In 2008, Biospherical Instruments embarked on a joint project with NASA to develop and deploy a state-of-the-art above-water radiometer system in support of current and next-generation ocean color satellite missions. The system—known as Optical Sensors for Planetary Radiant Energy (OSPREy)—will provide high-quality measurements of the sea, Sun, sky, and Moon at a lower cost and over a greater spectral range (305–1,640 nm) than alternative in-water approaches. The measurements will have a documented uncertainty, and will satisfy the accuracy requirements for the vicarious calibration and algorithm validation of ocean color satellites.
OSPREy is the subject of an entire NASA Technical Memorandum—"Optical Sensors for Planetary Radiant Energy (OSPREy): Calibration and Validation of Current and Next-Generation NASA Missions"—NASA TM #2012–215872, which was written by both NASA and BSI personnel. Individual subject-specific chapters from the TM, indicated with "(Chap#)", can be downloaded from their applicable paragraphs below and from the OSPREy System Configurations page.
OSPREy takes full advantage of Biospherical Instruments’ microradiometer technology to produce a suite of diverse instruments sharing a common architecture. Wavelength bands in the ultraviolet (UV), near infrared (NIR), and short-wave infrared (SWIR) will be used to enhance data products or improve atmospheric corrections of satellite data products such as water-leaving radiance.
Figure 1: Deployment of the OSPREy system with additional above- and in-water components recommended for operations: the two EPIC dyads mounted on top of an offshore tower; solar panels and a wind generator for providing power; the ocean color satellite to be validated; a telecommunications satellite for data telemetry; a shore-based calibration and logistical support facility, and regular quality control visits using a small boat. The latter includes the sampling of in-water optical profiles with the Compact-Optical Profiling System (C-OPS). OSPREy is the only commercially available satellite calibration and validation system. It is a modular system using Biospherical Instruments’ Enhanced Performance Instrument Class (EPIC) radiometers that are based on further reducing the measurement uncertainty of already very accurate microradiometers. The improvements are achieved by using redundant (dyad) sensors, instrument enhancements (e.g., thermal regulation), and higher accuracy calibrations. A typical deployment of an OSPREy system on an off-shore platform is shown in Figure 1.
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Figure 2: The OSPREy system concept showing the two dyads of sensors (denoted A and B), which each contain one radiance sensor mounted on a pointing system and one irradiance sensor with shadow band attachment. The OSPREy instrumentation (Chap1) combines a) new, microradiometer-based, high performance, fixed wavelength optical instruments with b) integrated commercially available hyperspectral spectrographs, c) shadowband irradiance technology, and d) automated pointing systems. The resulting hybrid instruments return high spectral resolution and large dynamic range in both radiance and irradiance data sets across a wide spectral range (UV to SWIR).
Typically, OSPREy is based on sets of dual radiometers called dyads (Figure 2). Each dyad is composed of one radiance and one irradiance instrument. The radiance instrument is mounted on an automated pan-tilt device to allow spectral measurements of the Sun, sky, ocean, or moon. (Chap5)
The irradiance instrument is equipped with a shadowband system (Figure 3a) to afford measurements of both the global and diffuse solar spectral irradiance. The radiance instrument (Figure 3b) is mounted on a tracker and includes a camera for viewing the solar disk to aid in tracking and alignment. A filter wheel extends the capability to include polarization studies. In addition to solar radiometry, the tracker can be used for sky radiance and water-leaving radiance measurements.
Figure 3a: Shadowband assembly mounted on an irradiance radiometer. The band can be programmed to track the Sun or sweep across the sky. Synchronous sampling with two dyads ensures quality assessment of data products; asynchronous sampling affords sampling independently of shadowband operations.
Figure 3b: The OSPREy OR-L deployed in sun photometer mode.
The combined measurements of the four instruments are used to derive an unprecedented number of near-simultaneous atmospheric and oceanic parameters. For example, the sun-tracking capability provides sun photometer data products, which are expected to improve the atmospheric correction of ocean color satellite retrievals, as well as the derivation of water-leaving radiances and associated data products in optically complex (coastal) waters.
OSPREy deployments will also include monthly calibration monitoring in the field with a portable light source. Radiometric measurements are traceable to the irradiance scale of the National Institute of Standards and Technology (NIST) via the OSPREy Transfer Radiometer (OXR).
OSPREy radiometers are based on EPIC radiometers, which are hybrid systems with both fixed wavelength microradiometers and a hyperspectral spectrograph (Figure 4). The instruments feature a cluster of 18 or 19 temperature-stabilized microradiometers covering the wavelength range of 340–1,640 nm (UV to SWIR) at a spectral bandwidth of 10 nm. (Chap2)
The systems combine the superiorities of microradiometer technology (e.g., large dynamic range, excellent temporal stability, low stray light, and a high scan speed of up to 20 Hz) with the spectral resolution of a spectrograph. The spectrographs (models MMS UV-VIS II and MMS1 from Zeiss) feature a rugged, monolithic design where all optical components are firmly cemented to a body of UBK7 glass, resulting in high wavelength stability and reliability. Radiance radiometers have a small field of view (2.5°) to allow accurate retrievals of aerosol optical depth.
The collector of the irradiance radiometer has a novel diffuser made of two layers of different materials with excellent cosine law agreement over the full wavelength range. Microradiometers and the spectrograph are temperature regulated to better than ±0.1°C. The microradiometer cluster is installed in an inner housing filled with dry nitrogen for optimum long-term stability of optical filters and electronics.
Figure 4: An OSPREy radiance radiometer. The instrument is equipped with two Gershun tube assemblies, one for the microradiometer cluster and one for the spectrograph, both with a field of view of 2.5°. Microradiometer channels cover the wavelength range of 340–1,640 nm. The spectrograph is equipped with a filter-wheel assembly, outfitted with neutral density filters to increase the dynamic range of the spectrograph, an opaque disk to monitor dark offsets and determine internal stray light, and polarization filters to measure the polarization state of sky and sea radiance. Pictures from a digital video camera mounted collinearly with the Gershun tubes are used to confirm the radiance sensors are pointing accurately at the Sun, but also confirm the absence of contaminants (e.g., clouds across the solar disk and floating debris on the sea surface). Using these data, small alignment errors (e.g., leveling) of the sun-tracking mode can be easily quantified and corrected.
OSPREy Transfer Radiometers
The radiance and irradiance scales of the OSPREy project are linked to the radiometric scale maintained by the SIRCUS facility of the National Institute of Standards and Technology (NIST). The SIRCUS scale is transferred to OSPREy field instruments via two OSPREy Transfer Radiometers (OXRs), which are currently being developed at Biospherical Instruments (Figure 5a).
One OXR is equipped with a cosine collector for irradiance measurements and the other with radiance front optics. As one of the first activities for the project, an OXR prototype was developed to provide a platform for evaluating many of the design aspects of EPIC radiometers, the OXR-E (Figure 5b). (Chap4)
Figure 5a: The prototype OXR. Figure 5b: The OXR-E having its calibration checked at NIST.
The default OSPREy configuration consists of two dyads, each composed of one EPIC radiance radiometer mounted on a pointing device and one EPIC irradiance instrument (Figure 2). Such a system provides maximum redundancy for demanding applications such as satellite validation. For less ambitious measurement tasks, more economical configurations can be offered (Figure 6).
The simplest configuration ("Starter System") consists of one radiance sensor mounted on a pointing system, filling a roll similar to SeaPRISM and providing data products relevant to atmospheric conditions and ocean color.
The "Minimum System" consists of a complete OSPREy dyad. The radiance and irradiance pair delivers global irradiance, shadowband data products, and redundancies in data processing, e.g., two methods for measuring direct solar irradiation.
The most demanding configuration ("Maximum System") is a "triad" consisting of one radiance and two irradiance instruments with different, but overlapping, spectral ranges with irradiance collectors optimized for the given range.
Figure 6: Configuration examples of EPIC sensors
Because all EPIC radiometers are based on microradiometers, the number of channels can be initially set to an affordable small number, and then the system can be expanded as funding or research demands increase without compromising the quality of the observational suite. This is possible, because the cost structure is basically on a per-channel basis, and the individual channels are modular machine-made components, rather than hard to fit hand-made components.
Scientific Merits of OSPREy
Global ocean color satellites provide a revolutionary capability for understanding Earth. The quality of satellite remote sensing data depends critically on the calibration and characterization of the instruments on orbit. Prelaunch characterizations often have large uncertainties due to budget constraints, the harshness of the launch environment, and space. Consequently, vicarious calibration using ground-based (more properly sea-truth) observations is a prerequisite for successful space-borne ocean color research.
The OSPREy concept is based on acquiring approximately 30 contemporaneous matchups of clear-sky in situ measurements and glint-free satellite images as rapidly as possible to derive a fixed set of calibration gains. In contrast to in-water systems for vicarious calibration, the above-water OSPREy instruments also collect important atmospheric data required for inversion algorithms such as aerosol optical parameters (e.g., wavelength-dependent optical depth, single scattering albedo) and cloud optical depth.
The large wavelength range of 340–1,640 nm provides new research opportunities. For example, UV wavelengths might be exploited to distinguish the absorption signals of colored dissolved organic matter (CDOM), chlorophyll a, detritus, and mineral concentrations, yielding new algorithms for coastal waters. Measurements at SWIR wavelengths help to separate atmospheric and oceanic contributions, because the ocean is radiometrically blacker in this spectral range. This will help to improve atmospheric corrections of satellite data. Data collected by OSPREy radiometers also can be used to quality assure and interpret data from in-water profilers and measurements of inherent optical properties, thus enhancing the value of these data sets. (Chap3)
Economical Benefits of OSPREy
Cost savings are achieved by relying on state-of-the-art commercial-off-the-shelf (COTS) instrumentation rather than one-of-a-kind custom-built hardware. Additional savings are realized by the above-water approach: a) offshore platforms and towers already exist and are maintained by other institutes and agencies; b) a tower provides a very stable platform and deterministic solar geometry; c) biofouling is significantly reduced; d) acquiring high-quality data at longer wavelengths is easier to accomplish; and e) the offshore structure offers a great deal of protection for the instruments.
Because all OSPREy radiometers are based on microradiometers, the number of channels can be initially set to an affordable small number, and then the system can be expanded over time as funding or research demands increase, without compromising the quality of the observational suite. In addition, if the spectral suite needs to change as the scientific objectives evolve, sensors built using microradiometers are much easier and cheaper to modify than hand-made (legacy) instruments.