Far-Infrared Spectroscopy of the Troposphere

SDL worked with the NASA Langley Research Center (LaRC) and the Smithsonian Astrophysical Observatory (SAO) to perform a balloon study called Far–Infrared Spectroscopy of the Troposphere (FIRST). The FIRST sensor is part of NASA’s Instrument Incubator Program (IIP) and is a precursor to a satellite based version that will provide daily global coverage.

FIRST is a long–wave imaging spectrometer that measures between 10–100 µm, filling a gap in the existing systems that monitored the Earth’s climate. Previously, NASA spacecraft measured the spectral distribution of the Earth's radiant energy that disperses into space from about 2 µm to 25 µm. FIRST has a noise equivalent temperature difference (NETD) goal of 0.2 K from 10 to 100 µm. The spectral resolution will allow simultaneous retrievals of temperature and water vapor profiles. A 10 x 10 array of 10 km IFOVs is desired to isolate clear and cloudy fields of view while providing the capability for daily global coverage.

The FIRST IIP sensor was developed in partnership with LaRC, SAO and SDL. SDL developed and fabricated the Fourier Transform Spectrometer (FTS) portion of the sensor. SAO developed and provided the pellicle beamsplitter film – a film capable of transmission and reflection over the extremely wide pass band of the FIRST sensor. LaRC provided the innovative focal plane array, which was based on Winston cone flux concentrator technology, and the focal plane’s helium dewar.

Drawing from 25 years of experience in interferometer technology, SDL engineered the FTS with radiometric calibration in the spectral range from 10 to 100 µm (1000 to 100 cm 1) at 0.6 cm–1 unapodized resolution. FTS incorporates a broad bandpass beamsplitter and a high–throughput optical and detector system. This key technology will allow for data acquisition from this critical range ensuring FIRST meets its primary objective.

SDL developed, integrated, and calibrated the instrument, and supported the balloon flight during the IIP portion of the program. The balloon was launched in June 2005 and the instrumentation successfully reported the vital and highly anticipated data. The resulting interferograms and encouraging comparisons with similar data from a coordinated satellite, has firmly established SDL as an industry leader in innovative space sensor technology.

Why study this range? The radiative balance of the troposphere, and hence climate, is dominated by the infrared absorption and emission of water vapor, particularly at far–infrared (far–IR) wavelengths longer than 15 µm. Half of the Earth’s outgoing long–wave radiation occurs beyond 15.4 µm (650 cm–1). Determining the distribution of water vapor and its far–IR radiative forcings and feedbacks in this range could resolve uncertainties in understanding and predicting future climate and aid in understanding the greenhouse effect.

FIRST launch image FIRST launch image
FIRST launch image