NASA''s small spacecraft technology initiative “Clark” spacecraft

The Small Satellite Technology Initiative (SSTI) is a National Aeronautics and Space Administration (NASA) program to demonstrate smaller, high technology satellites constructed rapidly and less expensively. Under SSTI, NASA funded the development of “Clark,” a high technology demonstration satellite to provide 3-m resolution panchromatic and 15-m resolution multispectral images, as well as collect atmospheric constituent and cosmic x-ray data. The 690-Ib. satellite, to be launched in early 1997, will be in a 476 km, circular, sun-synchronous polar orbit. This paper describes the program objectives, the technical characteristics of the sensors and satellite, image processing, archiving and distribution. Data archiving and distribution will be performed by NASA Stennis Space Center and by the EROS Data Center, Sioux Falls, South Dakota, USA.     

Receiver antenna scan rate requirements needed to implement pulsechasing in a bistatic radar receiver

Pulse chasing is a technique implemented by a bistatic or multistatic radar system that allows rapid and efficient search of a desired volume of space whereby the receiving antenna is made to follow or “chase” the transmitted pulse as it travels radially outward from the transmitter antenna. An expression for receiver antenna scan rate requirements is derived that corrects an error in the prior literature. The results give significantly reduced scan rates in the forward scatter region near the baseline showing that pulse chasing is more easily implemented using conventional analog beamformer phased array technology than was suggested by prior work     

Space Shuttle UHF communications performance evaluation

Computational investigations and experimental measurements were performed to evaluate the Space Shuttle UHF communication system performance for payload bay antenna at the proposed new location. To insure adequate communication coverage at relocated new location, the link margin for the Extravehicular Activity (EVA) astronauts and between Space Shuttle Orbiter and International Space Station (SSO-ISS) during rendezvous were analyzed. The multipath effects from the vehicle structures surrounding the antenna were investigated. The Radio Frequency (RF) electromagnetic radiation to the Orbiter Docking System (ODS) pyrotechnics was also analyzed to ensure the EMC/EMI compliances.     

Covariance Matrix for ?-?-? Filtering

Closed-form expressions are derived for the output noise response of an ?-?-? filter in steady-state operation with constant measurement noise statistics. To the author's knowledge such expressions have been unavailable until now. These formulas are useful for target tracking applications.     

Time-dependent thermal X-ray afterglows from GRBS

Time-dependent thermal X-ray spectra are calculated from physically plausible conditions around GRB. It is shown that account for time-dependent ionization processes strongly affects the observed spectra of hot rarefied plasma. These calculations may provide an alternative explanation to the observed X-ray lines of early GRBs afterglows (such as GRB 011211). Our technique will allow one to obtain independent constraints on the GRB collimation angle and on the clumpiness of circumstellar matter.     

Distance estimation at 60 GHz

An FM-CW radar front-end was fabricated in an integrated manner at 60 GHz by using the NRD guide. Main emphasis was placed on compactness in size and high-precise operation in performance. The fabricated radar consists of an FM Gunn oscillator, a balanced mixer, and a planar antenna fed by leaky NRD guide with a mechanically beam-scanning performance. All circuit components and the antenna were contained in a compact housing of 170 /spl times/ 140 mm in area and 25 mm in thickness, and thus, a thin type of millimeter-wave radar front-end was successfully developed. Moreover, an error of distance estimation was measured to be less than a distance of 0.7 m.     

Operational testing of valve regulated lead acid batteries incommercial aircraft

Valve regulated lead acid (VRLA) batteries provide electrical performance that is virtually identical to sintered plate nickel-cadmium battery systems. In addition, the VRLA batteries offer the user a no-maintenance battery and other enhanced features that make this a very desirable battery for aircraft applications. In field trials, where VRLA batteries were substituted for nickel-cadmium batteries, the VRLA provided the user with a high reliability turbine engine starting battery under a wide variety of climatic conditions     

Relationship between daily variation of cosmogenic nuclide Be-7 concentration in atmosphere and solar activities

Daily Be-7 concentrations in air at the height of 15 m are continuously observed at 38°15.2′N, 140°20.9′E, between 2000 and 2001. The average concentration and the relative standard deviation were 4.0 mBq/m³ and 50% in 2000–2001, respectively. The Be-7 concentrations increased 2.5% with the decrease in the sunspot numbers by 6.7% for the term of two years. From the power spectral analysis, the periodicity of 26 days is shown for the daily Be-7 concentrations. The folding analysis indicates that the time variation of the Be-7 concentration is similar to that of the ground-based neutron counting rate, and the phase delay for the minimum portion of Be-7 concentration was roughly 8 days to the maximum sunspot number. These results indicate that the Be-7 concentrations in the air at ground level have 26 day periodicity as a component of time variations and the time variation is caused by the solar modulation of galactic cosmic rays, which corresponds to the variation of the sunspot number due to the rotation of the sun.     

Generating requirements for complex embedded systems using State Analysis

It has become clear that spacecraft system complexity is reaching a threshold where customary methods of control are no longer affordable or sufficiently reliable. At the heart of this problem are the conventional approaches to systems and software engineering based on subsystem-level functional decomposition, which fail to scale in the tangled web of interactions typically encountered in complex spacecraft designs. Furthermore, there is a fundamental gap between the requirements on software specified by systems engineers and the implementation of these requirements by software engineers. Software engineers must perform the translation of requirements into software code, hoping to accurately capture the systems engineer's understanding of the system behavior, which is not always explicitly specified. This gap opens up the possibility for misinterpretation of the systems engineer's intent, potentially leading to software errors. This problem is addressed by a systems engineering methodology called State Analysis, which provides a process for capturing system and software requirements in the form of explicit models. This paper describes how requirements for complex aerospace systems can be developed using State Analysis, using representative spacecraft examples.     

Comments on the Communication and Data Problems Associated with a Mars Trip During a Conjunction Phase

The analysis and comments presented in this paper are meant to establish the general communication parameters associated with Martian flyby probes and with lander and manned vehicles. Fundamental data transfer problems are reviewed to define comparisons and trends of tradeoffs for future studies. Selected focal points are based upon the long propagation path length, with inherent time delays, and the high noise produced by the sun. These problems are magnified because large quantities of data must be obtained to satisfy the needs of the scientific community and the curiosity of an interested public. A comparison of two communication systems is provided: the microwave spectrum and the optical spectrum, as represented by the microwaves at 2.3 GHz and the laser at 6328 ?. A method of cost effectiveness or value received from space missions (a criterion of power input for data quantity received) is also presented.