TABLE C.1 Specifications of Spaceborne Instruments Capable of Measuring CO2

Specification

OCOa

GOSATb

SCIAMACHYc

AIRSd

IASIe

Tropospheric gases measured

CO2, O2

CO2, CH4, O2, O3, H2O

O3, O4, N2O, NO2, CH4, CO, CO2, H2O, SO2, HCHO

CO2, CH4, O3, CO, H2O, SO2

CO2, CH4, O3, CO, H2O, SO2, N2O

CO2 sensitivity

Total column including near surface

Total column including near surface

Total column including near surface

Midtroposphere

Midtroposphere

Horizontal resolution (km)f

1.29 × 2.25/5.2

FTS: 10.5/80-790

30 × 60/960

15/1,650

12/2,200

CO2 uncertainty (ppm)g

1-2

4

14

1.5

2

Instruments

3-channel grating spectrometer

CAI, SWIR/TIR Fourier transform spectrometer

8-channel grating spectrometer

Grating spectrometer

Fourier transform spectrometer

Viewing modes

Nadir, glint, target

Nadir, glint, target

Limb, nadir

Nadir

Nadir

Samples per day

500,000

18,700

8,600

2,916,000

1,296,000

Wavelength bandpass (μm)

0.757-0.772, 1.59-1.62, 2.04-2.08

0.758-0.775, 1.56-1.72, 1.92-2.08, 5.56-14.3

0.24-0.44, 0.4-1.0, 1.0-1.7, 1.94-2.04, 2.265-2.38

3.74-4.61, 6.20-8.22, 8.80-15.4

3.62-5.0, 5.0-8.26, 8.26-15.5

Signal/noise (nadir, 5% albedo)

>300 @ 1.59-1.62 μm, >240 @ 2.04-208 μm

~120 @ 1.56-1.72 μm, ~120 @ 1.92-2.08

<100 @ 1.57 μm

~2,000 @ 4.2 μm, ~1,400 @ 3.7-13.6 μm, ~800 @ 13.6-15.4 μm

~1,000 @ 12 μm, ~500 @ 4.5 μm

Orbit altitude

705 km

666 km

790 km

705 km

820 km

Local time

13:30 ± 0:1.5

13:00 ± 0:15

10:00

13:30

21:30

Revisit time/orbits

16 days/233 orbits

3 days/72 orbits

35 days

16 days/233 orbits

72 days/1,037 orbits

Launch date

Failed on launch

January 2009

March 2002

May 2002

October 2006

Nominal life

2 years

5 years

7+ years

7+ years

5 years

NOTES: AIRS = Atmospheric Infrared Sounder; CAI = Cloud and Aerosol Imager; FTS = Fourier transform spectrometer; GOSAT = Greenhouse gases Observing Satellite; IASI = Infrared Atmospheric Sounding Interferometer; OCO = Orbiting Carbon Observatory; SCIAMACHY = Scanning Imaging Absorption Spectrometer for Atmospheric Chartography; SWIR = short-wavelength infrared; TIR = thermal infrared.

aCrisp (2008); Crisp et al. (2008).

bAkihiko Kuze, Japan Aerospace Exploration Agency, personal communication, 2009; Hamazaki et al. (2007); Shiomi et al. (2007).

c<http://envisat.esa.int/instruments/sciamachy/>; Burrows et al. (1995); Noël et al. (1998); Buchwitz et al. (2005).

dAumann et al. (2003); Chahine et al. (2008).

ePhulpin et al. (2007); Crevoisier et al. (2009).

fInstantaneous field-of-view/Swath.

gThe uncertainty represents the estimate of random errors (e.g., the effects of detector noise) and additional systematic errors (e.g., bias caused by cloud and aerosol effects) unaccounted for or otherwise eliminated from the total error. Bias is reduced by successful validation efforts.

The GOSAT uncertainty is dominated by the precision (random errors). For OCO, Crisp et al. (2004) and Miller et al. (2007) discuss the observational system simulation experiments, including modeling of the OCO instrument performance characteristics, that led to an instrument design that would meet a measurement requirement of 1 part per million (ppm). The as-built OCO instrument performance was verified during prelaunch tests, which included direct solar observations. The analysis of the latter gave the best confirmation that the as-built instrument performance exceeded its design requirements.

The methods for bias reduction and validation are the same for GOSAT and OCO. Washenfelder et al. (2006) demonstrated the OCO validation concept and the essential role of ground-based measurements for meeting those objectives. Bösch et al. (2006) used these ground-based measurements to validate SCIAMACHY CO2. The GOSAT team also plans to use the same validation sites and instruments. OCO planned to include and use Aeronet measurements. The OCO validation plan purposely located ground-based validation measurements at Atmospheric Radiation Measurement (ARM) Program sites to capitalize on the wealth of ancillary atmospheric and surface measurements.

can be directly measured on shorter time scales. Time trends in oceanic CO2 at a single point are illustrated in Figure C.1. Most well-qualified oceanic CO2 datasets reside at the Department of Energy’s Carbon Dioxide Information Analysis Center.1

Early work to track the accumulated burden of anthropogenic CO2 in seawater relied on tracer data, such as bomb 14C. The use of tracers was necessary because of the high natural background level of dissolved CO2 in seawater, the complexity of the processes affecting its distribution, and the relatively small size of the anthropogenic signal, all of which combined to make direct observation an uncertain business. Today, there are widely available accurate standards and mea-



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