The reaction e+e~e+e m. m has been analyzed using 97 pb ' of data taken with the Crystal Ball detector at the DESY e e+ storage ring DORIS II at beam energies around 5.3 GeV. For the first time we have measured the cross section for yy~m. m. for n m invariant masses ranging from threshold to about 2 GeV. We measure an approximately flat cross section of about 10 nb for 8'=m 0 0 (0.8 GeV, which is below 0.6 GeV, in good agreement with a theoretical prediction 'tr n' based on an unitarized Born-term model. At higher invariant masses we observe formation of the ft(1270) resonance and a hint of the fo(975). We deduce the following two-photon widths: I rr(f, (1270)) =3.19+0. 1620 z, keV and I "(fo( 975)) (0.53 keV at 90% CL. The decayangular distributions show the m~system to be dominantly spin 0 for W &0.7 GeV and spin 2, helicity 2 in the f, (1270) region, with helicity 0 contributing at most 22% (90% C.L.).
[1] We present the first spaceborne observation of a Global Positioning System (GPS) signal reflected from the Earth's surface, specifically from the Pacific Ocean. This result is scaled to obtain the expected voltage signal-to-noise ratio (SNR) and altimetric accuracy for a generic GPS reflections altimetry mission and the current SAC-C and CHAMP missions. Cross-correlating a three-parameter phase model with both a 1-s and 4-s segment of spaceborne imaging radar-C (SIR-C) calibration data, recorded before and after a Galapagos Islands imaging pass, results in beam-limited signals having voltage SNRs of 10 and 334, respectively. Evidence for these results being reflected GPS signals includes: (1) The signals' temporal shapes agree closely with that predicted using a detailed scattering model, at two different observation geometries, and differ significantly from the expected direct signal shapes. (2) The signal in the 4-s data has a measured coherence time of 1.0 ms, which agrees closely with that expected for a reflected signal and is completely inconsistent with the direct signal's coherence properties. (3) The 1-and 4-s signals' voltage SNR is maximized by shifting the model frequency À2740 Hz and 497 Hz, respectively from that expected from their respective specular reflection points, or À2875 Hz and 690 Hz from the expected direct signal frequencies. These values agree with the À2900 Hz and 510 Hz Doppler frequency shifts expected from those points on the surface corresponding to the antenna's pointing direction, thus illustrating beam-steering effects on the surface. (4) Plausible hypotheses for the detected waveform being a corrupted direct signal, including second-order mismodeling effects, shuttle multipath effects, or a band-pass cutoff of the GPS spread spectrum, are shown to be inconsistent with the data. Space-based observations of reflected GPS signals, like the ones presented here, may enable a new class of ocean topography measurements unavailable from traditional altimeters, such as TOPEX/Poseidon, and perhaps surface wind vector measurements. Making such observations with sufficient SNR will require unusually large, high-gain antennas. The measurement presented here is scaled to assess the expected SNR for those applications. Because this result lies in a nonlinear scaling regime, the correct scaling equations are presented, and the expected signal strength from a generic GPS reflections altimetry mission is derived to illustrate the most important contributions to the signal SNR. An SNR estimate is also derived for the SAC-C and CHAMP missions, which are expected to make GPS reflection measurements in the near future. Finally, a qualitative wind speed determination is extracted from the observed signal.
[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.
Abstract.Differences in electromagnetic path delay, between direct Global Positioning System (GPS) signals and those reflected from the surface of Crater Lake, have led to lake surface height estimates with 2-cm precision in I second. This is the first high-precision altimetric demonstration with GPS from sufficient altitude (m 480 m) to probe fundamental experimental errors, which bear on future airand spaceborne passive GPS altimetry. It also serves as the first demonstration of a new approach to altimetric remote sensing in the coastal region, an area that is poorly measured by conventional radar altimetry. Time-series analyses suggest that tropospheric and thermal noise fluctuations dominate the altimetric error in this experiment. Estimating the differential delay from several simultaneously visible satellites may enable tropospheric error estimation and correction. Thermal noise on the reflected signal will be reduced with fully polarimetric observations and larger antenna apertures.
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