Pointing and Stability Requirements for Earth Orbital Experiments

Experiments for manned and unmanned space facilities of the next decade will impose stringent requirements for accurately pointing and/or maintaining stability of the space platforms over extended periods. Potential experiment programs identified to date in various NASA-and contractor-sponsored studies have been reviewed in order to define critical values and ranges for the following pointing/stability parameters: 1) pointing accuracy, 2) line-of-sight stability, 3) angular rate, and/or 4) acceleration level. The most stringent requirements were found to be in the research categories of: stellar and solar astronomy, earth observations, space physics, communications and navigation, and space biology.     

V: SEA LEVEL: Benefits of GRACE and GOCE to sea level studies

The recently published Third Assessment Reports of the Intergovernmental Panel on Climate Change have underlined the scientific interest in, and practical importance of past and potential future sea level changes. Space gravity missions will provide major benefits to the understanding of the past, and, thereby, in the prediction of future, sea level changes in many ways. The proposal for the GOCE mission described well the improvements to be expected from improved gravity field and geoid models in oceanography (for example, in the measurement of the time-averaged, or ‘steady state’, ocean surface circulation and better estimation of ocean transports), in geophysics (in the improvement of geodynamic models for vertical land movements), in geodesy (in positioning of tide gauge data into the same reference frame as altimeter data, and in improvement of altimeter satellite orbits), and possibly in glaciology (in improved knowledge of bedrock topography and ice sheet mass fluxes). GRACE will make many important steps towards these ‘steady state’ aims. However, its main purpose is the provision of oceanographic (and hydrological and meteorological) temporally-varying gravity information, and should in effect function as a global ‘bottom pressure recorder’, providing further insight into the 3-D temporal variation of the ocean circulation, and of the global water budget in general. This paper summaries several of these issues, pointing the way towards improved accuracy of prediction of future sea level change. This revised version was published online in August 2006 with corrections to the Cover Date.