Status
The Submillimeterwave Limb Sounder (SLS) is a heterodyne radiometer measuring thermal emission spectra near 640 GHz (for detection of ClO, HCl, and O3) and 604 GHz. (for detection of HNO3 and N2O) designed for use on high altitude balloons and aircraft. The instrument consists of five subsystems:
-optics which define the instrument field of view (FOV)
-radiometer front-ends which down converts incoming radiance signals
-intermediate frequency (IF) stage which selects and frequency shifts signal bands
-spectrometers which frequency resolve and detect the incoming power spectrum
-command and data handling which controls the instrument and transmits data to the ground
Limb scanning is accomplished by a flat mirror (~20 cm diameter) connected to a stepper motor (0.2 steps) and 14 bit position encoder. This mirror is also used for gain and zero calibration by viewing an absorber target located below the mirror and upward at 47° elevation angle to view the cold sky. A set of three off-axis parabolic reflectors form the instrument field of view (0.35 full width at half maximum) and couple limb radiance to the mixer input waveguide. These reflectors are oversized (~30 dB edge taper) to minimize side lobes in the FOV. Pointing and beam shape were verified by scanning the instrument FOV across the emission from a 600 GHz transmitter (multiplied output of a Gunn oscillator) located in the receiver optical far-field.
The radiometer front-end is an uncooled second harmonic mixer using a waveguide mounted Schottky diode. The radiometer is operated double side band (DSB), i.e., spectral features occurring symmetrically above and below the effective local oscillator frequency (637.050 GHz) appear together in the IF output spectrum. The diode is pumped at a 318.525 GHz. This source is generated by a tripled 106.175 GHz phase-locked InP Gunn oscillator and wave guide coupled to the mixer block. The mixer produces an IF output spectrum of 10.5 to 13 GHz, which corresponds to signals at the mixer input at 647.5 GHz to 650.0 GHz (in the radiometer upper side band) and 626.5 GHz to 624.1 GHz ( in the lower side band). The design of the 604 GHz radiometer system is similar to 637 GHz system but operates at a lower IF frequency of 2 to 3 GHz.
Diagram of the SLS frequency down-conversion scheme. RF signals enter the signal flow path through mixer feeds at the left of the diagram. At the right side, the signal flow enters a set of UARS MLS-type filterbank spectrometers where bands are further spectrally resolved, power detected, and digitized.
