RADIOMETRIC PROCESSING AND CALIBRATION OF EO-1 ADVANCED LAND IMAGER DATA B. L. Markham, Scientist Biospheric Sciences Branch NASA/GSFC Greenbelt, MD 20771 Brian.L.Markham@nasa.gov G. Chander, Scientist R. Morfitt, Systems Engineer D. M. Hollaren, Software Systems Lead J. Nelson, Systems Engineer Science Applications International Corporation (SAIC) Contractor to U.S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center Sioux Falls, SD 57198 gchander@usgs.gov rmorfitt@usgs.gov hollaren@usgs.gov jnelson@usgs.gov L. Ong, Scientist Science Systems and Applications, Inc EO-1 Mission Science Office NASA/GSFC Greenbelt, MD 20771 Lawrence.Ong@gsfc.nasa.gov J. A. Mendenhall, Technical Staff Member Advanced Space Systems and Concepts Group MIT Lincoln Laboratory 244 Wood Street Lexington, MA 02420-9185 mendenhall@ll.mit.edu ABSTRACT A radiometric and geometric processing system has been developed as part of the Advanced Land Image(r) Assessment System (ALIAS). A derivative of the ALIAS processing system has also been implemented operationally at EROS to generate Advanced Land Imager (ALI) Level 1 radiometrically and geometrically corrected data products. This system replaces the calibration pipeline previously used to generate Level 1R radiometrically corrected ALI data products at EROS. The revised calibration systems initially use the same radiometric calibration coefficients as previous systems, i.e., a fixed set of detector-by-detector gains established shortly after launch and biases measured shortly after each scene acquisition by closing the ALI s shutter. The data are scaled differently in the final products, however. The original calibration pipeline scaled the Level 1R data output by a fixed factor of 300, i.e., to obtain the radiance in mw/cm 2 sr m for a given pixel in any band, you divided the quantized calibrated data value by 300. To scale to W/m 2 sr m, a factor of 30 is used. The revised system uses a band specific multiplicative scaling factor of between 22 and 1100 as well as a band specific additive scaling factor for units of W/m 2 sr m. The band specific multiplicative scaling factor results in better preservation of the ALI s precision and dynamic range in going from raw to calibrated data, especially in the SWIR bands. This reduces the amount of high signal data being clipped by the rescaling process and reported as saturated in the calibrated data products. Setting the minimum valid data value to 1 DN in the output products combined with the additive factor results in all the valid data being positive values with negligible, if any, loss of valid data. Additional
work is underway to replace the current fixed set of detector gains with time-dependent functions that represent the gradual change in gain of the ALI detectors. INTRODUCTION The Advanced Land Imager (ALI) launched November 21, 2000 on the Earth Observing-1 (EO-1) satellite, is a technology demonstrator for future Landsat sensors (Ungar et al., 2003). As opposed to the traditional whiskbroom sensors, e.g., the Enhanced Thematic Mapper Plus (ETM+) on Landsat-7, the ALI is a pushbroom instrument. Also, the detector technology on ALI does not require cryogenic cooling for the Short Wave Infrared (SWIR) bands. The combination of increased dwell time on the detectors from the pushbroom technology which allows smaller optics while achieving significant improvement in signal to noise and the decreased cooling needs, leads to a significantly smaller, lighter and less expensive design. The ALI bands were designed to mimic the six standard ETM+ spectral bands 1, 2, 3, 4, 5 and 7; three new bands, 1p, 4p and 5p were added in order to more effectively address atmospheric interference effects and specific applications (Table 1). The ALI also incorporates a higher resolution (10m) and narrower spectral bandpass panchromatic band than the ETM+. The ALI uses wide-angle optics to provide a continuous field of view (FOV) without the use of a scan mirror. Although the ALI is designed to support a 15 FOV, only a 3 FOV was populated with linear detector arrays arranged in four Sensor Chip Assemblies (SCA s). The partially populated ALI provides a ground swath width of 37 km. Each SCA contains 320-element detector arrays that image in nine multispectral bands with 30 m spatial resolution, as well as a 960-element detector array with a 10 m spatial resolution that images a single panchromatic band. The ALI data are quantized to 12 bits and the saturation radiances are set to allow imaging targets of 100% diffuse reflectance up to the maximum solar elevation angles observed at nadir with ALI. A design similar to ALI is being considered for the Operational Land Imager (OLI) for the Landsat Data Continuity Mission (LDCM), the follow on to Landsat-7. The ALI data are being used by the LDCM project to develop an ALI Image Assessment System (ALIAS) as a precursor to the image assessment system to be developed for OLI. Algorithms developed for ALI image processing and characterization have been captured and documented. As part of the ALIAS effort, the ALI radiometric and geometric performance is being characterized over the lifetime of the mission. Historically, the EO-1 ALI data were processed at the U. S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center through the original EO-1 product generation system, also referred to as the EO-1 calibration pipeline. Many of the enhancements and changes developed as part of the ALIAS efforts have been incorporated into the operational EO-1 product generation system for ALI data, effective December 22, 2004. Among those changes incorporated are modifications to the scaling of the output data. This paper documents those changes to the radiometric scaling of the Level 1 data products. Table 1. EO-1 Advanced Land Imager Spectral Bands Band Bandpass (nm) Ground Sample Distance (m) Pan 480-690 10 1 433-453 30 1 450-515 30 2 525-605 30 3 630-690 30 4 775-805 30 4 845-890 30 5 1200-1300 30 5 1550-1750 30 7 2080-2350 30
CALIBRATION PIPELINE RADIOMETRIC PROCESSING AND SCALING The basic radiometric calibration equation in the ALI calibration pipeline is equation (1) of Evans and Viggh (1999). L λ = R n (C n C 0 ) (1) L is the spectral radiance in mw/cm 2 sr m (changed from pre-launch in-band units of W/m 2 sr) R n is the response coefficient from the calibration database for detector n in (mw/cm 2 sr m)/dn (inverse of the detectors gain (G) or responsivity) C n is detector n s response for a pixel in DN (12 bit integer) C 0 is the detectors average dark response or bias (B) measured with the shutter closed in DN The floating-point radiances produced by this calibration step are represented in the output data product as integers and are scaled by a factor of 300. Thus, to obtain a radiance value from the calibration pipeline data product, the output value is divided by 300. Thus the inherent quantization of the data products is 1/300 or 0.00333 mw/ cm 2 sr m (alternatively 1/30 or 0.0333 W/m 2 sr m). Per the standard Landsat terminology, in units of W/m 2 sr m, the calibration equations are: L λ = (Q B)/G (2) QCAL = 30* L λ or L λ = QCAL/30 (3) Q is the detectors response in DN (replacing C n ). QCAL is the calibrated pixel value in DN (16 bit signed integer). B is the detector bias in DN. G is the detector gain in DN/(W/m 2 sr m). UPDATED EO-1 PRODUCT GENERATION SYSTEM RADIOMETRIC SCALING The radiometric scaling of the ALI processed data was reexamined during the development of ALIAS. There were several considerations in the process: (1) Maintaining as much as possible of the usable dynamic range of each band as possible, while not exceeding a 16-bit output data product. This includes both not compressing the data so that precision is lost and not reducing the saturation radiances below what is required for Earth targets (2) Removing the potential for any negative numbers in the processed data, as the surveyed users preferred this and (3) Having distinguishable fill, real and saturated data values. Data Precision The precision of the unprocessed ALI was reviewed based the average operational response coefficients, R n, used in the ALI calibration pipeline for each band. As shown in column 2 of Table 2, the average step sizes (1/R n ) are larger than the calibrated data step size for all but bands 5 and 7. To retain the full precision of the band 5 and 7 data, the scaling factor of 30 needed to be increased for these bands, particularly band 7.
Table 2. EO-1 Cal Pipeline Raw and Processed Data Precision Band Average Raw Step Size Calibrated data step size (W/m 2 sr m/dn) (W/m 2 sr m/dn) Pan 0.16 0.033 1' 0.29 0.033 1 0.27 0.033 2 0.18 0.033 3 0.12 0.033 4 0.076 0.033 4' 0.062 0.033 5' 0.054 0.033 5 0.024 0.033 7 0.0058 0.033 Saturation Radiances Similarly, in processing the data, no loss in the saturation radiance was desired. Here, the average saturation radiance of the detectors on the sensor chip assembly with the highest average saturation radiance was used as the saturation radiance needed for the processed data. This radiance was increased by 10% to allow for degradation over the life of the mission (Table 3). Thus, in the processed data, the data should not saturate below these radiance values. Note that in several bands, e.g., 1 and 1, the saturation radiances are well above what will be seen for diffuse Earth targets. Table 3. EO-1 Processed Data Minimum Saturation Radiance Requirements Band Saturation Radiance (W/m 2 sr m) 100% Diffuse Solar (@22.5 SZA) (W/m 2 sr um) Pan 718.2 505.9 1 1393 535.8 1 1319 569.8 2 839.4 531.9 3 538.2 449.3 4 344.9 337.3 4 279.0 276.7 5 247.9 130.7 5 85.92 66.60 7 27.12 23.05 Minimum Radiances The user community desired processed data where the calibrated digital numbers were all positive. In order to not truncate digital values that were originally negative over dark targets due to detector noise, a bias needed to be introduced into the processed data. This bias or additive scaling coefficient was set so that the radiance equivalent to -5 times the standard deviations of the dark noise for the noisiest (2 DN was used as an approximation) and least sensitive detectors (minimum gain) for the Visible and Near-Infrared (VNIR) detectors was at 1 DN in the processed data (Table 4). For the SWIR detectors, -10 times the standard deviation was used. Any detector data values that do map to a DN value of less than 1 in the processed data are set to 1. The DN value of 0 is reserved as the fill value.
Final Scaling Parameters and Derived Minimum and Maximum Radiances Based on the minimum saturation radiances defined in Table 3 and the biases defined in the previous section, the minimum saturation radiances were set to a DN value of 30000, near the top end of the scale for 16 bit signed integer (32767). The derived scaling factors for the final calibration are shown in Table 4. Note that the processed data step size is in all cases smaller than the raw data average step size, preserving the data s precision (compare column 2 of Table 4 to column 2 of Table 2). The scaling equation is: L λ = M QCAL + A (4) M is the multiplicative factor in (W/m2 sr m)/dn. A is the additive factor in (W/m2 sr m) Table 4: ALI Processed Data Radiometric Scaling Coefficients Band Multiplicative Calibration Coefficient, M (W/m 2 sr m/dn) Additive Calibration Coefficient, A (W/m 2 sr m) Pan 0.024-2.2 1' 0.045-3.4 1 0.043-4.4 2 0.028-1.9 3 0.018-1.3 4 0.011-0.85 4' 0.0091-0.65 5' 0.0083-1.3 5 0.0028-0.6 7 0.00091-0.21 In terms of the traditional scaling coefficients supplied for Landsat data, LMIN, LMAX, QCALMIN, QCALMAX, (NASA, 2005): L = ((LMAX - LMIN)/(QCALMAX-QCALMIN)) * (QCAL-QCALMIN) + LMIN (5) QCALMAX = 32767 QCALMIN =1 LMIN and LMAX in Table 5. Note that the LMAX is in all cases is higher than the minimum saturation radiance specified in Table 2. Thus both the precision of the data as well as the saturation radiances are preserved.
ONGOING STUDIES As part of the ALIAS, the trends in the responses of the ALI instrument have been continued from the earlier studies of Mendenhall et al., (2003). To date lamp and lunar data for about 4.5 mission years have been processed, along with the previously processed solar data up until the date of the solar aperture failure in July 2002. Once the initial analysis of these data have been completed, the lifetime history of the ALI radiometric calibration will be described by a set of piecewise linear segments. It is anticipated that this initial analysis will be completed by the Fall of 2005 Table 5. ALI Radiometric Scaling Parameters LMIN LMAX (W/m 2 sr m) (W/m 2 sr m) Pan -2.18 784.2 1' -3.36 1471 1-4.36 1405 2-1.87 915.5 3-1.28 588.5 4-0.84 359.6 4' -0.641 297.5 5' -1.29 270.7 5-0.597 91.14 7-0.209 29.61 ACKNOWLEDGEMENTS This effort was supported by the Landsat Data Continuity Mission (LDCM) Project at NASA/Goddard Space Flight Center and U. S. Geological Survey at EROS. SAIC work at EROS performed under U.S. Geological Survey contract 03CRCN0001. REFERENCES Evans, J. B. and Viggh, H. E. M. (1999). Radiometric calibration pipeline for the EO-1 Advanced Land Imager. In Earth Observing Systems IV, SPIE Vol. 3750, pp 153-161. Mendenhall, J.A., Hearn, D. R., Lencioni, D. E., Digenis, C. J., and Ong, L. (2003), Summary of the EO-1 ALI performance for the first 2.5 years on-orbit. In Proc SPIE Vol 5151, pp 574-585. NASA. (2005). Landsat-7 science data users handbook. Online at http://ltpwww.gsfc.nasa.gov/ias/handbook/handbook_htmls/chapter11/chapter11.html#section11.3, NASA/GSFC, Greenbelt, MD. Ungar, S. G., Pearlman, J. S., Mendenhall, J.A., and Reuter, D. (2003). Overview of the Earth Observing One (EO- 1) mission. IEEE Trans. GeoSci and Rem. Sens., 41: 1149-1159.