Philip N. Slater

Reflectance- and radiance-based methods for the in-flight absolute calibration of multispectral sensors

Abstract Variations reported in the in-flight absolute radiometric calibration of the Coastal Zone Color Scanner (CZCS) and the Thematic Mapper (TM) on Landsat 4 are reviewed. At short wavelengths these sensors exhibited a gradual reduction in response, while in the midinfrared the TM showed oscillatory variations, according to the results of TM internal calibration. The methodology and results are presented for five reflectance-based calibrations of the Landsat 5 TM at White Sands, NM, in the period July 1984 to November 1985. These show a ±2.8% standard deviation (1 σ) for the six solar-reflective bands. Analysis and preliminary results of a second, independent calibration method based on radiance measurements from a helicopter at White Sands indicate that this is potentially an accurate method for corroborating the results from the reflectance-based method.

Evaluation of simplified procedures for retrieval of land surface reflectance factors from satellite sensor output

Abstract In response to the need for a simple atmospheric correction method and the consequent verification of such a method, an experiment was conducted to acquire a data set suitable for testing atmospheric correction procedures under a variety of atmospheric conditions. Several procedures, including radiative transfer codes (RTCs) with simulated atmospheres, image-based procedures and dark-object subtraction (DOS), were evaluated by comparing surface reflectance factors derived from Landsat Thematic Mapper (TM) digital data with low-altitude, aircraft-based measurements for seven dates over a 1-year period. Acceptable results, approximately ± 0.02 reflectance (1 σ RMS), were achieved based on an RTC with appropriate simulated atmospheres. The DOS technique was the least accurate method and, in fact, produced greater error in estimations of near-IR reflectance than no correction at all. Two hybrid approaches, which combined the image-based nature of DOS with the precision of an RTC, provided sufficient accuracy and simplicity to warrant consideration for use on an operational basis. Though these results were probably site-specific (characterized by relatively low aerosol levels and low humidity), they illustrate the feasibility of simple atmospheric correction methods and the usefulness of a diverse data set for validation of such techniques.

Discrimination of growth and water stress in wheat by various vegetation indices through clear and turbid atmospheres

Abstract Reflectance data were obtained over a drought-stressed and a well-watered wheat plot with a hand-held radiometer having bands similar to the MSS bands of the Landsat satellites. Data for 48 clear days were interpolated to yield reflectance values for each day of the growing season, from planting until harvest. With an atmospheric path radiance model and Landsat 2 calibration data, the reflectances were used to simulate Landsat digital counts (not quantized) for the four Landsat bands for each day of the growing season, through a clear ( ⋍100- km meteorological range) and a turbid ( ⋍10- km meteorological range) atmosphere. Several ratios and linear combinations of bands were calculated using the simulated data, then assessed for their relative ability to discriminate vegetative growth and plant stress through the two atmospheres. The results showed that water stress was not detected by any of the indices until after growth was retarded, and the sensitivity of the various indices to vegetation depended on plant growth stage and atmospheric path radiance.

Calibration of Space-Multispectral Imaging Sensors: A Review

Abstract This article is a review of the various methods in use, or under development, to calibrate space-multispectral-imaging-systems in the solar-reflective range. We introduce the subject by distinguishing between absolute and relative calibration, briefly discussing the use of scene and sensor models, and describing five calibration desiderata. We then briefly describe the different types of existing methods, highlighting their advantages and disadvantages, making the distinction between: preflight, onboard, and vicarious calibration. The different types of radiometric calibration: absolute, multitemporal, interband, and intersensor are mentioned with their related constraints. Finally, recommendations are made on how to improve these methods so that the scientific community may obtain remote-sensing data of the highest quality.

Mapping surface energy balance components by combining landsat thematic mapper and ground-based meteorological data☆

Surface energy balance components were evaluated by combining satellite-based spectral data with on-site measurements of solar irradiance, air temperature, wind speed, and vapor pressure. Maps of latent heat flux density (λE) and net radiant flux density (Rn) were produced using Landsat Thematic Mapper (TM) data for three dates: 23 July 1985, 5 April 1986, and 24 June 1986. On each date, a Bowen-ratio apparatus, located in a vegetated field, was used to measure λE and Rn at a point within the field. Estimates of λE and Rn were also obtained using radiometers aboard an aircraft flown at 150 m above ground level. The TM-based estimates differed from the Bowen-ratio and aircraft-based estimates by less than 12 % over mature fields of cotton, wheat, and alfalfa, where λE and Rn ranged from 400 to 700 Wm−2.

Vicarious Radiometric Calibrations of EOS Sensors

Abstract Four methods for the in-flight radiometric calibration and cross calibration of multispectral imaging sensors are described. Three make use of ground-based reflectance, irradiance, and radiance measurements in conjunction with atmospheric measurements and one compares calibrations between sensors. Error budgets for these methods are presented and their validation is discussed by reference to SPOT and TM results and shown to meet the EOS requirements in the solar-reflective range.

Bidirectional measurements of surface reflectance for view angle corrections of oblique imagery

Abstract An apparatus for acquiring bidirectional reflectance-factor data was constructed and used over four surface types. Data sets were obtained over a headed wheat canopy, bare soil having several different roughness conditions, playa (dry lake bed), and gypsum sand. Results are presented in terms of relative bidirectional reflectance factors (BRFs) as a function of view angle at a number of solar zenith angles, nadir BRFs as a function of solar zenith angles, and, for wheat, vegetation indices as related to view and solar zenith angles. The wheat canopy exhibited the largest BRF changes with view angle. BRFs for the red and the near-infrared (NIR) bands measured over wheat did not have the same relationship with view angle. NIR/Red ratios calculated from nadir BRFs changed by nearly a factor of 2 when the solar zenith angle changed from 20° to 50°. BRF versus view angle relationships were similar for soils having smooth and intermediate rough surfaces but were considerably different for the roughest surface. Nadir BRF versus solar-zenith angle relationships were distinctly different for the three soil roughness levels. Of the various surfaces, BRFs for gypsum sand changed the least with view angle (10% at 30°).

Three methods for the absolute calibration of the NOAA AVHRR sensors in-flight

Abstract Three different approaches are described for the absolute radiometric calibration of the two reflective channels of the NOAA AVHRR sensors. Method 1 relies on field measurements and refers to another calibrated satellite sensor that acquired high-resolution imagery on the same day as the AVHRR overpass. Method 2 makes no reference to another sensor and is essentially an extension of the reflectance-based calibration method developed at White Sands for the in-orbit calibration of Landsat TM and SPOT HRV data. Method 3 achieves a calibration by reference to another satellite sensor, but it differs significantly from the first approach in that no ground reflectance and atmospheric measurements are needed on overpass day. Calibration results have been obtained using these methods for seven NOAA-9 AVHRR images and for four NOAA-10 AVHRR images. A significant degradation in NOAA-9 AVHRR responsivity has occurred since the prelaunch calibration and with time since launch. The responsivity of the NOAA-10 AVHRR has also degraded significantly compared to the prelaunch calibration. The suitabilities of using Method 2 with the Rogers (dry) Lake site in California and using Methods 1 and 3 at White Sands are discussed. The results for Method 3, which requires no field measurements and makes use of a simplified atmospheric model, are very promising, implying that a reasonable in-orbit calibration of satellite sensors may be relatively straightforward.

Field calibration of reference reflectance panels

Abstract The measurement of radiation reflected from a surface must be accompanied by a near-simultaneous measurement of radiation reflected from a reference panel in order to calculate a bidirectional reflectance factor for the surface. Adequate calibration of the reference panel is necessary to assure valid reflectance-factor data. A procedure is described by which a reference panel can be calibrated with the sun as the irradiance source, with the component due to diffuse flux from the atmosphere subtracted from the total irradiance. Furthermore, the radiometer that is used for field measurements is also used as the calibration instrument. The reference panels are compared with a pressed polytetrafluoroethylene (halon) standard. The advantages of this procedure over conventional laboratory calibration methods are, first, that the irradiance and viewing geometry is the same as is used in field measurements and, second, that the needed equipment is available, or can be constructed, at most field research laboratories, including the press necessary to prepare the halon standard. A disadvantage of the method is that cloud-free sky conditions are required during the measurement period. The accuracy of the method is estimated to be 1%. Calibration results are given for four reference panels.

Uncertainties in the in-flight calibration of sensors with reference to measured ground sites in the 0.4-1.1 μm range

Abstract This article describes the error sources for three in-flight sensor calibration methods used by the Remote Sensing Group of the Optical Sciences Center at the University of Arizona. The three methods are the reflectance-, improved reflectance-, and radiance-based methods, which all reference the earth-atmosphere system. The sources of error or uncertainty for each method are discussed, and an estimate of the percent uncertainty associated with each source is made for conditions similar to those actually used for calibrations at White Sands, New Mexico. The results of in-flight calibrations are compared to those of the on-board lamp calibration system for a SPOT HRV camera.