Abstract

Passive remote sensing of airborne chemicals at infrared wavelengths may be limited by temporal fluctuations in atmospheric brightness temperatures δT(Δt). Brightness temperatures in two infrared spectral bands were simultaneously measured on clear and cloudy days along three lines of sights. For time windows Δt < 3–5 s, δTt) remained constant at the sensor noise level and rapidly increased as Δt increased. The fluctuation time scale for the cloudy day was longer than for the clear day. The long correlation time for T(t) limits the utility of signal averaging in improving detection signal-to-noise ratio (SNR). The simultaneous outputs of the two spectral channels during the clear day exhibited no spectral coherence at Δt < 3 s and limited coherence at Δt > 30 s. Measurements during the cloudy day were largely coherent. Consequently, band-by-band subtraction may have limited benefits.

© 2005 Optical Society of America

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References

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Air monitoring by spectroscopic techniqu (1)

P. L. Hanst and S. T. Hanst, �??Gas measurement in the fundamental infrared region,�?? in Air monitoring by spectroscopic techniques, M. W. Sigrist, ed. (Wiley, New York, NY, 1994).

Appl. Opt. (8)

Encyclopedia of analytical chemistry (1)

D. W. T. Griffith and I. M. Jamie, �??Fourier transform infrared spectrometry in atmospheric and trace gas analysis,�?? in Encyclopedia of analytical chemistry, R. A. Meyers, ed. (Wiley, Chichester, England, 2000).

J. Opt. Soc. Am. A. (1)

Ancellet, G. M. and R. T. Menzies, �??Atmospheric correlation-time measurements and the effects on coherent Doppler lidar,�?? J. Opt. Soc. Am. A. 4, 367-373 (1987).
[CrossRef]

Opt. Eng. (2)

S. Holland, R. Krauss, G. Laufer, "Demonstration of an uncooled LiTaO3-detector-based differential absorption radiometer for remote sensing of chemical effluents," Opt. Eng. 43, 2303-2311 (2004).
[CrossRef]

S. K. Holland, R. H. Krauss, G. Laufer, The effect of temperature on passive remote sensing of chemicals by differential absorption radiometry, to be published in Opt. Eng. (October 2005).

Opt. Express (1)

Opt. Lett. (1)

Other (5)

R. R. Beland, �??Propagation through atmospheric optical turbulence,�?? Ch. 2, Vol. 2, The infrared and electrooptical system handbook, ed. F. G. Smith SPIE Press, Bellingham, WA, USA, (1993).

R. E. Hufnagel, �??Propagation through atmospheric turbulence,�?? Ch. 6 The infrared handbook revised edition, ed. W. L. Wolfe and G. J. Zissis, The Infrared Information Analysis (IRIA) Center, Environmental Research Insitute of Michigan, USA (1985).

PcModWin 4.0 , MODTRAN atmospheric radiative transfer code, Ontar Corporation, North Andover, MA (2002).

S. M. Kay, Modern Spectral Estimation, Ch. 4, P T R Prentice Hall, Englewood Cliffs, New Jersey (1988).

A Papoulis and S. U. Pillali, Probability, random variables and stochastic processes, Ch. 12, McGraw-Hill, New York, 4th edition (2002).

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Figures (11)

Fig. 1.
Fig. 1.

Variation of the brightness temperature with time as measured in a clear day (afternoon) by detector 1 (thick lines) and detector 2 (thin lines) for line of sight pointing toward (1) a building at 0.5 km (blue), (2) a mountain at 6 km (red), (3) the horizon (green), and (4) a blackbody at room temperature in the lab (black). Note: the two detectors measurements are simultaneous for each line of sight but the measurements for the three lines of sight are not simultaneous and were taken within ~ 1 hour.

Fig. 2.
Fig. 2.

Same as Fig. 1 but for a cloudy day (morning). The measurements for the three lines of sight are not simultaneous and were taken within ~ 2 hours.

Fig. 3.
Fig. 3.

Variation of brightness temperature fluctuations δT with detection time window Δt as seen by channel 1 for a clear day using brightness temperature measurements shown in Fig. 1.

Fig. 4.
Fig. 4.

Variation of brightness temperature fluctuations δT with detection time window Δt as seen by detector 1 for a cloudy day using brightness temperature measurements shown in Fig. 2.

Fig. 5.
Fig. 5.

Variation of the time-lag autocorrelation function ρ with Δt for the results of detector 1 during the clear day tests.

Fig. 6.
Fig. 6.

Variation of the correlation coefficient ρ with Δt for the results of detector 1 during the cloudy day tests.

Fig. 7.
Fig. 7.

Transmission spectra of atmospheric water vapor (H2O and H2O continuum), O3, and CO2 along horizontal 5 km path as computed using MODTRAN for 1976 standard atmosphere, with 1 cm-1 resolution and 23 km visibility.

Fig. 8.
Fig. 8.

Power spectrum density of the brightness temperature of Fig. 1 (clear day, channel 1).

Fig. 9.
Fig. 9.

Variation with detection frequency of the coherence function between channels 1 and 2 for the clear day tests (Fig. 1).

Fig. 10.
Fig. 10.

Same as Fig. 10 but for the cloudy day tests (Fig. 2).

Fig. 11.
Fig. 11.

The standard deviation for the mean temperature σμi for the various one-minute sub-tests of the clear day experiment.

Tables (2)

Tables Icon

Table 1. System specifications for the dual MCT detectors

Tables Icon

Table 2. Coefficients of the polynomial fit of δT(Δt > 1 s) (Eq. 11) for the six outdoor tests.

Equations (15)

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S k = A k + B k W s ( T , λ k ) C k W C ( T C , λ k )
( σ N 2 ) i = 1 N j = 1 N ( T j μ i ) 2
μ i = 1 N T j j = ( i 1 ) N + 1 ( i 1 ) N + N
E [ σ N 2 ] = 1 n i = 1 n ( σ N 2 ) i
E [ σ N 2 ] = 1 n i = 1 n [ E ( T i 2 ) μ i 2 ] = 1 n i = 1 n E ( T i 2 ) 1 n i = 1 n μ i 2
1 n i = 1 n [ N μ i N μ ] 2 N 2 = σ 2 N [ 1 + 2 j = 1 N 1 ( 1 j N ) ρ j ] = 1 n i = 1 n μ i 2 μ 2
σ N 2 σ 2 σ 2 N [ 1 + 2 j = 1 N 1 ( 1 j N ) ρ j ]
ρ N = k = 1 m 1 [ T k μ ] [ T k + N μ ] σ 2 ( m N )
σ N 2 σ 2 σ 2 N ( 1 + ( N 1 ) ρ o )
{ ρ N 1 = K N j = 1 N 2 ( 1 j N ) ρ j 1 N 1 N K N = ( σ 2 δ T 2 ( Δ t N ) σ 2 ) N 2 1 2 } N > 1
log [ δ T ( Δ t ) ] = i = 0 4 β i log ( Δ t ) i
= β 0 + β 1 log ( Δ t ) + β 2 log ( Δ t ) 2 + β 3 log ( Δ t ) 3 + β 4 log ( Δ t ) 4
σ μ 2 σ 2 N [ 1 + 2 j = 1 N 1 ( 1 j N ) ρ j ]
ρ max 1 N Q 2
{ ρ N 1 = K N j = 1 N 2 ( 1 j N ) ρ j 1 N 1 N K N = ( σ μ N 2 E ( x 2 ) ) N 2 1 2 } N > 1

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