Collaborative Research: Development of GPS as a Soil Moisture Instrument

国家科学基金会支持发展主要用于地球物理研究的全球定位系统(GPS)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的全球定位系统数据可免费获得；这些数据正被用于许多非大地测量应用，包括大气和电离层研究。工程、遥测系统和档案服务已经存在，以支持这些连续运行的GPS网络。在这项研究中，将使用一种技术，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--。这项技术使用的是从持续运行的GPS站点下的土壤反射出来的信号。这些反射(多路径)本质上与地面的反射率相关--从而与土壤湿度--并且可以在GPS信噪比(SNR)数据中观察到。这种方法的一个潜在优势是，它可以通过明确测量存在于更细尺度上的土壤水分的变异性来估计约100平方米范围内的近地表土壤水分。PI已经将乌兹别克斯坦塔什干GPS站点的多路径反射幅度与陆地表面模型的土壤湿度进行了比较。虽然GPS多路径幅度和陆面模型土壤湿度的大小没有得到校准，但它们都随着每次降雨事件而急剧上升，并在所研究的70天期间缓慢下降。这项研究将进一步发展这项技术，并在科罗拉多州博尔德附近建立一个土壤湿度校准点。将对GPS接收器、天线和纪念碑进行测试，以确定哪些是土壤水分研究的最佳选择。将研究GPS多路径幅度与环境条件的相关性，包括温度、植被、土壤质地和土壤水分的垂直分布。估计的反射系数将与体积取样和水分反射计的现场测量结果进行比较。这项研究可能会产生更广泛的影响，为大陆和全球范围的气候和水文研究提供大量现有大地全球定位系统站点的每日土壤湿度估计。土壤水分测量对于研究过程、测试和校准模型以及预测水资源的可获得性至关重要。虽然存在土壤水分网络(例如http://okmesonet.ocs.ou.edu/)，)，但大多数仍然分布稀疏；世界上许多地区仍然缺乏标准化的仪器。基于GPS SNR的研究可以极大地增加水文和大气界可用的土壤湿度测量数量。该项目还将加强美国大地测量界、大气界和水文界之间的合作。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通过与水文学家合作开发并转移到公共领域。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.