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The development of a low gain Electronically Switchable Spherical Array Antenna is discussed. This antenna provides roughly 7 dBic gain for receive/transmit operation between user satellites and the Tracking and Data Relay Satellite System. When used as a pair, the antenna provides spherical coverage. The antenna was tested in its primary operating modes: directed beam, retrodirective, and Omnidirectional.
Kennedy Space Center (KSC) is situated in an area that experiences one of the world's highest rates of cloud-ground lightning strikes, about 600-2000 strikes per summer. Two lightning detection systems have been implemented, the Launch Pad Lightning Warning System (LPLWS) and the Lightning Location and Protection system (LLP). The LPLWS consists of field mills of eight vertically oriented stator sections mounted 10 in. above ground and alternately covered and uncovered as the rotor turns. Differential voltages between covered and uncovered sections furnish electric field amplitude and polarity data. Ten samples per second are telemetered to a central processing facility. The system is used during launch and landing. The LLP has high and low gain components, the former being two direction finder antennas with 100 m strike position finding accuracy, the latter featuring medium gain antennas for 500 m accuracy in locating strikes. The LLP system is used primarily to warn personnel of strike conditions and to lift warnings to avoid lost work time. Several experimental programs have been initiated for triggering lightning strikes and controlling their locations.
Antenna designs applicable to future satellite mobile vehicle communications are examined. Microstrip disk, quadrifilar helix, cylindrical microstrip, and inverted V and U crossed-dipole low gain antennas (3-5 dBic) that provide omnidirectional coverage are described. Diagrams of medium gain antenna (9-12 dBic) concepts are presented; the antennas are classified into three types: (1) electronically steered with digital phase shifters; (2) electronically switched with switchable power divider/combiner; and (3) mechanically steered with motor. The operating characteristics of a conformal antenna with electronic beam steering and a nonconformal design with mechanical steering are evaluated with respect to isolation levels in a multiple satellite system. Vehicle antenna pointing systems and antenna system costs are investigated.
A high gain, low noise corrugated feedhorn was designed and developed by JPL for use in Very Large Array (VLA) antennas, near Soccoro, New Mexico. The new feedhorn will enable the VLA to support the Voyager encounter of Neptune in August of 1989. This will significantly enhance the receiving capability of the United States for that historic event.
Operationally acquired Thematic Mapper and experimental MOMS-01 data are evaluated quantitatively concerning the systems spectral response and performance for geoscientific applications. Results show the two instruments to be similar in the spectral bands compared. Although the MOMS scanner has a smaller IFOV, it has a lower modulation transfer function performance for small, low contrast features as compared to Thematic Mapper. This deficiency does not only occur when MOMS was switched to the low gain mode. It is due to the CD arrays used (ITEK CCPD 1728).
This contribution reports on various aspects of the test and implementation of the new worldwide aeronautical satellite system. The system will be implemented in 1990 following several years of system design and equipment development. The full range of services and equipment will not be available immediately, but rather there will be a progressive phase-in over the first two years of service. Data services using low gain aircraft antennas are planned to be introduced first. Voice service, requiring high gain antennas is expected to begin on a full scale basis about May/June 1990. This contribution explains Inmarsat's approach to testing and authorization of equipment intended to make use of its satellites in this evolving situation. The approach is designed to permit progressive implementation of services while avoiding the need for large scale retrofitting of equipment which has already been introduced into service.
Stochastic robustness analysis guides the synthesis of robust linear quadratic Gaussian (LQG) regulators for a benchmark control problem. Probabilities of exceeding allowable design limits, including stability, setting time, and control usage, are estimated by Monte Carlo evaluation. Robust, low-gain compensators that fulfill objectives are designed by numerically minimizing quadratic functions of these probabilities. The method is straightforward and makes use of uncomplicated design principles.
We report on the development of a new high-dynamic-range two-stage Multi-Anode Microchannel Array (MAMA) imaging tube designed for improved high count rate performance at FUV and EUV wavelengths. The new two-stage MAMA tube employs two 25-mm-diameter format MCPs placed in tandem with a small gap between the plates. The front (input) MCP is designed to be a low-gain converter plate that supports an opaque photocathode and converts the detected photons to electrons, while the second (output) MCP is of higher conductivity and thus maintains the overall gain of the multiplier at high count rates. The second MCP is mounted in proximity focus with a (224 x 960)-pixel fine-fine coincidence MAMA array for high-spatial-resolution imaging studies. The applied voltage across each MCP can be controlled independently. We report on the gain and dynamic range performance characteristics of the two-stage MAMA tube in two different configurations: first, with the output MCP having moderate conductivity (about 100 MOhm); and second, with the output MCP having very high conductivity (about 2 MOhm). These results are compared and contrasted with those of the more conventional MAMA tube configuration which employs a single high-gain curved-channel MCP.
In February 1990, ESA began reactivation of the Giotto spacecraft, which had been dormant for the previous four years, via the omnidirectional low-gain antenna mounted atop the Giotto. By the end of April 1990, spacecraft temperatures had been reduced to levels at which the experiments could be activated; of the original complement of ten experiment modules, the magnetometer, energetic particle analyzer, and optical probe experiment were fully functional, and an additional three instruments were partly damaged but usable. A mission that targetted Comet Grigg-Skjellerup 214 million km from the earth was then put into effect.
The study reviews the research and development of a prototype laser used to study one possible method of short-pulse production and amplification, in particular, a pulsed Nd:YAG ring laser pumped by laser diode arrays and injected seeded by a 100-ps source. The diode array pumped, regenerative amplifier consists of only five optical elements, two mirrors, one thin film polarizer, one Nd:YAG crystal, and one pockels cell. The pockels cell performed both as a Q-switch and a cavity dumper for amplified pulse ejection through the thin film polarizer. The total optical efficiency was low principally due to the low gain provided by the 2-bar pumped laser head. After comparison with a computer model, a real seed threshold of about 10 exp -15 J was achieved because only about 0.1 percent of the injected energy mode-matched with the ring.
A coding system under consideration for the Galileo S-band low-gain antenna mission is a concatenated system using a variable redundancy Reed-Solomon outer code and a (14,1/4) convolutional inner code. The 8-bit Reed-Solomon symbols are interleaved to depth 8, and the eight 255-symbol codewords in each interleaved block have redundancies 64, 20, 20, 20, 64, 20, 20, and 20, respectively (or equivalently, the codewords have 191, 235, 235, 235, 191, 235, 235, and 235 8-bit information symbols, respectively). This concatenated code is to be decoded by an enhanced decoder that utilizes a maximum likelihood (Viterbi) convolutional decoder; a Reed Solomon decoder capable of processing erasures; an algorithm for declaring erasures in undecoded codewords based on known erroneous symbols in neighboring decodable words; a second Viterbi decoding operation (redecoding) constrained to follow only paths consistent with the known symbols from previously decodable Reed-Solomon codewords; and a second Reed-Solomon decoding operation using the output from the Viterbi redecoder and additional erasure declarations to the extent possible. It is estimated that this code and decoder can achieve a decoded bit error rate of 1 x 10(exp 7) at a concatenated code signal-to-noise ratio of 0.76 dB. By comparison, a threshold of 1.17 dB is required for a baseline coding system consisting of the same (14,1/4) convolutional code, a (255,223) Reed-Solomon code with constant redundancy 32 also interleaved to depth 8, a one-pass Viterbi decoder, and a Reed Solomon decoder incapable of declaring or utilizing erasures. The relative gain of the enhanced system is thus 0.41 dB. It is predicted from analysis based on an assumption of infinite interleaving that the coding gain could be further improved by approximately 0.2 dB if four stages of Viterbi decoding and four levels of Reed-Solomon redundancy are permitted. Confirmation of this effect and specification of the optimum four-level redundancy profile for depth-8 interleaving is currently being done.
New and refined models of radio signal propagation phenomena are needed to support studies of evolving satellite services and systems. Taking an engineering perspective, applications for propagation measurements and models in the context of various types of analyses that are of ongoing interest are reviewed. Problems that were encountered in the signal propagation aspects of these analyses are reviewed, and potential solutions to these problems are discussed. The focus is on propagation measurements and models needed to support design and performance analyses of systems in the Mobile-Satellite Service (MSS) operating in the 1-3 GHz range. These systems may use geostationary or non-geostationary satellites and Frequency Division Multiple Access (FDMA), Time Division Multiple Access Digital (TDMA), or Code Division Multiple Access (CDMA) techniques. Many of the propagation issues raised in relation to MSS are also pertinent to other services such as broadcasting-satellite (sound) at 2310-2360 MHz. In particular, services involving mobile terminals or terminals with low gain antennas are of concern.
A third antenna was added to the system. It is a terrestrial low-gain feed, to act as a veto for local interference. The 3-chip design for a 4 megapoint complex FFT was reduced to finished working hardware. The 4-Megachannel circuit board contains 36 MByte of DRAM, 5 CPLDs, the three large FFT ASICs, and 74 ICs in all. The Austek FDP-based Spectrometer/Power Accumulator (SPA) has now been implemented as a 4-layer printed circuit. A PC interface board has been designed and together with its associated user interface and control software allows an IBM compatible computer to control the SPA board, and facilitates the transfer of spectra to the PC for display, processing, and storage. The Feature Recognizer Array cards receive the stream of modulus words from the 4M FFT cards, and forward a greatly thinned set of reports to the PC's in whose backplane they reside. In particular, a powerful ROM-based state-machine architecture has been adopted, and DRAM has been added to permit integration modes when tracking or reobserving source candidates. The general purpose (GP) array consists of twenty '486 PC class computers, each of which receives and processes the data from a feature extractor/correlator board set. The array performs a first analysis on the provided 'features' and then passes this information on to the workstation. The core workstation software is now written. That is, the communication channels between the user interface, the backend monitor program and the PC's have working software.
In this paper, the feasibility of using an L-band low gain antenna (LGA) as a mobile terminal antenna for helicopters is described. The objective is to select the lowest cost antenna system which can be easily mounted on a helicopter and capable of communicating with a geosynchronous satellite. To ensure that all the antenna options are being considered, the steerable high gain reflector and medium gain array antennas as well as LGA are studied and compared in an exhaustive survey. The high gain reflector antenna in L-band is usually very large in size and heavy in weight. In addition, a bulky and expensive tracking system is needed to steer the antenna beam to the satellite direction. The medium gain antennas (including mechanically and electronically steered arrays) are also more expensive and less reliable than an LGA due to the addition of a beam steering system to track the satellite. The omni-directional LGA is simple, reliable, and inexpensive. It is typically ten times smaller than the medium gain antenna. This makes the position, selection, and mounting on the helicopter relatively easier. Therefore, the LGA is selected as a mobile terminal antenna for helicopters. Among the many LGA's (cross-dipole, helix, spiral, and slot antennas), the helix antenna is the most inexpensive. One can also change the size, shape, or pitch angle of the helix to optimize the gain in the desired direction. Therefore, the helix antenna is selected for further study. Both 2-arm and 4-arm helices are studied theoretically and experimentally to determine the antenna's performance and the scattering effects from the helicopter body and the blades. The multipath, Doppler, and Doppler rate issues as well as the periodic fading effects caused by the helicopter rotor blades will be briefly discussed in the paper.
The Galileo low-gain antenna mission has a severely rate-constrained channel over which we wish to send large amounts of information. Because of this link pressure, compression techniques for image and other data are being selected. The compression technique that will be used for images is the integer cosine transform (ICT). This article investigates the compression performance of Galileo's ICT algorithm as applied to Galileo images taken during the early portion of the mission and to images that simulate those expected from the encounter at Jupiter.
The Galileo Jupiter orbital mission using the Low Gain Antenna (LGA) requires a higher degree of spacecraft state knowledge than was originally anticipated. Key elements of the revised design include onboard buffering of science and engineering data and extensive processing of data prior to downlink. In order to prevent loss of data resulting from overflow of the buffers and to allow efficient use of the spacecraft resources, ground based models of the spacecraft processes will be implemented. These models will be integral tools in the development of satellite encounter sequences and the cruise/playback sequences where recorded data is retrieved.
The Wide Field and Planetary Camera 2 (WFPC2) on Hubble Space Telescope (HST) is providing unsurpassed imaging capabilities and supporting accurate stellar photometry over large fields of view at high angular resolution. I discuss a feature of the WFPC2 CCD systems that nominally limits the dynamic range attainable with single exposures: a 12 bit analog to digital converter that does not allow sampling of the CCD full-well depth even at the low gain (14e(sup -)/DN) setting. I demonstrate that accurate stellar photometry can be performed on stellar images that are strongly saturated. Two 40 s exposures in V and I bands on the old open cluster M67 are analyzed to demonstrate photometric capabilities with a dynamic range of over 12 mag from single exposures. New photometric results for both bright and faint objects in M67 are derived from the WFPC2 data.
The design of the transformer designated as T101 (061-0351) in the Filament/Bias module (061-0119) in the Planet-B NMS instrument was verified because of the differences from the GCMS and INMS instrument designs. A breadboard of a representation of the Hybrid 2301065, Bias Drive A driving a 2N3700 NPN transistor, with dual 75V secondaries, with loads, was used to test the circuit. The initial transformer design that was wound with bifilar secondaries was too unstable to test. The second 1408 transformer with a split bobbin and the feedback winding below the primary was also found to be unstable. (It was nearly impossible to keep the circuit from squeeging). The third transformer tested has the feedback on the outside of the resonant winding. The primary goal of the design was to have as tight a magnetic coupling as possible to the resonant winding, and as loose a coupling as possible to the primary. Further, the circuit AC ground is connected to the winding at the feedback end of the secondary winding. This transformer proved to be very stable - it is virtually impossible to make this design squeg. An emitter resistor (Rl29A) was added to this circuit, as referenced to the GCMS design, to protect Q102 from thermal runaway in the event of a turn on with a non- resonate circuit or load short. This was verified to protect Q102 for at least 30 seconds in the event of a short. Approximately 1% of the 4lmW input power is lost in this protection resistor under normal operation. The circuit was verified to operate normally when a radiated Q102 (2N3700), (low Beta) transistor was substituted for the normal 2N3700. It should be noted that the monitored drive voltage went to approximately 2.7V with this low gain transistor.