Mission: Formation Flying
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.
Formation flying enables CloudSat to track CALIPSO in a very precise way. After launch, maneuvers in the first days of the mission were performed to bring the CloudSat spacecraft in formation with CALIPSO, which, in turn, assumed 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 is monitored and periodically adjusted 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 orbits 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 overlaps 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 cross the equator within a few minutes of one another at around 1:30 p.m. local solar time. Since these missions 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.