Engineering topics
Publications and source records attributed to Purcell, G. H..
In this paper we describe the key technology challenges, the approach to resolving them through analysis and testbed activities, and the results of the testbed activities.
We present estimates for the mean bias of the TOPEX/POSEIDON NASA altimeter (ALT) and the Centre National d'Etudes Spatiales altimeter (SSALT) using in-situ data gathered at Platform Harvest during the first 36 cycles of the mission. Data for 21 overflights of the ALT and six overflights of the SSALT have been analyzed. The analysis includes an independent assessment of in-situ measurements of sea level, the radial component of the orbit, wet tropospheric path delay, and ionospheric path delay. (The sign convention used is such that, to correct the geophysical data record values for sea level, add the bias algebraically. Unless otherwise stated, the uncertainty in a given parameter is depicted by +/- sigma(sub x), where sigma(sub x) is the sample standard deviation of x about the mean.) Tide gauges at Harvest provide estimates of sea level with an uncertainty of +/- 1.5 cm. The uncertainty in the radial component of the orbit is estimated to be +/- 1.3 cm. In-situ measurements of tropopsheric path delay at Harvest compare to within +/- 1.3 cm of the TOPEX/POSEIDON microwave radiometer, and in-situ measurements of the ionospheric path delay compare to within -0.4 +/- 0.7 cm of the dual-frequency ALT and 1.1 +/- 0.6 cm of Doppler orbitography and radiopositioning integrated by satellite. We obtain mean bias estimates of -14.5 +/- 2.9 cm for the ALT and +0.9 +/- 3.1 cm for the SSALT (where the uncertainties are based on the standard deviation of the estimated mean (sigma(sub bar x/y), which is derived from sample statistics and estimates for errors that cannot be observed). These results are consistent with independent estimates for the relative bias between the two altimeters. A linear regression applied to the complete set of data shows that there is a discernable secular trend in the time series for the ALT bias estimates. A preliminary analysis of data obtained through cycle 48 suggests that the apparent secular drift may be the result of a poorly sampled annual signal.
We present preliminary estimates for the mean bias of the TOPEX/Poseidon NASA altimeter (ALT) and the CNES altimeter (SSALT) using in situ data gathered at platform Harvest during the first 36 cycles of the mission. Data for 21 overflights of the ALT and 3 overflights of the SSALT have been analyzed.
Signals transmitted by satellites of Global Positioning System (GPS) measure orientation of baseline on ship, aircraft, or other vehicle with accuracy. Two GPS antennas and receivers placed at well separated points on platform. Receivers measure positions of ends of baseline as functions of time. Output processor computes vector difference between two positions and determines orientation of baseline. Combined with conventional GPS data, orientation data allows more precise navigation and mapping and enhances calculations related to performance and control of vehicle.
Measurements of UT1 with a formal accuracy as low as 0.6 msec with only 6 hr of data and construction of a radio astrometric catalog of approximately 45 sources whose positions are known to better than 0.05 arcsec are described which were made possible by very long-baseline interferometry (VLBI) with the Deep Space Network (DSN). The characteristics and anticipated performance of the complete VLBI system being implemented within the DSN are discussed. It is noted that one of the capabilities of this system is the measurement of UT1 and polar motion at weekly intervals and that the determinations should be obtained with decimeter accuracy.
Long-baseline interferometry of the quasars 4C 39.25 and 3C 345 at 10.65 and 14.77 GHz shows that the centimeter radio source in each object is double, with component separations of 0.0020 arcsec (4C 39.25) and 0.0013 arcsec (3C 345 at 1974.5). For each source, the separation is the same at both frequencies, as well as similar to the structure observed at 7.85 GHz (and 5.0 GHz for 4C 39.25). The spectra of the individual components are derived and shown to vary with time approximately as expected for expanding self-absorbed synchrotron sources. The magnetic fields in the components are estimated to be as high as 0.1 gauss, but the structure of the sources appears to be unrelated to the magnetic-field orientation derived from low-resolution polarization measurements. The component separation in 4C 39.25 has not changed for several years, whereas 3C 345 shows rapid expansion.
The radio structure of the quasar 3C147 has been determined from multi-baseline VLBI data at 609 MHz using both a conventional method and a technique which uses the 'closure' phase information to produce a good approximation to a synthesis map of the source. The structure is similar to the central part of M87, with a bright core and a linear 'jet' of a projected length of about 1.5 kpc which is concentrated in bright 'knots'.
Results of extended VLBI observations at two epochs 15 months apart are compared for several sources suspected of showing decimetric flux variations. No evidence is obtained for changes in fringe amplitudes greater than 5%.
Explore the source record for details and available documents.
Observation of fringes from 31 compact radio sources, including eight known or suspected galaxies and 20 known or suspected QSSs, by using the Goldstack interferometer at lambda = 3.8 cm (d/lambda = 10 to the 8th power). Fringe visibility curves were obtained for nine sources showing structure on a scale of .001 sec of arc, and simple models are fitted to the data. Results for 3C 273 and 3C 279 are compared with data taken by Knight et al. (1971) at an earlier epoch. The apparent changes in brightness distribution of 3C 273 and 3C 279 are difficult to explain.