ARCSTONE Mission Objectives
The ARCSTONE instrument addresses two
primary objectives in satellite-based Earth observation:
- Improved Data Accuracy: The
mission will enhance the accuracy of measurements from Earth-viewing satellites
by establishing a lunar reflectance calibration ~10x better than exists
today. Increased data accuracy enables better weather/storm forecasting and
more accurate climate models. Some previously collected satellite datasets will
be recalibrated using ARCSTONE measurements, potentially improving the accuracy
of long-term environmental and climate models.
Figure 2: Hurricane Florence as seen from the International Space Station. Image courtesy of NASA.
- Simplified Future Satellite Missions: Lunar reflectance data from the ARCSTONE mission may eliminate the need for onboard calibration systems in some future satellite instruments, providing a long-term cost benefit.
Lunar Reflectance Calibration Methodology
Figure 3: Animation of the ARCSTONE instrument measuring solar and lunar signals. Courtesy of NASA/Tim Marvel/Blue Canyon Technologies.
ARCSTONE creates a lunar reflectance
calibration with near simultaneous measurements of the signal from both the sun
and the moon. This approach builds upon previous ground-based lunar reflectance
measurements, which had higher uncertainties due to atmospheric interference.
Operating above Earth's atmosphere, ARCSTONE
can measure lunar reflectance with significantly reduced uncertainty. The
resulting lunar reflectance dataset will enable the moon to be used as an
accurate “standard candle” calibration source, eliminating the need for
expensive, complex, and heavy onboard calibration systems in future
satellite-based instruments.
ARCSTONE Project Technical Challenges
The ARCSTONE project presented some significant
technical challenges, including:
- Dynamic Range Requirements: The instrument must accurately measure the brightness
of the moon and the brightness of the sun. The sun is ~1,000,000
times brighter than the moon.
- Orbital Environment Effects: Optical
systems in low Earth orbit experience degradation from radiation exposure,
highly reactive atomic oxygen, thermal cycling, and particle bombardment.
- Platform Constraints: The
instrument had to meet research-grade measurement requirements and still integrate
into a 6U CubeSat satellite bus. The space for the instrument, the control
computer, and the calibration system is about 9” x 9” x 3.5”—roughly the size
of a shoebox!
Addressing the ARCSTONE Technical Challenges
Resonon addressed these technical challenges
through several means:
- Novel Optical Design: The
system uses different integration times and measurement redundancy to achieve
the Signal-to-Noise Ratios (SNR) required for 0.5% accuracy.
- Common Optical Path:
Both solar and lunar measurements use the same optical components. This
approach ensures that any optical degradation impacts both solar and lunar measurement
sets equivalently, allowing degradation effects to cancel out when ratioing
lunar-to-solar measurements.
- Compact Design: After
many iterations, a simple but elegant design was created that fit into the 6U
CubeSat volume.
Figure 4: The ARCSTONE instrument built by Resonon, prior to being integrated into the CubeSat bus. Image courtesy of Resonon.
A Collaborative Effort
The ARCSTONE project was made possible via
the collaboration of multiple partners across different sectors:
- Industry: Resonon,
Quartus Engineering, and Blue Canyon Technologies (a subsidiary of Raytheon)
- Academic:
Laboratory for Atmospheric and Space Physics (LASP) at the University of
Colorado Boulder
- Government:
National Institute of Standards and Technology (NIST), United States Geological
Survey (USGS), and, of course, NASA