  We present a precise photometric calibration of the first 1.5 years of
science imaging from the Pan-STARRS1 survey (PS1), an ongoing optical survey of
the entire sky north of declination -30 degrees in five bands. Building on the
techniques employed by Padmanabhan et al. (2008) in the Sloan Digital Sky
Survey (SDSS), we use repeat PS1 observations of stars to perform the relative
calibration of PS1 in each of its five bands, solving simultaneously for the
system throughput, the atmospheric transparency, and the large-scale detector
flat field. Both internal consistency tests and comparison against the SDSS
indicate that we achieve relative precision of <10 mmag in g, r, and i_P1, and
~10 mmag in z and y_P1. The spatial structure of the differences with the SDSS
indicates that errors in both the PS1 and SDSS photometric calibration
contribute similarly to the differences. The analysis suggests that both the
PS1 system and the Haleakala site will enable <1% photometry over much of the
sky.

Instrumentation and Methods for Astrophysics