Nov 15, 2010

An interview with ESA scientist Dr Mark Drinkwater, Mission Scientist for the Gravity field and steady-state Ocean Circulation Explorer (GOCE) and CryoSat.

Estimated Article Reading Time: 8 min.

Could you describe ESA’s Earth Observation Programme?

ESA’s Earth Observation programme has an overall driving theme and this is embodied in the Living Planet Programme. It contains two thematic elements: Earth Explorer Missions and Earth Watch missions. The former are the Science-driven and research oriented missions, which address specific scientific themes, and the latter contain the combination of Operational Meteorological missions prepared in support of weather forecasting, together with the Global Monitoring for Environment and Security (GMES) missions known as the “Sentinels”, which are developed and implemented on behalf of the EC.

In total ESA is operating five Earth Observation missions (including ERS-2, Envisat, GOCE, SMOS, CryoSat-2) and is meanwhile in the process of developing five new Earth Explorer missions (three of which have A and B satellites) and six GMES Sentinels, and is supporting the preparation of two Meteorological missions (MTG and Metop Second Generation).

Overall, the goal of the Living Planet programme is to understand Earth as a system with a series of complicated interlocking processes that link the atmosphere, ocean, land surface, biosphere, cryosphere, and solid earth. These processes are for example the water cycle and the carbon cycle.

With the Earth Observation missions flying and in preparation we are targeting specific scientific problems with the Earth Explorer component of the programme. Earth Explorers are designed to help unlock fundamentally new knowledge with which to better understand the Earth System. Meanwhile, the operational satellites comprising the Operational meteorological and GMES Sentinel missions provide the backbone of our monitoring capability, reporting day to day on the health of the Earth system. The latter is the foundation to our forecasting capability, to preserving our environment, and to making educated policy decisions at the basis of a sustainable existence.

You’ve been Mission Scientist for GOCE (Gravity field and steady-state Ocean Circulation Explorer) for 10 years. Tell us about that mission.

A primary objective of GOCE is to map the Earth’s gravity field geoid at unprecedented resolution. GOCE is actually not the first satellite dedicated to providing a global gravity model. There have been a series of satellites over the last decade, each with slightly different measurement concepts – CHAMP, and the GRACE twin satellites, which make measurements of time-varying gravity.

GOCE is a single satellite flying as low possible and employing a revolutionary new instrument – a gravity gradiometer with six individual accelerometers. It is designed to focus on the first highly detailed, high-accuracy mapping of the Earth’s static gravity and geoid. It is complementary to GRACE, and revolutionary by comparison to CHAMP.

GOCE is important because of the number of revolutionary new elements including three pairs of the highest precision accelerometers in a three-axis configuration. Meanwhile, it is designed to fly as low as possible, such that the three pairs of accelerometers can measure gravity gradients (i.e. differences in gravity between each pair of accelerometers separated by 0.5metre distance). This combination of new features gives access to a high spatial resolution gravity field and geoid, which approaches 1Gal and 1-2cm radial accuracy, respectively, at a spatial resolution of approximately 100km.

GOCE was originally designed to last approximately 20 months. This was due to the fact that the satellite flies very low, and was based on predictions of the atmospheric drag. However, the solar activity has been particularly benign throughout the first year and a half of operation, meaning that the revolutionary ion propulsion system (which counterbalances the force of drag on the satellite) has needed to use less fuel than was originally predicted. The consequence is that the lifetime of the satellite may be extended. The current plan is to collect periodic intervals each of approximately 61 days of uninterrupted data. Until now we have three complete periods of 61 days – each allowing complete mapping of the global gravity. Ultimately all these intervals of data will be used together to make a high accuracy “static” representation of the mean global gravity field at smallest possible resolution (i.e. grid-spacing).

The benefit of having saved on-board fuel resources means that we may extend the GOCE mission lifetime and thus the science operations. Current estimates suggest there is sufficient fuel to extend operations until the end of 2013, and a request shall be made in November to the ESA Member States to extend the nominal period of operations – to allow the extension of the collection period for scientific datasets until at least the end of 2012.

What is the gravity field geoid, and why do we need to measure it so accurately?

The geoid is a reference surface (or so-called “equipotential”), along which the gravity potential is everywhere equal. This means that if you placed a ball anywhere on this surface, it would not roll to one side or the other. The way to imagine this is if you were to stop the worlds oceans from moving (i.e. no tides or currents), and look at how the water clings to the Earth under the influence of the Earth’s gravity. Because the gravity varies from place to place, what we see in the details of the geoid are hills and valleys, which represent the places of the strongest gravitational pull and the weakest gravitational pull, respectively.

The purpose of this reference surface is to be able to compare the details of the true ocean height (or so-called “dynamic topography”), as measured by radar altimeters on existing satellites such as Envisat, or Jason, with the geoid reference surface provided by GOCE. Motion of ocean water, due to tides and currents (i.e. ocean dynamics), redistributes ocean mass and causes variations in dynamic ocean surface topography, as compared with the geoid. Thus if we measure the true ocean surface and then subtract the geoid, what we see is the variations in height and slope (from place to place), due solely to the ocean movement. This allows us to gain a very accurate and detailed global view of the regional scale ocean currents. This is one of the scientific applications of the data.
Above: The latest geoid measurement by GOCE (with height scale in metres). Image: European Space Agency (ESA) – GOCE High Level Processing Facility.

The “imaginary” geoid also runs through the land surface (just beneath the surface of the continents), and is used as an accurate reference surface for measuring heights. When you make a measurement of height using a GPS, and subtract the geoid height, we obtain what is called an “orthometric height”. An accurate high-resolution geoid from GOCE, at 100km grid spacing, will enable us (for the first time), to unify independent height systems currently used by different nations throughout the world. Imagine you wanted to build a bridge or tunnel between different countries. This requires that you understand heights in different countries (typically measured above mean sea level). The problem is that mean sea level varies between each country due to gravity, and so a global geoid is the correct way by which geodetic height systems throughout the globe can be linked to one-another.

The geoid varies between maxima of around +85m (with highs over the central North Atlantic, and South-east Asia); and around -106m (south of Sri Lanka in the Indian Ocean).

How will the data from GOCE be used?

GOCE data users are typically from the main fields of largely scientific applications: oceanography, ice-sheet mass balance and sea-level rise, solid Earth physics, and geodesy. We have even had interest expressed from commercial interests – such as large oil companies, as well as national mapping agencies.

The details of the gravity gradients in such as the west-coast of South America, the Himalaya, south-east Asia and Antarctica indicate new information about the sub-surface structure in active plate margin locations (i.e. locations of active tectonics). In comparison with previous satellites, GOCE gives a much more detailed picture of gravity variations from place to place. But its main advantage is that its gravity gradients in three dimensions provide insight into details of the structure of the lithosphere and upper mantle which can be used, for instance, to better model processes in earthquake and volcanic regions.

GOCE data are extremely useful for studying earthquakes and volcanoes. The scientific users expect the mission to provide data with which to enable a better understanding of many processes below the surface of the Earth. Because gravity is directly correlated with the distribution of mass in the Earth’s interior, detailed mapping of the gravity gradients contribute to a better understanding of the structure and dynamics in Earth’s crust in locations where earthquakes and volcanoes are commonplace. Greater insight into the structure and sub-surface motion of tectonic plates at critical plate boundaries, such as the Andes, Indonesia, or even in Italy, is of significance for improving our predictive capabilities.

GPS derived geodetic heights currently do not rely on global gravity models – calculating instead height with respect to an idealised ellipsoidal reference surface or simplified geometrical model of the Earth. In the future our goal is that we can implement a unified scheme whereby everyone has access to “orthometric height” – or heights which are correctly referenced to the geoid. We are performing studies to look at this problem at continental scale (i.e. for USA and Europe), to understand first how to implement such a scheme. Naturally, this will rely on a global geoid model derived from the best-available satellite data – including GOCE.
Why does the Earth’s gravity field change over time?

Earth’s gravity field changes on different timescales due to mass being moved around by the atmosphere, the ocean, ground hydrology, deglaciation (and slow continental rebound after removal of the large ice sheets), sub-surface tectonic and volcanic processes, etc.

Though GOCE was not designed to measure time varying components of gravity, we will always have the possibility to intercompare GOCE gravity data collected at different times (and thus also models generated during different epochs or intervals) – as well as with with the mean static gravity field (computed from the entire record). This may later give indications about locations in which the gravity gradients are varying the most. Provided that there are uninterrupted series of high quality gravity gradient data, this may also give the possibility to compute accurate gravity field models over different intervals of time. This was not originally foreseen from the GOCE mission, but may allow us to study gravity variations over this extended mission lifetime.