CubeSat-Scale Hyperspectral Imager for Middle Atmosphere Investigations Rick Doe 1, Steve Watchorn 2, John Noto 2, Robert Kerr 2, Karl van Dyk 1, Kyle Leveque 1, and Christopher Sioris 3 1 SRI International 2 Scientific Solutions, Inc. 3 Environment Canada 2009 CUBESAT DEVELOPER S CONFERENCE, SAN LUIS OBISPO, CA
The Earth s Atmosphere 3 km
Characteristics of the Middle Atmosphere Coldest temperatures on Earth Convectively unstable Breaking gravity waves drive planetary wind circulation Competition between eddy and molecular diffusion Meteoric deposition of trace metals (Na, Fe, K) Inaccessible to satellites and balloons
Coldest Temperature in the Atmosphere PRESSURE (T) 10 5 10 4 10 3 3 km 10 2 10 1 10 0 10 1 180 210 250 290 350 TEMPERATURE ( K)
Mesospheric Temperature Minimum is Controlled by CO 2 IR Radiation 280 CO 2 cooling CO 2 cooling O 3 heating O 3 heating TEMPERATURE ( K) 220 160
Mesopause Temperature Minimum is a Harbinger of Climate Change CO 2 Doubling 3 km CO 2 Halving Roble and Dickinson, GRL, 16, 12, 1441-1444, 1989.
Gravity Waves in the Middle Atmosphere 3 km 87 km Middle atmosphere gravity waves couple energy and momentum fluxes from the troposphere to the thermosphere GW climatology helps improves our understanding of mean wind flow and tides, turbulence, heat and chemical transport, and higher altitude gravity waves (100 500 km)
Target Species for Middle Atmosphere Remote Sensing 3 km OH, Atomic Metals (Na & K), O, and O 2
Spectral Signatures are Sensitive to Mesopause Temperature Sodium (80-105 km) Molecular Oxygen (97 km)
Candidate Techniques for CubeSat-Based Middle Atmospheric Studies Imagers (limited spectral information) Traditional Slit Grating Spectrograph (large) Interferometer (luminosity advantage)
Classic Michelson Interferometer Peaks and Nulls in Fringe Pattern Probed by Moving Mirror Recovered Spectra is Fourier Transform of Fringe Pattern Moving Parts!
SHS Interferometer - No Scanning Required Tilted gratings replace mirrors to provide crossed wavefronts at exit aperture Wedge angle, grating density, and optical path adjusted to tune SHS to desired wavelength, resolution, and coverage Fringe pattern (Fourier Transform of Spectra) acquired without scanning Robust design
Spatial Heterodyne Spectrometer (SHS) Monolithic design can be locked down with no alignment issues or moving parts Fused silica design first created for space-based limb imaging of OH by NASA, U Wisconsin, and NRL SHS Instrument Operating on STPSat-1 since March 2007
In Hand Na-Optimized SHS Monolith 2 3.8 100 mm Light Baffle Light Baffle
Notional CubeSat HSI (CHSI) Layout Spatial Heteodyne Spectrometer Monolith CubeSat Envelope 100 mm Light Baffle Imaging Light Baffle Detector (CCD) Order Sorting Prefilter Aperture Slit
CubeSat HSI Orientation Cross-Track 100 mm Spatial Dimension INERTIAL FRAME Orbit Velocity Nadir Light Baffle Spectral Dimension Light Baffle IMAGE PLANE
Ideal SHS Fringe Pattern for Imaging Spectroscopy 100 mm Light Baffle Spatial Dimension Light Baffle Spectral Dimension
Prototype Sodium CHSI Field Test 100 mm Light Baffle Light Baffle
Raw Fringe Pattern from Prototype of Sodium CHSI 100 mm Light Baffle Light Baffle
Recovered Na Spectra from CHSI Prototype 589.0 nm 100 mm Light Baffle 589.6 nm Light Baffle
CubeSat-Optimized Na Monolith Design Grating Blk 1 PG Spacer 1 Prism 1 Spectral Coverage: 588.3 590.3 nm Spectral Resolution: 0.13 nm PW Spacer 1 15 mm BS Half 1 BS Half 2 PW Spacer 2 Prism 2 PG Spacer 2 Grating Blk 2 34 mm
CubeSat-Optimized Monolith Mass Model
Front Optic Selection 50 mm FL for 10 spatial field of view on grating Low-distortion Leica rangefinder lens 28 mm back focus distance sufficient to illuminate grating
Spatial Coverage
Starlight Xpress Lodestar CCD Camera Astronomy grade 752 X 580 pixel Sony ICX429AL CCD Low noise, 16-bit dynamic range, 10 Hz max sampling On-chip binning to enhance SNR and minimize telemetry Extensive Linux libraries for image processing
CCD Camera Performance 50 1700 0 1200 Dark Noise, 50º C 1 s Integration Read Noise, 50º C 1 ms Integration
Reimaging Optics Selection Transfers SHS fringe pattern to CCD focal plane Large aperture (F/1.4) to maximize sensitivity Close-up diopter added to decrease standoff distance Bolex 36-mm macro lens with +10 diopter adapter
Payload Miniaturization for 1.5 U Form Factor Miniaturized with Folding mirrors Folded electronics Repackaged lens
1.5 U CHSI Payload Model
Flight Processing Goals Optimize spectral and spatial binning for best science at acceptable telemetry and power loads Investigate on-orbit photometric correction (flat fields, dewarping) Perform on-orbit FFTs to extract band feature intensity Analyze recovered bands to extract temperature
Temperature Extraction from O 2 Spectra
CHSI Requires Nadir Orientation with CCD Spatial Axis Perpendicular to Ram Direction
Path Forward Design Integrated Cruciform Optimize foreoptic mechanical support Assess processing power associated with image manipulation Build vibration and thermal test unit