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information base against which we can gauge the significance of such an event. The ISY should actively encourage the major space agencies to ensure that:

• Long-term data sets are preserved ad remain accessible to users;

• The utility of archived data be preserved to maximize the possibility of its use for purposes which may differ from those originally intended.

• Geolocation and Spacecraft Position

  Of extreme importance in the application of remotely sensed data is the requirement for precise information which enables the geolocation of data. Geolocation of remotely sensed data is defined here as the attachment of Earth coordinates to the remotely sensed information.

  The elements in this process include a knowledge of the spatial coordinates of the spacecraft, the spacecraft attitudinal information (yaw, pitch, and roll) if available, and the sensor directional information in spacecraft coordinates. Too frequently one or more of the above ingredients is absent or degraded, and data users have been required to resort to ground control information to improve the geolocation of data for particular purposes.

  It would be far more satisfactory to be using ground information to provide the quality control of geolocation. The use of ocean data, for example, requires precision in geolocation since ground control features are not available.

CANDIDATES FOR DATA TESTING

  Possible candidates from which a short list of demonstration projects should be selected include:

• Detection of the Greenhouse Effect

  Considering future needs in the use of satellite data for studying a modelling global climate change, we propose for ISY a pathfinder test of the global information system. The program aimed to detecting the greenhouse effect is of international concern. It can also be designed to serve as a pathfinder experiment providing the basic model upon which future observing systems are designed. In addition, development of this data test requires that emphasis be given now to the design, preparation, and exchange of global data sets.

  In general, space observations are used in conjunction with in situ data and other ancillary information to provide the best level of accuracy and resolution. Frequently, in situ observations are the only available technique. In all cases there are three basic requirements for the data which must be maintained: long-term stability, calibration, and validation. These three requirements are of extreme importance if one is to attempt to observe and study climate change.

  A preliminary list of needed measurements includes the following: (a) climate change parameters - stratospheric temperature, tropospheric temperature, land and ocean surface temperature, global cloud cover, precipitation and soil moisture distributions, snow cover, ice sheet topography, and internal ocean temperature trends; and (b) climate forcing parameters- stratospheric and tropospheric trace gas trends including water vapor, solar irradiance, stratospheric aerosols, and surface albedo.

• Tropical Ocean and Global Atmosphere Program

  In the context of the TOGA Program, fields of surface wind, sea level, sea surface temperature, surface solar radiation, cloudiness, atmospheric water vapor, and indices of precipitation could be studied.

  ERS-1 will feature a scatterometer and an altimeter suitable for estimating the surface wind stress and sea level, respectively, which should be compared with ongoing in situ surface wind and subsurface temperature measurements. TOPEX/Poseidon will feature more precise measurements of sea level. Improvements in cloud drift winds from geostationary satellites would also be very significant in this context. There have already been intercomparisons of U.S. based techniques for satellite-derived estimates of sea surface temperature, but much better documentation and validation is needed of their use in practice. The Along Track Scanning Radiometer (ATSR) aboard ERS-1 will provide a unique opportunity to get better estimates of sea surface temperature and to validate the interpretations being placed on the International Satellites Cloud Climatology Program (ISCCP) data on global cloudiness.

  Microwave radiometers aboard MOS and DMSP and the 6.7 micron channels aboard GOES and METEOSAT could provide the first synoptic global analyses of atmospheric water vapor, but only if we learn how to integrate this information effectively into assimilation models. Microwave and infrared indices of precipitation are currently being assembled under the World Precipitation Climatology Project, but still need to be put in the context of estimates of surface evaporation and atmospheric moisture.

• Other Candidates

  In the context of the proposed IGBP initiative on terrestrial ecosystems, a global land survey suitable for estimating changes on a global scale over the next decades should be conducted. The LANDSAT Thematic Mapper and the Systeme Probatoire pour l'Observation de la Terre (SPOT) and the Japanese ERS-1 satellites all provide images of the land surface from which categories of present land use can be derived with varying degrees of precision. Combined with in situ measurements in selected areas, and subject to appropriate geographic sampling strategies, a baseline from which future changes can be estimated could be established



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