The Ocean Color Instrument (OCI) is the primary payload on NASA’s Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission. Its primary purpose is to enable new scientific studies of ocean biology, aerosols, and clouds. This paper describes the design of the instrument and its radiometric performance as measured during the prelaunch characterization campaign. OCI will be the first radiometer to provide hyperspectral (340nm-895nm) daily global coverage of top-of-atmosphere radiances. Seven multispectral bands cover wavelengths from 940nm to 2260nm. The spatial resolution is about 1.2km. OCI performance is optimized for ocean color applications, with a focus on high signal-to-noise ratio (SNR) at low radiance levels and high radiometric accuracy.

The Ocean Color Instrument (OCI) on NASA’s Plankton, Aerosol, Cloud, ocean Ecosystem mission is a hyperspectral imager with high SNR, precision and dynamic range, and with a very low striping artifact level in the 342-887 nm wavelength range with a spectral resolution of 5 nm in 2.5 nm steps, providing a significant technological advancement over previous ocean imagers. To achieve this, OCI is designed with specialized optical imaging and opto-electronic detection systems that push the boundaries of several state-of-the-art technologies. This paper provides an overview of these systems together with their achieved performances and discussions of their key design challenges.

The Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission [1] will extend and improve the data record of NASA’s satellite observations of global ocean biology, aerosols, and clouds. The Ocean Color Instrument (OCI) is the primary sensor on-board the PACE platform [2]. The OCI is a scanning radiometer with hyperspectral coverage from the ultraviolet (UV) to the near infrared (NIR) wavelength range and a fiber-coupled multiband filter spectrograph in the short-wave infrared (SWIR) spectral region. The OCI Flight Unit completed system level testing in November 2022 at the Goddard Space Flight Center (GSFC).This paper presents the spectral characterization and performance of the OCI Flight Unit. The OCI Flight spectral performance was determined to be within design specifications and the characterization was measured within specified uncertainties.

Scheduled for launch in January 2024, the PACE mission represents NASA’s next investment in ocean biology, clouds, and aerosol data records [1]. A key feature of PACE is the inclusion of an advanced satellite radiometer known as the Ocean Color Instrument (OCI), a global mapping radiometer that combines multispectral and hyperspectral remote sensing. This paper describes the methods used for pre-launch calibration of OCI and considerations to ensure the combination of Ground Support Equipment (GSE) and instrument effects meet uncertainty and performance requirements. General considerations when designing a calibration campaign are also discussed.

Scheduled for launch in January 2024, the Phytoplankton, Aerosol, Cloud, and ocean Ecosystem (PACE) mission represents NASA’s next investment in ocean biology, clouds, and aerosol data records [1]. A key feature of PACE is the inclusion of an advanced satellite radiometer known as the Ocean Color Instrument (OCI), a global mapping radiometer that combines multispectral and hyperspectral remote sensing. This paper describes the results of the prelaunch test campaign of the OCI Flight Unit. The measured OCI flight unit performance exceeded requirement thresholds in all critical areas. Overall, the performance of the OCI is excellent, and will allow the PACE science team to meet its science objectives.

Since Moderate Resolution Imaging Spectroradiometer Aqua's launch in 2002, the radiometric system gains of the reflective solar bands have been degrading, indicating changes in the system's optical throughput. To estimate the optical throughput degradation, the electronic gain changes were estimated and removed from the measured system gain. The derived optical throughput degradation shows a rate that is much faster in the shorter wavelengths than the longer wavelengths. The wavelength-dependent optical throughput degradation modulated the relative spectral response (RSR) of the bands. In addition, the optical degradation is also scan angle-dependent due to large changes in response versus the scan angle over time. We estimated the modulated RSR as a function of time and scan angles and its impacts on sensor radiometric calibration for the ocean science. Our results show that the calibration bias could be up to 1.8% for band 8 (412 nm) due to its larger out-of-band response. For the other ocean bands, the calibration biases are much smaller with magnitudes at least one order smaller.

Ocean color products such as, e.g., chlorophyll-a concentration, can be derived from the top-of-atmosphere radiances measured by imaging sensors on earth-orbiting satellites. There are currently three National Aeronautics and Space Administration sensors in orbit capable of providing ocean color products. One of these sensors is the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Aqua satellite, whose ocean color products are currently the most widely used of the three. A recent improvement to the MODIS calibration methodology has used land targets to improve the calibration accuracy. This study evaluates the new calibration methodology and describes further calibration improvements that are built upon the new methodology by including ocean measurements in the form of global temporally averaged water-leaving reflectance measurements. The calibration improvements presented here mainly modify the calibration at the scan edges, taking advantage of the good performance of the land target trending in the center of the scan.

The National Aeronautics and Space Administration ocean color products of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua have been reprocessed in 2009. This paper describes the changes to the calibration approach for MODIS Aqua. Due to a significant downward trend in the operational MODIS Aqua water-leaving radiances at 412 nm, the previous calibration approach was no longer sufficient. The new approach uses SeaWiFS water-leaving radiances to adjust the temporal trends of the radiometric calibration of MODIS Aqua bands at 412 and 443 nm. The adjustments to the temporal trends at the beginning of the scan are minor but are significant around nadir and at the end of scan (up to 5% at 412 nm and up to 1% for 443 nm). The remaining five bands (488 to 678 nm) are adjusted with regard to their scan-angle dependence only; no temporal correction is necessary. There is no indication that the sensor polarization sensitivity needs to be modified for MODIS Aqua.

The retrieval of aerosol properties from spaceborne sensors requires highly accurate and precise radiometric measurements, thus placing stringent requirements on sensor calibration and characterization. For the Terra/Moderate Resolution Imaging Spectroradiometer (MODIS), the characteristics of the detectors of certain bands, particularly band 8 [(B8); 412 nm], have changed significantly over time, leading to increased calibration uncertainty. In this paper, we explore a possibility of utilizing a cross calibration method developed for characterizing the Terra/ MODIS detectors in the ocean bands by the National Aeronautics and Space Administration Ocean Biology Processing Group to improve aerosol retrieval over bright land surfaces. We found that the Terra/MODIS B8 reflectance corrected using the cross calibration method resulted in significant improvements for the retrieved aerosol optical thickness when compared with that from the Multi-angle Imaging Spectroradiometer, Aqua/MODIS, and the Aerosol Robotic Network. The method reported in this paper is implemented for the operational processing of the Terra/MODIS Deep Blue aerosol products.

The NASA ocean color products of SeaWiFS and MODIS Aqua have been reprocessed in 2009 and 2010, respectively. This paper describes the changes to the calibration approach for MODIS Aqua. Due to a significant downward trend in the operational MODIS Aqua water-leaving radiances at 412nm, the previous calibration approach was no longer sufficient. The new approach uses SeaWiFS water-leaving radiances to adjust the temporal trends of the MODIS Aqua 412nm and 443nm water-leaving radiances. The remaining five bands (488nm to 678nm) are adjusted with regard to their scan angle dependence only. A similar approach has been applied to MODIS Terra before, but for MODIS Aqua there is no need to adjust the polarization sensitivity.

A bidirectional reflectance factor (BRF) model for urban areas for pixel sizes of more than 500 m/spl times/500 m is developed. Possible applications include albedo calculation, improvement of classification, and change detection algorithms, simulated global BRF maps, and refinement of atmospheric correction algorithms. The model combines the BRF effects at several scales (street grid, intermediate-sized objects, and microscale). The authors present modeling results as well as a comparison with measured data. The basic features of the urban BRF are the hotspot and the independence of its shape from wavelength in the range 450 nm/spl les//spl lambda//spl les/2300 nm. An analytical function that approximates the model is proposed for easy implementation and fast computation.