  The Primordial Anisotropy Polarization Pathfinder Array (PAPPA) is a
balloon-based instrument to measure the polarization of the cosmic microwave
background and search for the signal from gravity waves excited during an
inflationary epoch in the early universe. PAPPA will survey a 20 x 20 deg patch
at the North Celestial Pole using 32 pixels in 3 passbands centered at 89, 212,
and 302 GHz. Each pixel uses MEMS switches in a superconducting microstrip
transmission line to combine the phase modulation techniques used in radio
astronomy with the sensitivity of transition-edge superconducting bolometers.
Each switched circuit modulates the incident polarization on a single detector,
allowing nearly instantaneous characterization of the Stokes I, Q, and U
parameters. We describe the instrument design and status.
