Richard P. Santer

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.

Obtaining Surface Reflectance Factors from Atmospheric and View Angle Corrected SPOT-1 HRV Data

Abstract SPOT-1 High-Resolution Visible (HRV) multispectral (XS) and panchromatic data were acquired over an agricultural area on two consecutive days in June 1987, June 1988, and April 1989, at view zenith angles of approximately 23° and 10°. Digital data were converted to surface reflectance factors ( ρ s ) by use of the sensor calibration coefficients, measurements of atmospheric optical depth, and a radiative transfer model. View-angle corrections (Cv) were derived from ground-based measurements of bidirectional radiance of bare soil, and used to convert nadir ground- and aircraft-based measurements to off-nadir values ( ρ g and ρ a , respectively) for comparison with SPOT HRV data. The absolute error of ρ s values, relative to ρ g and ρ a , was less than 10% for most XS bands on all six days over the reflectance range 0.1-0.4. However, there was a systematic trend for ρ s estimates to be slightly higher than ρ g and ρ a measurements, particularly at low surface reflectances. The Cv coefficients were then applied to SPOT HRV data for a variety of cover types to assess the effectiveness of a simple, view-angle correction over a complex landscape. For rough, unvegetated surfaces, ρ s values that had originally differed by more than 0.09 in reflectance on the two days were brought to within 0.01 in all three XS bands. For vegetated surfaces, Cv appeared to be wavelength dependent; the soil-based Cv worked well for data in the red and green wavebands but overcorrected the near-IR data. The Cv correction overcompensated for view angle effects over planar surfaces (i.e., water and roads) in all wavebands.

Laboratory Calibration Of Field Reflectance Panels

A method used for calibrating field reflectance panels in the visible and shortwave infrared wavelength range is described. The directional reflectance factor of painted barium sulfate (BaSO4) panels is determined. The reference for this method is the hemispherical reflectance of pressed polytetrafluoroethylene (halon) powder prepared according to National Bureau of Standards (NBS) directions. The panels and a radiometer are mounted on rotation stages to measure the reflectance factor at different incidence and view angles. The sensor can be any laboratory or field filter radiometer small enough to mount on the apparatus. The method is used to measure the reflectance factors of halon and BaSO4 panels between 0.45 and 0.85 micrometers. These reflectance factors are compared to those measured by a field apparatus. The results agree to within 0.013 in reflectance at incidence angles between 15 and 75 degrees.

Absolute radiometric calibration of the noaa avhrr sensors

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 four NOAA-9 AVHRR images and for one NOAA-10 AVHRR image. 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 calibration of satellite sensors may be relatively straightforward.

ROSAS: a robotic station for atmosphere and surface characterization dedicated to on-orbit calibration

La Crau test site is used by CNES since 1987 for vicarious calibration of SPOT cameras. The former calibration activities were conducted during field campaigns devoted to the characterization of the atmosphere and the site reflectances. Since 1997, au automatic photometric station (ROSAS) was set up on the site on a 10m height pole. This station measures at different wavelengths, the solar extinction and the sky radiances to fully characterize the optical properties of the atmosphere. It also measures the upwelling radiance over the ground to fully characterize the surface reflectance properties. The photometer samples the spectrum from 380nm to 1600nm with 9 narrow bands. Every non cloudy days the photometer automatically and sequentially performs its measurements. Data are transmitted by GSM (Global System for Mobile communications) to CNES and processed. The photometer is calibrated in situ over the sun for irradiance and cross-band calibration, and over the Rayleigh scattering for the short wavelengths radiance calibration. The data are processed by an operational software which calibrates the photometer, estimates the atmosphere properties, computes the bidirectional reflectance distribution function of the site, then simulates the top of atmosphere radiance seen by any sensor over-passing the site and calibrates it. This paper describes the instrument, its measurement protocol and its calibration principle. Calibration results are discussed and compared to laboratory calibration. It details the surface reflectance characterization and presents SPOT4 calibration results deduced from the estimated TOA radiance. The results are compared to the official calibration.

BRDF and surface-surround effects on SPOT-HRV vicarious calibration

Typically, the surfaces used in radiative transfer codes to predict the radiance at a satellite sensor are assumed to be lambertian and spatially-homogeneous. Of course, surface bidirectionality and surface-surround effects are second order terms, but improvements in vicarious calibration procedures, as well as more challenging accuracy requirements, require that these effects be included. This paper examines the effect of surface BRDF and spatial inhomogeneity on retrieved calibration coefficients for the SPOT HRV cameras from the well-known reflectance-based calibration approach. This calibration method has primarily relied on test sites at White Sands Missile Range in New Mexico, USA and La Crau, France. BRDF effects are studied using multispectral measurements of the bi-directional reflectances of both sites used in a Fourier series expansion in azimuth to model the surface BRDF. This Fourier series expansion follows the architecture of the successive- orders-of-scattering radiative transfer code and is easily introduced as a new boundary condition. These BRDF models are used to reprocess past calibration data and the results are compared to those obtained by assuming the surface to be lambertian. In addition to surface BRDF, spatial- inhomogeneity of the surface reflectance is examined. The surrounding areas surface reflectance is derived from the SPOT imagery and included in the radiative transfer computations suing in house modifications to the 6S code. These results are compared to those which exclude adjacency effects.

Results of dark target vicarious calibration using Lake Tahoe

The ability to conduct in-flight absolute radiometric calibrations of ocean color sensors will determine their usefulness in the decade to come. On-board calibration systems are often integrated into the overall system design of such sensors and have claimed uncertainly levels from 2-3 percent, but independent means of system calibration are desirable to confirm that such systems are operating properly. Vicarious methods are an attractive means of this verification. Due to the high sensitivity of ocean color sensors, the use for bright reflectance surfaces often results in sensor saturation. Low reflectance targets, such as water bodies, should therefore be used. This paper presents the results of sensitivity studies of the reflectance- and radiance-based approaches when applied to a water target and method uncertainties for calibrations of the Sea-Viewing Wide Field-of-view Sensor (SeaWiFS). The paper also present the results of a field campaign which took place at Lake Tahoe in June 1995. This lake represents a typical oligotrophic water body and has the advantage of being located at a high elevation where tropospheric aerosol loading is low. Aircraft-based radiance data and surface measurements of reflectance are sued to calibrate SeaWiFS- simulated bands from Advanced VIsible and Infrared Imaging Spectrometer (AVIRIS) data. Atmospheric characterization is obtained using solar extinction measurements, surface-level atmospheric pressure readings, and columnar gaseous absorber amounts at sensor overpass. The measured radiances are transferred to the top of the atmosphere using a radiative transfer code which fully computes the contributions of multiple scattering by the atmosphere. The results are compared to those obtained form a laboratory-based calibration of AVIRIS.

Radiometer calibrations using solar radiation

Airborne radiometric instruments are often used to collect calibrated radiance data, whether for producing remotely- sensed imagery, for use in vicarious calibration, or for atmospheric correction. Typically, these radiometers are calibrated in a laboratory environment using source whose spectral outputs are traceable to some established, man-made standard. In the field, these devices are used with a different source: solar radiation. The use of solar radiation as a calibration source should therefore be considered when calibration radiometers used to collect energy in the solar reflective region. This paper presents a novel method of calibration which makes use of scattered solar radiation as the source. This technique is particularly applicable for airborne radiometers intended to view low-reflectance surfaces, since the magnitude and spectral distribution of the collected energy is very similar to that of skylight, especially at shorter visible wavelengths. The method is applied to visible and near-IR bands of a Barnes Modular Multispectral 8-channel Radiometer. A sensitivity study was performed for the method and an associated uncertainty analysis is presented. The calibration results are compared to a second, more established solar-based method whose source is directly- transmitted solar irradiance.

NOAA-11 channels 1 and 2 calibration in the SPACE software

The software SPACE, developed at JRC, Ispra, Italy, processes on a day-by-day basis the AVHRR images over Europe to forecast crop production which requires updated calibration of channels 1 and 2. The most suitable way to update calibration is to implement specific subroutines with, after identification and characterization, three Saharan desertic sites for inter-temporal calibration and the sunglint off-shore of Europe for an interband calibration. The paper also adresses the way to obtain absolute calibration and raises the problem of potential spectral shift of the sensor.

Solar aureole instrumentation and inversion techniques for aerosol studies: Part 1, system design and calibration

The in-flight calibration of satellite radiometers using ground truth measurements relies on the use of an atmospheric radiative transfer code. The accuracy of the calibration depends largely on the aerosol model used in the radiative transfer codes. In order to improve the calibrations, a camera system has been developed for the determination of the aerosol size distribution, index of refraction, and scattering phase function. In addition, the camera can be used to measure ozone and water vapor content. The camera uses a two dimensional silicon CCD array to image the sun and the solar aureole. A filter wheel provides eight spectral bands from 310 nm to 1045 nm. The camera is mounted on an altitude-azimuth mount for tracking the sun. An external computer allows automatic or manual data acquisition. This paper presents the design and calibration of the camera system. A companion paper presents the data collection and inversion techniques used to retrieve the parameters of interest.