GLAMR: Goddard Laser for Absolute Measurement of Radiance
The Goddard Laser for Absolute Measurement of Radiance (GLAMR) laboratory at Goddard Space Flight Center (GSFC) is a state-of-the-art spectral and radiometric sensor characterization facility. The work is based on the calibration methodology developed by National Institute of Standards and Technology (NIST) called Spectral Irradiance and Radiance Calibration using Uniform Sources (SIRCUS) . The primary objective of GLAMR is to transfer the advanced calibration technique of SIRCUS into an operational facility to make it better available for characterization of airborne and satellite sensors.
Traditional spectral and radiometric sensor characterization is typically accomplished with two distinct measurement steps: spectral scan using a collimated monochromator output for spectral characterization and measurement of a white-light integrating sphere for absolute radiometry. Limitations of the spectral characterization include: the collimated monochromator output can only reasonably characterize a small portion of the sensor’s focal plane; relatively wide bandwidth of monochromator output limits fidelity, and relatively large uncertainty of the monochromators spectral position compromise spectral knowledge. GLAMR combines spectral and radiometric characterization into one measurement sweep by creating a monochromatic extended source. This means that the monochromatic laser sources of GLAMR illuminate an integrating sphere with a large exit port which can fill the full aperture and field of view of most sensors. Spectral position of the light can be measured with tens-of-picometer accuracy and the output radiance can be measured to accuracies approaching 0.5% or better.
The GLAMR laser technology
GLAMR consists of several types of tunable lasers to span the solar reflective spectrum. Most of the spectrum can be produced using two homemade lithium triborate (LBO)-based optical parametric oscillators (OPO). Commercial off-the-shelf (COTS) systems are also used to cover a portion of the shortwave infrared (SWIR) spectral region. Several of the optical components of the optical cavities of the OPOs are outfitted with precision actuators that allow automated spectral tuning. The software developed to control these actuators also record the signal of radiometers that monitor radiance levels output from the integrating sphere onto the sensor under test.
Transfer of radiometric traceability
A set of unfiltered transfer radiometers is the linchpin of the radiometric traceability of GLAMR. Absolute calibration of the transfer radiometer allows GLAMR to achieve accurate absolute radiometric calibration of airborne and satellite sensors. Briefly, the transfer radiometer is calibrated by NIST using their Primary Optical Watt Radiometer (POWR). Traceability of the POWR calibration is through the helium-cooled electrical substitution radiometer that determines the amount of electric power needed to match the amount of optical power from the laser source used during the measurement. The end result is that the transfer radiometer is absolutely calibrated with an uncertainty <0.09% in most of the visible portion of the spectrum and 0.3-3% uncertainties in other spectral regions. A periodic calibration of the transfer radiometer at NIST facilities or a certified substitution radiometer is performed to maintain this level of accuracy. Once the transfer radiometers are received by GSFC, they are used to calibrate a set of monitoring radiometers permanently attached to the integrating sphere – called sphere monitors. The figure on the right shows the transfer radiometers in front of the integrating sphere and sphere monitors near the perimeter of the exit port of the sphere while illuminated with 412 nm light from GLAMR.
GLAMR is the light source
GLAMR is the light source enabling the transfer of the radiometric traceability of NIST’s POWR to the airborne/satellite sensor to via the transfer radiometers and sphere monitors. These efforts make use several lasers that spread the solar reflective portion of the electromagnetic spectrum and are being developed in the GSFC’s GLAMR Laboratory. Most recently, the Visible Infrared Imaging Radiometer Suite (VIIRS) for Joint Polar Satellite System-2 (JPSS-2) was characterized using GLAMR in August 2016. Future calibration plans that include GLAMR are Landsat 9 Operational Land Imager-2, Ocean Color Instrument on Plankton, Aerosol, ocean Ecosystem (PACE), CLARREO Pathfinder, and VIIRS on JPSS-2 and -3.
The GLAMR team composed of NASA and NIST personnel received the NASA Agency Group Achievement award in 2017 for “outstanding contribution to NASA’s excellence in sensor characterization to enable climate-level measurement accuracy.” Drs. McCorkel (NASA), Brown (NIST), Woodward (NIST), and McAndrew (NASA) also received a patent for this calibration methodology on June 13, 2017.