Collaborative Research: Development of GPS as a Soil Moisture Instrument

国家科学基金会支持开发主要用于地球物理研究的全球定位系统网络。 在与国际社会的广泛合作下，这些由国家科学基金会赞助的网络提供的全球定位系统数据可免费获得;这些数据正用于许多非大地测量应用，包括大气层和电离层研究。 工程、遥测系统和存档服务已经存在，以支持这些连续运行的GPS网络。对于这项研究，将采用一种技术，将现有的连续运行的GPS网络的使用扩展到一个新的科学界--水文界--。 这项技术使用的信号，反射了土壤下连续运行的全球定位系统网站。 这些反射（多路径）与地面的反射率-从而与土壤湿度-有着内在的联系，可以在全球定位系统信噪比数据中观测到。 这种方法的一个潜在优势是，它可以提供一个近地表土壤水分的估计在一个面积约100平方米，明确测量土壤水分的变化，存在于更精细的尺度。PI将来自乌兹别克斯坦塔什干GPS站点的多径反射幅度与来自陆面模型的土壤湿度进行了比较。 虽然GPS多路径振幅和陆面模型土壤湿度的幅度没有校准，他们都急剧上升，每次降雨事件后，缓慢下降，在70天的研究期间。 这项研究将进一步发展这项技术，并建立一个土壤水分校准网站附近的博尔德，CO. GPS接收器，天线和monumentation将进行测试，以确定哪些是最佳的土壤水分研究。 将研究全球定位系统多路径幅度对环境条件的依赖性，包括温度、植被、土壤质地和土壤湿度的垂直分布。 估计的反射系数将与体积取样和含水量反射计的现场测量值进行比较。这项研究有可能从大量现有的大地测量GPS站点为大陆到全球尺度的气候和水文研究提供每日土壤湿度估计。 土壤湿度测量对于研究过程、测试和校准模型以及预测水资源的可用性至关重要。 虽然有土壤湿度网络（如http：//okmesonet.ocs.ou.edu/），但大多数网络分布仍然很稀疏;世界上许多地区仍然缺乏标准化的仪器。 基于全球定位系统信噪比的研究可以大大增加水文和大气界可获得的土壤湿度测量数据的数量。 该项目还将加强美国大地测量，大气和水文社区之间的合作，这些技术也可以应用于国际数据集。 这项研究的大地测量技术将与水文学家合作开发，并将转让给公众。 一名研究生将参与这项研究，使用GPS和现场土壤湿度数据。

NSF has supported the development of Global Positioning System (GPS) networks primarily for geophysical studies. In a broad collaboration with the international community, GPS data from these NSF-sponsored networks are freely available; they are being used for many non-geodetic applications including atmospheric and ionospheric studies. Engineering, telemetry systems, and archiving services already exist to support these continuously operating GPS networks.For this research, a technique that expands the use of existing continuously operating GPS networks to a new scientific community---the hydrologic community---will be employed. This technique uses signals that reflect off soil beneath continuously operating GPS sites. These reflections (multipath) are intrinsically related to the reflectance of the ground -- and thus soil moisture - and can be observed in GPS signal-to-noise-ratio (SNR) data. A potential advantage of this method is that it could provide an estimate of near-surface soil moisture across an area of ~100 m2 by explicitly measuring the variability of soil moisture that exists at finer scales. The PIs have compared multipath reflection amplitudes from a GPS site in Tashkent, Uzbekistan to soil moisture from a land surface model. Although the magnitudes of GPS multipath amplitudes and the land surface model soil moisture are not calibrated, they both rise sharply following each rainfall event and slowly decrease over the 70-day period studied. This research will further develop this technique and create a soil moisture calibration site near Boulder, CO. GPS receivers, antennas, and monumentation will be tested to determine which are optimal for soil moisture studies. The dependence of GPS multipath amplitudes on environmental conditions, including temperature, vegetation, soil texture, and vertical distribution of soil moisture, will be studied. Estimated reflectance coefficients will be compared with in situ measurements from volumetric sampling and water content reflectometers. Broader Impacts The research could potentially provide daily soil moisture estimates from a large number of existing geodetic GPS sites for continental to global scale climate and hydrologic studies. Soil moisture measurements are critical to study processes, test and calibrate models, and forecast availability of water resources. Although soil moisture networks exist (e.g. http://okmesonet.ocs.ou.edu/), most are still sparsely distributed; many regions in the world still lack standardized instrumentation. GPS SNR based studies could greatly increase the number of soil moisture measurements available to the hydrologic and atmospheric communities. The project will also enhance collaborations between the geodetic, atmospheric, and hydrologic communities in the U.S. These techniques could also be applied to international datasets. The geodetic techniques for this research will be developed by collaborating with hydrologists and transferred to the public domain. A graduate student will be involved in the research, working both with GPS and in situ soil moisture data.