Launch Timing
Jason-2 was launched at 7:46 UTC (9:46 CET). The launch time of Jason-2 was designed to place the satellite in an orbit plane very close to, but not exactly the same as, the orbit plane of Jason-1. To achieve this, the launch had to occur between 7:47 and 7:56 UTC. This 9-minute time slot is what is called the launch window. If the launch had not been achieved within the launch window the opportunity would have been lost that day, and the launch would have been postponed till the next day.

The launch date was based only on the readiness of the satellite, the Delta launch vehicle and the launch range at Vandenberg Air Force Base. Jason-2 could have been launched any day between 15 June and 15 August, but for each day, the launch window would move a little, falling around 12 minutes earlier each day.

Launch Sequence
The launch sequence consisted of a set of delicate operations for both the launcher and the spacecraft bolted under the last stage fairing.

When the Delta II launched, its first-stage engine and its three strap-on boosters ignited at the moment of lift-off and the rocket rose vertically from the launch pad. Seven seconds later, the Delta II tilted towards the southeast, crossing the California coastline and heading upward and out over the open Pacific Ocean. Thirty-six seconds after lift-off, the launch vehicle reached the speed of sound, and 12 seconds later the vehicle reached its point of maximum aerodynamic stress. Sixty-four seconds after lift-off, the strap-on boosters burned out, and their spent casings were jettisoned approximately 99 seconds after lift-off.

The image on the left shows the launch trajectory with a timeline of events (larger version here, PNG, 262 KB). About four minutes and 24 seconds into the flight, the first-stage engine stopped firing as the launch vehicle passed west of Mexico’s Baja California peninsula. About eight seconds later, the first stage was discarded, and about five seconds later, the second stage ignited. Four minutes and 54 seconds after launch, the launch vehicle’s nose cone, or “fairing,” separated in two halves, like a clamshell, and fell away. At about 10 minutes and 27 seconds after lift-off, the second-stage engine temporarily stopped firing, and the launch vehicle and its payload coasted for 38 minutes and 23 seconds. At this point, it was heading south and passed between the tip of South America and Antarctica.

At about 48 minutes and 51 seconds after lift-off, the second-stage engine restarted, burning 24 seconds before shutting down. At this point, the spacecraft with the second stage of the Delta still attached was in a circular parking orbit between 1317 and 1329 kilometres above Earth. The Jason-2 satellite separation occured approximately 55 minutes after lift-off. Within minutes, the tracking station at Hartebeesthoek, South Africa, picked up Jason-2’s confirmation signal.

A forward-mounted camera system onboard the launch vehicle’s second stage was turned on prior to the spacecraft separation, when it was used to track the spacecraft separation and deployment of the spacecraft’s solar arrays. Jason-2 quickly deployed its solar array, powered up its instruments and rotated its altimeter to point to geodetic nadir – essentially straight down, the shortest path to the ocean.

The Tandem Mission
Jason-2 entered orbit about 10 to 15 km below Jason-1’s 1336 km altitude. After about three weeks, Jason-2 will manoeuvre to be placed on its operational orbit in order to be behind Jason-1 by one to ten minutes.

Then, as the two spacecraft fly in line together, they will be making nearly simultaneous measurements of the same sea surface from the same altitude, allowing direct comparison of the two measurements. Careful comparisons of the data will make sure that the instruments are calibrated exactly the same, a procedure expected to take about six months. Such cross-calibration is fundamental to establishing a long data record for global climate change studies.

When the cross-calibration process is complete, Jason-1 will be commanded to move aside to a parallel ground track midway between two adjacent Jason-2 ground tracks, increasing global data coverage twofold. Jason-2 will remain in place, now seamlessly continuing the data collection begun by Topex/Poseidon in 1992.