March 09, 2011

Satellites You Need to Know: Climb Aboard the A-Train

Today we are kicking off a new occassional series, "Satellites You Need to Know," which, as you may expect, explores satellites that we think you should know. It's important to understand what capabilities America needs to advance our interests in the natural security arena. And understanding our space assets is generally good practice for all security types.

The Afternoon Train, often called the “A-Train,” is a constellation of satellites that closely follow one another in an orbit that crosses the equator around 1:30 p.m. local time each day. The mission: understanding the earth's climate. The constellation currently is made up of five satellites; four that NASA operates - Aqua, CloudSat, CALIPSO (in cooperation with CNES) and Aura - along with one satellite, PARASOL, that is run by the French space agency CNES. The Glory satellite that Christine explored last week would have been A-Train’s sixth satellite. Two additional satellites, the GCOM-W1 and the OCO-2, are set to be launched in 2012 and 2013, respectively. With the satellites measuring different phenomena, scientists are able to get a fuller perspective on various factors affecting the climate.Image from A-Train

Aqua was launched into a low orbit from Vandenberg Air Force Base in California on May 4, 2002, and is the lead satellite in the constellation’s orbit. Its name comes from its mission which focuses primarily on collecting data on water in all its forms (sold, liquid and vapor), and during all phases of the water cycle. All together, the six instruments on board transmit 89 GB of data a day. Polar stations on the ground receive this data, and then transmit them to the Goddard Space Flight Center (GSFC) in Greenbelt, Maryland. Most of the data is processed at the GSFC while the remainder is sent to the Langley Research Center or Japan’s Earth Observation Center (EOC). Aqua’s mission life was originally scheduled to be only five years, but it continues to produce data.

The Aura satellite was the second of the A-Train satellites to be launched into orbit on July 15, 2004, but it is the last satellite in the constellation’s orbit, typically trailing Aqua by fifteen minutes. Aura collects information on the earth’s ozone and air pollution and, along with data provided by past satellites, allows scientists to assess how these have changed over the  decades. For example, this data has allowed the scientific community to conclude that between 1980 and 2000 ozone decreased by 3% on average, with this number reaching 50% around the Arctic region. In turn, this information is used to assess how factors such as ozone depletion and air pollution affect the earth’s climate. Similar to Aqua, Aura’s data is sent to polar stations that then transmitted to the GSFC. Its mission life was also originally scheduled to be in the five year range.

CALIPSO and CloutSat were launched together from Vandenberg Air Force Base on April 28, 2006. Their complementary missions (pdf) measure how clouds form and are distributed, and how this affects the earth’s climate. As NASA’s website makes clear, clouds have an enormous impact (pdf) on the earth’s climate as the key regulator of earth’s average temperature. Depending on the type of cloud and its distribution, it can have a cooling or warming affect on earth’s climate.  

CloudSat is the first satellite to capture clouds in a 3-D perspective. According to NASA, this data would allow scientists to “answer questions about how they [clouds] form, evolve and affect our weather, climate and freshwater supply.” CALIPSO, or the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, collects information on how aerosols affect the properties of clouds and climate. Its greatest contribution is being the first satellite that tracks the altitude of aerosols. The altitude of aerosols is highly consequential for determining how much, and in what ways, they will affect the climate. At low altitudes they are abundant but have a short duration as they get washed away by rain quickly. At higher altitudes, they are rare but travel across great distances, affecting the earth below them.  

PARASOL, or the Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar, was launched into orbit by the French Space Agency CNES in December 2004. Its instruments study how aerosols polarize the sun’s radiation, and how this “regulates” Earth’s radiation balance. Although originally conceived of as a two-year mission, it celebrated its sixth birthday in space last year. Beginning in December 2009, however, it has slowly been moving out of A-Train’s orbit. Glory would have assumed PARASOL’s spot in the A-Train’s mission had it not failed to reach orbit.

A-Train helps demonstrate the integral role earth observation satellites play in helping us better understand the climate and how it is changing. The complexity and complementary missions of each A-Train satellite also helps illustrate how big a loss Glory’s failure was for climate research. As we dig through the different systems and their uses, we’ll continue to provide updates on the satellites you need to know.

Photo: Image of dust storms off the coast of Africa, taken by A-Train satellites. Courtesy of NASA.