Delta 2 finally launches

by Chris Bergin

The Delta 2 rocket, carrying CloudSat and CALIPSO into the A-Train orbit, from US Air Force Base Vandenberg in California, finally launched this morning at 11:02am UK time, following multiple delays. covered the launch as a live event, with live updates and images – plus a free launch video replay is now available on the link below.



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Earlier attempts

With just 47 seconds left in the count on the first attempt, the Delta 2 with CloudSat and CALIPSO onboard suffered a launch scrub due to communications being lost between CALIPSO and its French assets.

Launch was scheduled for 11:02 UK Time. Another attempt was to be made in 24 hours, but that is off due to the unavailability of the ‘instrumentation aircraft’ on Saturday..

Sunday morning was hoping to bring another launch attempt. However, issues with the downrange caused the launch to be delayed yet again, and a fourth attempt on Monday morning was ruled out due to ‘crew rest requirements.’

The next attempt, with just a few minutes to go before launch, a weather balloon noted upper level winds were out of limits – leading to a scrub for the day. Another attempt was be made 24 hours later, which in turn was moved down another day due to bad weather.

The last attempt, on Thursday, was halted due to a sensor fault.

Preview (various NASA information)

The CloudSat and CALIPSO satellites will fly in orbital formation as part of what is known as the ‘A-Train’-a constellation of five (eventually six) Earth Observing satellites, including NASA’s Aqua and Aura satellites and France’s PARASOL satellite. (NASA’s Orbiting Carbon Observatory mission is planning to join the formation in a few years.)

Each of these missions has its own unique objectives and will improve our understanding of aspects of Earth’s climate. The data from the various satellites are synergistic; combining the observations of the CloudSat radar with information from CALIPSO and Aqua is a key aspect of the observing philosophy of the CloudSat mission.

The usefulness of data from CloudSat, CALIPSO, and the other satellites of the A-train will be much greater when combined. Joint observations will allow scientists to better understand the impact of aerosols on climate and how sources of local pollution affect air quality.

The combined set of measurements will also provide new insights into the distribution and evolution of clouds over the globe that will lead to improvements in weather forecasting and climate prediction.

The data will be used to study interactions between aerosols and clouds that may change the amount of sunlight they reflect and absorb, or enhance or suppress rainfall-subjects of current scientific debate.

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NASA’s Langley Research Center in Virginia is leading the CALIPSO mission, providing overall project management and collaborating with NASA’s Goddard Space Flight Center in Maryland, CNES, Ball Aerospace, Hampton University and the Institut Pierre Simon Laplace in France.

Formation flying enables CloudSat to track CALIPSO in a very precise way. After launch, maneuvers within the first 30-45 days of the mission will bring the CloudSat spacecraft in formation with CALIPSO, which is, in turn, assuming a formation position with respect to Aqua.

The three satellites are in very nearly the same orbit but with each satellite positioned along the orbit with separation distances from one another that remain relatively fixed. The CloudSat orbit will be adjusted and monitored to hold the CloudSat spacecraft at an approximate fixed distance from CALIPSO. The spacecraft will be controlled so that both sets of sensors, along with those of Aqua, view the same ground track for the majority of the time.

As the A-Train of satellites circles Earth, about 8 minutes pass between the time the first satellite (Aqua) and the last (Aura) passes over any given spot. Overall the string of satellites stretches across 3,000 kilometers (1,864 miles) in space, with each traveling about 7 kilometers per second (4.3 miles per second). The CloudSat radar trails Aqua by an average of about 60 seconds.

CloudSat will fly approximately 460 kilometers (286 miles) behind Aqua and much closer in front of CALIPSO-only around 93.8 kilometers (58.3 statute miles) away, which corresponds to 12.5 seconds average delay between lidar and radar measurements.

The chosen delay is a compromise between the desire to minimize the time delay between the radar and lidar measurements and the need to reduce the complexity in the implementation of formation flying.

In this way, the radar footprint will overlap the lidar footprint more than 50 percent of the time, creating coordinated and essentially simultaneous measurements. Maneuvers to maintain this circulation orbit will be carried out approximately weekly.

All of the five A-Train satellites will cross the equator within a few minutes of one another at around 1:30 p.m. local solar time. Since these missions will all fly in tandem, the set of satellites is referred to as a constellation, or formation.

Since this constellation is composed of missions with equator crossings in the early afternoon and also in the middle of the night (at about 1:30 a.m.), it is referred to as the Afternoon Constellation-to distinguish it from the Morning Constellation, consisting of Terra, Landsat-7, Satelite de Aplicaciones C and the New Millennium Program’s Earth Observing-1 spacecraft.

The term ‘A-Train’ comes from an old jazz tune, ‘Take the A-Train’, composed by Bill Strayhorn and made popular by Duke Ellington’s band, and has become a popular nickname for the Afternoon Constellation because Aqua is the lead member of the formation and Aura is in the rear.

The geometry of Aqua’s orbit (and thus, that of CloudSat and CALIPSO) has been selected to yield an orbital period such that the groundtrack repeats every 233 orbital revolutions or equivalently every 16 days, with a fixed orbital period of 98.88 minutes. The orbit is subject to small perturbations, mostly due to atmospheric drag on the satellite.

Aqua’s groundtrack is carefully aligned with the World Reference System (WRS-2) grid. CloudSat and CALIPSO use the same sun-synchronous, grid-aligned type of orbit as Aqua. The main difference is that the CloudSat/CALIPSO orbit is shifted 215 km (along the equator) east of Aqua’s equator crossing, in order to avoid a sun-glint problem that would impact observations by the CALIPSO satellite.


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