Spatial Heterodyne Spectroscopy for Atmospheric Remote Sensing

1. Spatial Heterodyne Spectroscopy for Atmospheric Remote Sensing C.R. Englert*, M.H. Stevens*, J.M. Harlander**, F.L. Roesler*** *Naval Research Laboratory, Washington, DC, USA **St. Cloud State University, St. Cloud, MN, USA ***University of Wisconsin Madison, Madison, WI, USA ASSFTS 12th Workshop Quebec City, Canada

2. Overview • Spatial Heterodyne Spectroscopy (SHS) • Why do we measure hydroxyl with SHS • SHIMMER on STPSat-1 • Outlook: SHS for the infrared

3. Spatial Heterodyne Spectroscopy: Basic Concept • Michelson: fixed diffraction gratings replace mirrors • Wavelength-dependent Fizeau fringe pattern recorded by imaging detector • Zero spatial frequency at Littrow wavenumber (σo) • Spectral resolution determined by dispersive elements • Interferometric throughput • Spectral range determined by detector sampling

4. SHS Sample Measurements Monochromatic Source Interferogram Spectrum Multiple Emission Line Source Harlander et al., Applied Optics, 42, 2829-2834, 2003.

5. SHS Properties • Interferometer has no moving parts • Fieldwidening possible with fixed prisms • High throughput (like standard FTS + fieldwidening) • High resolution in small package • No changing scene (scintillation) noise • Optical defects can be corrected in data processing • Alignment and fabrication tolerances are relaxed • Bandpass/Resolution trade-off driven by detector array and imaging optics • Very robust design possible (“monolithic interferometer”) • Imaging capability • No intrinsic wavelength standard

6. Why do we measure Hydroxyl (OH) • First and to date only global scale measurement of OH in the middle atmosphere: Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI, NRL) – Coming soon: AURA-MLS results (2.5 THz) Summers M.E., et al, Science, 277, 1967-1970, 1997. Conway et al., Geophys. Res. Let., 27, 2613-2616, 2000. Summers et al., Geophys. Res. Let, 28, 3601-3604, 2001.

7. Resolution ~0.1nm Resolution ~0.024nm Why do we measure hydroxyl with UV-SHS • Need for high resolving power because the OH emission is superimposed on the complex, Rayleigh scattered, solar background. • Small size, weight, power consumption. Compared to MAHRSI almost an order of magnitude. Now suitable for small satellites. • High throughput and 1D imaging results in higher S/N and avoids scanning Model Calculations of the limb signal for a tangent height of ~72km Red: Rayleigh scattered background Black: OH resonance fluorescence

8. Space Shuttle Proof of Concept Flight SHIMMER-Middeck STS-112, October 2002 PI: Joel Cardon (NRL) Instrument was operated by an astronaut via a laptop computer. The interferometer consists of a 7kg steel construction holding the optical components with interferometric tolerances. Three orbits of atmospheric data resulted from this flight allowing the assessment of on orbit performance, scattered light, flight-data analysis, etc. SHIMMER-MIDDECK is a joint program between NRL and the DoD Space Test Program

9. SHIMMER-MIDDECK Results Limb Profile Hydroxyl Emission Spectrum High Resolution Solar Spectrum Cardon et al., SHIMMER on STS-112: Development and Proof-of-Concept Flight, AIAA Paper: 2003-6224, 2003.

10. SHIMMER-STPSat-1 … contains the first monolithic SHS interferometer Optics & Photonics News, January 2004 Applied Optics, May 20, 2003 SHIMMER-STPSat-1 is a joint program between NRL and the DoD Space Test Program

11. SHIMMER-STPSat-1 Interferometer CCD Exit Optics Anamorphic Telescope Earth’s Limb SHIMMER STPSat-1 Optical Design SHIMMER STPSat-1 Optics Assembly

12. 100 km d 65 km V  30 km   v h 0 km • h  560 km • v  8 km/s • d  2572 km • v  1.6°   68.2° SHIMMER-STPSat-1 Observation Geometry

13. SHIMMER-STPSat-1 (northern Summer orientation) Mission duration: > 1 year 5-10% PMC occurrence 75km tangent point location Satellite ground track Ground terminator 75km Terminator Local time precession of the orbit: ~30 min per day

14. Outlook: SHS for the Infrared Typical past SHS applications: • UV • Very high resolution … exploiting mainly SHS advantages in throughput, size, and relaxed tolerances. Longer wavelengths (e.g. LWIR) typically pose new sets of requirements. Is SHS the right technique to use? – Depends on the application! One needs to consider: Resolution/bandpass requirements Throughput advantage  Multiplex disadvantage Imaging requirements Stability of pointing platform (changing scene, vibration environment) Availability of imaging detectors Potentially required cooling of optics etc.

15. Summary • Middle atmospheric OH measurements on a global, seasonal scale are currently not available, but necessary to answer numerous questions posed by the analysis of the existing OH data. • Spatial Heterodyne Spectroscopy (SHS) allows the design of small, lightweight, rugged UV spectrometers facilitating such a measurement from a small satellite. • SHIMMER STPSat-1 will be launched in 2006 to measure middle atmospheric OH on a global, seasonal scale. • SHS instruments for wavelengths longward of the visible are in preparation