Absolute Calibration of Landsat Instruments Using the Moon

Absolute Cali bration Landsat Instruments Using the Moon

Hugh H. Kieffer and Robert L. Wildey U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, AZ 86001

ABSTRACT:A lunar observation by Landsat could provide improved radiometric and geometric calibration of both the Thematic Mapper and the Multispectral Scanner in terms of absolute radiometry, determination of the modulation transfer function, and sensitivity to scattered light. A pitch of the spacecraft would be required.

INTRODUCTION tions of its brightness with phase angle in the range near zero (significant effect) and with libration (small HE MOONPRESENTS AN OBJECT whose diameter is T approximately 194 pixels for the TM (Thematic effect) are well known, both for the entire Moon Mapper) and 95 x 72 pixels for the MSS (Multi- and for individual areas on it. The brightness of the spectral Scanner). It has the following unique prop- Moon relative to the Sun has been determined erties: it is within the dynamic range of both instru- through the use of standard stars. The variability of ments, it is surrounded by a black field in both the Sun as measured over the past 25 years has been reflective and thermal bands, and its surface-bright- less than 0.3 percent. The Moon can thus serve as ness distribution is better known than that of any an excellent standard for determination of time vari- other natural object at which these instruments ations of an instrument response, for calibration be- could safely be pointed. tween various spacecraft instruments, and as an ab- A single observation of the Moon by Landsat-4 or solute reflectance standard. Landsat-5 could accomplish three evaluations of in- A calibration in terms of reflectivity requires only strument performance that are impossible through that the specific intensities of the Sun and the Moon the Earth's atmosphere: (1) radiometric calibration be established in any self-consistent system; the to the order of 2 percent; (2) determination of off- system currently best understood is that of the stan- axis response (sensitivity to scattered light); and (3) dard stars. One path from this system to absolute measurement of the MTF (modulation transfer func- radiometry (MKS units) is through Willstrop's cali- tion) for mathematically sharp boundary. bration of the system of stellar magnitudes and These observations could be made in flight, after colors using a standard lamp (Willstrop, 1960). the occurrence of the uncertain changes associated with launch and initial stabilization of operation in DISCUSSION space. Such observations would have two significant Quantitative terrestrial imaging is significant pri- engineering impacts: temporary reorientation of the marily for the implied values of reflectance (pho- spacecraft away from nadir pointing, and assurance tometric function multiplied by normal albedo); the of the required S-band and X-band communications absolute heterochromatic specific intensity in w/ in this attitude. (cm2 sr) is rarely of direct significance. At present, If appropriate conventional nadir-pointing data the absolute calibrations of the TM and MSS are (near-simultaneous coverage of a high-contrast based on use of a standard lamp prior to launch, scene) are available to allow cross-calibration be- which involves an uncertainty of 3 to 5 percent. tween the Landsat instruments, either spacecraft Reduction to reflectivity information requires the could be used for the absolute radiometric lunar acceptance of a completely independent absolute calibration; conventional scene data could be used spectral calibration of the Sun, which is of similar to transfer this calibration between the two space- uncertainty. Both of these absolute calibrations can craft. The MTF observation could probably be as- be circumvented by placing satellite imaging on the sumed to hold for both instruments. However, the system of stellar magnitudes and colors through the off-axis response (scattered light sensitivity), which acquisition of an image of the full Moon nearer to can be greatly affected by contamination of the op- zero phase-angle than 4". This calibration method tics, would hold only for the spacecraft used in the would enable the production of images as quanti- lunar observation. tative reflectivity arrays with absolute precision of The absolute spectral reflectivity properties of the order of 1 to 2 percent. A lunar observation can the Moon can be considered constant. The varia- calibrate the Landsat instruments. In reducing a

PHOTOCRAMMETRICENGINEERING AND REMOTE SENSING, 0099-1112/85/5109-1391W2.25/0 Vol. 51, No. 9, September 1985, pp. 1391-1393. 6 1985 American Society for Photogramnretry and Remote Sensing PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING. 1985 normal terrestrial scene to surface albedos, how- specific-intensity measurements as admitted ever, one must still contend with the variable at- through a small diaphragm covering but a tiny frac- mospheric radiance. tion of its surface are within the spread of apparent A lunar calibration is valuable because the stan- magnitudes of calibration stars. These measure- dard stars have luminosity constancy to a measuring ments, all of which are obtained in the linear range threshold of about 1 percent or better; solar lumi- of telescope-assisted photomultipliers, constitute nosity has been carefully measured during the past the data set directly tied to the system of stellar decade and varies only about 0.1 percent. In addi- magnitudes and colors. In addition, through an ef- tion, the reflective properties of the Moon are fort that integrates point-sequential photoelectric stable to an even greater-precision on any geologi- photometry, electronography, and photography, cally short time-scale. many photometric images have been obtained of the Measurements of the total solar irradiance over Moon within a few degrees of zero phase-angle. This the last 5 years have reached levels of absolute ac- work has enabled determination of normal albedo curacy on the order of 0.1 percent; the measure- distribution over the lunar nearside with a seleno- ments indicate variations of this magnitude over graphic precision exceeding the angular resolution short periods (days to weeks) and a possible gradual of the TM by a factor of 3, along with a well-deter- decrease of about 0.01 percentlyear (Willson, 1984). mined photometric function for this angular range Earth-orbiting spacecraft measurements between (Wildey, 1976, 1977). 1980 and 1983 indicate a decrease of 0.08 percent. The spectral reflectivity of several locations on Rocket and spacecraft measurements indicate con- the Moon has been measured from 0.3 to 2.5 pm stancy from 1969 to 1976 of better than 0.3 percent. with 0.03 pm resolution. These spectra show a basic Measurements from 1902 to 1964 had uncertainty similarity. Normalized to 1.0 at 0.56 pm, they in- greater than 1percent and did not detect solar vari- crease nearly linearly from 0.5 at 0.3 pm to 1.4 at ability. (See the detailed review by Willson, 1984.) 1.1 pm (McCord et al., 1972). Spectral ratios of dif- There are no measurements indicating temporal ferent terrain types typically show variations less variations of the bidirectional spectral reflectance of than 10 percent and are known to 1 percent. Much the Moon. Considering the estimated rates of im- of the lunar nearside has been mapped with silicon- pact-crater formation in the Earth-Moon system, vidicon imaging systems from 0.38 to 1.05 pm the area of bright lunar rays, and the contrast be- (Johnson et al., 1977; McCord et al., 1979). The tween these bright rays and dark maria, we estimate Moon exhibits a significant variation in reflectivity that the Moon darkens at a rate of about lo-' per- with phase angle (Lane and Irvine, 1973); this vari- cent per year between large cratering events. The ation is well known for times near full Moon lunar surface is probably nearer to steady-state than (Wildey, 1976) which correspond also to the phase indicated by this rate in terms of the micro-mete- of the spectral mapping. orite gardening process that influences its optical Lunar spectral reflectivities on a relative scale at properties. Thus the time-variation of lunar reflec- wavelengths from 0.65 to 2.5 pm have been ob- tance is assumed to be negligible. tained recently (McCord et al., 1981) and are tied The Sun presents the most serious problem in the to laboratory lneasurements of the spectral reflec- absolute calibration of albedos, regardless of method tivity of Apollo 16 soil samples. Beyond about 2.0 employed. Because it is almost a million times pm, a small correction for lunar thermal emission brighter than the full Moon, optical/mechanical must be amlied. .L deamplification is always necessary before transduc- Approximately one-tenth of the lunar nearside is tion to the signal-form actually measured. Whereas more than 50 percent brighter than the average of the solar constant, integrated over all wavelengths, the nearside, and can be relied upon for higher DN has been accurately measured by spacecraft- (digital quantization level) tie points. The DN levels mounted, active cavity radiometers (Willson, 1984), expected for an area 1.5 times brighter than the the low-resolution spectral radiance of the Sun is average near-full Moon for the TM (255 DN Max) best determined in comparison with standard stars and MSS (127 DN Max) bands are: and is expressed as the apparent magnitude of the Sun at a distance of 1.0 A.U. in a number of standard spectral band passes. The same standard stars TM MSS are used in measurements of lunar brightness. The stability of solar luminosity and the standard stars Band DN Band DN assures that any Landsat calibration tied to the system of stellar magnitudes and colors, regardless of when they were obtained, could be revised by any future redetermination of the apparent magnitude of the Sun. Although the full Moon is over ten thousand times brighter than the brightest star, the direct LUNAR LANDSAT CALIBRATION

Owing to the large number of samples on the with a Fraunhofer diffraction response for a finite Moon and also to the presence of approximately 1 circular detector in a Cassagrain optical system of DN noise, the instrument response could be deter- the appropriate dimensions. (See Chase et al., 1978, mined to the order of 0.1 DN, which is more than especially Figure 6). adequate to support the radiometric absolute accu- A time window for carrying out this calibration racy at these response levels. occurs once a month, and lninilnuln uneclipsed If the Landsat instruments were to obtain a lunar phase (optimum) occurs twice a year. The slew past image close to full Moon, maps of lunar reflectivity the Moon should be in pitch, and the spacecraft (the product of normal albedo and photometric func- should pass the Moon at a rate of 33"Imin (9.6 mradl tion) for the wavelength passbands used by the TM sec) in order to create a conventional image; any and MSS could be computed for the time of the rate from 1/30 to twice this rate is adequate for cal- Landsat observations. Thus the absolute radiance ibration purposes. For the absolute radiometric cal- calibration in terms of reflectivity could be obtained ibration, the time of year must be taken into ac- directly. count, as the eccentricity of the Earth's orbit con- The mid-infrared brightness temperature of the tributes a * 3.4 percent variation in solar irradiance; center of the full Moon is near 400" K; the radiance the orbit of the Moon around the Earth corresponds within TM band 6 would be about 2.3 times that at to only one-haIf percent variation in irradiance. the TM saturation temperature of 320" K. Only areas near the lunar east limb would be within the REFERENCES dynamic range of the TM, but even these could not Chase, S. C., Jr., Engle, J. L., Eyerly, H. W., Kieff'er, be used for a reliable radiometric calibration owing H. H., I'alluconi, F. D., and Schofield, I)., 1978. Vi- to the strong lateral temperature gradients. king Infrared Thermal Mapper: Applied Optics, v. 17, pp. 1243-1251. Near full phase, the Moon represents a circular Johnson, T. I!,Mosher, J. A,, and Matson, D. L., 1977. source one-half degree in diameter. At the nominal Lunar Spectral Units: A Northern Hemispheric slew rate, each detector of the TM or MSS would Mosaic: Proceecliitgs, 8th Lunar Science Conference, cut more than 10 parallel chords across the Moon. pp. 1013-1028. Reconstruction of the lunar limb in this image would Lane, A. P., and Irvine, W. M., 1973. Monochromatic allow location of the lirnb relative to the sampling Phase Curves and All)edos for the Lunar Disk: Astro- pattern to a precision better than 0.1 pixel. A large noinicrrl Jorrrt~al,v. 78, no. 3, pp. 267-277. set of samples could be selected that represents ap- McCord, T. B., Charette, M. P., Johnson, T. \'., Le- parent offsets of the limb by increments less than bofsky, L. A., Pieters, C., and Adams, J. B., 1972. 0.2 pixel (MSS) or 0.1 pixel (TM). The relation be- Lunar Spectral Types: Journal of Geophysicrrl Re- tween response and lirnb offset for such pixels would search, v. 77, no. 8, pp. 1349-1359. yield the near-field response to a knife-edge source McCord, T. B., Clark, R. G., Hawke. B. R., McFadden, from which the MTF could be determined. L. A,, and Owel~sl,y,P. D., 1981. Xfoon: Near-In- The off-axis response could be determined with frared Spectral Reflectance, A First Good Look: one-half degree angular resolution by transforming Journal of Geophysical Resecrrcli, v. 86, no. B11, pp. the full-scene image of the Moon in annular ele- 10883-10892. ments to construct a radial response function. Al- McCord, T. B., Gral)ow, M., Feierberg, .M. A., Mac- though the ideal response (if all the optic elements Laskey, D., and Pieters, C., 1979. Lunar Multispec- tral Maps, Part 11: The Lunar Nearside: lcctrus. v. 37, have remained clean) to the off-axis Moon would be pp. 1-28. less than DN, thousands of salnples could be av- 1 Wildey, R. L., 1976. The Lunar Geometric Albedo and eraged within the one-half degree resolution ele- the Magnitude of the Full Mvon: The Obsercrrtorcj, ment. Here, the existence of instrument noise is an v. 96, pp. 235-239. advantage. Without noise, the response over most , 1977. A Digital File of the Lunar Normal Albedo: of the image would be constant at the nolninal dark The Moon, v. 16, no. 2, pp. 231-277. level. However, where the instrument noise level Willson, K. C., 1984. Sleasurement of Solar Total Irradi- is one-half degree DN or more, valid statistical av- ante and Its \Tarial)ility: Sprrce Science Recietc, v. 38, erages could be obtained that would be virtually nos. 3-4, pp. 203-242. unaffected by the discreet digitization levels. Willstrop, R. V, 1960. Absolute Measures of Stellar Ra- For the TM thermal band, which is approximately diation: Monthly Notices of the Royal Astronotnical defraction limited, these results can be compared Society, v. 121, pp. 17-26.