6,247 research outputs found
The Palomar Testbed Interferometer
The Palomar Testbed Interferometer (PTI) is a long-baseline infrared
interferometer located at Palomar Observatory, California. It was built as a
testbed for interferometric techniques applicable to the Keck Interferometer.
First fringes were obtained in July 1995. PTI implements a dual-star
architecture, tracking two stars simultaneously for phase referencing and
narrow-angle astrometry. The three fixed 40-cm apertures can be combined
pair-wise to provide baselines to 110 m. The interferometer actively tracks the
white-light fringe using an array detector at 2.2 um and active delay lines
with a range of +/- 38 m. Laser metrology of the delay lines allows for servo
control, and laser metrology of the complete optical path enables narrow-angle
astrometric measurements. The instrument is highly automated, using a
multiprocessing computer system for instrument control and sequencing.Comment: ApJ in Press (Jan 99) Fig 1 available from
http://huey.jpl.nasa.gov/~bode/ptiPicture.html, revised duging copy edi
Getting expert systems off the ground: Lessons learned from integrating model-based diagnostics with prototype flight hardware
As an initial attempt to introduce expert system technology into an onboard environment, a model based diagnostic system using the TRW MARPLE software tool was integrated with prototype flight hardware and its corresponding control software. Because this experiment was designed primarily to test the effectiveness of the model based reasoning technique used, the expert system ran on a separate hardware platform, and interactions between the control software and the model based diagnostics were limited. While this project met its objective of showing that model based reasoning can effectively isolate failures in flight hardware, it also identified the need for an integrated development path for expert system and control software for onboard applications. In developing expert systems that are ready for flight, artificial intelligence techniques must be evaluated to determine whether they offer a real advantage onboard, identify which diagnostic functions should be performed by the expert systems and which are better left to the procedural software, and work closely with both the hardware and the software developers from the beginning of a project to produce a well designed and thoroughly integrated application
The PRIMA fringe sensor unit
The Fringe Sensor Unit (FSU) is the central element of the Phase Referenced
Imaging and Micro-arcsecond Astrometry (PRIMA) dual-feed facility and provides
fringe sensing for all observation modes, comprising off-axis fringe tracking,
phase referenced imaging, and high-accuracy narrow-angle astrometry. It is
installed at the Very Large Telescope Interferometer (VLTI) and successfully
servoed the fringe tracking loop during the initial commissioning phase. Unique
among interferometric beam combiners, the FSU uses spatial phase modulation in
bulk optics to retrieve real-time estimates of fringe phase after spatial
filtering. A R=20 spectrometer across the K-band makes the retrieval of the
group delay signal possible. The FSU was integrated and aligned at the VLTI in
summer 2008. It yields phase and group delay measurements at sampling rates up
to 2 kHz, which are used to drive the fringe tracking control loop. During the
first commissioning runs, the FSU was used to track the fringes of stars with
K-band magnitudes as faint as m_K=9.0, using two VLTI Auxiliary Telescopes (AT)
and baselines of up to 96 m. Fringe tracking using two Very Large Telescope
(VLT) Unit Telescopes (UT) was demonstrated. During initial commissioning and
combining stellar light with two ATs, the FSU showed its ability to improve the
VLTI sensitivity in K-band by more than one magnitude towards fainter objects,
which is of fundamental importance to achieve the scientific objectives of
PRIMA.Comment: 19 pages, 23 figures. minor changes and language editing. this
version equals the published articl
Workshop on Advanced Technologies for Planetary Instruments, part 1
This meeting was conceived in response to new challenges facing NASA's robotic solar system exploration program. This volume contains papers presented at the Workshop on Advanced Technologies for Planetary Instruments on 28-30 Apr. 1993. This meeting was conceived in response to new challenges facing NASA's robotic solar system exploration program. Over the past several years, SDIO has sponsored a significant technology development program aimed, in part, at the production of instruments with these characteristics. This workshop provided an opportunity for specialists from the planetary science and DoD communities to establish contacts, to explore common technical ground in an open forum, and more specifically, to discuss the applicability of SDIO's technology base to planetary science instruments
Taming and Leveraging Directionality and Blockage in Millimeter Wave Communications
To cope with the challenge for high-rate data transmission, Millimeter Wave(mmWave) is one potential solution. The short wavelength unlatched the era of directional mobile communication. The semi-optical communication requires revolutionary thinking. To assist the research and evaluate various algorithms, we build a motion-sensitive mmWave testbed with two degrees of freedom for environmental sensing and general wireless communication.The first part of this thesis contains two approaches to maintain the connection in mmWave mobile communication. The first one seeks to solve the beam tracking problem using motion sensor within the mobile device. A tracking algorithm is given and integrated into the tracking protocol. Detailed experiments and numerical simulations compared several compensation schemes with optical benchmark and demonstrated the efficiency of overhead reduction. The second strategy attempts to mitigate intermittent connections during roaming is multi-connectivity. Taking advantage of properties of rateless erasure code, a fountain code type multi-connectivity mechanism is proposed to increase the link reliability with simplified backhaul mechanism. The simulation demonstrates the efficiency and robustness of our system design with a multi-link channel record.The second topic in this thesis explores various techniques in blockage mitigation. A fast hear-beat like channel with heavy blockage loss is identified in the mmWave Unmanned Aerial Vehicle (UAV) communication experiment due to the propeller blockage. These blockage patterns are detected through Holm\u27s procedure as a problem of multi-time series edge detection. To reduce the blockage effect, an adaptive modulation and coding scheme is designed. The simulation results show that it could greatly improve the throughput given appropriately predicted patterns. The last but not the least, the blockage of directional communication also appears as a blessing because the geometrical information and blockage event of ancillary signal paths can be utilized to predict the blockage timing for the current transmission path. A geometrical model and prediction algorithm are derived to resolve the blockage time and initiate active handovers. An experiment provides solid proof of multi-paths properties and the numeral simulation demonstrates the efficiency of the proposed algorithm
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