  The Prime Extragalactic Areas for Reionization and Lensing Science (PEARLS),
a JWST GTO program, obtained a set of unique NIRCam observations that have
enabled us to significantly improve the default photometric calibration across
both NIRCam modules. The observations consisted of three epochs of 4-band
(F150W, F200W, F356W, and F444W) NIRCam imaging in the Spitzer IRAC Dark Field
(IDF). The three epochs were six months apart and spanned the full duration of
Cycle 1. As the IDF is in the JWST continuous viewing zone, we were able to
design the observations such that the two modules of NIRCam, modules A and B,
were flipped by 180 degrees and completely overlapped each other's footprints
in alternate epochs. We were therefore able to directly compare the photometry
of the same objects observed with different modules and detectors, and we found
significant photometric residuals up to ~ 0.05 mag in some detectors and
filters, for the default version of the calibration files that we used
(jwst_1039.pmap). Moreover, there are multiplicative gradients present in the
data obtained in the two long-wavelength bands. The problem is less severe in
the data reduced using the latest pmap (jwst_1130.pmap as of September 2023),
but it is still present, and is non-negligible. We provide a recipe to correct
for this systematic effect to bring the two modules onto a more consistent
calibration, to a photometric precision better than ~ 0.02 mag.
