10,660 research outputs found
Design, modeling, and analysis of multi-channel demultiplexer/demodulator
Traditionally, satellites have performed the function of a simple repeater. Newer data distribution satellite architectures, however, require demodulation of many frequency division multiplexed uplink channels by a single demultiplexer/demodulator unit, baseband processing and routing of individual voice/data circuits, and remodulation into time division multiplexed (TDM) downlink carriers. The TRW MCDD (Multichannel Demultiplexer/Multirate Demodulator) operates on a 37.4 MHz composite input signal. Individual channel data rates are either 64 Kbps or 2.048 Mbps. The wideband demultiplexer divides the input signal into 1.44 MHz segments containing either a single 2.048 Mbps channel or thirty two 64 Kbps channels. In the latter case, the narrowband demultiplexer further divides the single 1.44 MHz wideband channel into thirty two 45 KHz narrowband channels. With this approach the time domain Fast Fourier Transformation (FFT) channelizer processing capacity is matched well to the bandwidth and number of channels to be demultiplexed. By using a multirate demodulator fewer demodulators are required while achieving greater flexibility. Each demodulator can process a wideband channel or thirty two narrowband channels. Either all wideband channels, a mixture of wideband and narrowband channels, or all narrowband channels can be demodulated. The multirate demodulator approach also has lower nonrecurring costs since only one design and development effort is needed. TRW has developed a proof of concept (POC) model which fully demonstrates the signal processing fuctions of MCDD. It is capable of processing either three 2.048 Mbps channels or two 2.048 Mbps channels and thirty two 64 Kbps channels. An overview of important MCDD system engineering issues is presented as well as discussion on some of the Block Oriented System Simulation analyses performed for design verification and selection of operational parameters of the POC model. Systems engineering analysis of the POC model confirmed that the MCDD concepts are not only achievable but also balance the joint goals of minimizing on-board complexity and cost of ground equipment, while retaining the flexibility needed to meet a wide range of system requirements
Performance of a family of omni and steered antennas for mobile satellite applications
The design and performance of a family of vehicle antennas developed at JPL in support of an emerging US Mobile Satellite Service (MSS) system are described. Test results of the antennas are presented. Trends for future development are addressed. Recommendations on design approaches for vehicle antennas of the first generation MSS are discussed
Microstructure, Mechanical Property and Biocompatibility of Porous Ti-Nb-Zr Alloys Fabricated by Rapid Sintering using Space Holder
Space holder method can easily control Young’s modulus due to control the pore size, distribution and shape. In this study, porous Ti-Nb-Zr biomaterial which is not included poison elements was successfully fabricated by powder metallurgy using space holder of NH4HCO3 and foaming agent of TiH2. The consolidation of powder was conducted by spark plasma sintering process (SPS) at 850 °C under 30MPa conditions. The effect of space holder contents on pore size and distribution of Ti-Nb-Zr alloys was observed by optical microscope (OM) and scanning electron microscope (SEM). As a result of microstructure observation, a lot of pore was uniformly distributed in the sintered Ti-Nb-Zr alloys. Cell cultivation experiments were conducted using cell cultivation experimental. The porous Ti-Nb-Zr alloys were fabricated successfully with 30% pore ratio and 50-60GPa of Young’s modulus. Biocompatibility of porous Ti-Nb-Zr alloys is similar to Ti-6Al-4V alloy
Generative Modeling of Residuals for Real-Time Risk-Sensitive Safety with Discrete-Time Control Barrier Functions
A key source of brittleness for robotic systems is the presence of model
uncertainty and external disturbances. Most existing approaches to robust
control either seek to bound the worst-case disturbance (which results in
conservative behavior), or to learn a deterministic dynamics model (which is
unable to capture uncertain dynamics or disturbances). This work proposes a
different approach: training a state-conditioned generative model to represent
the distribution of error residuals between the nominal dynamics and the actual
system. In particular we introduce the Online Risk-Informed Optimization
controller (ORIO), which uses Discrete-Time Control Barrier Functions, combined
with a learned, generative disturbance model, to ensure the safety of the
system up to some level of risk. We demonstrate our approach in both
simulations and hardware, and show our method can learn a disturbance model
that is accurate enough to enable risk-sensitive control of a quadrotor flying
aggressively with an unmodelled slung load. We use a conditional variational
autoencoder (CVAE) to learn a state-conditioned dynamics residual distribution,
and find that the resulting probabilistic safety controller, which can be run
at 100Hz on an embedded computer, exhibits less conservative behavior while
retaining theoretical safety properties.Comment: 9 pages, 6 figures, submitted to the 2024 IEEE International
Conference on Robotics and Automation (ICRA 2024
Ground-state electric quadrupole moment of 31Al
Ground-state electric quadrupole moment of 31Al (I =5/2+, T_1/2 = 644(25) ms)
has been measured by means of the beta-NMR spectroscopy using a spin-polarized
31Al beam produced in the projectile fragmentation reaction. The obtained Q
moment, |Q_exp(31Al)| = 112(32)emb, are in agreement with conventional shell
model calculations within the sd valence space. Previous result on the magnetic
moment also supports the validity of the sd model in this isotope, and thus it
is concluded that 31Al is located outside of the island of inversion.Comment: 5 page
On A two-variable p-adic l_q function
We prove that a two-variable p-adic l_q-function has the series p-adic
expansion which interpolates a linear combinations of terms of the generalized
q-Euler polynomials at non positive integers. The proof of this original
construction is due to Kubota and Leopoldt in 1964, although the method given
this note is due to WashingtonComment: 11S8
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