Atmosphere and Climate Explorer Plus Looking at the Horizon: Innovative Atmospheric Sounding using Active Inter-satellite Cross-link Signals

The Atmosphere and Climate Explorer Plus (ACE+) is an atmospheric sounding mission using radio occultation techniques. The mission is a response to ESA ́s 2nd Call for Earth Explorer Opportunity Missions in 2001 and was ranked no. 1 out of 25 candidate missions. ACE+ will considerably advance our knowledge about atmosphere physics and climate change processes. The mission will demonstrate a highly innovative approach using radio occultations for globally measuring profiles of humidity and temperature throughout the atmosphere and stratosphere. The baseline constellation of 4 small satellites, tracking L-band GPS/GALILEO signals and X/K-band LEO-LEO cross-link signals, will be launched in 2 counter-rotating orbits with 2 satellites in each at 650 and 850 km respectively. The system design is aimed to optimise the science return by proper distribution of the observations in space and time. The LEO-LEO cross-link instrument is driven by accuracy requirements that drive antenna design, transmitter power, and transmitter and receiver stability. Spacecraft design is driven by the relatively high power consumption of the instruments, and their pointing requirements. Satellite characteristics include a mass of 160 kg, and available power of 125 W. Low cost launch is envisaged to be with Rockot or START-1, and can take place in 2006-2007. Finally, the ground segment processes data from all satellites, including GPS satellites, for distribution to the meteorological community.