precise GlobM Positioning System (GPS) geodesy improves the •ccur•cy and precision of the estimated quantities, and that the estimated gradients resemble real atmospheric moisture gradients observed with a water vapor radiometer (WVR).Using a low elevation angle cutoff, combined with a model of the atmospheric delay gradient as a random walk process leads to 19.5% and 15% average improvement in radial and horizontal site position repeatabihties, respectively, relative to a current state-of-the-art estimation strategy that does not model horizontal gradients and imposes high elevation angle cutoff. The agreement between estimated values of zenith wet delay from collocated GPS receivers and WVRs was improved by at least 25%. Merely lowering the elevation angle cutoff improves the repeatabihty of the radial component of the site's position vector but tends to degrade the repeatability of the horizontal components of the position vector if troposphere gradients are not properly modeled. The estimates of wet delay gradients from a collocated GPS receiver and a WVR at Onsala, Sweden, seem to be correlated over timescales as short as 15 min. The agreement in azimuth between the GPS-based and the WVRbased gradients was at the 10 ø level, for significant gradients. The GPS was found to under-estimate the magnitude of the gradients by about 60% relative to the WVR-based gradients. The abihty to sense atmospheric moisture gradients from a single GPS receiver increases the useful information content from networks of GPS receivers by providing additional spatial information for weather forecasting apphcations.
[1] We present the first two aircraft Global Positioning System (GPS)-reflection altimetry measurements, the most precise GPS ocean-altimetry measurement, and demonstrate the altimetric precision and spatial resolution necessary to map mesoscale eddies. Our first experiment demonstrated a 14-cm precision single-satellite ocean altimetry measurement while our more recent experiment demonstrates 5 cm altimetric precision with 5-km spatial resolution. The new results show significant improvement over our previous effort, due to improved modeling, greater aircraft altitudes and velocities, improved receiver positioning, and better experimental control. Plans to further reduce speckle and refine models to obtain 5-cm altimetric accuracy are presented.
Photofragment spectroscopy of CH212 at a wavelength of 266 nm shows evidence for a two-photon, twostep dissociation process. The first photon absorption causes transition to a repulsive excited state of BI symmetry that breaks up rapidly to yield highly internally excited CHJ radicals and an I atom. On average, between 80% and 90% of the effective available energy for this process appears in CHJ rovibrational excitation. Once formed, a sizable fraction of the CH 2 1 fragments may absorb a second photon, dissociating to CH 2 radicals and another I atom. Energy distribution analysis indicates that the CH 2 fragments are both translationally and internally excited, although quantitative estimation of their rovibrational excitation is complicated by the possibility of electronic excitation of one or more fragments in the two-step process. An approximate analysis of the CH 2 fragment recoil angular distribution allows a crude resolution of the CH 2 1 fragment internal excitation into its rotational and vibrational components. In the absence of electronic excitation, 40% of the CH 2 1 internal energy is found to be rotational and 60% vibrational, qualitatively consistent with the picture of impulsive recoil of a semirigid radical. Kinetic analysis of the dependence of CH 2 1 and CH 2 fragment production on photolyzing light intensity permits extraction of the cross section for second photon absorption. From this, it is estimated that the extinction coefficient for CH 2 1 radical absorption at 266 nm is 30 I mole-I em-I. uPresumably, at longer wavelengths ground state molecular iOdine could be formed.
Photofragment spectroscopy of acetyl iodide (CH3COI) at 266 nm shows evidence of a two-step dissociation into three products. Initial photodissociation yields an iodine atom and a highly internally excited acetyl radical containing on average 80% of the energy available in the dissociation process. The transition dipole moment of the initial absorption lies in the C–C–O plane near the C–I bond axis, and breakup of the photoexcited acetyl iodide is rapid, on the order of 10−13 sec. The acetyl fragment is found to live for several rotational periods, ∼10−11 sec. Its subsequent decomposition to CO and CH3 can be modeled by statistical theories of unimolecular decay. It is found that an average 30% of the energy residing in the acetyl radical ultimately appears as relative translational energy of the CO and CH3 fragments.
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