EAGER: Collaborative Research: Mapping Melting Glacial Surfaces with GNSS Reflectometry

Part I: Nontechnical
Global navigation satellite systems (GNSS) such as the Global Positioning System (GPS) are continuously transmitting signals toward Earth. While many people may be familiar with using the GPS signals for positioning and navigation, these signals are also usable for sensing Earth’s environment. Ice and snow surfaces are continuously awash with radio signals broadcast from GNSS. When the signal bounces off the ice or snow surface and then arrives at a receiver, it acts as a form of radar, in which the radar transmitter is free, covers the globe, is always on, and is unaffected by precipitation. This work will build and deploy a GNSS reflectometry (GNSS-R) system specifically to detect reflections off glaciated surfaces. The goal of the work is to find out how the signal changes depending on surface type, and specifically, whether using GNSS as a radar can be effective for monitoring snow and ice melt and freeze on a glaciated surface. In this system, two GNSS antennas and receivers will be used, one facing upward for positioning, and one directed downward to collect the surface reflections. Setting up the GNSS-R system near the ice runways on the McMurdo Ice Shelf, near to the US McMurdo Station, Antarctica, the system will monitor for variations in the signal as it reflects off alternately surface ice, meltwater, and snow. With camera images and lidar surveys at the site will relate the GNSS “radar” signal and the area it bounced from (knowable from geometry because the GNSS satellite and receiver locations are known) to the surface type. If GNSS-R is developed to the point of being comparable to or better than existing ways of characterizing frozen surfaces, it would find a niche in applications ranging from local ablation monitoring to assessment of aircraft runway safety.

Part II: Technical Description
The proposed research aspires to answer the question: Can global navigation satellite system (GNSS) reflectometry (GNSS-R) be used to reliably map snow-cover, ice, and surface water in a harsh glaciated environment at high spatio-temporal resolution? Our working hypothesis is that GNSS-R can differentiate among cold snow, wet snow, bare ice, wet ice, and surface water in a way that will yield observations that can inform how glacial surfaces accumulate and ablate. This project will test this hypothesis by conducting GNSS-R instrument design, field trial and signal processing, and comparison with other methods, including the single-antenna interferometric reflectometry (GNSS-IR) method currently in use. The objective is to develop GNSS-R instrumentation and data-processing techniques as an effective high-spatiotemporal-resolution method of characterizing the composition of snow, firn and melting ice surfaces relevant to climate change on the Antarctic Ice Sheet. The GNSS-R receiver system will capture the signal after it has interacted with the surface (glaciated in this case), in order to infer variable compositions of the surface. Passive radar return intensity will be used to characterize the surface type, whether snow, firn, ice, or water. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

第一部分：非技术概述
全球导航卫星系统（Global Navigation Satellite Systems, GNSS）如全球定位系统（Global Positioning System, GPS），会持续向地球发射信号。多数人或许熟知利用GPS信号开展定位与导航，但此类信号亦可用于感知地球环境。冰雪表面始终处于GNSS广播的无线电信号覆盖范围内。当信号经冰或雪面反射后抵达接收机时，便构成了一种雷达探测形式——其发射源无需额外搭建，可覆盖全球且持续运行，不受降水影响。本研究将搭建并部署一套专门用于探测冰川表面反射信号的GNSS反射测量法（GNSS reflectometry, GNSS-R）系统。本研究的目标在于探究信号随表面类型产生的变化，具体而言，即验证利用GNSS作为雷达能否有效监测冰川表面的积雪与冰层融化及冻结情况。该系统将采用两套GNSS天线与接收机：一套朝上用于定位，另一套朝下以收集地表反射信号。研究团队将在南极洲美国麦克默多站（US McMurdo Station）附近的麦克默多冰架（McMurdo Ice Shelf）冰跑道附近搭建这套GNSS-R系统，监测信号经交替出现的冰面、融水与雪面反射后的变化。结合现场拍摄的相机影像与激光雷达（lidar）勘测结果，可将GNSS"雷达"信号与其反射区域（通过GNSS卫星与接收机的已知位置，可借助几何关系确定反射区域）与表面类型建立关联。若GNSS-R技术发展至可媲美甚至优于现有冻融表面表征方法的水平，便能在从局部消融监测到飞机跑道安全评估等诸多应用中占据一席之地。

第二部分：技术描述
本拟开展的研究旨在解答如下问题：全球导航卫星系统反射测量法（GNSS-R）能否以高时空分辨率可靠绘制冰川严酷环境下的积雪、冰层与地表水分布图？我们的工作假设为：GNSS-R可区分干雪、湿雪、裸冰、湿冰与地表水，所得观测结果能够为冰川表面的积累与消融过程提供参考。本项目将通过开展GNSS-R仪器设计、野外试验与信号处理，并与现有方法（包括当前投入使用的单天线干涉反射测量法（GNSS-IR））进行对比，以验证这一假设。本研究的目标在于开发GNSS-R仪器与数据处理技术，以此作为一种高效的高时空分辨率表征方法，用于分析南极冰盖相关气候变化背景下的积雪、粒雪与融冰表面组成。GNSS-R接收机系统将捕获与地表（本研究中为冰川表面）相互作用后的信号，以此推断地表的可变组成。将利用无源雷达回波强度来表征表面类型，即区分积雪、粒雪、冰层或水体。本资助项目契合美国国家科学基金会（National Science Foundation, NSF）的法定使命，且通过了基金会的学术价值与更广泛影响评审标准，被认定为值得支持的项目。