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NASA NTRS2010

A Model-Based Probabilistic Inversion Framework for Wire Fault Detection Using TDR

Time-domain reflectometry (TDR) is one of the standard methods for diagnosing faults in electrical wiring and interconnect systems, with a long-standing history focused mainly on hardware development of both high-fidelity systems for laboratory use and portable hand-held devices for field deployment. While these devices can easily assess distance to hard faults such as sustained opens or shorts, their ability to assess subtle but important degradation such as chafing remains an open question. This paper presents a unified framework for TDR-based chafing fault detection in lossy coaxial cables by combining an S-parameter based forward modeling approach with a probabilistic (Bayesian) inference algorithm. Results are presented for the estimation of nominal and faulty cable parameters from laboratory data.

Schuet, Stefan R.↗

NASA NTRS2008

TDR Using Autocorrelation and Varying-Duration Pulses

In an alternative to a prior technique of time-domain-reflectometry (TDR) in which very short excitation pulses are used, the pulses have very short rise and fall times and the pulse duration is varied continuously between a minimum and a maximum value. In both the present and prior techniques, the basic idea is to (1) measure the times between the generation of excitation pulses and the reception of reflections of the pulses as indications of the locations of one or more defects along a cable and (2) measure the amplitudes of the reflections as indication of the magnitudes of the defects. In general, an excitation pulse has a duration T. Each leading and trailing edge of an excitation pulse generates a reflection from a defect, so that a unique pair of reflections is associated with each defect. In the present alternative technique, the processing of the measured reflection signal includes computation of the autocorrelation function R(tau) identical with fx(t)x(t-tau)dt where t is time, x(t) is the measured reflection signal at time t, and taus is the correlation interval. The integration is performed over a measurement time interval short enough to enable identification and location of a defect within the corresponding spatial interval along the cable. Typically, where there is a defect, R(tau) exhibits a negative peak having maximum magnitude for tau in the vicinity of T. This peak can be used as a means of identifying a leading-edge/trailing-edge reflection pair. For a given spatial interval, measurements are made and R(tau) computed, as described above, for pulse durations T ranging from the minimum to the maximum value. The advantage of doing this is that the effective signal-to-noise ratio may be significantly increased over that attainable by use of a fixed pulse duration T.

Lucena, Angel↗

NASA NTRS2000

Shape Measurement of Large Aerospace Structures Using Sensitivity Electrical TDR Distributed Strain Sensor

Electrical time domain reflectometry (ETDR) sensing technique can be best described as "closed-loop radar," where the information is derived from the reflections of a voltage pulse sent through a transmission medium. The ETDR sensing technique is a well-developed method and has been widely used to locate and evaluate discontinuities in long coaxial power transmission cables. The ETDR technique provides a true distributed sensing capability which can not only sense the distributed loading condition of the structure but also can pin-point the location of disturbance, such as the locations of stress concentration and structural damages. Proof-of-concept experiments have been conducted using photoelastic specimens with embedded commercial coaxial cables, i.e., RG85/U and RG174, to demonstrate the stress/strain sensing capability of ETDR sensors for structural health monitoring application. Although the test results showed that the ETDR sensor signals capture specimen deformation pattern both in bending and tension and indicate the location and type of crack damages of the photoelastic specimen; yet, the low signal-to-noise ratio of the sensor signal smears the details of the strain measurement that the ETDR signals can convey. A high-sensitivity ETDR coaxial strain sensor prototype newly developed at Clark Atlanta University will be presented. The construction of the prototype sensing cable as well as its electrical properties relevant to distributed strain sensing application will be shown in details. Test results of the sensitivity and tension responses of the ETDR signal of the prototype sensor will be presented and compared with those of commercial coaxial cables. Promising potentials of the ETDR distributed strain sensing method for shape measurement application of large aerospace structures will also be demonstrated using long slender beam with surface-bonded ETDR distributed strain sensor.

Lin, Mark W.↗

NASA NTRS1998

Southern Great Plains 1997 hydrology experiment: The spatial and temporal distribution of soil moisture within a quarter section pasture field

Understanding the spatial and temporal distribution of soil moisture near the soil surface is important to relate ground truth data to remotely sensed data using an electronically scanned thinned array radiometer (ESTAR). The research was conducted at the A-ARM EF site in the Little Washita Watershed in Chickasha Oklahoma. Soil moisture was measured on a 100 x 100-m grid on a quarter section (0.8 km by 0.8 km) size field where the DOE A-ARM SWATS is located. This site has several drainage channels and small ponds. The site is under four different land use practices, namely active pastureland, non-grazed pastureland covered with thick grass, forest area covered with trees, and a single residential area. Soil moisture was measured with a Time Domain Reflectometry (TDR) Delta-T 6-cm theta-probe and gravimetric soil moisture (GSM) technique for the top 6 cm of the soil depth. A fourth order polynomial equation was fitted to each probe calibration curve. The correlation between TDR and GSM measurement technique ranges from 0.81 to 0.91. Comparison of the spatial and temporal distribution of soil moisture measured by the TDR and GSM techniques showed very strong similarities. Such TDR probes can be used successfully to replace the GSM techniques to measure soil moisture content rapidly and accurately with site specific calibration.

Tsegaye, T.↗

NASA NTRS2004

Improved Method of Locating Defects in Wiring Insulation

An improved method of locating small breaches in insulation on electrical wires combines aspects of the prior dielectric withstand voltage (DWV) and time-domain reflectometry (TDR) methods. The method was invented to satisfy a need for reliably and quickly locating insulation defects in spacecraft, aircraft, ships, and other complex systems that contain large amounts of wiring, much of it enclosed in structures that make it difficult to inspect. In the DWV method, one applies a predetermined potential (usually 1.5 kV DC) to the wiring and notes whether the voltage causes any arcing between the wiring and ground. The DWV method does not provide an indication of the location of the defect (unless, in an exceptional case, the arc happens to be visible). In addition, if there is no electrically conductive component at ground potential within about 0.010 in. (approximately equal to 0.254 mm) of the wire at the location of an insulation defect, then the DWV method does not provide an indication of the defect. Moreover, one does not have the option to raise the potential in an effort to increase the detectability of such a defect because doing so can harm previously undamaged insulation. In the TDR method as practiced heretofore, one applies a pulse of electricity having an amplitude of less than 25 V to a wire and measures the round-trip travel time for the reflection of the pulse from a defect. The distance along the wire from the point of application of the pulse to the defect is then calculated as the product of half the round-trip travel time and the characteristic speed of a propagation of an electromagnetic signal in the wire. While the TDR method as practiced heretofore can be used to locate a short or open circuit, it does not ordinarily enable one to locate a small breach in insulation because the pulse voltage is too low to cause arcing and thus too low to induce an impedance discontinuity large enough to generate a measurable reflection. The present improved method overcomes the weaknesses of both the prior DWV and the prior TDR method.

Greulich, Owen R.↗

NASA NTRS1994

A PC based time domain reflectometer for space station cable fault isolation

Significant problems are faced by astronauts on orbit in the Space Station when trying to locate electrical faults in multi-segment avionics and communication cables. These problems necessitate the development of an automated portable device that will detect and locate cable faults using the pulse-echo technique known as Time Domain Reflectometry. A breadboard time domain reflectometer (TDR) circuit board was designed and developed at the NASA-JSC. The TDR board works in conjunction with a GRiD lap-top computer to automate the fault detection and isolation process. A software program was written to automatically display the nature and location of any possible faults. The breadboard system can isolate open circuit and short circuit faults within two feet in a typical space station cable configuration. Follow-on efforts planned for 1994 will produce a compact, portable prototype Space Station TDR capable of automated switching in multi-conductor cables for high fidelity evaluation. This device has many possible commercial applications, including commercial and military aircraft avionics, cable TV, telephone, communication, information and computer network systems. This paper describes the principle of time domain reflectometry and the methodology for on-orbit avionics utility distribution system repair, utilizing the newly developed device called the Space Station Time Domain Reflectometer (SSTDR).

Pham, Michael↗

NASA NTRS2002

NASA Hybrid Reflectometer Project

Time-domain and frequency-domain reflectometry have been used for about forty years to locate opens and shorts in cables. Interpretation of reflectometry data is as much art as science. Is there information in the data that is being missed? Can the reflectometers be improved to allow us to detect and locate defects in cables that are not outright shorts or opens? The Hybrid Reflectometer Project was begun this year at NASA Ames Research Center, initially to model wire physics, simulating time-domain reflectometry (TDR) signals in those models and validating the models against actual TDR data taken on testbed cables. Theoretical models of reflectometry in wires will give us an understanding of the merits and limits of these techniques and will guide the application of a proposed hybrid reflectometer with the aim of enhancing reflectometer sensitivity to the point that wire defects can be detected. We will point out efforts by some other researchers to apply wire physics models to the problem of defect detection in wires and we will describe our own initial efforts to create wire physics models and report on testbed validation of the TDR simulations.

Lynch, Dana↗

NASA NTRS2013

Technology Development Roadmap: A Technology Development Roadmap for a Future Gravitational Wave Mission

Humankind will detect the first gravitational wave (GW) signals from the Universe in the current decade using ground-based detectors. But the richest trove of astrophysical information lies at lower frequencies in the spectrum only accessible from space. Signals are expected from merging massive black holes throughout cosmic history, from compact stellar remnants orbiting central galactic engines from thousands of close contact binary systems in the Milky Way, and possibly from exotic sources, some not yet imagined. These signals carry essential information not available from electromagnetic observations, and which can be extracted with extraordinary accuracy. For 20 years, NASA, the European Space Agency (ESA), and an international research community have put considerable effort into developing concepts and technologies for a GW mission. Both the 2000 and 2010 decadal surveys endorsed the science and mission concept of the Laser Interferometer Space Antenna (LISA). A partnership of the two agencies defined and analyzed the concept for a decade. The agencies partnered on LISA Pathfinder (LPF), and ESA-led technology demonstration mission, now preparing for a 2015 launch. Extensive technology development has been carried out on the ground. Currently, the evolved Laser Interferometer Space Antenna (eLISA) concept, a LISA-like concept with only two measurement arms, is competing for ESA's L2 opportunity. NASA's Astrophysics Division seeks to be a junior partner if eLISA is selected. If eLISA is not selected, then a LISA-like mission will be a strong contender in the 2020 decadal survey. This Technology Development Roadmap (TDR) builds on the LISA concept development, the LPF technology development, and the U.S. and European ground-based technology development. The eLISA architecture and the architecture of the Mid-sized Space-based Gravitational-wave Observatory (SGO Mid)-a competitive design with three measurement arms from the recent design study for a NASA-led mission after 2020-both use the same technologies. Further, NASA participation in an ESA-led mission would likely augment the eLISA architecture with a third arm to become the SGO Mid architecture. For these reasons, this TDR for a future GW mission applies to both designs and both programmatic paths forward. It is adaptable to the different timelines and roles for an ESA-led or a NASA-led mission, and it is adaptable to available resources. Based on a mature understanding of the interaction between technology and risk, the authors of this TDR have chosen a set of objectives that are more expansive than is usual. The objectives for this roadmap are: (1) reduce technical and development risks and costs; (2) understand and, where possible, relieve system requirements and consequences; (3) increase technical insight into critical technologies; and (4) validate the design at the subsystem level. The emphasis on these objectives, particularly the latter two, is driven by outstanding programmatic decisions, namely whether a future GW mission is ESA-led or NASA-led, and availability of resources. The relative emphasis is best understood in the context of prioritization.

Laser Interferometry Space Antenna↗

NASA NTRS1972

Tracking and data relay satellite system configuration and tradeoff study. Volume 5: TDRS spacecraft design, part 1

A dual spin stabilized TDR spacecraft design is presented for low data rate (LDR) and medium data rate (MDR) user spacecraft telecommunication relay service. The relay satellite provides command and data return channels for unmanned users together with duplex voice and data communication channels for manned user spacecraft. TDRS/ground links are in the Ku band. Command links are provided at UHF for LDR users and S band for MDR users. Voice communication channels are provided at UHF/VHF for LDR users and at S band for MDR users. The spacecraft is designed for launch on the Delta 2914 with system deployment planned for 1978. This volume contains a description of the overall TDR spacecraft configuration, a detailed description of the spacecraft subsystems, a reliability analysis, and a product effectiveness plan.

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NASA NTRS1982

Possible applications of time domain reflectometry in planetary exploration missions

The use of a time domain reflectometer (TDR) for planetary exploration is considered. Determination of the apparent dielectric constant and hence, the volumetric water content of frozen and unfrozen soils using the TDR is described. Earth-based tests were performed on a New York state sandy soil and a Wyoming Bentonite. Use of both a cylindrical coaxial transmission line and a parallel transmission line as probes was evaluated. The water content of the soils was varied and the apparent dielectric constant measured in both frozen and unfrozen states. Advantages and disadvantages of the technique are discussed.

Heckendorn, S.↗

NASA NTRS2012

Calibration Procedure for Measuring S-Parameters in Balun Applications on 150-ohm High-Speed Cables

In the radiofrequency (RF) world, in order to characterize cables that do not conform to the typical 50-omega impedance, a time domain reflectometer (TDR) would probably be the simplest and quickest tool to attain this goal. In the real world, not every engineer has a TDR at their disposal; however, they most likely have a network analyzer available. Given a generic 50-omega vector network analyzer (VNA), we would like to make S-parameter measurements for non-50-omega devices (DUTs). For that, we utilize RF balanced/unbalanced transformers (called baluns for short), which are primarily used to match the impedance between the two VNA ports and the DUT's input and output ports, for the two-port S-parameter measurements.

Theofylaktos, Onoufrios↗

NASA NTRS2021

Terminal Descent Radar System Testbed for Future Planetary Landers

Terminal Descent Radars (TDR), or landing radars, have been an integral element of Guidance, Navigation and Control (GN\&C) sensor suites of robotic exploration missions to the Moon and Mars. As plans for new, exciting exploration missions to the Moon, Mars and other planetary bodies are being developed, there is a need for a new generation of TDRs that are smaller, consume less power and are less expensive than previous sensors. The challenge of designing such a landing sensor is twofold: the first is to have well-vetted software tools that allow us to explore the design space for a particular mission scenario and analyze performance of relevant radar architectures. The second challenge is to reduce mass and power requirements of a landing radar without compromising reliability and performance. New design approaches that address these challenges need to be tested and demonstrated in realistic Entry-Descent-Landing (EDL)/Deorbit-Descent-Landing (DDL) scenarios. In this paper, we describe a TDR testbed developed at the Jet Propulsion Laboratory. The testbed is a closed-loop design, analysis and verification capability used to design and evaluate the next generation of landing radars for a variety of EDL/DDL scenarios.

Tope, Michael↗

NASA NTRS2021

Terminal Descent Radar System Testbed for Future Planetary Landers

Tope, Michael↗

NASA NTRS1972

Ku-band high gain antenna

An exposed four element array of 12.8 m diameter elements mounted on a common pedestal is the recommended ground antenna configuration in support of the 2 GHz bandwidth Ku-band downlink from tdr satellites. The array provides three channel phase monopulse tracking capability with only listening feeds in each element. The array is as cost effective as a single aperture antenna and offers significant advantages in tracking and reliability.

Deerkoski, L. F.↗

NASA NTRS1973

Tracking and Data Relay Satellite System configuration and tradeoff study. Volume 2: Delta 2914 launched TDRSS, Configuration 2. Part 2: Final Report, 22 August 1972 - 1 April 1973

Configuration data and design information for a Delta 2914 launched configuration with greatly enhanced telecommunication service over the Part I Delta 2914 configuration is contained. The overall system definition, operations and control, and telecommunication service system, including link budgets are discussed. A brief description of the user transceiver and ground station is presented. A final section includes a summary description of the TDR spacecraft and all the subsystems. The data presented are largely in tabular form.

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NASA NTRS1973

Tracking and Data Relay Satellite System configuration and tradeoff study. Volume 3: Atlas Centaur launched TDRSS. Part 2: Final Report, 22 August 1972 - 1 April 1973

Configuration data and design information for the Atlas Centaur launched configuration are presented. Overall system definition, operations and control, and telecommunication service system, including link budgets, are discussed. A brief description of the user telecommunications equipment and ground station is presented. A summary description of the TDR spacecraft and all the subsystems is included. The data presented are largely in tabular form. A brief treatment of an optional configuration with enhanced telecommunications service is described.

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NASA NTRS1973

Tracking and Data Relay Satellite System configuration and tradeoff study. Volume 4: Space shuttle launched TDRSS. Part 2: Final Report, 22 August 1972 - 1 April 1973

Configuration data and design information for the space shuttle launched configuration is presented. The overall system definition, operations and control, and telecommunication service system including link budgets are discussed. A brief description of the user transceiver and ground station is presented. A final section includes a summary description of the TDR spacecraft and all the subsystems. The data presented are largely in tabular form.

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NASA NTRS1972

Tracking and data relay satellite system configuration and tradeoff study. Volume 3: TDRSS configuration and data summary, part 1

A reference handbook of configuration data and design information is presented. It treats the overall system definition, operations and control, and telecommunication service system including link budgets. A brief description of the user transceiver and ground station is presented. A final section includes a summary description of the TDR spacecraft and all the subsystems. The data presented are largely in tabular form for easy reference.

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