The Geostationary Fourier Transform Spectrometer

Abstract

The Geostationary Fourier Transform Spectrometer (GeoFTS) is an imaging spectrometer designed for an earth science mission to measure key atmospheric trace gases and process tracers related to climate change and human activity. The GeoFTS instrument is a half meter cube size instrument designed to operate in geostationary orbit as a secondary "hosted" payload on a commercial geostationary satellite mission. The advantage of GEO is the ability to continuously stare at a region of the earth, enabling frequent sampling to capture the diurnal variability of biogenic fluxes and anthropogenic emissions from city to continental scales. The science goal is to obtain a process-based understanding of the carbon cycle from simultaneous high spatial resolution measurements of carbon dioxide (CO 2 ), methane (CH 4 ), carbon monoxide (CO), and chlorophyll fluorescence (CF) many times per day in the near infrared spectral region to capture their spatial and temporal variations on diurnal, synoptic, seasonal and interannual time scales. The GeoFTS instrument is based on a Michelson interferometer design with a number of advanced features incorporated. Two of the most important advanced features are the focal plane arrays and the optical path difference mechanism. A breadboard GeoFTS instrument has demonstrated functionality for simultaneous measurements in the visible and IR in the laboratory and subsequently in the field at the California Laboratory for Atmospheric Remote Sensing (CLARS) observatory on Mt.Wilson overlooking the Los Angeles basin. A GeoFTS engineering model instrument is being developed which will make simultaneous visible and IR measurements under space flight like environmental conditions (thermal-vacuum at 180 K). This will demonstrate critical instrument capabilities such as optical alignment stability, interferometer modulation efficiency, and high throughput FPA signal processing. This will reduce flight instrument development risk and show that the GeoFTS design is mature and flight ready.

Near-infrared remote sensing of Los Angeles trace gas distributions from a mountaintop site

“…The mountaintop viewing geometry offers two significant advantages relative to direct solar measurements from ground sites in the boundary layer: enhanced sensitivity to composition changes within the boundary layer due to the long low-altitude optical path lengths (tens of km), and the capability to independently measure the composition within the free troposphere to provide background reference information. Apart from differences in viewing angles, the mountaintop vantage point also closely approximates the observations from geostationary orbit by space missions that will be launched later this decade (Key et al, 2012).…”

Stray Light Analysis and Suppression for an Infrared Fourier Imaging Spectrometer

“…The point source transmittance (PST) is a transfer function that is commonly used to describe the effects of stray light on optical systems [31]. The PST is equal to the total amount of stray light incident on the focal plane of an optical system divided by the total amount of light incident on the optical system that enters the pupil, and it can be calculated as follows:…”

Design and integration of an adaptive controller for a Fourier Transform Spectrometer

“…The CIRIS design benefits from the ability to operate without the need for a reference laser, a feat which greatly reduces the system's physical and (as illustrated above) electronic footprint. Compared to existing state-of-the-art designs such as those seen in [3], [10], [17], [18], [19], [20], [21], [22], [23], CIRIS and its controller show promise of achieving similar performance with more compact optics and reduced electronic complexity.…”