Surface reflectance measurements in the UV froman airborne platform. Part 1

D. D. Doda and A. E. S. Green

The reflectance of naturally occurring surfaces is measured from a twin engine Cessna 402B aircraft bothspectrally (0.29-0.40 /im) with a compact double monochromator and broadband (0.29-1.2 gim) with a UVenhanced photodiode. The measurement system, which is computer controlled and electrically isolatedfrom the aircraft, consists of upward and downward looking hemispheric diffusers, filters, a rotating 900 mir-ror, a focusing lens, and a double monochromator/PMT or the UV photodiode. Measurements are takenat several altitudes enabling the empirical determination of backscatter and attenuation effects on the re-flectance. The results are presented for pine forest canopy, green farmland, open ocean, and brown farm-land as a function of wavelength and altitude.

1. Introduction

The spectral reflectance of a naturally occurringsurface, an intrinsic characteristic of that surface, hastaken on new importance in the light of the presentgeneration of earth monitoring satellites and the currentwealth of both atmospheric measurements and calcu-lations. In general, the numerous algorithms for eval-uating the total ozone overburden, the ozone profile, theearth radiation budget, etc. from satellite data all re-quire knowledge of the surface reflectance over a par-ticular spectral range.' Several of these methods inferthe surface reflectance from the satellite data by the useof an iterative procedure.2' 3 However, if the surfacereflectance was accurately known in the requiredspectral region, the uncertainty in the calculationswould be reduced, and possibly the iterative method-ology could be used to infer additional information fromthe measurements.

Inherent in any satellite, balloon, or aircraft mea-surement of reflectance is the atmospheric interactionwith the reflected radiance from the surface. This in-teraction is dependent on numerous atmospheric pa-rameters such as the ozone and aerosol contributions,the solar zenith angle, the wind direction and velocity,the haze layer height, temperature, and pressure, all ofwhich cannot be known at any one time. This atmo-spheric interaction has led many researchers 4-1 ' to make

The authors are with University of Florida, Physics Department,Gainesville, Florida 32611.

Received 5 Febuary 1980.0003-6935/80/132140-06$00.50/0.© 1980 Optical Society of America.

reflectance measurements from the ground. Theirresults yield reflectances for various types of vegetation,soil, desert sand, water, and snow. In addition, oth-ers12-23 have utilized aircraft and balloon platforms tomake reflectance measurements over desert sand, water,and snow. And finally, laboratory measurements2 4 25

have been performed to measure the reflectance ofcertain types of vegetation along with soil and sandsamples. Typically, none of the nonlaboratory studiesto date have had the spectral resolution or in some casesthe spectral range necessary for the current needs ofatmospheric research.

This paper is primarily concerned with the evaluationof the spectral hemispheric reflectance in the near UV(0.29-0.40 gim) as measured from an aircraft platform.Measurements over a pine forest, green farmland,brown farmland, and the open ocean are made at severaldifferent altitudes permitting correction for atmo-sphenic effects and the determination of the groundreflectance itself. In addition, a broadband (0.29-1.20-,m) hemispheric reflectance is evaluated.

II. Instrumentation

Traditionally, the reflectance of natural surfaces hasbeen measured by using upward and downward lookinghemispheric and beam sensing pyranometers or filter/channel radiometers suspended from various types ofbooms or attached in various ways to different types ofaircraft. In contrast to this approach, the system de-veloped for this study used one single detector thatmeasures both the upward and downward irradiancesfrom an aircraft platform. The entire instrumentationplatform centers around the system's optical path,

2140 APPLIED OPTICS / Vol. 19, No. 13 / 1 July 1980

CARGODOOR FRAME

cZ~

INSTRUMENTATIONDARK HOUSING

SHOCKMOUNTS

RODS

HEMISPHERICDIFFUSER

BAFFLES

FRONTSURFACEROTATINGMIRROR

LENS AND'FILTER

HOLDER

BAFFLES

HEMISPHERICDIFFUSER

Fig. 1. Schematic drawing of the modified cargo door.

which is structured into the cargo door of a twin engineCessna 402B aircraft, as shown in Fig. 1.

The front of the instrumentation package consists ofupward- and downward-looking shaped BaSO4 coatedquartz hemispheric diffusers. These have a good cosineresponse for angles <750 but deviate by 5% or more atangles >800. The diffusers are mounted on 4.2-cm(1.75-in.) diam black anodized, baffled tubes connectedto holders for transfer lenses and filters. A 5-cm (2-in.)diam front surface rotating mirror alternately directsthe downwelling or upwelling radiation to the spec-trometer/detector combination. The spectrometer isa Jobin-Yvon DH-10 double monochromator, employ-ing two concave holographic gratings and a computercontrollable wavelength drive. The system is typicallyoperated with a 2-nm bandpass using an EMI 9781UBFL PMT as the detector. The spectrometer/de-tector assembly is enclosed in a light-tight dark boxsecured to the optical path structure. In addition, forthe broadband work an EG&G 444B UV photodiodesensitive over the 0.11-1.2-,um detector range or effec-tive 0.29-1.2-gm atmospheric range is used. The de-tector output for both arrangements is routed through

a Keithley 414A picoammeter to an eight-channel dif-ferential 12-bit analog-to-digital converter (ADC),which is sampled under computer control.

A Commodore Pet 2001-8K memory microcomputercontrols the entire aircraft instrument platform.Programmed in basic and machine language, the Pet,through a hardware interface, controls the wavelengthdrive of the spectrometer, rotates the front surfacemirror, and samples, reduces, and stores data in hardcopy form for security and on cassette tapes for lateranalysis. The final data analysis is performed on aPDP-11/34 minicomputer after data transfer from thePet cassettes to the PDP magnetic disks, where thelarger capacity 64K memory enables the proper foldingof filter responses, system responses, and absolute cal-ibrations.

Additional support equipment includes a high voltagepower supply for the PMT; two dc power modules, I15and +5 V, to power the hardware interface and ADC;a dc to ac 1000-W inverter; two deep cycle 12-V dcbatteries that comprise a 24-V dc battery pack; a matrixprinter for hard copy output; and a 7.50/step bipolarcomputer controllable stepper motor coupled directlyto the rotating mirror. The entire system (shownschematically in Fig. 2) is electrically isolated from thatof the aircraft. Fully charged, the system is capable of8 h of continuous operation without recharging. Anovernight charging is then required to return the batterypack to full power.

The entire aircraft instrumental system can be in-stalled in -2 h with the experimental cargo door hingedin place and the electronics rack, microcomputer, andbattery pack all secured to the passenger seat rails of theaircraft. The system has been flight tested and ap-proved by the FAA for flights on any Cessna 402B air-craft.

Ill. Methodology

Hemispheric spectral irradiance measurements, bothupward and downward, are made at several differentaltitudes above a given target surface area. The ratioof these measurements yields a combination of thesurface reflectance and the atmospheric backscattercontribution at the different altitudes provided that thereflecting surface is homogeneous and scatters isotro-pically. In the course of the measurement considerablecare was taken to choose homogeneous target sites.

Fig. 2. Schematic overview of the data acquistion and controlsystems.

1 July 1980 / Vol. 19, No. 13 / APPLIED OPTICS 2141

.16

.14

.12

WUzI-

Il-Ja:

.10

.08 I-

.06 F

.04 -

.02 _.

300 320 340 360 380 400WAVELENGTH (NM)

Fig. 3. Spectral reflectance results over pine forest with the averagelatitude, longitude, solar zenith angle, and altitude in feet given foreach data set. The * represent the empirically extrapolated

reflectance at the surface.

Typically scattering varies in directional characteristicsfrom surface to surface with a truly Lambertian surfacebeing rare in nature. However, recent studies2 2 indicatethat at 0.52 gm green farmland (i.e., pastureland) andpine forest canopy (i.e., coniferous forest) scatter iso-tropically out to -75° from the zenith. The directionalnon-Lambertian properties of a reflecting surface in theUV region will be considered in later ground-basedstudies. For the present study we assume that thetarget site i homogenous and Lambertian and that thereflected radiance is isotopic.

To separate out the atmospheric contribution weanalyze the measured irradiances, both upward anddownward, in a linear least squares sense as a functionof altitude, extrapolate back to ground level, and cal-culate resulting reflectances. This linear extrapolationmethod is consistent with the theoretical altitude de-pendent results of Braslau and Dave26 and Green etal. 27 Their calculations from 0 to 10,000 ft show thatboth the upward and downward irradiance are linearwith altitude from 0.29 to 0.36 gm over the range of re-flectances (0.0-0.8) and of solar zenith angles (0.0-60.00).

This empirical determination of the reflectance hasseveral distinct advantages: (a) it evaluates and sub-tracts out the atmospheric effects of aerosols, ozone, andthe Rayleigh interaction along with any temperatureand pressure contributions without assuming any at-mospheric model for these contributions other thanrequiring a high linear correlation; (b) it allows for the

Table 1. Spectral Reflectance Result RAIIItude(f) Given for Various Surfaces with RGND the EmpiricallyExtrapolated Reflectance at the Surface

A

PineForest

B

OpenOcean A

C

OpenOcean B

D

GreenFarmland

E

BrownFarmland

A R1 10 0 RGND R1 0 00 RGND R10 00 RGND R0 85 0 R1000

300.0 -- -- 10.88 8.06 8.91 -- 4.49302.0 3.85 -- 9.34 7.68 10.19 5.96 -- 4.30305.9 3.75 -- 7.77 5.13 8.00 5.34 4.50 4.25310.0 4.75 1.95 8.35 6.39 6.73 4.92 3.94 4.35312.5 4.83 2.13 8.87 6.73 7.76 5.51 4.19 4.20317.5 5.24 2.06 9.44 7.20 8.07 5.93 4.25 3.90320.0 4.43 1.28 8.89 6.80 7.28 5.28 4.26 3.95330.0 3.85 1.19 9.05 8.05 7.14 5.63 4.71 3.70331.2 4.03 1.81 9.99 7.68 7.64 5.86 4.13 3.59339.9 4.32 2.24 9.97 7.60 8.26 6.50 4.31 3.72343.3 3.59 1.73 9.60 7.91 7.79 6.88 4.13 3.51350.0 4.35 2.80 9.97 8.07 8.02 7.18 3.84 3.08360.0 3.87 2.34 10.55 9.18 7.76 6.86 3.72 3.03370.0 4.30 3.11 9.75 8.64 7.94 7.16 3.75 3.10380.0 3.76 2.62 10.54 8.19 7.97 7.18 4.49 2.86390.0 3.17 2.02 10.00 8.65 7.78 7.07 4.16 2.63400.0 3.18 2.30 9.66 9.39 8.30 7.34 3.96 3.18

2142 APPLIED OPTICS / Vol. 19, No. 13 / 1 July 1980

PINE FOREST ZA ALTITUDE (FT1

LAT. * 30.3 N � 36.S* 7350PINE FOREST

LAT. 0.3 N

LONG. 82.3'W

ZA ALTITUDE FT)

a 36.6, 7350

0 4.7 4350

0 46.7' 1100

I I I I I I I I I I

determination of the total irradiance as a function ofaltitude; and (c) the extrapolated results are easilycomparable to measurements made at the ground.

IV. Results

A. Pine Forest

Spectral data samples were taken at altitudes of 1100,4350, and 7350 ft from 302 to 400 nm on 16 May 1979.The altitude dependent spectral reflectance, averagesolar zenith angle, and average latitude and longitudeare given in Fig. 3 and Table I, column A. These resultsshow a spectral reflectance of 3.17-5.24 + 0.42% at 1100ft with the atmospheric backscatter interaction beingapparent on inspection of the increased reflectance asa function of altitude and decreasing wavelength. Thelinearly extrapolated reflectance at the ground, alsogiven in Fig. 3 and Table I, column A, is 1.19-3.11% overthe wavelength range. The broadband data taken at4350 and 6350 ft shows reflectances of 13.90 + 0.04% at4350 ft and 13.26% for the linearly extrapolated reflec-tance at the ground. This entire data run was per-formed on a morning flight (9:03-11:07 EST) with verylittle turbulence, reasonable visibility, no clouds, andwith the haze layer top at 4000 ft.

B. Open Ocean

Spectral data samples were taken on 7 Sept. (openocean A) and 22 Oct. (open ocean B) 1979 at altitudesof 500 (22 Oct. only), 1000, 3000, 5000, and 8000 ft oneach day. The results shown in Figs. 4 and 5 and TableI, columns B and C, indicate spectral reflectances of

OPEN OCEAN A ZA ALTITUDE (FT)

LAT. * 29.7 N 0 23.6 6000

.22 : i . 1| 4 LONG. * 61.1 W & F4.4 5000

.20 3

.18

o 16z

U

w~ .14

.12

.10

.08

300 320 340 360 380 400WAVELENGTH (NM)

Fig. 4. Spectral reflectance results over the open ocean with the

305.9-nm surface results equaling 0.052 (see Fig. 3 caption).

.22

.20

.18

.16

.|Uz4U .14

U.

.12

.10

.08

.06

300 320 340 360 380 400WAVELENGTH (NM)

Fig. 5. Spectral reflectance results over the open ocean with

305.9-nm surface results equaling 0.049 (see Fig. 3 caption).

7.77-10.88 + 0.89% (open ocean A) and 6.73-10.19 t0.53% (open ocean B) at 1000 feet with the linearly ex-trapolated reflectances being 5.13-9.39 and 4.92-7.34%,respectively, over the wavelength range. The broad-band data taken at 1000, 3000, and 5000 ft on 8 Sept.and at 1000, 5000, and 8000 ft on 22 Oct. yield reflec-tances of 4.60 + 0.06 and 4.74 t 0.06% at the 1000 ftlevel with 4.43 and 4.58% being the linearly extrapolatedreflectance at the ground. These results for the openocean differ by a solar zenith angle variation that willbe looked at in detail in future work.

The data run performed on 7 Sept. involved a middayflight (11:24-12:08 EST) with very little turbulence, noclouds, and a high haze layer, topping at 6700 ft. Therun on 8 Sept. was a morning flight (10:23-11:57 EST),once again with little turbulence, no early clouds, butsome cumulus buildup on the horizon toward the endof the run and a haze layer boundary at 4300 ft. Therun on 22 Oct. was a morning through midday flight(10:10-1:02 EST) with clear sky, little turbulence, abuilding low level haze, and the haze layer boundary at5000 ft. The general ocean surface structure for all dataruns was relatively calm with very few or no white capsvisible. In addition, the data taken on 7 Sept. werecoupled with the Coastal Data Network (1979) of theDepartment of Coastal Engineering at the Universityof Florida, 2 8 which measured an average swell heightand period of 65 cm and 11 sec during the course of thereflectance data runs.

1 July 1980 / Vol. 19, No. 13 / APPLIED OPTICS 2143

10 y0I-.U

l .08

.06

.04_

.02-

I I I I I I I I I I 300 320 340 360 380 400

WAVELENGTH (NM)

Fig. 6. Spectral reflectance results over green farmland (see Fig. 3caption).

C. Green FarmlandSpectral data samples were taken on 8 Sept. at 850,

2850, and 4850 ft; however, due to a monochromatordrive malfunction the 4850-ft data were invalid. Theresults shown in Fig. 6 and Table I, column D, indicatea spectral reflectance at the 850-ft level of 3.72-4.71 +0.48%. The linear extrapolation was not carried out dueto the limited data set. The broadband data from 16May taken at 4350 and 6350 ft yield reflectance valuesat 4350 ft of 16.92 0.08% with the linearly extrapolatedvalue at the ground being 14.11%. The data run char-acteristic for these flights are given above.

D. Brown Farmland

This spectral data set was performed. on 22 Oct. ataltitudes of 1000, 2000, and 2920 ft with the resultingspectral reflectance of 2.63-4.35 0.37% at 1000 ftshown in Fig. 7 and Table I, column E. The linear ex-trapolation methodology indicated a very poor corre-lation between the altitude dependent data points at-tributed to the limited altitude range and the increasinglow level haze. The broadband results indicate a re-flectance of 19.30 0.20% at 1000 ft with the linearlyextrapolated results for the broadband data showing ahigh correlation and a value of 16.77%.

V. Discussion

The measurements made from an aircraft platforminvolve numerous variables that must be considered toensure the minimization of any systematic errors.Listed below are several such considerations.

.16

.14

UZ .,o

W .08

.06

.04

.02 f-

300 320 340 360 380 400WAVELENGTH (NM)

Fig. 7. Spectral reflectance results over brown farmland (see Fig.3 caption).

A. Calibration

The wavelength calibration of the double mono-chromator was performed immediately before and afterevery flight with the shift observed to be <0.1 nm.

The filter responses, throughput, and the cosine re-sponse of the hemispheric diffusers were measuredusing a secondary standard that we directly cross-cali-brated with our FEL-80 NBS calibrated 1000-W irra-diance standard. In addition, the error in the totalmeasured irradiances due to the deviation of the dif-fusers from a true cosine response is <2.5% as estimatedfrom Hitzfelder et al. 29

B. Flight Characteristics

The Cessna 402B aircraft flying at 110 mph orientsthe optical path in a vertical direction. At this velocitywith the autopilot in control of the aircraft and innonturbulent conditions, the observationally estimatedvariation in the attitude is <2° in pitch and roll, and thealtitude variation is <10 ft. In addition, the Cessna402B is equipped with DME and VOR distance mea-suring and range finding equipment that yields theaircraft's geographic position in terms of the radial angleand linear distance from a VOR center.

C. Data SamplingThe data sampling runs typically require 3.5-5.0 min

of flight time. This is equivalent to a 6.4-9.2-mile dataflight path. The sampling rate is 10-25 samples/wavelength (2-5-sec integration), depending on theextent and homogeneity of the target site. A normal

2144 APPLIED OPTICS / Vol. 19, No. 13 / 1 July 1980

BROWN FARMLAND ZA ALTITUDE FT)LAT. * 7.8aN 4 49.1 2920

LONG. * 75.6'W 0 49.4 2000

0 49.8 1000

.12 I-

__ I I I I I I I I I I

data run samples wavelengths from 290 to 400 nm;however, the data collected at <300 nm is usually in thenoise of the system and is questionable. Moreover, thestart and end VOR/DME coordinates, altitude, atti-tude, heading, velocity, outside ambient temperature,an estimate of the turbulence, and the haze layerboundary are recorded on every data run.

D. Terrain Description

The pine forest data were measured in northernFlorida over a densely populated forest with only a fewsmall access roads visible. The green farmland targetsite was in northwestern Florida. It consisted of auniform sectional farm area, totally green in coloration,with an occasional tree- or brush-lined farm division andaccess road. The brown farmland data were accumu-lated on the eastern shore of Virginia over recentlyfurrowed potato farms. There were several autumnfoliage tree-lined farm divisions and a few occasionalfarm houses. The open ocean data were measured offthe east coasts of Florida and Virginia. In both cases,the seas were calm, and the data runs were performedover nonboundary layer areas in the open ocean be-tween 10 and 20 miles off each coast.

E. Ground Reflectance

The linear extrapolation methodology is consideredvalid and employed when the correlation between thealtitude dependent irradiance measurements is >0.90.Typically it is found that the upward irradiance has a>0.98 correlation. The downward irradiance correla-tion is more varied (0.90-0.99) and more dependent onatmospheric conditions. The best results were obtainedwhen the day was cloud-free with a uniform nonbuildinghaze layer and when the data were taken at several al-titudes spanning the 0-10,000-ft range.

VI. Conclusion

A unique computer controlled aircraft based remotesensing system has been developed, tested, and utilizedto measure the reflectance of naturally occurring sur-faces. To date, the spectral and broadband hemi-spheric reflectance of a pine forest, green farmland,brown farmland, and the open ocean have been mea-sured. The data are collected at several altitudes al-lowing for the empirical determination of the reflectanceat the ground. In addition, the measurement scenarioyields the altitude dependence of the upwelling anddownwelling irradiance. These measurements coupledwith future work on snow, sand, and possibly cloudsshould enable researchers to reduce the number of un-knowns in both theoretical and experimental radiativetransfer calculations.

The authors thank L. M. Garrison and J. W. Hooverfor their helpful discussions during the early stages ofthe program and gratefully acknowledge W. E. Steeger,senior design machinist, for his contributions in allphases of construction. We also thank Mel Turner forlocating and facilitating the installation of the Cessna402 door. This work was supported by NASA under theNimbus-7 programs contract NAS5-22980.

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