(4C)_Final_SOW__Full_Sky_Hemispherical_Spectral_Polarization_and_Radia.docx

Statement of Work Full Sky Hemispherical Spectral Polarization and Radiance Sensor

1. Description:

The Naval Research Laboratory is soliciting proposals for a full sky hemispherical-type camera system that will measure the absolute spectral radiance and polarization of the natural downwelling light field. It will be capable of autonomous operation and collect data semi-continuously throughout the daytime (dawn to dusk) at programmable intervals. At a minimum, it will be able to collect the S0, S1, and S2 Stokes parameters, in roughly 10 degree sampling intervals, over the 350 nm to 1050 nm wavelength range. The system must include the camera, which includes collecting optics and detector(s), acquisition hardware and software, processing software and a field-deployable, weatherproof enclosure.

As this is an instrument that can be fabricated in a number of ways using COTS (commercial off the shelf) parts, some possible implementations are provided as guidance. These possible implementations are presented as examples, however the contractor is encouraged explore any design that would meet the requirements. One possible implementation of the system is to use an optical fiber/lens system to collect the polarized spatial samples as shown in figure 1. These fibers could then pipe the light to an imaging spectrometer, for example, and all spatial samples measured simultaneously. Another possible implementation would be to collect one polarized spatial sample using three fiber fed spectrometers and use a motorized stage to step through the required number of samples for the full sky. As a last example, pushbroom imaging spectrometers with polarizers on their foreoptics could be used to scan across the sky.

Figure : Possible implementation of the system using optical fibers at all spatial locations an d piping their output to an imaging spectrometer. Here there are three fibers at each spatial location with polarizers placed in front of each fiber at relative angles of 0, 45, and 90 degrees Fish eye lens Fibers with polarized inputs Imaging spectrometer w ith shutter for darks Downwelling light

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2. Specifications:

The sensor parameters are specified in detail in this section, and where applicable, are tabulated in the specification matrix, table 1.

Table 1: Specification matrix for some of the requirements and goals

Spatial Sampling:

Required- The system must collect light within the nominal 180° hemispherical field-of-view of the sky, with a minimum sample spacing of 10° intervals along the elevation direction, and 15° along the azimuth direction. The minimum elevation angle must be no larger than 10° above the horizon.

Each individual sample will have a minimum solid angle of 5° half-angle resolution.

Goal -Less than or equal to 5° intervals in elevation direction and less than or equal to 10° in azimuth direction with a 2° half-angle resolution.

Temporal sampling:

Required- The system must be able to repeat the measurements and store the data for a full-sky map every 10 minutes Goal- The system must be able to repeat the measurements for a full-sky map every 5 minutes

Maximum sampling time:

Required- All angular measurements for a single full-sky map must be made in no more than 4 minutes.

Goal – All angular measurements for a single full-sky map must be made in less than 1 minute Time stamp:

Required- Each angular measurement must be time stamped to within 10 millisecond of GPS time.

Goal- Each angular measurement must be time stamped to within 1 millisecond of GPS time.

Direct sun measurement:

Required - the system must either: a) Avoid measurement of the direct solar beam (occulting the solar disk or avoiding the solar direction completely), or b) ensure any measurement of the direct solar beam does not affect the measurement of the sky radiances at other angles, and should not damage the sensor. The system must autonomously adapt to changing solar position during the day.

Spectral Sampling:

Required- The system must make measurements in the spectral range from 350-1050 nm, at a sampling resolution of no more than 10 nm with a full-width half maximum (FWHM) bandwidth of 15 nm. All spectral samples for each spatial sample must be acquired simultaneously. As noted above, it is not required that all spatial samples are sampled simultaneously.

Goal -spectral range from 350-2500 nm at a sampling resolution of 3 nm with a FWHM bandwidth of 5 nm in the visible wavelengths, and no more than a 10 nm sampling resolution with a FWHM bandwidth of 20 nm in the short-wave infrared wavelengths.

Collection of Dark Data;

Required – the system must be capable of being shuttered so that data may be collected with no light striking the detectors.

Polarization:

Required -The system must be capable of measuring the SO, S1, and S2 elements of the Stokes vector. All polarization measurements must be made in the required time.

Signal Fidelity:

Required -Table 2 shows the required Signal-to-Noise Ratio (SNR) in the SO component at a given signal level. The S1 and S2 elements of the Stokes vector need to measure degree of linear polarization to an accuracy of 2 percent.

Goal - The Goal SNR for the SO component is also shown in table 2. The S1 and S2 elements of the Stokes vector need to measure degree of linear polarization to an accuracy of 0.5 percent.

Saturation:

Required- The sensor must not saturate for signal levels at ten times the signal levels in table 2. The saturation requirements must be satisfied when running the instrument using settings that also meet the SNR requirements.

Goal- sensor must not saturate for signal levels at a thousand times the signal levels in table 2. The saturation goal must be satisfied when running the instrument using settings that also meet the SNR requirements.

Table 2: Downwelling spectral radiance and required/goal signal to noise ratios. Modeled spectral radiance values assume 30 degree solar zenith angle, and coastal aerosols, with the sensor viewing at 60 degrees elevation, and 180 degrees in azimuth (i.e. directly away from sun). Note that the minimum requirement is the required SNR for wavelengths only between 350-1050nm.

Electrical:

Required - The system must be able to operate at standard 120V at 50-60 Hz. Goal- Battery operation for at least one day of measurements.

Operating Temperature Range:

Required -The system must be fully operational over the ambient temperature range of 5-40° C with desired range of -10 to 45° C.

Goal- The system must be able to operate under conditions of full sunlight in at least 40° C weather.

Weather Resistance:

Required - The system will be operated in a coastal ocean environment, and must demonstrate features that minimize any potential harmful effects due to presence of saltwater (e.g. sea spray). The system should be weather-resistant, but does not need to operate under heavy rain.

Acquisition/Control Hardware/Software and Processing/Analysis Software/Hardware:

The hardware for control and operation of the system must be capable of being operated under field conditions. A graphical user interface (GUI) based control software is required. The system must be capable of storing a full, 16 hour, day’s worth of data with the storage of a week's worth of data as a goal.

-Real-time capabilities and display must include diagnostics indicating data quality to ensure 'good ' data is being collected, and the operating condition of each component.

-Post Processing: Processing software must provide a data quality assurance report. In addition, software must collect, display, store, and process raw data and provide the capability to retrieve the Stokes parameters for each angle.

3. Period of Performance:

Delivery shall be no longer than (365 days) from date of award.

4. Warranty:

A minimum, warranty of one year from receipt of all hardware components of the system is required.