Vicarious Calibration of Earth Observing Nanosatellite Sensors

The Naval Research Laboratory (NRL) established a “Regional Coastal Oceanography with Nanosatellites” (ReCON) project which is exploring the ability for high-resolution nanosatellites to monitor coastal, estuarine, riverine, and other maritime environments in support of U.S. Navy operations. The project initially is using data from the almost 200 Planet “Dove” nanosatellites which fly in “flocks” acquiring remotely sensed data from sunlight reflecting off the earth surface. Traditionally, earth observing and orbiting sensors can be vicariously calibrated using numerous in situ data sensors including the Marine Optical Buoy (MOBY), which is the standard calibration site maintained by NOAA and used for vicarious calibration of past and present ocean color satellite sensors (SeaWiFS, MERIS, OCM, MODIS, VIIRS, Sentinel-3, GOCI). The usefulness of remotely sensed ocean color data is determined by the accuracy of the computed water leaving radiances (Lw), which are the basis for the generation of remote sensing reflectance and other inherent and apparent optical property products. The accuracy of the Lw computation is effected by the atmospheric correction process, which removes atmospheric radiance components from the top of the atmosphere radiance measurement (Lt). The vicarious calibration process adjusts the prelaunch laboratory calibration to improve the accuracy of the actual in-orbit Lw measurements.

The vicarious calibration process does this by generating updated gain values for each wavelength band. It has two phases, a forward phase and an inverse phase. In the forward phase, the Lt value for each wavelength band recorded by the sensor is multiplied by a gain factor of 1. Then in the atmospheric correction process, the light scattering components, including Rayleigh radiances (Lr) and aerosol radiances (La), are subtracted from Lt to generate the water leaving radiances (Lw) and normalized water leaving radiances (nLw). During this phase, the values of these light scattering components are saved. During the inverse phase, the nLw values from the in situ sensor are added to the stored Lr and La components, resulting in a vicarious top of the atmosphere radiance value (vLt). This is the top of the atmosphere radiance value that after the atmospheric correction will result in nLw values that closely match the in situ nLw values. Therefore, the ratio of the vLt/Lt for each wavelength band is a gain factor that when multiplied by the sensor’s Lt will generated an adjusted top of the atmosphere radiance value. When using this adjusted Lt as the sensor’s top of the atmosphere radiance, after atmospheric correction the modified nLw values will more closely match the nLw values observed by the in situ sensor. Therefore, the set of vLt/Lt ratios for all of the wavelength bands is the vicariously calibrated gain set that will be multiplied with the sensor’s spectral Lt values so that the derived spectral nLw values are accurate.

Although, the Dove sensors are intercalibrated before launch, a vicarious calibration process does need to be performed for these sensors. The difficulty for the vicarious calibration of these nanosats lies in the sheer number of Dove sensors and the need for them to accurately measure water leaving radiances and maintain good intercalibration. A database will be development to keep track of information about each Dove, including the vicarious calibration metadata and gains. The metadata will include the in situ source for the sensors’ vicarious calibration. It is expected that only a limited number of Doves will pass over the MOBY site at a regular frequency. However, other in situ data sources are available (BOUSOLLE, etc). Therefore, the in situ data source for the calibration will be included in its database record. If a lack of in situ data is available for a particular Dove sensor, it may be necessary to use nLw values from a coincident Dove scene as the in situ data for the vicarious calibration. The volume of Dove sensors will provide challenges for maintaining a well calibrated suite of sensors for ocean color product generation and will require a mixture of well-established methodologies and database information storage techniques.