Map Making with Differential Data WMAP observes temperature
differences between points separated by ~141° on the sky. Maps
of the relative sky temperature are reconstructed from the
difference data using a modified form of the algorithm adopted
by COBE-DMR. The algorithm WMAP uses to reconstruct sky maps
from differential data is an iterative one. In the limit that
the instrument noise is white (uncorrelated from one sample to
the next in time) the algorithm is mathematically equivalent to
a least squares fit of a set of temperature differences to a set
of map pixel temperatures. However, the implementation does not
require the evaluation or inversion of large matrices and has a
very intuitive interpretation as follows. The actual signal WMAP
measures is the temperature difference between two points on the
sky, DT = T(A)-T(B), where T(A) is the temperature seen by feed
A, and likewise for B. Feed A can be thought of as viewing the
sky while feed B can be thought of as viewing a comparative
reference signal, o