The future shape of a Greenland GNSS observation network, 2017

The effort to study polar regions using modern observational techniques is formally known and funded under the Polar Earth Observing Network (POLENET). In Greenland, the GNET project, or Greenland Network, was developed to establish a network of GPS receivers operating continuously and autonomously on stable bedrock around Greenland. The purpose of the January 26-27, 2017 workshop was to discuss the future of this network. We were especially focused on documenting how GNET is used now, how the network could evolve, and to ask what scientific questions motivate its future. GNET data have served many purposes, such as providing ground truth for predictive models for post glacial rise/fall of bedrock adjacent to the ice sheet that in turn, play an essential role in correcting satellite gravity and altimetry based estimates of ice mass balance on decadal time scales. In addition to enumerating past successes of GNET, we examined the value that the network may provide for new science, such as tropospheric and ionospheric mapping, gaining new insights concerning surface mass balance, ice dynamics and ocean tidal mapping among others. This workshop report is designed to capture some of these new explorations. There is an emphasis placed upon the components of ice sheet surface mass balance in this report. This is motivated by three recent science breakthroughs: Since about 2006, the negative mass balance of the Greenland ice sheet is dominated by melt processes Recent findings demonstrate that GNET have sensitivities to the loading components of the various elements of the surface mass balance The zenith-delays measured in the carrier phase of the electromagnetic pulses received at the stations are capable of significantly improving hindcast models of precipitation, fundamental to surface mass balance The primary recommendations of this report are: Continue to support the continuous and autonomous operation of the current configuration of GNET. The spatial distribution and long time series of observations has enabled a wealth of scientific discovery, and extending these time series enables new science Maximize the utility of the current data by encouraging low-latency access to data Encouraging new uses of GNET data, such as tropospheric zenith delay analyses to improve atmospheric models in Greenland and use of these data for modeling the ionosphere If possible, densify the current network to better resolve those areas of maximum gradient in glacial isostatic adjustment (GIA), and/or regions of rapid glacier change

采用现代观测技术开展极地地区研究的工作，正式命名为极地地球观测网络（Polar Earth Observing Network，简称POLENET）并获得资助。在格陵兰地区，GNET项目（又称格陵兰网络）旨在构建一套连续自主运行的GPS（全球定位系统）接收站网络，部署于格陵兰周边的稳定基岩之上。2017年1月26日至27日举办的研讨会，旨在探讨该网络的未来发展方向，重点围绕梳理GNET的当前应用现状、探讨网络的演进路径，以及研讨驱动其未来发展的科学问题。

GNET数据已服务于诸多研究场景，例如为冰盖周边基岩冰后升降的预测模型提供地面实测基准，而此类模型反过来在修正基于卫星重力与测高数据的十年尺度冰量平衡估算中发挥着核心支撑作用。除了梳理GNET过往的研究成果外，本次研讨还探讨了该网络在新兴科学研究中的应用价值，包括对流层与电离层测绘、获取地表质量平衡相关的全新认知、冰盖动力学研究以及海洋潮汐测绘等领域。本研讨会报告旨在记录这些新兴探索方向，其中重点聚焦冰盖地表质量平衡的组成要素，这一聚焦源于三项近期科学突破：其一，自2006年左右起，格陵兰冰盖的负质量平衡主要由融化过程主导；其二，最新研究表明，GNET对地表质量平衡各组成要素的负荷效应具备敏感性；其三，接收站接收到的电磁脉冲载波相位中的天顶延迟数据，可显著提升降水后报模型的精度，而降水模型是地表质量平衡研究的核心基础。

本报告提出的核心建议如下：
1. 持续支持GNET当前配置的连续自主运行：其观测的空间分布特性与长期时间序列已催生大量科学发现，延长观测时间序列将助力更多新兴科学研究；
2. 通过推动数据的低延迟共享访问，最大化现有数据的应用价值；
3. 鼓励GNET数据的创新应用场景，例如利用对流层天顶延迟分析优化格陵兰地区的大气模型，以及将此类数据用于电离层建模；
4. 若条件允许，对现有网络进行站点加密扩充，以更好地解析冰川均衡调整（Glacial Isostatic Adjustment，简称GIA）梯度最大区域以及冰川快速变化区域。