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Temporal variability of GNSS-Reflectometry ocean wind speed retrieval performance during the UK TechDemoSat-1 mission
Remote Sensing of Environment ( IF 13.5 ) Pub Date : 2020-06-01 , DOI: 10.1016/j.rse.2020.111744
Matthew Lee Hammond , Giuseppe Foti , Christine Gommenginger , Meric Srokosz

Abstract This paper presents the temporal evolution of Global Navigation Satellite System Reflectometry (GNSS-R) ocean wind speed retrieval performance during three years of the UK TechDemoSat-1 (TDS-1) mission. TDS-1 was launched in July 2014 and provides globally distributed spaceborne GNSS-R data over a lifespan of over three years, including several months of 24/7 operations. TDS-1 wind speeds are computed using the NOC Calibrated Bistatic Radar Equation algorithm version 0.5 (C-BRE v0.5), and are evaluated against ERA5 high resolution re-analysis data over the period 2015–2018. Analyses reveal significant temporal variability in TDS-1 monthly wind speed retrieval performance over the three years, with the best performance (~2 m∙s−1) achieved in the early part of the mission (May 2015). The temporal variability of retrieval performance is found to be driven by several non-geophysical factors, including TDS-1 platform attitude uncertainty and spatial/temporal changes in GPS transmit power from certain satellites. Evidence is presented of the impact of the GPS Block IIF Flex mode on retrieved GNSS-R wind speed after January 2017, which results in significantly underestimated ocean winds over a large region covering the North Atlantic, northern Indian Ocean, the Mediterranean, the Black Sea, and the Sea of Okhotsk. These GPS transmit power changes are shown to induce large negative wind speed biases of up to 3 m∙s−1. Analyses are also presented of the sensitivity of TDS-1 wind speed retrieval to platform attitude uncertainty using statistical simulations. It is suggested that a 4° increase in attitude uncertainty can produce up to 1 m∙s−1 increase in RMSE, and that TDS-1 attitude data do not fully reflect actual platform attitude. We conclude that the lack of knowledge about the GNSS-R nadir antenna gain map and TDS-1 platform-attitude limits the ability to determine the achievable wind speed retrieval performance with GNSS-R on TDS-1. The paper provides recommendations that accurate attitude knowledge and a good characterisation of GNSS-R nadir antenna patterns should be prioritised for future GNSS-R missions.

中文翻译:

英国 TechDemoSat-1 任务期间 GNSS-Reflectometry 海洋风速反演性能的时间变化

摘要 本文介绍了英国 TechDemoSat-1 (TDS-1) 任务三年期间全球导航卫星系统反射计 (GNSS-R) 海洋风速反演性能的时间演变。TDS-1 于 2014 年 7 月发射,提供全球分布式星载 GNSS-R 数据,寿命超过三年,包括几个月的 24/7 运行。TDS-1 风速使用 NOC 校准双基地雷达方程算法版本 0.5 (C-BRE v0.5) 计算,并根据 2015-2018 年期间的 ERA5 高分辨率再分析数据进行评估。分析显示 TDS-1 月风速反演性能在三年内具有显着的时间变化,在任务的早期(2015 年 5 月)取得了最佳性能(~2 m∙s−1)。发现检索性能的时间可变性是由几个非地球物理因素驱动的,包括 TDS-1 平台姿态不确定性和某些卫星 GPS 发射功率的空间/时间变化。有证据表明 GPS Block IIF Flex 模式对 2017 年 1 月后检索到的 GNSS-R 风速的影响,导致北大西洋、印度洋北部、地中海、黑海等大片区域的海风被严重低估,还有鄂霍次克海。这些 GPS 发射功率变化被证明会引起高达 3 m∙s−1 的大负风速偏差。还使用统计模拟分析了 TDS-1 风速反演对平台姿态不确定性的敏感性。建议姿态不确定性增加 4°可使 RMSE 增加 1 m∙s−1,并且 TDS-1 姿态数据不能完全反映实际的平台姿态。我们得出的结论是,缺乏对 GNSS-R 天底天线增益图和 TDS-1 平台姿态的了解限制了在 TDS-1 上使用 GNSS-R 确定可实现的风速反演性能的能力。该论文提供了建议,即未来 GNSS-R 任务应优先考虑准确的姿态知识和对 GNSS-R 天底天线方向图的良好表征。我们得出的结论是,缺乏对 GNSS-R 天底天线增益图和 TDS-1 平台姿态的了解限制了在 TDS-1 上使用 GNSS-R 确定可实现的风速反演性能的能力。该论文提供了建议,即未来 GNSS-R 任务应优先考虑准确的姿态知识和对 GNSS-R 天底天线方向图的良好表征。我们得出的结论是,缺乏对 GNSS-R 天底天线增益图和 TDS-1 平台姿态的了解限制了在 TDS-1 上使用 GNSS-R 确定可实现的风速反演性能的能力。该论文提供了建议,即未来 GNSS-R 任务应优先考虑准确的姿态知识和对 GNSS-R 天底天线方向图的良好表征。
更新日期:2020-06-01
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