Colocation

Satellite measurements made onboard different A-train satellite platforms may be intercompared by mapping the points to the same geolocation on the Earth's surface.

(See Great Circle Distance & Laws of Cosines for more details)

Below is a diagram showing a measurement points / pixels undertaken by the CERES satellite instrument. These points lie along the orbital track of MLS onboard the Aura satellite platform. A red line indicates the +/-12km across-track ground track of MLS, where satellite measurements are suitable for comparison with CERES onboard the Aqua platform. The Aura and Aqua satellites overlie the same footprint on the Earth's surface within a 15 minute time window, making close measurement comparisons possible for cloudy scenes of atmosphere.

The CloudSat and CALIPSO satellites form the central formation of the A-train and are just seconds apart. This makes their detailed measurements of clouds easily inter-comparable and enables the scientific user to neglect the effects of atmospheric dynamics. CloudSat and CALIPSO were currently 200km off-set from the A-train track, so that comparisons with MODIS and CERES were limited, but were reconfigured in space during Summer 2008 and are now 7/8 minutes behind Aqua. The intersection of satellite measurements with MODIS and CERES footprints now allow detailed cloud property comparisons from the A-Train instruments.

The figure below shows the MLS central track during the period 2300:2305UTC over the tropical Pacific coast of Central-America during the equinox period of September 19th, 2004. Overlayed is a set of colocated measurements undertaken by CERES onboard the Aqua satellite. The color scale refers to the outgoing thermal emission in W/m2. The color scale ranges from deep blue within convective cores of the tropics with emission at ~100W/m2, to red values in the subsidence regions of ~400W/m2. Intersatellite-comparisons undertaken from the A-Train are very useful in demonstrating a clear, direct relationship between clouds and the radiation budget. These comparisons include infering links between MLS ice water content measurements undertaken from the Aura satellite with CERES measurements of longwave emission at the top-of-atmosphere undertaken onboard the Aqua platform.