NASA Earth Observing System (EOS) Program began circa 1986. The director of the Earth Observing Directorate at NASA, Shelby Tilford, wanted to make a series of satellites to observe the whole world several times a day with high accuracy. The three satellites were EOS Terra (to observe the surface), EOS Aura (to observe the chemistry), and EOS Aqua (to observe water). On Earth, water is the most abundant and most important greenhouse gas. NASA launched the EOS Aqua satellite on May 4, 2002. TRW (now NGST) built the Aqua spacecraft.
The orbit of the Aqua satellite is polar sun-synchronous. At the same time very day, Aqua passes overhead with the Sun in the same position. This is important because clouds change altitude with time of day, and having the clouds at different altitudes on different days would make day-to-day comparisons difficult. The satellite’s ascending node is 1:35 p.m. and descending node is 1:35 a.m. This means that Aqua passes over the equator going North at 1:35 p.m. (ignoring the tilt of the Earth’s axis), and going South at 1:35 a.m.
An altitude of 705 kilometers gives an orbital period of 98.8 minutes, completing approximately 14.5 orbits per day. The repeat cycle period is 233 orbits (16 days) with a ground track repeatability of ±20 kilometers.
Aqua shares an orbit with several other satellites. The entire constellation of satellites is the A-train (‘A’ for ‘Afternoon’), with each satellite following the next. In approximately 2022, Aqua will run out of sufficient propellant to safely maintain its position in the A-train, so NASA will lower the orbit to avoid a possible collision with other A-train satellites.
Since many instruments share the Aqua spacecraft, each Aqua instrument has independent scan mirrors.
Other Instruments on Aqua
- AMSR-E – Advanced Microwave Scanning Radiometer-EOS
- MODIS - Moderate Resolution Imaging Spectroradiometer
- CERES - Clouds and the Earth's Radiant Energy System.
The instruments on the Aqua spacecraft provide several complementary data products. Compared to AIRS, AMSR-E provides higher spatial resolution measurements of sea surface temperature, total column water vapor (no profile information), cloud liquid water, and detection of sea ice, but AMSR-E retrievals are available over ocean only. CERES measures the total upwelling radiation in the longwave and shortwave and uses MODIS to assist in the calculation. CERES infrared outgoing longwave radiation (OLR) agrees well with OLR from AIRS. MODIS and AIRS are often used together to characterize cloud properties. MODIS has much finer spatial resolution than AIRS, but MODIS is less sensitive to ice clouds because of coarser spectral resolution. When AIRS infrared spectra are combined with microwave spectra from AMSU and HSB, the AIRS retrieval system provides ‘cloud-cleared’ high vertical resolution temperature and water vapor profiles, trace gases, and cloud and surface properties. Microwave instruments within the AIRS suite are less sensitivity to clouds, but have broader vertical weighting functions, so microwave-only retrievals are of coarser vertical resolution but wider spatial coverage than the standard AIRS products.