The role of hardware in-loop motion simulation for Indiansatellites

Testing of hardware packages of sensors, actuators, and control electronics in the control loop for the IRS and SROSS satellites using motion simulators is reported. Simulation exercises and test results are described. Both air-bearing platforms and servo tables were used. The problems observed varied from interface incompatibilities and logic-level mistakes to onboard software malfunctions. unit-to-unit differences, and controller gain parameters. The importance of testing with hardware in a dynamic environment, is demonstrated     

A novel approach for the SARSAT system

A novel method for determining the location of a 406-MHz emergency locator transmitter (ELT) for the search and rescue satellite aided tracking (SARSAT) system is proposed. The technique uses information from a geostationary satellite (Geosat) link, as well as the conventional polar orbiting satellite links. The method is based on solving a system of equations to determine the range from the ELT to the polar orbiting satellite (Polarsat) for each signal transmitted. Only two pulses need to be received from the ELT in order to obtain a location estimate. The main difference between the procedure outlined and the global positioning system (GPS) is that accurate time information is not available in the transmissions since aircraft and marine vessels seldom have accurate time standards onboard     

406 MHz ELT signal spectra for SARSAT

The signal format and spectral properties of the 406-MHz emergency locator transmitter (ELT) used in the search and rescue satellite aided tracking (SARSAT) system are examined. The ELT improves location estimate accuracies and can relay information about the particular aircraft and its problem, by means of the digitally modulated message fields. It is shown that due to the RF signal frequency characteristics and the Doppler shift, processing must be performed over a frequency band of approximately 25 kHz. Through the use of the fast Fourier transformation (FFT), the frequency spectrum of the ELT is analyzed, taking account of effects due to noise, multiple simultaneously received signals, and Doppler shift. It is demonstrated that the FFT provides an effective means for detecting and recognizing the presence of one or more ELT signals over this 25-kHz frequency band. Some recommendations are made to improve the spectral characteristics and the performance of the ELT     

Improved tracking with mode-S data-linked velocity measurements

The use of downlinked airspeed and magnetic heading data to enhance tracking in mode-S equipped air traffic control (ATC) systems is examined. A tracker performing satisfactorily during straight line flight as well as during steep maneuvers is discussed. The filter copes easily with longitudinally accelerating targets and is suitable for tracking low-velocity targets like helicopters in all phases of flight. The filter assumes that the target flies in a circular path from sample to sample, which results in nonlinear system equations. The filter is suitable for implementation in three-dimensional tracking systems, particularly on the vertical axis, where target velocities are usually small     

Cosmic X-ray observations performed with a gas scintillation spectrometer

Observations of the spectrum of CAS-A in the X-ray energy range 3 to 10 keV have been made using two gas scintillation proportional counters flown on an Aries rocket. Analysis of the data from the 180 second observation has resolved the emission features at 7 keV, which results from transitions in highly ionised iron.     

The active sun

A brief summary is given of observations which will be required to investigate further the structure and energy balance of active regions.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.     

The Goddard Range and Range Rate Tracking System: Concept,design and performance

This paper is concerned with the concept, evolution and performance analysis of a high-accuracy, near-earth and cislunar cooperative target tracking system, known as the Goddard Range and Range Rate System.This system combines the advantages of harmonic (or sidetone) and pseudo-random coded ranging signals in a highly effective and versatile manner, operable either as an all-harmonic system in near-earth orbital tracking or as a hybrid system for tracking more distant spacecraft. The system also combines the utilization of the two types of signals in a very attractive technique for speeding up the process of acquiring the ambiguity resolving code component in tracking spacecraft at cislunar and translunar distances.The theoretical analysis of system performance and errors is followed by a summary of performance data gathered to date by operating GRARR systems on a number of NASA missions.     

Observations of the profiles of solar UV emission lines and their analysis in terms of the heating and production of the corona

Observations of the solar spectrum have been made between 1200–2200 with high spectral resolution. The results were obtained with an all-reflecting echelle spectrograph carried by a stabilized Skylark rocket launched in April 1970. Measurements of the profiles of a number of emission lines due to Siii, Cii, Siiii and Civ formed in the temperature range 10⁴-10⁵ K, indicate ion energies which are considerably in excess of the electron temperatures derived from the ionization balance. Since the ion/electron relaxation time is very short the observed ion energies cannot correspond to an ion temperature and hence a non-thermal mechanical energy component exists in the transition zone.It is postulated that the non-thermal energy component represents the actual mechanical energy responsible for the heating of the corona, and, that, it is propagated as an acoustic wave. On this basis and with a preliminary estimate of the reflection from the transition zone, a flux of 3 × 10⁵ erg cm ^-2 s ^-1 is established as entering the corona. This value is in agreement with estimates of the total energy loss from the corona due to conduction, radiation and the solar wind, thus establishing a gross energy balance.Theoretical calculations are currently underway to establish the physical nature of the atmosphere which would result from such a propagating flux. At the present time this has been carried out for an atmosphere in hydrostatic equilibrium and the energy balance equation solved. A preliminary temperature structure which results is shown in Figure 1, together with the derived distribution in electron density. This gives a corona of the right temperature and density but the observed structure deviates in detail from those derived from an analysis of the solar XUV spectrum.     

Evolution of coronal mass ejection/shock system in interplanetary space

The evolution of coronal mass ejection/shock system is investigated by numerically solving the usual set of two-dimensional single-fluid polytropic magnetohydrodynamic equations from 1 R_s to 1 AU in the meridian plane. The simulation result reveals that the coronal mass ejection/shock system formed near the sun evolves into the magnetic cloud/shock system near the earth’s orbit through the following three phases: the initial formation, the dominant latitudinal expansion and the similar expansion.