candidate concepts. Earth Explorers are research missions dedicated to specific aspects of our Earth environment whilst demonstrating new technology in space. Earth Explorer missions focus on the atmosphere, biosphere, hydrosphere, cryosphere and the Earth's interior with the overall emphasis on learning more about the interactions between these components and the impact that human activity is having on natural Earth processes. The Biomass mission concept is set to become the next in a series of satellites developed to further our understanding of Earth. The mission aims to take measurements of forest biomass to assess terrestrial carbon stocks and fluxes for a better understanding of the carbon cycle. 1) 2) 3) 4)
Background: The primary environmental science challenge in the early 21st century is to improve our understanding of how global changes are affecting the Earth System, and the feedbacks within this system, in order that human societies can assess their likely impacts and adopt ways to mitigate and adapt to them. Deeply embedded in the functioning of the Earth system is the carbon cycle, which consists of intermeshed processes by which carbon is exchanged between the atmosphere, land and ocean (Figure 1). Quantifying this global-scale cycle is fundamental to understanding many of the dramatic changes taking place on Earth because of its close connection with both fossil fuel combustion and land use change. These are the two most significant drivers of global change, leading to increases in atmospheric CO2 and the associated global warming (IPCC, 2007). 5)
Terrestrial processes play a crucial role in the carbon cycle through the carbon uptake and respiration associated with plant growth, emissions due to the disturbance of natural processes (e.g. wildfires) and anthropogenic land use change. There is strong evidence that over the last 50 years the terrestrial biosphere has acted as a net carbon sink, removing from the atmosphere approximately one-third of the CO2 emitted in the process of fossil fuel combustion (Canadell et al., 2007).6) However, the status, dynamics and evolution of the terrestrial biosphere are the least well understood and most uncertain elements of the carbon cycle.
Consequently, the UN Framework Convention on Climate Change (UNFCCC) has identified biomass as an Essential Climate Variable that is needed to reduce uncertainties in our knowledge of the climate system (GCOS, 2004). 7) While global observation programs for most terrestrial ECVs (Essential Climate Variables) are advanced or evolving, there is currently no such effort for biomass (Houghton et al., 2009). 8) In addition, the sequestration of carbon in forest biomass is the only mechanism for mitigating climate change recognized under the Kyoto Protocol, other than reduced emissions.
Scientific Objectives: The Biomass mission will address a fundamental gap in our understanding of the land component of the Earth system, which is the status and the dynamics of Earth's forests, as represented by the distribution of forest biomass and its changes. With accurate, frequent and global information on these forest properties at a spatial scale of 200 m, it will be possible to address a range of critical issues with far-reaching scientific and societal consequences. In particular, the Biomass mission will help to:
- reduce the large uncertainties in the carbon flux due to changes in land use
- provide scientific support for international treaties, agreements and programs such as the UN's REDD (Reducing Emissions from Deforestation and Forest Degradation in Developing Countries) program
- improve understanding and predictions of landscape-scale carbon dynamics
- provide observations to initialize and test the land element of Earth system models
- provide key information for forest resources management and ecosystem services.
The Biomass mission will explore Earth's surface for the first time at the P-band wavelength, making observations that could have a wide range of as yet unforeseen applications, such as for mapping subsurface geological features in deserts in support of palaeohydrological studies and in ice sheets, and the surface topography of areas covered by dense vegetation.
Figure 1: The global carbon cycle for 2000–09 showing estimates of the net amount of carbon (Gt) that cycles between the atmosphere, ocean and land every year (green boxes), and the amounts of carbon stored in the atmosphere and in the land (data from the Global Carbon Project). The amount of carbon stored in forest biomass is the least understood component of this cycle (image credit: ESA/EOGB)
Biomass addresses one of the most fundamental questions in our understanding of the land component in the Earth system, namely the status and the dynamics of forests, as represented by the distribution of biomass and how it is changing. Gaining accurate and frequent information on forest properties at scales that allow changes to be observed will mean that the scientific community is equipped to address a range of critical issues with far-reaching benefits for science and society. Moreover, Biomass will greatly improve our knowledge of the size and distribution of the terrestrial carbon pool. And provide much-improved estimates of terrestrial carbon fluxes. In addition, the mission responds to the pressing need for biomass observations in support of global treaties such as the UN REDD+(Reducing Emissions from Deforestation and forest Degradation) initiative – an international effort to reduce carbon emissions from deforestation and land degradation in developing countries. These mission objectives respond directly to the specific scientific challenges in ESA's Living Planet Program (Ref. 18). 9) 10) 11) 12) 13) 14)
In addition, the measurements made by Biomass offer the opportunity to map the elevation of Earth's terrain under dense vegetation, yielding information on subsurface geology and allowing the estimation of glacier and ice-sheet velocities, critical to our understanding of ice-sheet mass loss in a warming Earth.
Biomass also has the potential to evolve into an operational system, providing long-term monitoring of forests – one of Earth's most important natural resources.
Before the launch of the BIOMASS P-band SAR satellite, foreseen in 2021, only spaceborne L-band data from ALOS PALSAR until 2011 and ALOS-2 (launch on May 24, 2014) are available for biomass studies. 15)
The primary scientific objectives of the Biomass mission are to determine the distribution of above-ground biomass in the world forests and to measure annual changes in this stock over the period of the mission to greatly enhance our understanding of the land carbon cycle. To achieve these objectives, the Biomass sensor will consist of a P-band (435 MHz) Synthetic Aperture Radar (SAR) in side-looking geometry with full polarimetric and interferometric capabilities. The main architectural elements of the Biomass mission are shown in Figure 2. 22) 23) 24) 25) 26) 27) 28)
Biomass will provide global maps of forest biomass stocks at a spatial resolution in the order of 4 ha, once a year over the life of the five-year mission. These maps will greatly improve on existing forest inventories and give vastly improved information for managing Earth's forest resources.
Biomass will also provide maps of biomass change, which can be linked to disturbance, degradation, land-use change, forest growth. In addition, the full resolution of the instrument of around 0.25 ha will be used to detect deforestation; linking this to the coarser resolution maps of biomass will allow associated carbon losses to be estimated at scales commensurate with the processes of land-use change. The Biomass observation requirements have been derived from these high level objectives (Table 2).
Table 2: Biomass mission observation requirements
Biomass will be based on a P-band polarimetric SAR mission with controlled inter-orbit distances (baselines) between successive revisits to the same site. At each acquisition, the radar will measure the scattering matrix, from which the backscattering coefficients (equivalent to radar intensity) will be derived in each of the different linear polarization combinations, i.e. HH, VV, HV & VH (where H and V stand for horizontal and vertical transmitted and received), and the inter-channel complex correlation. For interferometric image pairs, the system will provide the complex interferometric correlation (coherence) between the images at each linear polarization. PolInSAR (Polarimetric interferometric SAR) coherence and PolSAR (Polarimetric SAR) backscatter observations provide independent, complementary information that can be combined to give robust, consistent and accurate retrieval of biomass. In addition tomography techniques, using a multi-baseline polarimetric SAR acquisition, will be used to complete the knowledge of the vertical structure of the forest (Figure 3). By exploiting these capabilities, Biomass will build up a unique archive of information about the world's forests and their dynamics (Ref. 24).
Figure 3: Biomass observation principle based on three complementary techniques (image credit: ESA)
1) Horizontal mapping: In polarimetric mode, after calibration and correction for ionospheric effects, each BIOMASS pixel measures the scattering matrix, from which the backscattering intensity will be derived in each of the linear polarization combinations, i.e. HH, VV, HV & VH, where H and V stand for horizontal and vertical transmitted and received signals. For a forest canopy, the P-band radar waves penetrate deep into the canopy, and their interaction with the structure of the forest (through volume scattering, surface scattering or double bounce scattering mechanisms) differs between polarizations. P-band SAR is particularly sensitive to large forest constituents, such as the trunk and large branches, where most of the biomass resides, and polarizations can be chosen to minimize the contribution from the ground and effects arising from topographic and soil moisture variation. Hence P-band polarimetric measurements can be used to map AGB (Above Ground Biomass), as demonstrated from airborne data for temperate & boreal forests and tropical forest.
2) Height mapping: Using repeat revisits to the same location with controlled inter-track distances, the BIOMASS SAR system will measure the polarimetric complex interferometric correlation between image pairs, from which it is possible to estimate the height of scattering in the forest canopy as a function of polarization (PolInSAR).This allows canopy height to be derived, assuming a model for the vertical structure of scatterers in the forest. Numerous airborne experiments over temperate, boreal and tropical test sites have shown that forest height can be mapped with accuracy comparable to that of airborne lidar. A crucial factor here is that at the long wavelength used by BIOMASS, temporal coherence is preserved over much longer timescales than, for example, at L-band. This is because BIOMASS is sensitive to larger structures in the canopy, which more are more stable; in addition, the longer wavelength makes the phase less sensitive to small motions of the dominant scatterers. BIOMASS will be first spaceborne radar sensor providing large scale height maps using PolInSAR, although application of the technique from space has been demonstrated using Shuttle Imaging Radar (SIR-C) L-band data.
3) 3-D mapping: The P-band frequency used by BIOMASS is low enough to ensure penetration through the entire canopy, even in dense tropical forests. As a consequence, resolution of the vertical structure of the forest will be possible using tomographic methods from the multi-baseline acquisitions to be made by BIOMASS. This is the concept of SAR tomography, which has been implemented with airborne systems and will be available for the first time using space with the BIOMASS mission. When the vertical resolution is less than half the forest height, it is possible to split the vertical distribution of the backscatter intensity into a number of layers, without assuming any prior knowledge about the forest vertical structure. As expected, the bottom layer contains mainly ground scattering and the backscatter from this layer is very weakly correlated with AGB. However, in two different tropical forest sites in French Guiana, the backscatter from a layer at about 30 m above the ground was found to be strongly correlated with AGB, up to biomass densities of 450-500 t/ha, allowing the production of wide area biomass maps. Findings from airborne data are expected to carry across to BIOMASS, despite the coarser spatial and vertical resolution available in BIOMASS tomography.
The technical description of the Biomass mission is derived from the preparatory activities and shows how candidate implementation concepts can respond to the scientific requirements. The system description is mainly based on the results of the work performed during two parallel Phase A system studies by two industrial consortia, led by Astrium DS Ltd. and Thales Alenia Space, Italy. Consequently, two implementation concepts, marked A and B, are described in what follows.
The space segment comprises a single spacecraft carrying a P-band SAR, operating in stripmap mode in a near-polar, sun-synchronous quasi-circular frozen orbit at an altitude of 634–666 km, depending on the different mission phases. The baseline Vega launcher will inject the satellite into its target orbit. The orbit is designed to enable repeat-pass interferometric acquisitions throughout the mission's life. The baseline is different for the interferometric (in the order of 2 km at equator) and the tomographic phases (below 1 km at equator).
Acquisitions are made at dawn/dusk, i.e. 06:00/18:00 local time (at the equator), to minimize the adverse influence of the ionosphere on the radar signal. The Biomass mission will last five years and will comprise a tomographic phase with a duration of 1 year followed by the interferometric phase.
The strategy for meeting the baseline requirement is based on the selection of an orbit with a ‘controlled drift', flying the satellite in an orbit where the altitude is slightly higher or lower than that of the exact repeating orbit. The coverage build-up for the observation concept is shown in Figure 2 and combined roll of the satellite to acquire 3 consecutive interferometric images, with controlled drift of the orbit.
Figure 4: Coverage strategy (image credit: ESA)
The Biomass space segment comprises a single LEO (Low Earth Orbit) satellite platform carrying the SAR instrument. The SAR antenna is based on a large deployable reflector (12 m circular projected aperture) with an offset feed array and a single-beam. The satellite configuration is strongly constrained by the accommodation of the very large reflector antenna inside the Vega launcher. This large antenna must be folded for launch and deployed in orbit to form a stable aperture throughout the mission's life.
The overall configuration of Concept A is shown in Figure 3 and is compatible with COTS (Commercial-Off-The-Shelf) reflectors from the US manufacturers Harris Corporation (HC) and Northrop Grumman (NG). For Concept A, the reflector is illuminated by a 3 x 2 array of cavity-backed circular microstrip radiators, which is mounted onto to the –Y wall of the satellite at the lower end (not visible in the figures).
Figure 5: Artist's rendition of the BIOMASS satellite configurations (image credit: Airbus DS)
Harris Corporation has been selected by Airbus Defence and Space UK, the builder of the Biomass satellite, to provide a 12-meter deployable reflector and precision boom assembly for this carbon-monitoring craft (Figure 5). The Harris deployable reflector is a major component of the SAR antenna and enables the Biomass satellite to obtain a high level of map accuracy not attainable by ground measurement techniques alone. With more than 80 reflectors in orbit, Harris is the leading supplier of large reflector apertures and deployable mesh reflector-feed antenna systems. 30)
• November 9, 2017: Microwave radio signals – able to pass freely through Earth's atmosphere as well as empty space – play a role in just about everything, including mission telemetry and telecommands, satellite services and broadcasting, navigation and timing signals and radar, along with other forms of active remote sensing. 31)
- ESA's Biomass mission is designed to track status and dynamics of tropical forests using P-band radar. Because this frequency is so low, a vacuum tube amplifier would be too heavy and bulky for the type of satellite we envisage. Instead Biomass has baselined solid state amplifiers using novel high-power semiconductor gallium nitride, harnessed for space through the ESA-led ‘GaN Reliability Enhancement and Technology Transfer Initiative' (GREAT2) consortium.
- So, while the GaN technology has already been qualified, what we need to do is put that into a hermetically sealed package that can be flown in space – specially tailored to avoid any risk of electrical discharge or similar operating risks – and then put through rigorous lifetime testing to ensure reliability, arriving at a guaranteed mean time to failure.
- After the transistor is packaged at solid-state power amplifier (SSPA) level, we're concerned about things like overall electrical performance and thermal dissipation, checking that waste heat is carried away without affecting component reliability – along with all the usual qualification steps of vibration, shock and thermal vacuum testing. The mission needs six SSPAs in total, each of these with three packaged GaN transistors inside. Nevertheless we are qualifying a larger amount of packaged transistors to have a safe number of parts.
- Today that 6 x 6 mm prototype chip is now fabricated – also harnessing GaN – putting together a high-power amplifier, low-noise amplifier, a transmit-receive switch and a calibration coupler: what would normally involve an individual chip or circuit for each of these functions. The concept is applicable to any frequency band. In this case, we demonstrated the concept in C-band (for Sentinel-1) and it yielded three times more output power than the current amplifier on board these satellites, with savings of about 40% in terms of size.
- There were all kinds of challenges in integrating all these functions on such a small chip. For instance, having the high-power amplifier beside the low-noise amplifier – the heat from the former could for instance compromise the performance of the latter, but we were able to come up with system-level solutions, such as switching off elements when not in use.
Figure 6: Natanael Ayllon, ESA payload engineer, showing a prototype transmit/receive module on a single chip (image credit: ESA - SJM Photography)
• October 17, 2017: Thales Alenia Space has signed a contract with Airbus Defence and Space GmbH to develop the feed array system for the antenna on the European Space Agency's Biomass spacecraft. This equipment is essential to guarantee the full satellite performance. 32)
• October 2016: The SRR (System Requirements Review) was conducted in the summer of 2016. A successful SRR is an important step in the Project's life cycle because it begins the procurement of the individual satellite components and the build-up of the full industrial consortium. Ground-based and airborne campaigns to collect data to support the algorithm development and validation are being conducted, underpinned by a study to tackle the end-to-end performance calibration of a P-band synthetic aperture radar system in the presence of the ionosphere. 33)
• May 3, 2016: ESA and Airbus Defence and Space UK signed a €229 million contract on 29 April to build the next Earth Explorer: the Biomass satellite, due to begin its mission in 2021. The satellite will provide global maps of how much carbon is stored in the world's forests and how this stock is changing over time, mainly through the absorption of carbon dioxide, which is released from burning fossil fuels. Biomass will also provide essential support to UN treaties on the reduction of emissions from deforestation and forest degradation. 34)
- The spacecraft will carry the first spaceborne P-band synthetic aperture radar to deliver exceptionally accurate maps of tropical, temperate and boreal forest biomass that are not obtainable by ground measurement techniques. The mission will collect frequent information on global forests to determine the distribution of above-ground biomass in these forests and measure annual changes. The five year mission will witness at least eight growth cycles in the worlds' forests.
- By using a P-band SAR(Synthetic Aperture Radar), the mission will use all-weather imaging from space to estimate forest biomass. Biomass will also be able to measure paleo aquifers in desert regions to find new water sources in arid regions as well as contribute to observations of ice sheet dynamics, subsurface geology and forest topography. Because Biomass will see through the forest canopy to the ground, terrain height maps will be provided, improving current Digital Elevation Models in densely forested areas. Biomass data will also support REDD+(Reducing Emissions from Deforestation and Forest Degradation), a UN climate change initiative aimed at reducing emissions due to deforestation, by systematically monitoring forests in vulnerable areas with no need for ground intervention.
Launch: A launch of the Biomass spacecraft is planned in 2021 on a Vega vehicle from Kourou.
Orbit: Sun-synchronous near circular dawn-dusk orbit (LTAN of 6:00/18 hours), altitude of ~666 km, depending on the different mission phases. The orbit is designed to enable repeat pass interferometric acquisitions throughout the mission's life and to minimize the impact of ionospheric disturbances. The baseline observation principle is based on double-baseline interferometric acquisitions, with a repeat cycle of 17 days.
The strategy for meeting the interferometric baseline requirement is based on the selection of an orbit with a‘controlled drift'. The amount of drift between successive orbital cycles is chosen to match the interferometric baseline requirement. In practice, the baseline is achieved by flying the satellite in an orbit where the altitude is slightly higher or lower than that of the exact repeating orbit. Because of this small drift, the resulting orbit will have a quasi-repeat cycle of 17 days for the baseline interferometric phase.
A double-baseline interferometric mode provides two interferometric acquisitions with temporal decorrelation within the requirements in order to improve the retrieval accuracy. As shown in Figure 7, this mode consists of a set of three acquisitions with a fixed baseline to retrieve the forest height, while the orbit repeat cycle is kept to a minimum to ensure good temporal coherence between acquisitions spaced by two repeat cycles.
In such a way, each of the three swaths is imaged over three repeat cycles, before the satellite is rolled to observe the next one. The complete coverage is therefore achieved by matching the overall combined interferometric swath (obtained after nine repeat cycles) of 160 km with the orbit fundamental interval, achieving an orbit repeat cycle of 17 days and a global coverage in just 5 months for the baseline interferometric phase.
Figure 7: Double-baseline interferometry using three interleaved swaths (major cycle). The blue lines represent the swaths, while the filled blocks are the areas where interferometric acquisition can be performed. The grey blocks represent acquisitions in the adjacent ground intervals (image credit: ESA)
Sensor complement: (P-SAR)
P-SAR (P- Synthetic Aperture Radar)
P-SAR operates in a stripmap mode with a swath illuminated by a single antenna beam, i.e. an imaging configuration similar to that of the ERS-1/2 SAR. Global coverage is obtained by the interleaved stripmap operations among three complementary swaths as described previously. The beam re-pointing is performed through a roll maneuver of the spacecraft, as there is ample time over the poles for such operations. This solution using the spacecraft rolling was preferred over the possibility of electronic beam switching due to its simplicity (Ref. 24).
Instrument Concept A: A single sideband transmit pulse (linear FM) is generated, up-converted, amplified and sent to the polarization switch in the CEU (Central Electronics Unit). The polarization switch, operating at low power level, toggles between the V and H transmit channels at each pulse repetition interval. The modulated transmit pulse is then amplified in the corresponding polarization channel in the transmit unit, routed to the circulator unit and radiated through the feed array. The HPA (High Power Amplifier) is made of three SSPAs (Solid-State Power Amplifiers) in parallel, each delivering a peak RF power of 120 W with 10 % duty cycle and PRF (Pulse Repetition Frequency ) of 3000 Hz on average. Because of the concentration of high peak power after the power combiner, multipaction must be avoided by an appropriate circuit design of the radar front-end part, between the HPA output and the power divider in the feed array.
In reception, the echo signals (V and H) are routed through the circulator unit to the receive unit where filtering and amplification are performed. They are then routed to the CEU for analog-to-digital/down-conversion, data compression and packetization. In addition, the LNA (Low Noise Amplifiers) are protected by a limiter at their inputs against possible strong interference signals emitted by the SOTR (Space Objects Tracking Radars) and wind profilers.
The ICU (Instrument Control Unit) receives commands and information from the platform computer. It sets up the instrument operation parameters, controls image acquisitions, relays telemetry information and manages fault/limit checking and takes action where appropriate. It also maintains the instrument time reference, synchronized to an on-board GPS (Global Positioning System) clock. The instrument power unit converts the 28 V DC unregulated power supply from the platform to appropriately conditioned DC power for all the electronics units, as well as provides the heater power for instrument thermal control. The instrument mass (including margin) is 202 kg with the NG reflector, and 275 kg with the HC reflector. The maximum required DC power is 463 W for both reflector options and the maximum data rate is 115 Mbit/s prior to compression.
Figure 8: Illustration of instrument concept A (top) and concept B (bottom), image credit: ESA
Instrument Concept B: The linear transmit pulse is split in the BFN (Beam Forming Network) and routed to two parallel transmit chains and amplified. A polarization switch is placed after the HPA, in order to select the transmit polarization in each of the TRUs (Transmit/Receive Units), which delivers a peak RF power of 120 W with 12 % duty cycle and a PRF of 3050 Hz on average. The two pairs of radiators (upper and lower) are fed separately by the respective TRUs and illuminate the reflector. Splitting the power in two parallel transmit channels helps avoiding potential multipaction problems.
In reception, the echo signals from the two radiator pairs are filtered and amplified in four parallel receive chains (TRU-1: V and H and TRU-2: V and H). Those are recombined in the BFN to form the V and H signals and routed to the CEU. They are finally down-converted and digitized, followed by data compression and packetization. As any amplitude or phase imbalances between the channels would affect the beam pattern, the channel stability is ensured by appropriate design of the TRUs, i.e. of the HPAs and of the LNAs. An additional phase equalization can be foreseen for compensating relative phase drifts due to aging (included in the CEU). A limiter and an isolation switch at the LNA input protect them against possible strong interference signals. The instrument mass (including margin) is 206 kg. The maximum required DC power is 221 W and the maximum data rate is 117 Mbit/s prior to compression.
Both concepts use a single-offset reflector antenna system consisting of a feed array and a large deployable mesh reflector with a circular projected aperture diameter of 11.5 m – 12 m, depending on the concept. The selected configuration is characterized by a relatively short focal length in order to minimize the distance between the spacecraft and the reflector. Because of this, the reflector, when illuminated by a linearly polarized spherical wave from the feed, would produce a significant cross-polar radiation (12–15 dB below the co-polar peak gain) in its main beam, which has the form of a difference pattern (narrow null along the principal elevation plane). To comply with the cross-polarization ratio requirement, a pre-compensation technique is then implemented at the level of the feed. Stacking of the patches is necessary to achieve a sufficient bandwidth at the level of the feed subsystem (<10 MHz). The feed assembly is made of multilayer sandwich structure, consisting of metalized carbon or Kevlar-fiber-reinforced plastic sheets and Kevlar honeycomb or Rohacell foam core, thus low mass. Concept A uses three pairs of radiators with tapered excitation in elevation, whereas only two pairs of radiators with equal excitation are used for Concept B.
Figure 9: Feed array consisting of 3 x 2 stacked circular patches and body-mounted on the satellite Concept A (left); Deployable feed array consisting of 2 x 2 stacked square patches on a support structure Concept B (right), image credit: ESA
The radio frequency and digital electronics of the Biomass SAR instrument use well-established technologies thanks to the low radar frequency (UHF band) and narrow system bandwidth (6 MHz). However, the combination of the low frequency and high peak RF power increases the risk of multipaction. Therefore, a number of specific risk-retirement activities were undertaken and specific measures were implemented in the radar front-end design.
Figure 10: P-SAR viewing geometry (image credit: BIOMASS Team)
P-band Antenna Feed S/S (Subsystem):
The BIOMASS SAR P-Band antenna consists of a large deployable reflector antenna system with an offset geometry that results in high cross-polar level from the reflector, this is compensated with an adequate design of the reflector Feed S/S to generate a "cross-polar" pattern with proper amplitude and phase to cancel the reflector cross-polar down to admissible values. 36)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).