IMOS - Satellite Remote Sensing - Ocean colour Sub-Facility

The Aqua satellite platform carries a MODIS sensor that observes sunlight reflected from within the ocean surface layer at multiple wavelengths. These multi-spectral measurements are used to infer the concentration of chlorophyll-a (Chl-a), most typically due to phytoplankton, present in the water. The data are produced from the near real time (nrt) data stream formed by combining data from all the available direct broadcast reception stations in Australia (Alice Springs, Melbourne, Townsville, Perth, Hobart). The data are presented as a sequence of granules, each with a maximum size corresponding to 5 minutes of data, and beginning on UTC-5 minute boundary (eg. 04:05, 04:10, 04:15). The granules have been remapped from satellite projection into a geographic (Latitude/Longitude axes) projection and are formatted as CF-compliant netCDF files. It should be noted that the data are not processed until the definitive spacecraft ephemeris becomes available, usually 12-24 hours after the overpass. This means that the geolocation should be of a uniformly high standard.There are multiple retrieval algorithms for estimating Chl-a. These data use the OC3 method recommended by the NASA Ocean Biology Processing Group and implemented in the SeaDAS processing software l2gen. The radiometric sensitivity of the MODIS sensor is evolving continuously during its mission and is being monitored regularly. The SeaDAS software uses tables of calibration coefficients that are updated periodically. From time to time upgrades to the algorithms and thand/or the format of the calibration tables are required, in which case a new version of SeaDAS is released. These data are initially being produced using SeaDAS 6.1, then SeaDAS 6.4 and eventually with SeaDAS 7.x. The data processed with each different version can be distinguished by the digits at the end of the root URL for the data; for example l2oc.nrt.61 is level-2 ocean colour (l2oc), using the near real time (nrt) data stream, produced using SeaDAS 6.1.