FIRSC is a Fourier transform spectrometer (FTS) configured as a Martin-Puplett polarizing interferometer. In a Martin-Puplett configuration, dihedral (roof-top) mirrors replace the usual flat mirrors of a Michelson interferometer and a linear polarizing grid is the beamsplitter. The instrument can operate in intensity or polarization measuring mode. The Martin-Puplett configuration provides two separate input and output ports defined by reflection/transmission from input and output polarizer elements. The measured spectrum is the difference of two input ports consisting of the nadir scene and a controlled temperature blackbody source. The two output ports provide the two spectral channels. The basic instrument operation is described in the article by Vanek et al. (Appl. Opt. 40, 2169-2175, 2001). More pictures of FIRSC in the lab are available here.
FIRSC operates in the unpressurized instrument pod of the Scaled Composites Proteus aircraft. The Proteus altitude of 56,0000 ft (17 km) exceeds most summer tropopause and cirrus heights. In addition the absence of a pressure window and cabin humidity significantly reduces calibration uncertainties. The instrument has a stepper motor controlled input scene mirror to periodically view hot and cold sources for calibration.
The FIRSC instrument and measurement parameters are summarized in the table below.
|Channels||Channel 1: 10-50 cm-1|
|Channel 2: 80 - 135 cm-1|
|Spectral Resolution||0.10 cm-1 (unapodized)|
|Max Optical Path Diff.||+- 5 cm|
|Scan Time||4 sec|
|Input Aperture||5.4 cm diameter|
|Field of View||0.03 radian|
|Footprint||150 m at 5 km range|
|Detectors||Channel 1: Bolometer @ 0.3 K|
|Channel 2: Ge:Ga PC @ 4.2 K|
|Channel 1 NEdT||1.2 K rms at 30 cm-1 varying as freq-2|
|Channel 2 NEdT||1.2 K rms|
|Power||10A @ 120VAC|
The only measurement parameter change planned for CRYSTAL compared to the past AFWEX mission is to extend the bolometer channel 1 band from a 35 cm-1 cutoff to 50 cm-1. In the current Proteus instrument configuration, the control and data acquisition operates autonomously. A compact PC and Windows 2000 operating system integrates all the control, data acquisition, GPS, and CCD camera acquisition. A 5.6 GB optical disk accommodates flight data durations up to 8 hr. The data is obtained by double-sided scans of optical path difference 5 cm in 4 s with a 1 s reset time, or 12 scans per minute. At the typical Proteus speed of 150 m/s, the scan spacing along the flight track is about 0.75 km. However, the instantaneous beam footprint is much smaller, expanding in size by about 30 m/km to cloud top. The standard data sequence in an autonomously operating mode consists of 10 hot calibration scans, 10 cold target scans, and 380 nadir scene scans. This 400 scan cycle repeat time is approximately 35 min.
Establishing the absolute calibration for scene brightness temperature is an on-going process. A major purpose of the unpressurized operation is the reduction of instrument humidity and avoiding the need for a thick cabin pressure window. The current installation uses a thin (10 micron) polypropylene port window to avoid excessive air exchange for temperature stability as well as to prevent accumulation of ground moisture contamination. The absolute calibration has both random (precision) and systematic uncertainties. Our present calibration identifies three precision uncertainties arising from the detector noise (1.2 K at 30 cm-1 varying as inversely as frequency squared), from the precision of the blackbody temperatures (0.5 K), and the window radiance correction (0.5 K), which sum to an rms of 1.5 K. However, in preliminary comparisons of recent AFWEX spectra to atmospheric model spectra an additional fitting error of about 2 K is indicated. This additional error appears as varying small spectral features on a spectral slope. Thus we currently estimate the precision of about 2.5 K. The principal systematic uncertainly is the absolute emissivity of the calibration blackbodies. The blackbody targets are a carbon-filled epoxy having an inverted pyramidal surface. This material has been developed by the research group of Prof. Peter Ade of the Univ. of Wales for submm applications. Based on specular reflectivity measurements the emissivity is 0.99 with an uncertainty of 0.01 or 1%. Thus the current uncertainties in absolute calibration can be summed up as a precision of 2.5 K plus a systematic rms of 2.5 K, for a total absolute rms in brightness temperature of 3.5 K.
FIRSC is scheduled to fly on the Proteus aircraft with NAST-I and NAST-M during CRYSTAL-FACE in July 2002.
FIRSC flew on the Proteus during the ARM FIRE Water Vapor Experiment (AFWEX) in December 2000. Most of the AFWEX flights were in clear sky, but the last flight on December 8 (UTC) occurred above patchy cirrus from 6 to 10 km in altitude. A poster (250KB PDF) presented at the AMS Satellite Meteorology conference in Madison (10/17/01) describes analysis of FIRSC data from the AFWEX experiment.
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