June 18, 2008

Jason-2 all set for the off

The Jason-2 oceanography satellite is now in the home straight and set for launch Friday 20 June. Before reaching final orbit, the satellite will go through a sequence of steps just after launch and during the beginning-of-life phase of its mission. Jason-2 teams are watching launch preparations advance in California with much relief after a series of setbacks over the last 2 weeks. First, successive delays in the launch of the GLAST satellite pushed back the departure of Jason-2, since the same ground teams were preparing both satellites; and then poor weather prevented the satellite from being mated with the launcher as planned.

18 juin 2008

But all obstacles in its way have now been cleared and Jason-2 is sitting atop the 2nd stage of the Delta II launcher, patiently awaiting lift-off on 20 June.

Cooked all round

55 minutes into the flight, the satellite will separate from the launcher and then turn on all axes like an irregularly-shaped stone as it is placed in a temporary orbit 10 km below that of Jason-1. Its solar arrays will then start to unfurl and Jason-2 will go into “barbecue mode”, alternately exposing its different sides to the sunlight.

Over the next 3 days the Satellite Control Centre in Toulouse, France, will activate and check out instruments.

The last crucial phase for CNES teams will consist in positioning Jason-2 just minutes behind Jason-1 in a nearly identical orbit. The two satellites will fly in tandem for 6 to 9 months, during which Jason-2’s instruments will be cross-calibrated with Jason-1’s.

Orbital trade-offs

The more often a satellite revisits the same point on the globe, the fewer points it sees and vice-versa. The chosen orbit is therefore a trade-off between revisit frequency and spatial resolution. Jason-2 will orbit at high altitude (1,336 km) to alleviate the effects of atmospheric and gravity disturbances and to facilitate orbit determination, inclined 66° to the equator to cover almost all of Earth’s ice-free sees. This means it will return to the same point every 10 days, flying a trajectory optimized to study large-scale ocean variability.

When the cross-calibration process is complete, Jason-1 will be moved aside to a parallel ground track midway between two adjacent Jason-2 ground tracks to increase global data coverage and optimize spatial resolution. Jason-1 and Jason-2 data will also be combined with data from Envisat.

Watch the Jason-2 launch live on 20 June from 9.10 a.m. CET on this site or on our oceanography blog.