The Solar Occultation For Ice Experiment (SOFIE) was launched onboard the Aeronomy of Ice in the Mesosphere
(AIM) satellite on 25 April 2007, and began science observations on 14 May 2007. SOFIE conducts solar occultation
measurements in 16 spectral bands that are used to retrieval vertical profiles of temperature, O3, H2O, CO2, CH4, NO,
and polar mesospheric cloud (PMC) extinction at 11 wavelengths. SOFIE provides 15 sunrise and 15 sunset
measurements each day at latitudes from 65°-85°S and 65°-85°N. This work describes the SOFIE experiment and
shows preliminary retrieval results based on observations from the initial months on-orbit.
SOFIE (Solar Occultation for Ice Experiment) is a 16-channel radiometer that was launched into a polar orbit on
NASA's Aeronomy of Ice in the Mesosphere (AIM) spacecraft on 25 April 2007. An in-depth jitter analysis was
performed to verify that the spacecraft could meet the pointing requirements. The analysis was based on an integrated
modeling capability which combines structural dynamics with dynamic ray tracing to determine the motion of the
boresight on the focal plane array (FPA) in the presence of disturbances. Two approaches were used and compared: a
frequency-based analysis and a time-based analysis. For the frequency approach, the spacecraft provider determined the
peak amplitude of the disturbance motions within 10% of each SOFIE modal frequency. The transmissibility factor Q
between disturbance motion input and boresight motion output was determined for each degree of freedom and modal
frequency. The disturbance amplitudes were then multiplied by each Q and summed over all frequencies and degrees of
freedom. For the time-based analysis, the disturbance time histories were applied directly to the integrated model to
generate the motions of the boresight ray on the FPA. The resulting motions were input to the sun sensor simulation to
determine if the sun tracking algorithm could stay in fine track mode, or lose lock and jump to coarse track mode. As
expected, the jitter from the frequency-based analysis was worse than the time-based analysis due to the implied
assumption that the disturbance frequencies lined up exactly with the modal frequencies. Even so, the worst-case result
met the requirement of 35 arcsec peak-peak jitter. The sun sensor simulation showed that the algorithm would still
remain in fine-track mode and not lose lock even under the worst-case condition. Actual on-orbit data is presented that
verifies the validity of the analysis.
The Solar Occultation For Ice Experiment (SOFIE) is scheduled for launch onboard the Aeronomy of Ice in the
Mesosphere (AIM) satellite in March 2007. SOFIE is designed to measure polar mesospheric clouds (PMCs) and the
environment in which they form. SOFIE will conduct solar occultation measurements in 16 spectral bands that are used to retrieve vertical profiles of temperature, O3, H2O, CO2, CH4, NO, and PMC extinction at 10 wavelengths. Thirty
occultations are observed each day covering latitudes from 65° - 85°S and 65° - 85°N. The PMC measurements are
simultaneous with temperature and gas measurements that are unaffected by PMC signal. This data set will be the first
of its kind, and allow new advancements in the understanding of the upper mesosphere.
The SOFIE pointing control system (PCS) locates and tracks the top edge of the sun and periodically scans the solar disk for calibration. Primary hardware components are a steering mirror assembly (SMA), sun sensor, vibration isolation system (VIS), and associated electronics. The SMA has a 100-Hz control bandwidth and is capable of ±1.6 mechanical degree deflection in azimuth and elevation axes. The sun sensor uses a 1024x1024 pixel, radiation-hardened focal plane array and coarse and fine tracking algorithms to report the solar centroid and edge positions to the PCS. The PCS control law uses this information to command the SMA. A change in launch loads necessitated the development of the VIS, which features passive viscoelastic damping to protect the SMA. A rapid prototyping methodology was used to develop the control laws for the inner SMA feedback loop and outer PCS feedback loop. The methodology features integrated end-to-end modeling of structural dynamics, controls, and optics; automatic C-code synthesis from block diagrams; real-time hardware-in-the-loop (HIL) testing; and the ability to change control parameters "on the fly." Extensive testing of the PCS shows stable pointing performance of about 2 arcsec in the presence of 60-arcsec disturbances, compared to the requirement of 15 arcsec.
Accurate simultaneous retrievals of temperature and pressure are key to retrieving high quality mixing ratio profiles from occultation sensors. Equally important is accurate determination of the vertical separation between measurement points. Traditionally, these tasks are complicated by platform motion and CO2 model errors. We present a new approach that is independent of platform motion and CO2 concentration, using inexpensive modern 2D focal-plane arrays and an innovative refraction-angle measurement. This provides both accurate temperature retrievals and precise vertical separation of measurement samples, greatly improving the quality of mixing ratio retrievals. We show recent studies demonstrating the expected performance of the SOFIE instrument (Solar Occultation For Ice Experiment) to be launched as part of the AIM (Aeronomy of Ice Mission) in September 2006. This system will have the ability to retrieve accurate temperature, through mild particulate contamination (such as volcanic aerosol and cirrus) from cloud-top to stratopause, independent of mixing ratio knowledge. Additional CO2 absorption channels will provide retrieved temperature and CO2 mixing ratios through the mesosphere and into the lower thermosphere.
Proc. SPIE. 1961, Visual Information Processing II
KEYWORDS: Optical transfer functions, Point spread functions, Statistical analysis, Imaging systems, Sensors, Image restoration, Image acquisition, Digital imaging, Visual information processing, Systems modeling
This paper provides a common mathematical framework for analyzing image fidelity losses in rectangularly and hexagonally sampled digital imaging systems. The fidelity losses considered are due to blurring during image formation, aliasing due to undersampling, and imperfect reconstruction. The analysis of the individual and combined effects of these losses is based upon an idealized, noiseless, continuous-discrete-continuous end-to-end digital imaging system model consisting of four independent system components: an input scene, an image gathering point spread function, a sampling function, and an image reconstruction function. The generalized sampling function encompasses both rectangular and hexagonal sampling lattices. Quantification of the image fidelity losses is accomplished via the mean-squared-error (MSE) metrics: imaging fidelity loss, sampling and reconstruction fidelity loss, and end-to-end fidelity loss. Shift-variant sampling effects are accounted for with an expected value analysis. This mathematical framework is used as the basis for a series of simulations comparing a regular rectangular (square) sampling grid to a regular hexagonal sampling grid for a variety of image formation and image reconstruction conditions.