  We improve predictions of the cosmic microwave background (CMB) power
spectrum induced by cosmic strings by using source terms obtained from
Nambu-Goto network simulations in an expanding universe. We use three
high-resolution cosmic string simulations that cover the entire period from
recombination until late-time $\Lambda$ domination to calculate unequal time
correlators (UETCs) for scalar, vector and tensor components of the
cosmic-string energy-momentum tensor. We calculate the CMB angular power
spectrum from strings in two ways. First, to aid comparison with previous work,
we fit our simulated UETCs to those obtained from different parameter
combinations from the unconnected segment model and then calculate the CMB
power spectra using these parameters to represent the string network. Second
and more accurately, we decompose the UETCs into their corresponding
eigenvalues and eigenvectors and input them directly into an Einstein-Boltzmann
solver to calculate the power spectrum for each of the three simulation time
periods. We combine the three simulations together, using each of them in its
relevant redshift range and we obtain overall power spectra in temperature and
polarisation channels. Finally, we use the power spectra obtained with the
latest Planck and BICEP2 likelihoods to obtain constraints on the cosmic string
tension.
