The WMAP satellite ( Wilkinson Microwave Anisotropy Probe ) was an American mission launched on 30 June 2001. It made very precise measurements of the cosmic microwave background.
The WMAP satellite was made up of two practically identical optical units that observed the sky in opposite directions. This made it possible to eliminate the systemic asymmetries that could have been due to the orientation of the satellite when pointing in whatever direction it was observing. Each optical system focused the area of sky being examined on to antennas tuned to five narrow frequency bands. Furthermore, the output of each antenna was separated by two orthogonal polarisations. Some antennas analysed in parallel directions, others in directions at 45°.
This provided complete maps of the sky in each band together with polarisation data. About 98.7% of the data was useful, the rest being unusable because of various malfunctions. 0.1% of the data was collected while passing in front of a planet, and this data was used for calibration rather than for map making.