Advanced Very High Resolution Radiometer Calibration Research Articles

Intercalibrating the MODIS and AVHRR Visible Bands Over Homogeneous Land Surfaces

Sensor intercalibration has been widely performed over low-latitude deserts and circumpolar regions where surface conditions are generally well characterized. This letter proposes a new method for intercalibrating visible bands of Moderate Resolution Imaging Spectroradiometer (MODIS) and Advanced Very High Resolution Radiometer (AVHRR) over homogenous land surfaces that differ in brightness, in order to improve calibration results. The method is based on the extended simultaneous nadir overpass events. Initially, the spectral band adjustment factors are calculated using surface reflectances in the MODIS 552 and 645-nm bands. Sensor differences related to atmospheric effects are corrected using MODIS atmospheric parameters and MODIS/AVHRR sun-target-sensor geometries. In view of excellent MODIS calibration accuracy, the residual MODIS–AVHRR difference may result from AVHRR calibration bias. A preliminary validation study over radiometric sites shows <1.0% uncertainty for the high-gain calibration and <2.0% uncertainty for the low-gain calibration. The proposed method allows sensor intercalibration over varying land surfaces, and contributes to a collection of sensor calibration and intercalibration results of AVHRR sensors.

A Consistent AVHRR Visible Calibration Record Based on Multiple Methods Applicable for the NOAA Degrading Orbits. Part II: Validation

AbstractConsistent cross-sensor Advanced Very High Resolution Radiometer (AVHRR) calibration coefficients are determined using desert, polar ice, and deep convective cloud (DCC) invariant Earth targets. The greatest AVHRR calibration challenge is the slow orbit degradation of the host satellite, which precesses toward a terminator orbit. This issue is solved by characterizing the invariant targets with NOAA-16 AVHRR observed radiances that have been referenced to the Aqua Moderate Resolution Imaging Spectrometer (MODIS) calibration using simultaneous nadir overpass (SNO) observations. Another benefit of the NOAA-16 invariant target–modeled reflectance method is that, because of the similarities among the AVHRR spectral response functions, a smaller spectral band adjustment factor is required than when establishing calibrations relative to a non-AVHRR reference instrument. The sensor- and band-specific calibration uncertainties, with respect to the calibration reference, are, on average, 2% and 3% for channels 1 and 2, respectively. The uncertainties are smaller for sensors that are in afternoon orbits, have longer records, and spend less time in terminator conditions.The multiple invariant targets referenced to Aqua MODIS (MITRAM) AVHRR calibration coefficients are evaluated for individual target consistency, compared against Aqua MODIS/AVHRR SNOs, and selected published calibration gains. The MITRAM and SNO relative calibration biases mostly agree to within 1% for channels 1 and 2, respectively. The individual invariant target and MITRAM sensor relative calibration biases are mostly consistent to within 1% and 2% for channels 1 and 2, respectively. The differences between the MITRAM and other published calibrations are mostly attributed to the reference instrument calibration differences.

A Non-Stationary 1981–2012 AVHRR NDVI3g Time Series

The NDVI3g time series is an improved 8-km normalized difference vegetation index (NDVI) data set produced from Advanced Very High Resolution Radiometer (AVHRR) instruments that extends from 1981 to the present. The AVHRR instruments have flown or are flying on fourteen polar-orbiting meteorological satellites operated by the National Oceanic and Atmospheric Administration (NOAA) and are currently flying on two European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) polar-orbiting meteorological satellites, MetOp-A and MetOp-B. This long AVHRR record is comprised of data from two different sensors: the AVHRR/2 instrument that spans July 1981 to November 2000 and the AVHRR/3 instrument that continues these measurements from November 2000 to the present. The main difficulty in processing AVHRR NDVI data is to properly deal with limitations of the AVHRR instruments. Complicating among-instrument AVHRR inter-calibration of channels one and two is the dual gain introduced in late 2000 on the AVHRR/3 instruments for both these channels. We have processed NDVI data derived from the Sea-Viewing Wide Field-of-view Sensor (SeaWiFS) from 1997 to 2010 to overcome among-instrument AVHRR calibration difficulties. We use Bayesian methods with high quality well-calibrated SeaWiFS NDVI data for deriving AVHRR NDVI calibration parameters. Evaluation of the uncertainties of our resulting NDVI values gives an error of ± 0.005 NDVI units for our 1981 to present data set that is independent of time within our AVHRR NDVI continuum and has resulted in a non-stationary climate data set.

Post calibration of channels 1 and 2 of long-term AVHRR data record based on SeaWiFS data and pseudo-invariant targets

Advanced very high resolution radiometer (AVHRR) data provide a global, long-term, consistent time series with favorable spectral and spatial resolution. Such measurements are necessary for climate change studies. However, significant problems with the AVHRR calibration have been seen in two separate sources of early AVHRR data — lack onboard calibration and observed trends and biases when the AVHRR radiances are compared with other accurate top-of-atmosphere (TOA) radiances sources. In this paper, new calibration coefficients for channels 1 and 2 of the AVHRR sensors onboard 16 NOAA satellites from TIROS-N, NOAA-6 to NOAA-19, and Metop-A, B are reported. The calibration slopes are derived based on several pseudo-invariant sites endorsed by the Committee on Earth Observation Satellites (CEOS). Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) channels 6 and 8 radiance data are applied in characterizing the bidirectional reflectance distribution functions (BRDF) of the TOA reflectance over the employed targets, and MODTRAN simulations are carried out to parameterize the spectral channel adjustment factors (SBAFs) between SeaWiFS and each AVHRR sensor to eliminate the spectral response differences over different targets considering the impacts of observing geometries and gaseous absorption. The BRDFs and SBAFs are then applied to filtered AVHRR time-series to generate time series of calibration slopes presented in the form of quadratic polynomials as a function of time and applied in the recalibration of AVHRR data records. Comparisons with calibration coefficients generated from other products exhibit preferable validity and consistency, and notable incoherence and trend in the TOA reflectance time series of the L1B AVHRR data over each site from all the launched NOAA series polar orbiting satellites are largely eliminated after the recalibration. The long-term and inter-sensor consistent AVHRR data records after this post calibration can provide valuable insight into global climate changes since 1978.

Inter-calibration of polar imager solar channels using SEVIRI

Abstract. Accurate calibration of satellite imagers is a prerequisite for using their measurements in climate applications. Here we present a method for the inter-calibration of geostationary and polar-orbiting imager solar channels based on regressions of collocated near-nadir reflectances. Specific attention is paid to correcting for differences in spectral response between instruments. The method is used to calibrate the solar channels of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on the geostationary Meteosat satellite with corresponding channels of the Moderate Resolution Imaging Spectroradiometer (MODIS) on the polar-orbiting Aqua satellite. The SEVIRI operational calibration is found to be stable during the years 2004 to 2009, but offset by −8, −6, and +3.5 % for channels 1 (0.6 μm), 2 (0.8 μm), and 3 (1.6 μm), respectively. These results are robust for a range of choices that can be made regarding data collocation and selection, as long as the viewing and illumination geometries of the two instruments are matched. Uncertainties in the inter-calibration method are estimated to be 1 % for channel 1 and 1.5 % for channels 2 and 3. A specific application of our method is the inter-calibration of polar imagers using SEVIRI as a transfer instrument. This offers an alternative to direct inter-calibration, which in general has to rely on high-latitude collocations. Using this method we have tied MODIS-Terra and Advanced Very High Resolution Radiometer (AVHRR) instruments on National Oceanic and Atmospheric Administration (NOAA) satellites 17 and 18 to MODIS-Aqua for the years 2007 to 2009. While reflectances of the two MODIS instruments differ less than 2 % for all channels considered, deviations of an existing AVHRR calibration from MODIS-Aqua reach −3.5 and +2.5 % for the 0.8 and 1.6 μm channels, respectively.

Derive a MODIS-Based Calibration for the AVHRR Reflective Solar Channels of the NOAA KLM Operational Satellites

The Advanced Very High Resolution Radiometer (AVHRR) has been on board the National Oceanic and Atmospheric Administration (NOAA) polar-orbiting satellites (POSs) beginning with the Television Infrared Observation Satellite N launched in October 1978. Since then, a series of AVHRR sensors have collected over 30 years of continuous daily global observations. Recently, the NOAA K, L, and M POSs have provided an improved monitoring of the Earth's environment. The AVHRR instrument carried on NOAA-15 (NOAA-K) was launched in May 1998. It is followed by NOAA-16 to NOAA-18 (NOAA-L, NOAA-M, and NOAA-N), Metop-A, and NOAA-19 (NOAA-N'). All of them have been operating to date. An accurate and consistent calibration for the AVHRR reflective solar channels has been challenging as there is no onboard calibrator and vicarious calibration often needs to accumulate large number of reliable observations to derive any meaningful long-term trends. In this paper, we use the Committee on Earth Observation Satellites-endorsed calibration/validation Libya-4 (28.55 N, 23.39 W) desert site to track the long-term stability of reflective solar channels of the NOAA KLM AVHRR. The Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Terra and Aqua platforms is used as a reference to recalibrate the NOAA KLM trends. This study is focused on evaluating the calibration accuracy for the visible and near-infrared channels of each AVHRR instrument using MODIS channels 1 (620-670 nm) and 2 (841-876 nm). A site-specific bidirectional reflectance distribution function developed based on observations made by MODIS is used to normalize AVHRR-observed reflectances. Impacts of atmospheric water vapor on AVHRR-to-MODIS reflectance ratios are corrected with measured total water-vapor contents derived using the split-window temperature difference technique. Finally, MODIS-based AVHRR calibration results, which track the AVHRR on-orbit change, are applied on top of the AVHRR prelaunch values and presented in the form of quadratic polynomials as a function of time. All recalibrated AVHRR reflectances are compared with those provided from a Level 1B product. Additional validation is performed using NOAA-17 AVHRR observations acquired over Antarctic Dome Concordia site where the impact due to atmospheric water vapor is believed to be extremely small.

A Physical Method for the Calibration of the AVHRR/3 Thermal IR Channels. Part II: An In-Orbit Comparison of the AVHRR Longwave Thermal IR Channels on board MetOp-A with IASI

Abstract Obtaining stable and accurate satellite radiances for climate change research requires extremely high standards for satellite calibration. Many satellite sensors do not currently meet the accuracy criteria, especially heritage sensors such as the Advanced Very High Resolution Radiometer (AVHRR), which shows scene temperature–dependent trends and biases of up to 0.5 K. Recently, however, a detailed study of the AVHRR/3 prelaunch data showed significant problems with both the calibration algorithm and the prelaunch data and indicated that the inherent accuracy of the AVHRR may actually be quite high. A new approach has been suggested that fixed many of the issues with the current (operational) calibration, but has not yet been applied to the in-orbit case. In this paper the behavior of the AVHRR in orbit is examined and compared to the operational AVHRR radiances from the Meteorological Operation (MetOp)-A with those based on the new calibration to radiances derived from the Infrared Atmosphere Sounding Instrument (IASI). It is shown that the current AVHRR calibration does indeed introduce large (0.5 K) biases, but these biases are remarkably stable. It is further shown that, with some modification related to differences between the prelaunch test environment and the in-orbit environment, a physically based AVHRR calibration can match IASI to better than ~0.05 K, which is an order of magnitude better than what is currently available. Finally, it is shown that, while the new calibration is capable of providing accurate and stable radiances for the nadir view, off-nadir biases of up to 1.5 K still exist at the largest zenith angles and at the coldest scene temperatures (~210 K). For surface temperature determination, however, the scan angle bias is very small (&amp;lt;0.02 K), implying that the new AVHRR calibration will provide a significant improvement to, for example, sea surface temperature measurements, one of the Global Climate Observing System (GCOS)-designated essential climate variables.

Calibrations for AVHRR channels 1 and 2: review and path towards consensus

The over three-decade-long data record from the Advanced Very High Resolution Radiometer (AVHRR) is ideal for studies of the Earth's changing climate. However, the lack of on-board calibration requires that the solar channels be recalibrated after launch. Numerous calibration studies have been conducted, but significant differences remain among the calibrations. This study is one effort to outline a path towards consensus calibration of the AVHRR solar channels. The characteristics of the polar orbiting satellites bearing the AVHRRs, the AVHRR instruments and data are described as they are related to calibration. A review of past and current calibration studies is also presented and examples of their lack of consensus shown. A list of consensus items is then provided that, if followed by the AVHRR calibration community, should bring the various calibration methods to within the small percent difference required for long-term climate detection.

Removal of Contaminated Pixels from the Desert Target for AVHRR Vicarious Calibration

Abstract Desert-based vicarious calibration plays an important role in generating long-term reliable satellite radiances for the visible and near-infrared channels of the Advanced Very High Resolution Radiometer (AVHRR). Lacking an onboard calibration device, the AVHRR relies on reflected radiances from a target site, for example, a large desert, to calibrate its solar reflective channels. While the radiometric characteristics of the desert may be assumed to be stable, the reflected radiances from the target can occasionally be affected by the presence of clouds, sand storms, vegetation, and wet surfaces. These contaminated pixels must be properly identified and removed to ensure calibration performance. This paper describes an algorithm for removing the contaminated pixels from AVHRR measurements taken over the Libyan Desert based on the characteristics of consistent normalized difference vegetation index (NDVI) land-cover stratification. An NDVI histogram-determined threshold is first applied to screen pixels contaminated with vegetation in each individual AVHRR observation. The resulting analyses show that the vegetation growth inside the desert target has a negligibly small impact on the AVHRR operational calibration results. Two criteria based on the maximum NDVI compositing technique are then employed to remove pixels contaminated with clouds, severe sand storms, and wet sand surfaces. Compared to other cloud-screening methods, this algorithm is capable of not only identifying high-reflectance clouds, but also removing the low reflectance of wet surfaces and the nearly indifferent reflectance of severe dust storms. The use of clear pixels appears to improve AVHRR calibration accuracy in the first 3–4 yr after satellite launch.

Systematic corrections of AVHRR image composites for temporal studies

For quantitative studies of vegetation dynamics, satellite data need to be corrected for spurious effects. In this study, we have applied several changes to an earlier advanced very high resolution radiometer (AVHRR) processing methodology (ABC3; [Remote Sens. Environ. 60 (1997) 35; J. Geophys. Res.-Atmos. 102 (1997) 29625; Can. J. Remote Sens. 23 (1997) 163]), to better represent the various physical processes causing contamination of the AVHRR measurements. These included published recent estimates of the NOAA-11 and NOAA-14 AVHRR calibration trajectories for channels 1 and 2; the best available estimates for the water vapour, aerosol and ozone amounts at the time of AVHRR data acquisition; an improved bidirectional reflectance algorithm that also takes into consideration surface topography; and an improved image screening algorithm for contaminated pixels. Unlike the previous study that compared the composite images to a single-date AVHRR image, we employed coincident TM images to approximate the AVHRR pixel field of view during the data acquisition. Compared to ABC3, the modified procedure ABC3V2 was found to improve the accuracy of AVHRR pixel reflectance estimates, both in the sensitivity (slope) of the regression and in r 2. The improvements were especially significant in AVHRR channel 1. In comparison with reference values derived from two full TM scenes, the corrected AVHRR surface reflectance estimates had average standard errors values of ±0.009 for AVHRR C1, ±0.019 for C2, and ±0.04 for NDVI; the corresponding r 2 values were 0.55, 0.80, and 0.50, respectively. The changes in ABC3V2 were not able to completely remove interannual variability for land cover types with little or no vegetation cover, which would be expected to remain stable over time, and they increased the interannual variability of mixed forest and grassland. These results are attributed to a combination of increased sensitivity to interannual dynamics on one hand, and the inability to remove all sources of noise for barren or sparsely vegetated northern land cover types on the other.

Recalculated pre-launch saturation temperatures of the AVHRR 3.7 μ m sensors on board the TIROS-N to NOAA-14 satellites

Previously published values of pre-launch Advanced Very High Resolution Radiometer (AVHRR) 3.7 w m channel saturation brightness temperatures (Robinson 1991) show inter-satellite differences as large as 10 K between AVHRRs on board National Oceanic and Atmospheric Administration (NOAA) satellites from TIROS (Television Infrared Observation Satellite)-N to NOAA-11. We redid the computations using pre-launch AVHRR calibration data up to NOAA-14. Our results show that 3.7 w m channel pre-launch saturation temperatures for the TIROS-N, NOAA-6, -8, -9, -10, -11, -12 and-14 satellites are within ∼0.5 K of each other while NOAA-7 differs by ∼1 K from the others. We believe that the AVHRR 3.7 w m channel pre-launch saturation temperatures listed in Robinson (1991) are actually the ypre-launch saturation temperatures for the AVHRR 10.8 w m channel. To check our hypothesis, we computed TIROS-N to NOAA-11 10.8 w m channel pre-launch saturation temperatures; the values are within 0.5 K of the values listed in Robinson (1991).

On the use of long‐term global data of land reflectances and vegetation indices derived from the advanced very high resolution radiometer

The availability of advanced very high resolution radiometer (AVHRR) time series of global shortwave data for the past two decades motivated many scientists to investigate interannual variability and trends in land surface conditions. For these studies the observed change in radiances due to two varying factors, namely, sensor responsivity and illumination conditions, must be known a priori because of the degradation of AVHRR shortwave channels and the orbit drift of afternoon spacecraft. The current work analyzes the behavior of global land AVHRR shortwave time series data for the last 12 years, processed using postlaunch calibration, and investigates their usefulness for the monitoring of global land surface processes. Its focus is on verifying the postlaunch calibrations for the AVHRR sensors on board NOAA 11 and 14. It is assumed that the NOAA 9 AVHRR calibration is correct so that the changing illumination effects can be parameterized based on its data. After accounting for the illumination effects, the residual trends in data, averaged over global deserts and rain forests, are attributed to calibration discrepancies. In particular, NOAA 11 calibration was found to yield only small residuals, whereas NOAA 14 calibration produced significant unrealistic global increase in both reflectances and vegetation indices. The artificial trends caused by the combination of calibration residuals and satellite‐orbit drift should be removed to alleviate their misidentification as real trends in Earth's climate system and to make statistical studies of anomalies more reliable. This study draws attention to the above aspects of time series analysis with the available global AVHRR data and suggests ways to improve these data for interannual variability studies.