An FTS for Climate Monitoring
The crucial requirement for climate monitoring from space is to achieve the required accuracy and, even more importantly, to demonstrate in flight that this accuracy has been achieved. We summarize here a system based upon simple Fourier transform spectrometers (FTSs) that satisfies this requirement. It is based upon three general principles:
- simplicity of design: the fundamental comparison between black-body standards and the observed scene should be as uncomplicated as possible.
- redundancy in essential systems, combined with on-board diagnostics.
- parallel laboratory and field activities together with continuous engagement of the climate community in a critique of procedures and results.
The instrument configuration discussed here employs two small FTSs. The two instruments are bore-sighted on Earth’s surface. Each has its own electronics, can view space over a 45º range of angles and has two independent black bodies. Several in-flight diagnostic features are included. The key attributes of the instruments are shown in Table 1.
The experimental physics and technology that underlie this design have been developed by many researchers. The key elements include research on thermodynamic (Nicholas, 1995) and practical thermometry (Rusby et al., 1991), blackbody standards (Mason et al., 1996; Fowler 1995; Best et al., 1997), calibration procedures (Revercomb et al., 1993, 1997; Kannenberg et al., 1998; Villemaire et al., 1997), and detector developments that improve linearity (Whatamore 1986; Porter 1981). In addition, the special requirements of climate monitoring admit simplified optical design by minimizing the number of optical elements, diminishing alignment sensitivity, compensating for thermal gradients, and reducing off-axis response. Simplicity of design facilitates the inclusion of redundant calibrations, a pragmatic means to achieve climate Benchmark requirements (Keith and Anderson, 2001). The key strategic point is that each experimental variable that determines measurement accuracy is checked on-orbit by redundant but independent methods (Anderson et al., 2003).
| Table 1. Key instrument attributes | |
| Attribute | Value |
|---|---|
| FTSs | Two |
| blackbody accuracy | >< 0.1 K |
| spectral range | 225-600 cm-1 |
| spectral resolution (unapodized) | >0.6 cm-1 |
| aperture | 2.5 cm |
| field of view (half angle) | 65 mrad |
| footprint | 100 km |
The particular instrument configuration discussed here (Figure 4) draws upon studies of small satellites (Anderson et al., 1996), atmospheric spectroscopy (Hu et al., 1999), redundant calibrations (Dykema et al., 1999), flight intercomparisons (Hu et al., 2000) and detector technologies (Keith et al., 2001).
Figure 4. Instrument configuration. A key design feature is the use of dual FTSs bore-sighted on the same nadir footprint. Each instrument is a four-port, cube-corner FTS, each with two blackbodies that overdetermine the calibration, and with a deep space view over 45° to calibrate polarization errors.
