Christof Huebner

Spatial time domain reflectometry and its application for the measurement of water content distributions along flat ribbon cables in a full-scale levee model.

Spatial time domain reflectometry (spatial TDR) is a new measurement method for determining water content profiles along elongated probes (transmission lines). The method is based on the inverse modeling of TDR reflectograms using an optimization algorithm. By means of using flat ribbon cables it is possible to take two independent TDR measurements from both ends of the probe, which are used to improve the spatial information content of the optimization results and to consider effects caused by electrical conductivity. The method has been used for monitoring water content distributions on a full-scale levee model made of well-graded clean sand. Flood simulation tests, irrigation tests, and long-term observations were carried out on the model. The results show that spatial TDR is able to determine water content distributions with an accuracy of the spatial resolution of about ±3 cm compared to pore pressure measurements and an average deviation of ±2 vol % compared to measurements made using another independent TDR measurement system.

Modelling of electromagnetic wave propagation along transmission lines in inhomogeneous media

Time domain reflectometry (TDR) is a well-established method for the measurement of moisture in various materials, especially soils. Standard waveform analysis usually provides the average water content along the length of the TDR probe, while more sophisticated methods are required to reconstruct the spatial water content profile. A reconstruction algorithm has been developed which uses one- or two-sided reflection data to calculate capacitance and conductivity distributions from which water content profiles can be derived. Several examples demonstrate the performance of the algorithm under various conditions in lossless and lossy materials.

On the Feasibility of Pressure Profile Measurements With Time-Domain Reflectometry

Many applications in geotechnical engineering require knowledge of total pressure distributions. So far, only measurements at single points at individual locations of interest can be carried out with conventional geotechnical measurement devices. Time-domain reflectometry (TDR) has already been used to assess the amount of shearing at multiple distinct locations along coaxial cables grouted in a rock or soil mass. However, pressure profile determination has not been investigated up to now. A novel approach has been investigated to determine pressure profiles from the time-domain reflection data of transmission lines. Mechanical forces change the distance between rubber-insulated conductors of transmission lines and, therefore, affect the spatial distribution of capacitance and inductance. These variations lead to partial reflections of an incident step pulse from which physical parameter distributions along the inhomogeneous transmission line are reconstructed. A reconstruction algorithm that was originally developed for soil moisture profile determination (Spatial-TDR), was adopted and applied in laboratory experiments. A further experiment illustrates TDR signals that can be expected in actual applications under different load conditions.

Long‐range wireless sensor networks with transmit‐only nodes and software‐defined receivers

Wireless sensor networks for environmental monitoring and agricultural applications often face long-range requirements at low bit-rates together with large numbers of nodes. This paper presents the design and test of a novel wireless sensor network that combines a large radio range with very low power consumption and cost. Our asymmetric sensor network uses ultralow-cost 40 MHz transmitters and a sensitive software dened radio receiver with multichannel capability. Experimental radio range measurements in two dierent outdoor environments demonstrate a single-hop range of up to 1.8 km. A theoretical model for radio propagation at 40 MHz in outdoor environments is proposed and validated with the experimental measurements. The reliability and delity of network communication over longer time periods is evaluated with a deployment for distributed temperature measurements. Our results demonstrate the feasibility of the transmit-only low-frequency system design approach for future environmental sensor networks. Although there have been several papers proposing the theoretical benets

Miniaturized FPGA-Based High-Resolution Time-Domain Reflectometer

Time-domain reflectometry (TDR) is a well-known measurement principle for evaluating frequency-dependent electric and dielectric properties of various materials and substances. Although TDR is a proven method, the high price for TDR measurement equipment and complex laboratory setups is often a limiting factor for cost-sensitive applications or large-scale field experiments, where a large number of TDR meters is required. This paper reports on the development of a new miniaturized low-cost TDR meter capable of sampling a repetitive rectangular waveform, which is used as an excitation signal. The developed sampling circuit is based on a digital delta modulator (DM) and allows for capturing the waveform of a repetitive measurement signal. A 1-MHz signal can be captured with a virtual sampling resolution of 1 ps within a measurement interval of 1 s. The generated pulses have a rise time of 2 ns and can be captured with an amplitude resolution of approximately 10 bit and an accuracy of approximately 8 bit. The developed digital DM architecture is implemented inside a small field programmable gate array and integrated into a miniaturized low-power TDR meter prototype for battery-powered outdoor applications. The captured measurement data are stored on integrated micro-SD card memory and can be read out either via a Universal Serial Bus, an RS-485 bus system, or a wireless interface. The TDR meter is controlled by an integrated microcontroller and a real-time clock and therefore can operate completely independent from any additional control setup. The TDR meter targets applications within the field of geoscience and agricultural monitoring, where large-scale measurement systems are required.

Error control strategies for transmit-only sensor networks: A case study

Wireless sensor networks using transmit-only sensor nodes are a promising new approach for applications such as environmental monitoring because they can provide high network scalability and long life-time at low cost. But achieving reliable data delivery in a transmit-only network is a challenging problem because there is no feedback channel for reporting lost or damaged packets. Taking a systems approach, this paper identifies three specific error control strategies that are applicable in this design space. The strategies are drawn from the areas of temporal diversity, spatial diversity and coding-based methods. An empirical investigation of these strategies is performed using two case study deployments in indoor and outdoor settings, running from two weeks to several months. In both case studies data replication in successive packets and receiver diversity performed best. In most cases, but not all, coding-based methods offered a poor balance between overhead and efficiency.

Vicarious calibration for remotely sensed hydro power water resource

As part of the natural resource management (NRM) the exploitation and assessment of water resources from melted snow, the snow water equivalent (SWE) for hydro power generation can proceed only by remote sensing. The evaluation algorithm EQeau is already introduced for this assessment. This connects the remotely data to the SWE exploiting the seasonal thermal resistance change of the soil and snow cover. On the other hand this technique needs representative vicarious ground calibrations, especially that of the snow density. At this end the devices used for the ground truth measurements are too small in comparison with a remote sensing pixel size and are not suitable for continous monitoring. The new method and device senses a more than 50 times larger measuring volume than the usual ones, its linear extension can be compared by a pixel side. The sensor is an unshielded flat band cable which will be embedded by snow fall and remains and measures there during the entire winter season. With time domain reflectometry (TDR) and low frequency measurements on this long sensor one can determine the density, the most important input for the calibration of the remotely sensed data. The method contributes to a better prognoses for avalanche and flood warning as well, because it measures the snow liquid water content also. The method and instrument are installed in four consecutive winter seasons.

Subsurface Sensing, Subsurface Aquametry

An attempt is started to define the terms of aquametry and its adjective subsurface. Subsurface Sensing represents a field of scientific research and development character, which is much wider than that of the classical non-destructive material testing in industrial production. It covers the 2 or 3D mapping of distributed parameters, broad application fields, the interactions of waves and beams with matter, signal propagation in media, almost all sensing techniques like radar, optical, acoustic and nuclear sensors as well as the measurements of material properties. Dielectric sensing dominates in the water content determination, i.e. in aquametry. The problems of aquametry are critically commented: Proposals of free and bound water definitions are made. The volume-related water content determination is preferred. The reference method, i.e. oven drying, impedes the development. The calibration or relation to the water content in indirect measurements is discussed. Optimum measuring frequency is treatedas well. Subsurface Aquametry is introduced as a development of Microwave Aquametry and Dielectric Aquametry. Also, dielectric water research is involved. By providing included and excluded examples and accepting the basic features of common Aquametry, the subject is defined with a call for contributions.

TDR application for moisture content estimation in agri-food materials

Cereals and legumes are invaluable resources, as they represent the raw materials of many foods and beverages. Cereals are also largely used as livestock feeds, thus indirectly influencing the quality of dairy products and meat. Because of their key role in human nutrition and well-being, the safety and quality of these agri-food materials are extremely important topics in food science. In this regard, moisture content is one of the ultimate factors influencing the quality, safety and price of the final food product; hence, it is crucial to monitor water content of materials in the food production line. Starting from these considerations, this paper describes the use of time domain reflectometry (TDR) for both in-line and off-line moisture content sensing of agrifood materials. In particular, after a brief description of the basic principles of TDR, two representative application cases for moisture content measurements of agri-food materials are reported and commented on.

Controlling the irrigation process in agriculture through elongated TDR-sensing cables

In this work, time domain reflectometry (TDR) is used in conjunction with an innovative type of low-cost, wirelike diffused sensor for diffused soil water content monitoring in agriculture. Thanks to its wire-like configuration, these passive, maintenance-free sensors can be rolled out and buried along the perimeter/path to be monitored (for example, along a row of plants/trees). A single sensor can be long up to 150 meters, and several sensors can be simultaneously installed in a large area, thus creating a diffused sensor network which can be controlled through a single measurement instrument. To assess the practical feasibility of the proposed TDR-based system, a practical experiment was carried out by employing a bifilar, wire-like sensing element (SE) for monitoring soil water content in a watermelon cultivation. For comparative purposes, also in view of continuous remote monitoring, measurements were carried out through three TDR instruments with different specifications and costs.