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A tangential far infrared interferometer/polarimeter system has been upgraded for the measurement of two-dimensional electron density and toroidal magnetic field profiles on the National Spherical Torus Experiment (NSTX). The system employs a three-wavelength configuration that simultaneously produces interferometric phase shift and Faraday rotation polarimetric data from a single detector for each channel. Three CH3OH lasers (λ=119 μm) including Stark-tuned laser cavities are optically pumped by a CO2 laser tuned to 9.7 μm. For high time-resolution measurements, the modulation frequency in the heterodyne interferometer is operated at ∼ 7 MHz via the Stark-tuned laser, serving as the local oscillator in this system. Unwanted phase changes introduced by mechanical vibrations originating from various sources were eliminated by extensive work including a specially designed vibration-free stand for the retroreflector. The system is designed to have a full remote control capacity using the LABVIEW program via fiber links and real time signal processing circuits with a tracking receiver technique. A high IF frequency and a low noise fringe counting system enabled the system to measure wide-band density fluctuations such as microturbulence and magnetohydrodynamics. In this paper, a comparison study of the measured electron density with Thomson scattering data and para/diamagnetism effects measured via polarimetry signal and compared with the calculated value using the EFIT equilibrium code are presented. A recent upgrade plan together with the technical details of the system development will also be described.