Diffusion analysis of track loss in clutter

In tracking a target through clutter, the selection of incorrect measurements for track updating causes track divergence and eventual loss of track. The plot-to-track association algorithm is modeled as a Markov process and the tracking error is modeled as a diffusion process in order to study the mechanism of track loss analytically, without recourse to Monte Carlo simulations, for nearest-neighbor association in two space dimensions. The time evolution of the error distribution is examined, and the connection of the approach with diffusion theory is discussed. Explicit results showing the dependence of various performance parameters, such as mean time to lose track and track half-life, on the clutter spatial density are presented. The results indicate the existence of a critical density region in which the tracking performance degrades rapidly with increasing clutter density. An optimal gain adaptation procedure that significantly improves the tracking performance in the critical region is proposed     

Worldwide direct audio broadcasting from space : Promoting ‘Glasnost’ with DBS-A

About 100 countries now pursue their ‘public diplomacy’ interests by broadcasting audio programmes via shortwave to 400 000 000 radio listeners. However signals broadcast by surface-based shortwave transmitters are limited in coverage, reliability and quality, and shortwave broadcasting suffers from increasingly difficult operating and political problems. Space technology could now begin to be employed to create common-carrier, common-user (Intelsat- and Inmarsat-like) space-based systems which could provide a truly excellent, low-cost, direct-broadcast audio service throughout the world. The world's governments could thereby use space in a new way to promote ‘Glasnost-Openness’ for ‘the benefit of all mankind’.     

Effects of exercise equipment on the microgravity environment.

Numerous types of exercise equipment have flown on manned space flights to evaluate and maintain crew members' physical condition while on orbit. Vibrations associated with the use of some exercise equipment cause concern among microgravity scientists who are usually looking for a quiescent environment in which to run their experiments. We discuss the impact of aerobic (bicycle ergometer, treadmill) and non-aerobic (resistance devices) exercise on the microgravity environment of the Space Shuttle Orbiters and the Mir Space Station. In general, characteristic vibration disturbances due to ergometer exercise show the pedalling frequency at 2.5 to 3 Hz and the crew members' body rocking side-to-side at about half the pedalling frequency. For treadmill exercise, the footfall frequency on the treadmill platform can be clearly seen in the 1 to 2 Hz range, along with upper harmonics. The use of resistance exercise devices does not typically cause vibrations. Several vibration isolation systems used on the Orbiters and planned for the International Space Station are introduced. Finally, the responses of specific experiments to exercise vibrations are outlined.     

Distribution of general noncentral positive definite quadratic formin K-dimensions

The authors present a series solution using Hermite polynomials to the long-standing problem of computing the probability P that positive definite noncentral quadratic form d(x) of a Gaussian random vector x∈R satisfies d( x)⩽r² for any given r∈R. This problem has wide applications in radar, tracking, air traffic control, etc. The fast-converging series solution presented is very accurate and can be performed rapidly using the recursion relations for Hermite polynomials     

Shock Tube Instrumentation Techniques for Study of Hypervelocity Entry Problems

Shock tube instrumentation techniques developed for the study of convective heat transfer and the radiative properties of high temperature gases at conditions simulating hypervelocity entry into planetary atmospheres are described. An electrically heated helium driven shock tube is used; measurement methods suitable for conventional shock tubes must be modified before they can be used in the hypervelocity shock tube. Extremely high shock velocities necessary for proper simulation are associated with very short test times imposing requirements for fast response instrumentation systems. Methods for evaluating test gas quality are illustrated. Techniques described have been applied to several studies; typical results are shown and instrumentation requirements for the extension of aerothermodynamic investigation in the hypervelocity regime are outlined.     

The New Horizons Spacecraft

The New Horizons spacecraft was launched on 19 January 2006. The spacecraft was designed to provide a platform for seven instruments designated by the science team to collect and return data from Pluto in 2015. The design meets the requirements established by the National Aeronautics and Space Administration (NASA) Announcement of Opportunity AO-OSS-01. The design drew on heritage from previous missions developed at The Johns Hopkins University Applied Physics Laboratory (APL) and other missions such as Ulysses. The trajectory design imposed constraints on mass and structural strength to meet the high launch acceleration consistent with meeting the AO requirement of returning data prior to the year 2020. The spacecraft subsystems were designed to meet tight resource allocations (mass and power) yet provide the necessary control and data handling finesse to support data collection and return when the one-way light time during the Pluto fly-by is 4.5 hours. Missions to the outer regions of the solar system (where the solar irradiance is 1/1000 of the level near the Earth) require a radioisotope thermoelectric generator (RTG) to supply electrical power. One RTG was available for use by New Horizons. To accommodate this constraint, the spacecraft electronics were designed to operate on approximately 200 W. The travel time to Pluto put additional demands on system reliability. Only after a flight time of approximately 10 years would the desired data be collected and returned to Earth. This represents the longest flight duration prior to the return of primary science data for any mission by NASA. The spacecraft system architecture provides sufficient redundancy to meet this requirement with a probability of mission success of greater than 0.85. The spacecraft is now on its way to Pluto, with an arrival date of 14 July 2015. Initial in-flight tests have verified that the spacecraft will meet the design requirements.     

On analytical evaluation of glint error reduction for frequency-hopping radars

Analytical techniques are presented for evaluating the glint error reduction of frequency-agile tracking radars employing discrete frequency-hopping (FH). The techniques can be used to compare the tracking accuracies obtained with different procedures for frequency selection. The cases of random, cyclic, and hybrid FH are worked out in detail. An illustration using a simple alpha-beta tracking filter indicates that the preferred type of frequency selection usually depends on the filter gains used in a specific application.<>     

Continuous-time ECV and ECA track filters with colored measurementnoise

Analytical solutions are presented for two continuous-time track filters with correlation in both the measurement errors and the target maneuvers. The measurement error model is a first-order Markov process, characterized by a correlation time (γ) and a position error variance (r). The target motion is described by one of two dynamic models: the exponentially correlated velocity (ECV) model, and the exponentially correlated acceleration (ECA) model. For both models, the steady-state velocity variance exhibits resonant behavior. The analytical solutions include as special cases several previously published results     

Tracking targets with constant heading and variable speed

Two estimation procedures for tracking targets moving with variable speed in a fixed direction are presented and shown to be maximum likelihood estimators under differing sets of assumptions. Monte Carlo simulations are used to compare the accuracy of each method in a realistic tracking scenario. For miss-distance and heading estimation, it is often more accurate to make an approximate choice of speed profile than to make no choice at all