John C. Burton

Comparison of algorithms for computing the two-dimensional discrete Hartley transform

Three methods have been described for computing the two-dimensional, discrete Hartley transform. Two of these employ a separable transform; the third method, the vector-radix algorithm, does not require separability. In-place computation of the vector-radix method is described. Operation counts and execution times indicate that the vector-radix method is fastest.

High precision refraction measurements by solar imaging during occultation: results from SOFIE

A new method for measuring atmospheric refraction angles is presented, with in-orbit measurements demonstrating a precision of +/-0.02 arcsec (+/-0.1 microrad). Key advantages of the method are the following: (1) Simultaneous observation of two celestial points during occultation (i.e., top and bottom edges of the solar image) eliminates error from instrument attitude uncertainty. (2) The refraction angle is primarily a normalized difference measurement, causing only scale error, not absolute error. (3) A large number of detector pixels are used in the edge location by fitting to a known edge shape. The resulting refraction angle measurements allow temperature sounding up to the lower mesosphere.

On the determination of biases in satellite‐derived temperature profiles

Comparisons are presented between Nimbus 7 LIMS (Limb Infrared Monitor of the Stratosphere) mapped temperatures and both Datasonde and sphere in situ rocketsonde temperature measurements. With this approach up to 666 LIMS/Datasonde pairs were obtained for various pressure levels to look for small biases in LIMS temperatures as a function of altitude, latitude and season. Between 10-1 hPa LIMS and Datasonde agree everywhere to better than +/- 2 K with the exception of a warm bias of about 3 K at 2 hPa at high latitudes. However, LIMS is colder than the Datasonde by about 4 K at 0.4 hPa and by about 8-10 K at 0.1 hPa. When compared with the more accurate sphere temperatures the bias at 0.1 hPa is reduced by nearly one-half. These results indicate that the LIMS zonal mean constituent profiles are nearly free of temperature bias, except perhaps at 0.1 hPa.

Highly efficient and athermal 1550nm-fiber-MOPA-based high power down link laser transmitter for deep space communication

We demonstrate highly efficient, 1.5um-fiber-amplifier, optimized for athermal and reliable operation. High efficient operation is sustained for a wide range of pulse-position-modulation (16 to 128-ary PPM) formats with pulse widths varying from 8nsec to 0.5nsec. System achieves 6W average and ~1kW peak power with 8nsec pulses and 3Ghz linewidth. Stimulated Brillion scattering is managed by use of LMA fiber in final stage and precise linewidth control while maintaining the required diffraction limited, and (PER>20dB) polarized output. System maintains performance for ambient temperatures 10-50°C.

Improvements in Nimbus 7 limb infrared monitor of the stratosphere ozone profiles as obtained with updated spectral line parameters and radiance algorithms

Ozone distributions from the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) experiment of 1978–1979 are generally in good agreement with other concurrent ozone measurements in the middle and upper stratosphere, but not at lower altitudes. LIMS ozone is too large below about the 15-hPa (or millibar) level, particularly at lower latitudes. A new LIMS ozone distribution is presented for 1 day of profiles, May 5, 1979, obtained with an improved forward radiance algorithm and using the spectral line parameters for the 9- to 10-μm region in the HITRAN 92 compilation. However, we also divided our single day of retrieved ozone mixing ratio profiles by a suggested factor of 1.051, which makes them compatible with the ozone distributions from several of the UARS experiments. Our revised distribution still agrees with the archived LIMS ozone to better than ±5% above about the 15-hPa level, but there is a significant decrease for the revised ozone in the lower stratosphere. That decrease approaches 25% at 50 hPa in the tropics. In general, the revised LIMS ozone is now in accord with the comparison data sets for the stratosphere, except for the lowest levels (Umkehr layer 3 or 63–127 hPa), where the effects of interfering species and small biases in LIMS temperatures are most pronounced. It is concluded that the current 9- to 10-μm ozone line list is adequate for obtaining good quality ozone mixing ratio profiles from satellite broadband limb-infrared measurements.

Highly-efficient, high-energy pulse-burst Yb-doped fiber laser with transform limited linewidth

A 1um fiber laser outputting high energy (<1mJ) pulse-bursts with high peak powers (<15kW) and narrow linewidth (<300MHz) is an attractive pump source for tunable periodically poled crystal (PPx) based OPA’s which are used in gas sensing, imaging and communication applications. Here a turn-key 1064nm PM Yb-doped fiber amplifier capable of generating high pulse burst energies with transform limited linewidth is presented. The ~20W average power capable laser is optimized for high energy (0.5-2mJ) and high peak power (<10kW) operation at low duty cycles (<0.1%). The laser is capable of operating at <10x the saturation energy level of the final stage gain fiber and achieves a high level of pulse-to-pulse peak power uniformity within pulse-burst. Stimulated Brillion Scattering (SBS) limited micro pulse energy up to 40uJ is achieved and SBS dependence on micro pulse width and separation are characterized. High wall plug efficiency (<20%) for the FPGA controlled system is maintained by temporal and spectral ASE suppression and by spreading the necessary pulse pre-shaping losses (~12dB) to three different amplitude modulation points in the amplifier chain.

Temperature, pressure and high-fidelity pointing knowledge for solar occultation using 2D focal plane arrays

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.

Sounding the upper mesosphere using broadband solar occultation: the SOFIE experiment

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.

Rectangularly and hexagonally sampled imaging-system-fidelity analysis

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.

Highly efficient, high energy 1.5 μm pulsed fiber laser with precise linewidth and wavelength control of individual pulses

High power, high energy pulsed fiber laser with precise control of individual pulse width and wavelength is an enabling source for coherent imaging and communication applications. Here a turn-key 1550 nm PM fiber amplifier generating 22 μJ pulse energy with near transform limited linewidth (600 MHz) is presented. Individual pulse wavelengths and pulse widths can be controlled with 30-120 pm wavelength separation and 2-10 nsec pulse width. The 22 W average power laser, based on COTS Er and ErYb doped LMA PM-fibers is optimized for high peak power (< 4 kW), low duty cycle (~0.1%) operation while maintaining diffraction limited beam quality (M2 < 1.1). High wall plug efficiency (<10%) for the FPGA controlled system is maintained by temporal and spectral ASE suppression. Pulse energies are limited by Stimulated Brillion Scattering and Four Wave Mixing. Dependence of the fiber nonlinearities on pulse width and wavelength separation is characterized.