TERAHZ LIMB SOUNDER (TLS) IS A LOW-MASS LOW-POWER HIGH SPECTRAL RESOLUTION HETERODYNE SPECTRO-POLARIMETER OPERATING IN THE.TERAHZ (THZ) FREQUENCY REGIME DESIGNED TO REMOTELY MEASURE THERMOSPHERIC WIND TEMPERATURE AND ATOMIC OXYGEN DENSITY PROFILES.FROM A LOW-EARTH ORBIT PLATFORM. IT RESOLVES THE DOPPLER LINE SHAPES OF THE ATOMIC OXYGEN EMISSIONS AT 2.06 THZ (145 MICRON) AND.4.7 THZ (63 MICRON) AND USES THEIR POLARIMETRIC PROPERTIES TO IMPROVE ITS MEASUREMENT PRECISIONS. UNDER THE SUPPORT OF PREVIOUS G/IDES PROGRAM (2/2013-1/2016) AND H-TIDS PROGRAM (3/2016-2/2019) WE HAVE DESIGNED AND FABRICATED A RECEIVER PROTOTYPE FOR THE 2 THZ CHANNEL OF TLS AND SUCCESSFULLY DEMONSTRATED ITS TECHNOLOGY FEASIBILITY AND PERFORMANCE. WHILE THE 2 THZ CHANNEL OF TLS IS OPTIMIZED TO PROVIDE LOWER THERMOSPHERIC MEASUREMENTS WE PROPOSE HERE TO DEVELOP THE 4 THZ CHANNEL AND DEMONSTRATE ITS