CLARREO Instruments: Advancement in Absolute Calibration of Earth Observing Instruments
CLARREO requirements were used to develop instrument designs with the goal of reducing instrument size to minimize mass, power, and cost.
A wide range of mission orbits, spacecraft, and launch vehicle designs were considered to optimize the requirements.
Prototype designs were developed for all of the instruments,
with similar designs being used to verify calibration accuracy tests in collaboration with NIST.
The CLARREO instrument suite includes:
- Infrared (IR) Spectrometer
- Reflected Solar (RS) Imaging Spectrometer
- Radio Occultation (RO)
Infrared (IR) Instrument
The CLARREO IR instrument will obtain spectrally resolved infrared radiance (IR)
emitted from Earth to space with an accuracy of 0.065 K (k = 2, or 95% confidence),
and traced to the SI standard for the kelvin. Because calibration is central to the design of the infrared sensor,
the CLARREO calibration approach differs in a fundamental way from previous missions.
Flight instruments are typically calibrated on the ground before launch, and then post
-launch performance is validated through intensive calibration/validation field campaigns
and by comparison with existing instruments. The approach for the CLARREO infrared sensor is
to include a verification suite that provides SI-traceable calibrations on-orbit.
The verification suite, as shown in Figure 1, includes: a variable temperature deep
cavity blackbody to check radiance calibration accuracy over a range of target temperatures;
phase change cells to provide SI-traceable temperature sensor calibration; heated collars to
verify blackbody cavity emissivity; a quantum cascade laser and integrating sphere to verify
instrument line shape and provide an independent blackbody emissivity check; and a second deep
space view to quantify instrument polarization sensitivity. The full capability instrument concept
description that was developed for Mission Concept Review (MCR) in November 2010 is available
in the CLARREO SDT Report (Section 4.3 Infrared Sensor Concept). Work is also ongoing to develop
a smaller, more compact instrument design that can obtain a portion of the CLARREO-defined science at substantially reduced cost.
Figure 1: (a) CLARREO IR measurement concept with verification suite. Typical observation sequence is zenith space view-calibration blackbody-nadir-zenith space view-calibration blackbody-verification view, where the verification view may be the verification blackbody, integrating sphere, or off-zenith space view. The quantum cascade laser (QCL) provides a monochromatic source for illumination of the integrating sphere and a high intensity source for checking the blackbody cavity reflectivity. (b) The IR instrument packaging concept.
Reflected Solar (RS) Instrument
The CLARREO RS instrument design represents an advance in absolute calibration over existing approaches.
The RS benchmark measurement is spectrally resolved nadir reflectance of solar radiation from Earth to space with an accuracy of 0.3% (k = 2).
The percentage is relative to the mean spectral reflectance of the Earth of about 0.3.
Figure 2 demonstrates the approach for the reflected solar spectrometer and its use of the Moon
as a reference for stability in orbit, the Sun with multiple attenuators to verify instrument
nonlinearity of gain across the Earth viewing dynamic range, and ability to directly scan deep space
to verify instrument offsets. Spectral response is verified using solar spectral absorption line features.
One of the critical differences of this instrument relative to others in orbit is its ability to
point the entire instrument at Earth, Sun (every 2 weeks), Moon (monthly at 5 to 10 degree phase angle), or deep space.
This eliminates the need for scanning mirrors with angle dependent calibration uncertainties, and allows the
use of depolarizers to reduce polarization sensitivity to the required levels. Scanning the instrument
view across lunar and solar disks provides images suitable for verifying stray light performance.
Note that the calibration of the reflected solar is in terms of reflectance units. Conversion to
absolute radiance can be done using the spectral total solar irradiance provided by instruments such
as the Total Solar Irradiance Spectrometer (TSIS) with expected
absolute accuracy of 0.25%. The full capability instrument concept description that was developed
for Mission Concept Review (MCR) in November 2010 is available in the
CLARREO SDT Report
(Section 4.2 Reflected Solar Sensor Concept).
Figure 2: CLARREO concepts for improved SI-traceable absolute accuracy in orbit. The verification of nadir spectral reflectance accuracy relies on rotating the entire instrument to view the Moon at constant phase angle as a single-level stable reflectance source, similar to the SeaWiFS approach, the Sun in combination with filters and precision apertures for nonlinearity determination, and the use of depolarizers to control polarization sensitivity.
Global Navigation Satellite System–Radio Occultation (GNSS-RO) Receiver
The Global Navigation Satellite System Radio Occultation (GNSS-RO) instrument provides a measurement of the frequency shift of GNSS signals as they move through the limb of the atmosphere, with an accuracy of 0.06% (k = 2) for a range of altitudes from 5 to 20 km in the atmosphere. This active limb-sounding is tied directly to the time standards of navigation satellites (such as GPS, Galileo, and GLONASS) and is therefore directly traceable to the international standard of the second. Radio occultation measurements provide the refractivity of the atmosphere, which is directly related to pressure, temperature, and water vapor.
The basic concept of the RO measurements is shown in Figure 3. The CLARREO GNSS-RO instrument uses occulting GNSS satellites to measure atmospheric refractivity through Doppler shifts. It observes the change of delay of the transmitted GNSS constellation satellite signal through the atmosphere as it sets or rises behind the Earth's limb. The change of delay, measured as a Doppler frequency shift, is a function of the slowing and bending of the GNSS signal, and so it is translated to a bending angle, α, as shown in Figure 3. A vertical refractivity profile is created at the tangent point, and allows for reconstruction of the temperature, pressure and humidity in the neutral atmosphere.
The GNSS-RO instrument design is based on instruments that have flight heritage, augmented with improved technologies in next generation designs to meet the CLARREO needs. The CLARREO GNSS-RO receiver is currently envisioned to be a derivative of the TriG (Tri-GNSS) receiver being developed by the Jet Propulsion Laboratory. The Current Best Estimate (CBE) receiver dimensions are 30x30x20 cm3. This configuration is anticipated to provide for collecting both rising and setting occultation data that meets the CLARREO requirements over the entire mission lifetime. The full capability instrument concept description that was developed for Mission Concept Review (MCR) in November 2010 is available in the CLARREO SDT Report (Section 4.4 GNSS-RO Instrument Concept).
Figure 3: Concept of the CLARREO GNSS-RO measurements.