Solar calibration performance in Landsat multispectral scanners Jack C. Lansing, Jr.

Hughes Santa Barbara Research Center, 75 Coromar Drive, Goleta, California 93117. Received 29 August 1985 0003-6935/86/030333-02$02.00/0. © 1986 Optical Society of America A statement recently appeared in this journal that the

Landsat 1, 2, and 3 Multispectral Scanner (MSS) solar cali­bration method "was never successful."1 This statement was based on actual experience with Landsat 1 data only. As reported by Horan et al.,2,3 the Landsat 1 MSS solar calibra­tion was indeed seriously degraded after launch, but the Landsat 2 calibration performed satisfactorily. Landsat 3 solar calibration was also observed to perform satisfactorily.4

The solar calibration data were never used to verify the ground system output, partly because the calibration proce­dures developed on Landsat 1 data using calibration lamps gave satisfactory results.

An initial reason for not applying the solar calibration stemmed from a wide variation in duration of the sun signal due to a spacecraft attitude control problem. The horizon scanner used to stabilize spacecraft attitude produced erro­neous signals when the spacecraft was near the north termi­nator crossing, which is just the orbit position where the sun calibrator must provide data. After this latter effect was taken into account, the sun calibrator data showed consis­tent and stable performance.5 Unfortunately, that result was not documented except in laboratory notebook form, as far as this author can determine.

Another clarification concerns the statement in Ref. 3: "One should note that the ground calibration signals (which were performed on Landsat 1 only) were theoretically cor­rected to an exoatmospheric value." The two sets of ground tests (at Table Mountain, CA and Valley Forge, PA) were done with the Engineering model MSS only not the Landsat 1 flight model.

It is of interest to review the comparisons in Table I of Ref. 3 using more complete information on the two sets of tests. At the Table Mountain tests6 measurements of the solar spectral irradiance were made with some of the instruments which had been used in a NASA program of airborne solar measurements.7 These instruments were operated by the experimenters who had been responsible for them in the airborne program, and the data collected were also reduced by the same workers.8 The atmospheric transmission used to correct the MSS sun signals was the ratio of the irradiance from the solar measurements to the exoatmospheric irradi­ance listed in Ref. 7.

The ground tests at Valley Forge viewed the sun through ~ 2.2 air masses (vs 1.4-1.5 at Table Mountain) from a low-altitude location and used an auxiliary mirror to bring the sun to the instrument. The atmospheric transmission was only estimated in these tests.9 Because of the direct mea­surement of irradiance, the direct sun viewing, and the re-

1 February 1986 / Vol. 25, No. 3 / APPLIED OPTICS 333

Table I. Solar Calibrator Performance

duced atmosphere, the Table Mountain tests are considered to be more accurate. Table I shows the results from these data, corrected to outside the atmosphere, compared to the values calculated from Ref. 7 exoatmospheric solar irradi-ance and the MSS radiometric calibration, which is refer­enced to National Bureau of Standards spectral irradiance standards.

The original calculations which used Ref. 7 for irradiance above the atmosphere also used Ref. 10 for the ratio of solar disk center radiance to mean radiance, necessary because the MSS IFOV is much smaller than the sun. The table entries under "Engineering model" are the means of measurements through all four apertures, referred to outside the atmo­sphere. The ratios of these means to the calculated values are also shown here.

The recalculated values used a different source of solar disk center radiance11 considered by some to be of high accuracy.12 The Landsat 2 results in orbit are repeated here, and the ratios are shown for both the original calculations and the recalculation. While the latter appear closer to unity, the MSS calibration accuracy is not sufficient to pre­sume that this is evidence for the accuracy of Ref. 11.

References 1. W. A. Hovis, J. S. Knoll, and G. R. Smith, "Aircraft Measure­

ments for Calibration of an Orbiting Spacecraft Sensor," App. Opt. 24, 407 (1985)

2. J. J. Horan, D. S. Schwartz, and J. D. Love, "Partial Perfor­mance Degradation of a Remote Sensor in a Space Environment, and some Probable Causes," App. Opt. 13, 1230 (1974).

3. J. J. Horan, D. S. Schwartz, and J. D. Love, "In Orbit Sun Calibration Performance of Landsat-2," Appl. Opt. 14, 2053 (1975).

4. J. A. Balla, Goddard Space Flight Center; personal communica­tion (1985).

5. W. White, Goddard Space Flight Center; personal communica­tion (1976).

6. V. T. Norwood, L. R. Fermelia, and G. A. Tadler, "Multispectral Scanner System for ERTS; Four-Band Scanner System," Final Report Vol. 1, Hughes Aircraft Co. for NASA Goddard Space Flight Center, Aug. 1972, NASA STAR Accession N73-25474.

7. M. P. Thekaekara, R. Kruger, and C. H. Duncan, "Solar Irradi­ance Measurements from a Research Aircraft," Appl. Opt. 8, 1713 (1969).

8. A. T. Mecherikunnel and C. H. Duncan, "Total and Spectral Solar Irradiance Measured at Ground Surface," Appl. Opt. 21, 554 (1982).

9. D. S. Schwartz, General Electric; personal communication (1985).

10. M. Minnaert, "The Photosphere," in The Sun, G. P. Kuiper, Ed. (U. Chicago Press, Chicago, 1953).

11. H. Neckel and D. Labs, "The Solar Radiation Between 3300 and 12500 Å," Sol. Phys. 90, 205 (1984).

12. C. Frolich, "Data on Total and Spectral Solar Irradiance: com­ments," Appl. Opt. 22, 3928 (1983).

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