  We describe the PDFI_SS software library, which is designed to find the
electric field at the Sun's photosphere from a sequence of vector magnetogram
and Doppler velocity measurements, and estimates of horizontal velocities
obtained from local correlation tracking using the recently upgraded FLCT code.
The library, a collection of Fortran subroutines, uses the "PDFI" technique
described by Kazachenko et al. (2014), but modified for use in spherical,
Plate-Carr\'ee geometry on a staggered grid. The domain over which solutions
are found is a subset of the global spherical surface, defined by
user-specified limits of colatitude and longitude. Our staggered-grid approach,
based on that of Yee (1966), is more conservative and self-consistent compared
to the centered, Cartesian grid used by Kazachenko et al. (2014). The library
can be used to compute an end-to-end solution for electric fields from data
taken by the HMI instrument aboard NASA's SDO Mission. This capability has been
incorporated into the HMI pipeline processing system operating at SDO's JSOC.
The library is written in a general and modular way so that the calculations
can be customized to modify or delete electric field contributions, or used
with other data sets. Other applications include "nudging" numerical models of
the solar atmosphere to facilitate assimilative simulations. The library
includes an ability to compute "global" (whole-Sun) electric field solutions.
The library also includes an ability to compute Potential Magnetic Field
solutions in spherical coordinates. This distribution includes a number of test
programs which allow the user to test the software.
