Introduction
The European Space Agency (ESA) has dedicated a substantial part of its programmes to observing the Earth since the launch of its first meteorological mission Meteosat in 1977. Following this mission, the subsequent series of Meteosat satellites, the ERS-1 and ERS-2 missions and, more recently, Envisat, the largest Earth observation (EO) satellite ever built, have provided a wealth of data about the Earth, its climate and changing environment. In the mid-Nineties ESA refocused its approach about future EO missions to include, on one hand, smaller science-driven missions and, on the other hand, more application-oriented operational missions. Consequently, the ESA Living Planet Programme now comprises two main components: an EO science and technique demonstration element, carried out with the Earth Explorer (EE) missions of which we will provide some interesting examples, and an Earth Watch (EW) element comprising meteorological missions for EUMETSAT as well as missions for long-term monitoring of the environment. The latter missions are part of the Global Monitoring for Environment and Security (GMES) initiative, in partnership with the European Commission. All ESA EO missions are coordinated within the GEOSS framework defined by the GEO.
Earth Explorers (EE) are proposed and selected jointly with the scientific community to answer key issues linked to the scientific challenges of the programme [ESA SP-1304]. Six EE are now under development and scheduled for launch between 2008 and 2013: the Gravity field and steady-state Ocean Circulation Explorer (GOCE), described below, for launch in early 2008; the Soil Moisture and Ocean Salinity (SMOS) mission, also described below, for launch in 2008; ADM-Aeolus (Atmospheric Dynamics Mission), for launch in 2009 and aimed at proving the Doppler wind lidar sensing technique to determine vertical profiles of wind speed up to 20 km height; Cryosat-2, for launch in 2009 (Cryosat was lost at launch in 2005) and aimed at mapping variations of the sea-ice thickness and mass; Swarm, for launch in 2010 and aimed at accurately surveying the sources of the Earth’s magnetic field; and EarthCARE (Cloud, Aerosols and Radiation Explorer), for launch in 2013 and aimed at characterizing clouds and aerosols to determine their influence on climate change.
We will take now a closer look at two EE, GOCE and SMOS, both for launch in 2008, as they provide excellent examples of how the development of novel EO technologies enables advances in the Earth sciences. It is worth noting that the EW missions, which include both geostationary satellites (the current Meteosat second generation and the forthcoming third generation) and low-Earth orbiting satellites (MetOp for meteorology and the GMES Sentinels), though based on well-proven EO techniques, also require injection of new technologies to increase performance and operational robustness.
Many geophysical phenomena influence the value of ‘g’. GOCE will determine ‘g’ to the 5th decimal digit at all latitudes, except near the poles.
The GOCE Mission
The data from GOCE will be used to determine the Earth gravitational field to an accuracy of 10-5 ms-2, i.e. about one ppm of the gravitational acceleration at the Earth surface, as well as the geoid, the hypothetical equipotential surface of the oceans at rest under the only action of gravitation, to 1-2 cm accuracy, with a high spatial resolution (~100 km). An improved geoid model is crucial to derive very accurate measurements of ocean circulation, sea-level changes and terrestrial ice dynamics, all of which are fundamental to climate change. Figure 1 shows a view of the satellite as well as sketches of gravity field models indicating, on an amplified scale, the improvements in the knowledge of gravity anomalies (i.e., the deviations of the gravity field from that of a reference ellipsoid) obtained from recent missions (CHAMP, GRACE) and expected from GOCE. A better knowledge of gravity anomalies will contribute to an enhanced understanding of the Earth’s interior, such as the dynamics of earthquakes and volcanism.
Besides precise satellite tracking with the Global Positioning System (GPS), GOCE will use satellite gravity gradiometry, a new sensing technique that is the key to measuring gravity at high spatial resolution. The gradiometry principle is sketched in the animation, where the proof masses (gray) of six spring-mass type accelerometers are perturbed by mass anomalies (red) due to gravitational attraction. The principle is again recalled in the Figure 2. The five independent components of the gravity gradient tensor, which is symmetric and traceless, are retrieved from the measurements of the tiny differential accelerations between three pairs of well-shielded proof masses. Each proof mass is at the core of an ultra-sensitive accelerometer. As shown in the Figure 2 inset, the tensor components are very weak. To reach the mission goals they must be known with an error spectrum < 0.01 E Hz-1/2 (1 E (Eoetvoes) = 10-9 s-2) in the measurement bandwidth of 0.005 – 0.1 Hz.
To achieve this, each accelerometer reaches a sensitivity of ~10-12 ms-2 Hz-1/2. The six accelerometers of the GOCE gradiometer are arranged around the satellite centre of mass, with baselines of 0.5 m. Each proof mass is electrostatically suspended and kept very precisely at the centre of the accelerometer electrode cage. Capacitive displacement sensors (CDS) using electrode pairs measure the position and orientation of the proof mass, so that all its degrees-of-freedom are closed-loop controlled (via a digital processor), with force actuation provided by electric fields applied using the same electrodes. The CDS are crucial to the accelerometer sensitivity. They are based on resonant capacitive bridges designed to obtain quantum non-demolition displacement measurements of extreme sensitivity (~6 pm Hz-1/2 in GOCE). The electrostatic suspension voltages provide the measurements of the forces required to maintain the proof mass at the centre of the electrode cage. The measurements from the six accelerometers, after processing and removal of perturbing accelerations, yield then the gravity gradient measurements.
GOCE will fly in a sun-synchronous orbit at a very low altitude (~250 km) where air drag is relatively strong, so active compensation in the flight direction is necessary for both orbit altitude maintenance and gradiometric performance. Electric propulsion with modulatable 20-mN low-noise ion thrusters will support this. Thrusters and other key satellite elements are visible in Figure 3.
Besides the accelerometers and the drag compensation system, the whole satellite presents many other technical challenges. For instance, considering again the gradiometer, the baseline must be stable to ~10-12 m over periods up to 200 s. The carbon-carbon structure on which the accelerometers are mounted will therefore be kept extremely stable in temperature, which is accomplished by a multi-stage thermal control and filtering scheme, with the gradiometer outer panel actively controlled to < 0.01 K. The gradiometer operation requires no moving parts in the entire satellite to avoid any disturbances that may be caused by, e.g., variations in satellite self-gravity and micro-vibrations (from e.g. flow valves). In addition, the gradiometer measurements are used by the satellite drag-free and attitude control system, so tightly coupling satellite platform and payload and making their operations more complex.
Only a small part of the technical challenges of GOCE and of the corresponding technological advances can be outlined here. However, progress in the determination of the gravity field can only be achieved by conquering such challenges and the scientific benefits that will be reaped from this will definitely warrant the effort.
The SMOS Mission
The SMOS mission will measure soil moisture (SM) over land and ocean surface salinity (OS) by means of L-band microwave imaging radiometry. Assuming the surface temperature to be known by other means, the brightness temperature measured by the radiometer will provide the surface emissivity, from which SM and OS can be retrieved. Better knowledge of the global distributions of SM and OS will enhance weather forecasting, climate and extreme event predictions. SM plays a key role in the water cycle and in vegetation monitoring. The water and energy fluxes at the surface/atmosphere interface strongly depend upon SM since SM drives evaporation, infiltration and runoff. SM also determines the rate of water uptake by vegetation. Observation of SM is a target of the ‘water’, ‘weather’ and ‘disasters’ themes of the GEOSS. OS plays an important role in the Northern Atlantic sub-polar area, where intrusions with low salinity influence the deep thermohaline circulation and the heat transport. OS is a key variable for the water cycle and for the coupled ocean-atmosphere models.
SMOS, which will fly in a sun-synchronous at 763 km altitude, will deliver SM data to an accuracy of 4 % (volumetric) every three days, with a spatial resolution of 50 km. The SMOS observations of the brightness temperature at multiple incidence angles will allow one to retrieve SM in the presence of vegetation. L-band radiometer measurements over the ocean are sensitive to surface salinity, though perturbing factors (roughness, wind, etc.) have to be accounted for. Salinity in the oceans varies from ~30 to ~40 gram per kg of water and SMOS will measure it to ~0.1 gram/kg at a spatial resolution of 2* x 2* every 10 days. These goals will be met by spatially and temporally averaging observations, which requires keeping any systematic errors very low, also through frequent calibrations.
Operating a radiometer at L-band provides the maximum sensitivity of emissivity to both SM and OS. However, antenna diameters of several meters are required to meet the spatial resolution requirement. A real-aperture antenna poses many practical difficulties when global coverage within a few days is needed. Microwave imaging by aperture synthesis, a 2D interferometric approach to radiometry inspired by the techniques developed in radio astronomy, provides an effective solution. With this approach, many small antenna/receiver units are geometrically arranged in order to sample the signal that would have been received by a real-aperture antenna. This sampling in the spatial domain by a ‘thinned’ antenna array provides a scanning capability through a wide swath through signal processing. Apart from the on-board cross-correlation of the signals from the receivers, such processing is conveniently carried out on ground.
For SMOS the aperture synthesis is based on a Y-shaped symmetrical antenna array and therefore the payload includes three deployable antenna arms connected to a central hub. The central hub is 1.3 m in diameter with the three arms deployed extending up 8 m in diameter. The arms, whose carbon-fiber structure provides strong thermo-elastic stability, are deployed in a synchronized manner shortly after launch. Each arm has three segments, each containing six L-band radiometers. An additional four radiometers in the central hub complement each line of 18 radiometers per arm. Finally, there are three total power (noise-injection) radiometers placed in the central hub, for a total of 69 antenna elements. To ensure performance all antenna/receiver elements are electrically very similar. Temperature gradients are actively controlled to ensure a thermal gradient < 1 K across any arm segment.
Each element uses a patch antenna in multi-layer microstrip technology handling both H and V polarizations and with a wide beamwidth (~65o). The latter would imply a large area in view, however, because of the antenna array geometry and of the interferometric measurement principle, the field-of-view is a hexagon-like shape about 1000 km across. In each receiver the signal is filtered to precisely the same band, namely 1404 – 1423 MHz in a protected region of the spectrum. It is then amplified, frequency down-converted and finally sampled and converted to a digital signal.
Several custom-made high-performance MMIC’s are used for these operations and for fine-tuning the receiver similarity. Each receiver output data stream is transmitted at ~130 Mb/s to the central correlator unit using optical fiber links, employing, among others, 74 laser diodes. Given the number of antenna/receiver elements, the cross-correlations to derive the brightness temperature images imply the use of several thousand correlators. These are implemented in nine custom-made digital ASIC’s in 0.35 micron CMOS radiation-tolerant technology.
An on-board calibration subsystem, based on distributed noise injection, provides a correlated noise reference to calibrate the noise temperature and relative phase characteristics between the receivers. The noise-injection radiometers will measure the antenna temperature and the amplitude of the noise injected by the calibration system, using also the cold sky for absolute calibration. To view the cold sky the satellite pointing will be changed from Earth-fixed to inertial pointing periodically.
SMOS will address key issues of the Earth’s water cycle of crucial relevance for the study of climate, while also demonstrating a new space observation technique, 2D interferometric radiometry. Even at the relatively low frequency of 1.4 GHz, this is a challenging concept since a large number of parameters and items, ranging from antenna and signal processing aspects to control and thermo-structural ones, had to be very carefully accounted for in the design and development work to ensure the mission performance.
Conclusion
Two missions carrying novel payloads into space for the very first time – a gravity gradiometer on GOCE and a synthetic aperture radiometer on SMOS – will be launched in 2008 to determine geophysical parameters not measured so far and other parameters to much better performance than achieved hitherto, opening the way to new Earth science advances and applications.
Pierluigi Silvestrin
Head of Future Missions Division
Earth Observation Programmes Directorate
European Space Agency
Acknowledgement: Graphics by AOES Medialab for ESA