The ACE+ Mission: An Atmosphere and Climate Explorer based on GPS, GALILEO, and LEO-LEO Radio Occultation

The European Space Agency (ESA) has recently, in May 2002, selected the Atmosphere and Climate Explorer (ACE+) radio occultation (RO) mission as the top priority mission out of 25 proposed Earth Explorer Opportunity Missions, which had addressed all areas of Earth system science. The ACE+ constellation of 4 Low Earth Orbit (LEO) satellites utilizes GPS, GALILEO, and LEO-LEO signals for RO sounding of fundamental atmospheric variables such as humidity and temperature.

ACE+ will acquire near 5000 GPS/GALILEO RO soundings per day and demonstrate the novel LEO-LEO concept collecting about 230 LEO-crosslink soundings per day. Following confirmation after the ongoing phase A study, expected before mid 2004, the ACE+ development is scheduled to last until 2007, with launches in 2007/08 followed by a 5 years operational phase.

The primary ACE+ mission goals are focused on climate and include, based on precise monitoring of climatic variations and trends in temperature, humidity, and geopotential heights, improved climate change detection and attribution, improved understanding of climatic feedbacks, validation of the simulated mean climate and its variability in global climate models, improvement — via data assimilation — of process parameterizations in climate models, and detection of variations in external forcings of climate.

Additional important goals relate to numerical weather prediction, atmospheric processes research, and space weather. The key innovation compared to similar (earlier) missions (e.g., COSMIC) is the novel use of GALILEO and LEO-LEO signals. Especially the LEO-LEO signals placed at 3 frequencies within 9-23 GHz, from center to wing of the 22 GHz water vapor absorption line, will for the first time allow for RO measurements of humidity in the troposphere without temperature-humidity ambiguity.

For example, the LEO-LEO data have the potential to furnish much needed upper troposphere humidity profiles with an unprecedented accuracy of better than 5% in specific humidity. The paper provides an introduction to the ACE+ mission along the above lines, with emphasis on the scientific rationale and the novel LEO-LEO capability, including first humidity and temperature retrieval performance results.