Stability and Control of a Trap Mode in Synchronous Orbit

The possibility of the existence of a trap mode in synchronous orbit is investigated. It is shown that when such a mode exists, certain parameters of the satellite orbit may soon attain undesired equilibrium values, thus complicating the normal stationkeeping operation of the spacecraft. A state-dependent control force which eliminates any such trap mode in the satellite orbit is then investigated.     

Limiting the Impulsive Noise in a PLL

This paper analyzes the operation of a phase-locked loop (PLL) preceded by a bandpass limiter (BPL) in the presence of impulsive noise. It is shown that the effect of the limiter consists essentially in a change of the statistics of the pulse strengths of the noise, so that the behavior of a BPL + PLL can be deduced from that of a simple PLL by suitable adjustment of the noise model. It is found that the limiter greatly enhances the PLL performance by reducing both the phase-error variance in the loop and the probability of cycle slippage. Finally, the design of the filter of the BPL is discussed, resulting in the conclusion that the best results are obtained by using a singletuned RLC circuit.     

Comparison of quiet-Sun radiances measured by CDS and SUMER on SOHO

Space Science Reviews - Since the beginning of the SOHO (Solar and Heliospheric Observatory) mission an intercalibration programme was carried out which included simultaneous observations of the...     

Probability Density of the Click Duration in an Ideal FM Discriminator

The expression of the probability density function PClick(T) of the duration r of the clicks is determined as a function of 1) the expected number of clicks per second, Felick 2) the expected number of times per second that the noise component x(t) in phase with the carrier has instantaneous amplitude larger than that of the carrier itself 3) the probability density function peross(T) of the duration of the time intervals in which x(t) < -A 4) the probability that the values of the noise component in quadrature with the carrier will be of opposite sign at the extremes of time intervals of duration v. Using for Pcross(T) an approximate formula that holds fairly well in the range of the values of the signal-to-noise power ratio ? usually encountered, PIick(T) has been calculated for several values of a and for two shapes of the noise spectrum, Gaussian and rectangular. Finally, an expression is given for the mean value of T.     

Simultaneous COMPTON GRO and optical monitoring of AGN

During two 14-day periods in January 1992 and March 1992, several AGN which have been in the wide field of view of COMPTEL and EGRET at that time were monitored optically for variations in intensity and polarization. We report on first results of this correlated optical and CGRO observations.     

Optical focussing with soft X-rays and in the extreme ultraviolet

Space Science Reviews -     

Development and performance of a position sensitive proportion counter for the rosat project

A multiwire proportional counter capable of imaging soft X-rays has been constructed. The position of an incident photon is obtained by measuring the induced charge signals on two orthogonal sets of cathode wire grids. The anode signal is used as trigger and energy signal. At a photon energy of 0.94 keV the achieved position resolution is 300 μm (FWHM), and the energy resolution is 45% (FWHM). With a five sided anticoincidence the background rejection efficiency is 90–95%.A small version of the counter has successfully been flown twice on sounding rocket payloads. X-ray images of the supernova remnants Puppis A and Cassiopeia A have been taken. As the prime focal plane detector for the ROSAT project we are building a version with an aperture of 80 mm.     

Wind structure and small-scale wind variability in the stratosphere and mesosphere during the November 1980 energy budget campaign

Between November 6 and December 1, 1980, a series of rocket observations obtained from two sites in northern Scandinavia as part of the Energy Budget Campaign indicated that significant vertical and temporal changes in the wind structure were present and were noted to coincide with different geomagnetic conditions, i.e., quiet and enhanced. This series of observations represents for the first time the largest amount of data ever gathered at high latitudes over such a short interval of time. It is observed that prior to November 16, the meridional wind component above 60 kilometers was found to be positive (southerly) while the magnitude of the zonal wind component increased with altitude. After November 16 the meridional component became negative (northerly) and the magnitude of the zonal wind component was noted to decrease with altitude. Time-sections of the perturbations of the zonal wind show the presence of vertically propagating waves which suggest gravity wave activity. These waves increase in wavelength from 3–4 kilometers near 40 kilometers to over 12 kilometers near 80 kilometers. The observational techniques employed at Andoya, Norway, and ESRANGE in Sweden, consisted of chaff foil, chemical trails, inflatable spheres, and parachutes.