TES is one of four science instruments aboard NASA's Aura
satellite, which was launched from Vandenberg Air Force Base, California on July 15, 2004. The satellite flies at an altitude of 705 km (438 miles) in an orbit that takes it near Earth's North and South Poles. With each orbit, the spacecraft advances 22° westward. After 233 orbits (16 days), it is back at its starting point and the pattern repeats. Thus, every 16 days, Aura re-examines the same portions of the atmosphere, and its instruments are able to measure changes that may have occurred in each sampled area. The satellite and its instruments are scheduled to perform their atmospheric studies for five years.
Aura is one of a series of satellites in NASA's Earth Observing System (EOS), which supports understanding of the Earth as an integrated system by observing its land surfaces, biosphere, atmosphere and oceans. EOS, in turn, is the principal element of Earth Science Enterprise, an international effort to understand our planet's climate system.
Aura is in a "sun-synchronous" orbit that brings the satellite over each latitude at the same local "solar mean time" each day. For example, each time Aura crosses the equator, the local solar mean time is about 1:43 PM. This means that the sun is as close as possible to the same angle (given the constant changing of the sun's angle due to the tilt of Earth's axis) each time TES is over a given point on the ground. Sunlight plays a critical role in many of the chemical reactions that affect air quality, and keeping the sun at a nearly constant angle for each observation of the same location simplifies interpretation of the data.
Some of Aura's instruments look ahead of the spacecraft, some look behind it, and some look straight down. All four observe approximately the same column of air within about 13 minutes, though they differ somewhat as to the altitudes they cover, so they complement each other's findings.
In principle, TES is capable of observing chemicals at any altitude, but in practice it specializes in the portion of the atmosphere that extends from the ground up to the middle of the stratosphere, to an altitude of about 32 km. This includes the entire troposphere, the lowest part of the atmosphere, which stretches from the stratosphere to Earth's surface.
TES observes both straight down (nadir view) and at a sideways angle (limb view) behind the satellite. Limb viewing provides a much longer path through the atmosphere, and looking through a larger mass of air improves the chances of observing sparsely distributed substances that might be missed in the nadir view. Limb viewing's sideways angle also makes it easier to determine the altitudes of the observed substances. But limb viewing is very susceptible to interference (only rarely does the line of sight reach the surface). Nadir viewing is less impacted by clouds, but looking straight down makes it more difficult to identify altitudes.