The rationale for this project is that sea ice thickness is a key climate indicator at high latitudes which seems to undergo significant changes, but this parameter is a real challenge to observe and there is an urgent need for improved measurement techniques. Results from global climate models indicate that global warming in the next decades will be most pronounced in the Arctic region, causing a net melting of the sea ice. Ice area and thickness are the two fundamental parameters for estimation of ice mass balance, which shows the response of sea to climate change. In addition to the climate aspect, sea ice thickness is also important to observe for practical operations such as ice navigation, sea transportation and offshore drilling. Scientific objectives and approach: The overall objective of SITHOS is to develop European monitoring systems for sea ice thickness and related parameters for climate change detection, support to sea transport, offshore operations as well as environmental protection in polar regions. SITHOS will include pre-launch validation experiments for CRYOSAT, a European Satellite to be launched in 2004 with global sea ice thickness observation as a key objective. SITHOS will therefore use the following approaches: 1. satellite altimeter data provides regular data on ice freeboard and thickness averaged over a 100 by 100 km grid and with monthly repetition; 2. aircraft surveys with laser and GPS - and possibly electromagnetic induction - can provide profiles of topography, freeboard and thickness on opportunity basis, over distances from a few hundred to thousand km depending on type of aircraft (AL-GPS) 3. use of electromagnetic induction and laser (EML) mounted on icebreakers or other vessels of opportunity sails through the ice, and on helicopters. Typical distances covered by ice-going vessels are a few hundred km. 4. use of Upward Looking Sonars (from submarines and AUV) to be used for mapping of ice draft along tracks (ULS) 5. time series of ice thickness from a cluster of automatic ice stations (AIS) deployed on ice floes in the interior of the Arctic, possibly as part of the International Arctic Buoy Programme 6. improve sea ice modelling by use of the new data sets and define the requirements for sea ice thickness and other ice data from modelling perspective Expected impacts Improved methodology to measure sea ice thickness by combination of several observing techniques. The metodology will be important for monitoring changes of sea ice thickness in the future. New data sets will be available to document the status of sea ice thickness in the Arctic which will be important for verification of the global warming effect which is predicted to become more pronounced in the Arctic compared to other parts of the world.