The developments of the microradiometer and the OSPREy systems led to several new classes of instruments, which are about to be offered for sale. Because microradiometers are the modular building blocks for these novel radiometers, it is easy to make changes to the instrumentation as scientific objectives evolve.
There are four classes of instruments that can be built using microradiometers (Figure 1):
- Standardized Technologies and Architecture for Radiometry (STAR),
- Expandable Technologies for Radiometric Applications (XTRA),
- OSPREy Transfer Radiometer (OXR), and
- Enhanced Performance Instrument Class (EPIC).
Although each sensor within each class uses a high-quality microradiometer as the basic optical detector, there are significant differences in the packaging and ancillary technologies that determine the final capabilities of the sensors.
Figure 1.: The STAR, XTRA, OXR, and EPIC instrument classes.
The XTRA sensors have a wide range of functions, because they are fully functional with all STAR and EPIC devices, i.e., pointing units, GPS
, and ancillary (meteorological) sensors. The most sophisticated, and therefore the most capable, sensors are the OXR
and EPIC instruments. These more advanced sensors are hybrid units with both fixed wavelength microradiometers and a hyperspectral spectrograph (Chap7)
. The significantly superior capabilities of the former are used to ensure the proper registration (or calibration) of the latter.
In the case of the STAR and XTRA sensors, five system configurations are envisioned (Figure 2). The simplest or basic system uses the minimum number of sensors and manual pointing to provide a low-cost starting point for above-water radiometry. The addition of a second sensor provides redundancy and simultaneous sampling, which yields enhanced quality assurance (QA) and data products. Automated pointing from the pan-tilt unit adds unattended sampling scenarios, which when combined with ancillary sensors like the shadowband, yield a variety of new data products. As the multiple sensor system is made more complete, redundancy minimizes risk and sampling scenarios using synchronous and asynchronous protocols further enhances data products and QA.
Figure 2: Configuration examples of STAR and XTRA sensors.
The culmination of autonomous control is the inclusion of additional sensors, so the automated functions can be used safely on a moving platform, in this case a research vessel. The extra sensors also allow for improved QA, which results in better data products. A bow mounting system provides an easy mechanism to quickly remove the sensor suite in the event of severe weather (so it cannot be damaged), as well as the capability for quick and accurate reinstallation. Additional mounting options for tall (but accessible) superstructures or telescoping masts
provide alternative solutions for measurements that are not contaminated by the presence of the sampling platform.
Maximum spectral and ancillary information are obtained with the EPIC sensors, with the latter permitting unprecedented QA opportunities. For example, the video camera provides a picture of the radiance target, so the presence of clouds across the solar disk or floating debris on the sea surface can be properly detected and the data flagged. The scale of configurations for EPIC sensors (Figure 3) is similar to the STAR and XTRA classes, but the emphasis is more on accuracy, simply because the sensors were designed for it. The starter system is a single, but automated, radiance sensor that can asynchronously view the Sun, sky, sea, and Moon. The addition of a solar irradiance sensor improves the data product suite, starts to add redundancy for some of the measurements, and improves the QA possibilities. High spectral accuracy in both the radiance and irradiance observations is achieved using two irradiance sensors with shadowbands and optimized cosine collectors.
Figure 3: Configuration examples of EPIC sensors.
The operational system is based on two dyads composed of a radiance and irradiance sensor in each, with shadowband attachments. This system provides a significant redundancy to minimize risk (data loss from sensor malfunction), enhanced data products from synchronous sampling scenarios, and detection of contaminants from asynchronous sampling. It is this kind of system that is envisioned for ocean color vicarious calibration work, because the emphasis is to create a network of systems covering both the Northern and Southern hemispheres, so the most effective compromise between cost and capability is required. For applications that require one-of-a-kind solutions with maximum risk reduction and the highest quality data possible—which might be needed for specialized aspects of vicarious calibration—triad sensor systems are anticipated. The triads ensure no compromises in data quality or products, because the cosine collectors are optimized for two different spectral ranges. (Chap8)