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	This artists concept shows the TOPEX/Poseidon and Jason-1 tandem mission.

More than sixteen years after the launch of TOPEX/Poseidon, and more than seven years after the launch of Jason-1, OSTM/Jason-2 is scheduled for lift-off in June 2008 to continue the important time series of sea surface height measurements. After launch, OSTM/Jason-2 will move into position along the same ground track and ahead of Jason-1, and the two spacecraft will make measurements in tandem.

As with the TOPEX/Poseidon and Jason-1 Tandem Mission, which lasted more than four years, the near simultaneous measurements from the same altitude of the same sea surface locations by the Jason-1 and OSTM/Jason-2 altimeters will allow scientists to compare and correlate the two altimetry measurements. These careful comparisons ensure that OSTM/Jason-2 continues adding seamlessly to the sixteen years of TOPEX/Poseidon and Jason-1 data. This long, continuous time series of data is critical for improving climate prediction capabilities.

OSTM/Jason-2 will enter orbit 10 kilometers (6 miles) below Jason-1. It will be between 1 and 10 minutes away from the older spacecraft and in very nearly the same orbit. Three onboard instruments, the Laser Retroreflector Array, the GPSP receiver and the DORIS instrument, enable the spacecraft operators to locate OSTM/Jason-2 very accurately relative to the ocean's surface.

Soon after launch, OSTM/Jason-2 will deploy its solar array, power-up its instruments and orient its altimeter to point straight down to geodetic nadir, the shortest path to the ocean. OSTM/Jason-2 will then be commanded to execute a series of maneuvers to raise its orbital altitude to match that of Jason-1. At this stage, it will be ahead of the older satellite by about 60 seconds. At orbital speeds of 8 kilometers per second, this may be as much as 500 kilometers (300 miles) away. OSTM/Jason-2 will be positioned to move along the same ground track as Jason-1. These delicate maneuvers should take between 20 and 40 days.

Once it is in position, OSTM/Jason-2 and Jason-1 will cross-calibrate their instruments. This period will last up to six months, during which the two data sets will be rigorously compared and correlated.

At the end of the calibration phase the OSTM/Jason-2 measurements will correspond with those of its two predecessors, Jason-1 will then maneuver to change its orbital position relative to OSTM/Jason-2. In this tricky procedure the older spacecraft will be commanded to move onto a parallel ground track mid-way between two adjacent OSTM/Jason-2 ground tracks.

Double-teaming the satellites in this way will once again increase our global data two fold, as did the TOPEX/Poseidon-Jason-1 Tandem Mission. For as long as Jason-1 remains in good health, these altimetry twins will be doubling the amount of science data collected and providing a more detailed look at smaller-scale ocean phenomena, such as eddies and tides in the coastal areas and shallow seas, than is possible with just one satellite, and will bring us closer to solving the global climate puzzle.

For additional information on the tandem mission, watch the tandem mission animation (1 MB Flash animation).