Development of field radiometers for satellite calibration and validation.

An Optical Sensors for Planetary Radiance Energy (OSPREy) system consists of radiance radiometers mounted on computer-controlled pointing systems and irradiance radiometers equipped with shadowbands. Synchronous and asynchronous sampling modes measure the sea, Sun, and sky, across a wide spectral range---the ultraviolet (UV), through the visible (VIS) and near-infrared (NIR), and into the short-wave infrared (SWIR)---to derive an unprecedented number of near-simultaneous atmospheric and oceanic parameters. This new sampling capability is designed to provide vicarious calibration and algorithm validation capabilities which will also improve the following: a) the atmospheric correction of ocean color data, b) the accuracy in separating the living and nonliving components of seawater, c) the derivation of water-leaving radiances and associated data products in optically complex (coastal) waters, and d) the understanding of the interaction between the ocean and atmosphere.

Supplementing the emphasis on oceanic radiometry are ancillary sensors to broaden and characterize atmospheric properties and to safeguard the sensors during severe weather or to disable acquisition during inclement weather. The measurements will have a documented uncertainty satisfying the accuracy requirements for calibration and validation activities for both ocean color and atmospheric satellites. All optical measurements are traceable to the irradiance scale of the Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) facility operated by the National Institute of Standards and Technology (NIST). The traceability is maintained with special OSPREy Transfer Radiometers (OXRs) that have already been built plus a lamp library of NIST and secondary lamp standards that has already been established.

A typical OSPREy calibration and validation system consists of two radiance radiometers mounted on pointing systems or trackers, and two irradiance radiometers that are equipped with shadowbands for measuring global and diffuse solar irradiance. The two pairs of different radiometers, or dyads (Fig. 1) are a key design element of the OSPREy concept as is the modularity of the architecture. The latter allows a system to be configured in the most cost-effective manner for the science objectives and available resources. For example, an OSPREy system may consist of only one radiance unit mounted on a tracker for the validation of satellite aerosol data products (a starter system), or the same unit might be deployed on an offshore platform with an irradiance sensor and also provide oceanic validation products (a minimum system).

OSPREy systems are built with Enhanced Performance Instrument Class (EPIC) instruments, which are based on modifying recently developed commercial-off-the-shelf (COTS) sensors. EPIC radiometers are hybrids that include 18 (irradiance) or 19 (radiance) fixed-wavelength filter channels based on microradiometers, plus a thermally regulated spectrograph with an optical fiber front-end (Fig. 2). Other modifications to produce EPIC sensors involved the following:

1. Redesigned radiance entrance optics for a smaller field of view (FOV) of 2.5° in compliance with sun photometry requirements;

2. Thermal stability control to increase filter and detector stability and reduce drift;

3. A new irradiance diffuser allows a single cosine collector to span the UV to SWIR (305–1,670 nm).

EPIC radiance sensors are also equipped with a digital video camera plus a seven-position filter wheel in line with the fiber-optics. The filter wheel permits hyperspectral polarimetry (three polarizers), direct Sun viewing (neutral density filter), stray-light correction (340 nm cut-on filter), and dark current measurements (solid disk). The dynamic range of the microradiometers, approximately 9.5 decades, permits viewing of all natural targets for both radiometric sensor types including the Sun and the Moon. The digital camera is used in lieu of a quadrant detector to accurately point the sensor at the center of the Sun. The camera also provides an image of the radiance FOV, so the presence of clouds across the solar disk, or floating debris on the sea surface, can be properly detected. Picture recognition algorithms, which are integral to the system control software, are used to flag data accordingly. OSPREy trackers are COTS units, which allow radiance sensors to be pointed in arbitrary directions with a speed of up to 50°/s.

An OSPREy system is a COTS state-of-the-art, autonomous, radiometric observing system supporting a) cost-conscious atmospheric and oceanographic research, and b) calibration and validation activities for current and next-generation ocean color and atmospheric satellites, including NRC Decadal Survey and Climate-Centric Architecture missions. A summary depiction of an OSPREy system deployed on an offshore platform is presented in Fig. 3 - recalling that the platform-perturbation field must be properly understood to avoid contamination in the radiometric data.

Fig. 1.

OPRey images

The OSPREy system concept showing two dyads (A and B), each containing one irradiance sensor (with shadow band attachment) and one radiance sensor. The pedestal mount is for illustrative purposes as are the pointing angles and shadow band positions.

Fig. 2.

An EPIC radiance sensor showing the 19 microradiometers arranged in a central cluster with the the gershun tube assembly to the left and the aggregator electronics to the right. The spectrograph with thermal regulation connects to the filter wheel assembly with a fiber-optic cable and has its own gershun tube assembly. The digital camera has an exit aperture on the other side of the filter-wheel assembly. The end cap also contains the spectrograph support electronics.

Fig. 3.

 

Deployment of the OSPREy system with many of the components required for operations: two EPIC dyads at the top of an offshore tower measuring the atmosphere and the ocean; an ocean color satellite sampling the on- to off-shore gradient (green to blue) in productivity; the Sun and Moon as calibration targets; a telecommunications satellite for data telemetry from the platform sensor suite (including meteorological sensors), which are all powered by solar panels and a wind generator; a shore-based calibration and logistical support facility, including test capabilities at a pier; and monthly validation (including in-water AOP profiles) and maintenance visits with a small boat.