NOAA-20 VIIRS DNB Aggregation Mode Change: Prelaunch Efforts and On-Orbit Verification/Validation Results IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing

The Visible Infrared Imaging Radiometer Suite (VIIRS) on-board the National Oceanic and Atmospheric Administration-20 (NOAA-20, previously named Joint Polar Satellite System-1 or J1) satellite was successfully launched in late 2017, following six years of a successful operation by its predecessor on the Suomi National Polar-Orbiting Partnership (S-NPP) satellite. NOAA-20 VIIRS day/night band (DNB) adopts a new on-board aggregation option (Op21), which is different from S-NPP DNB (using Op32), to mitigate high non-linearity at high scan angles, observed in its radiometric response during prelaunch test. As a result, NOAA-20 VIIRS DNB has a larger scan angle at the end of scan (~60.5°) and exhibits a unique feature, i.e., ~600 km extended Earth view (EV) samples, compared to S-NPP DNB and other VIIRS bands. VIIRS geolocation (GEO) algorithm and geometric calibration parameters were analyzed in-depth and subsequently modified to accommodate the NOAA-20 VIIRS DNB aggregation mode change. The GEO code change was tested using S-NPP data; S-NPP DNB simulated J1 DNB radiance and limited J1 prelaunch test data. After the launch, it was further verified using NOAA-20 VIIRS on-orbit observations. Our results show that the prelaunch VIIRS GEO code change performs well. GEO validation results using nighttime point sources show that NOAA-20 DNB GEO errors are comparable to those for S-NPP DNB over the nominal EV range, with averaged nadir equivalent GEO errors less than 200 m after on-bit updates. Over the extended EV samples (scan angle > 56.06°), the averaged GEO errors are less than 500 m. Moreover, NOAA-20 VIIRS DNB radiometric calibration performance is comparable to S-NPP.

美国国家海洋和大气管理局-20（National Oceanic and Atmospheric Administration-20, 简称NOAA-20，前身为联合极轨卫星系统1号（Joint Polar Satellite System-1, 简称J1））卫星搭载的可见光红外成像辐射仪套件（Visible Infrared Imaging Radiometer Suite, 简称VIIRS）于2017年末成功发射，其前身苏奥米国家极轨伙伴卫星（Suomi National Polar-Orbiting Partnership, 简称S-NPP）上的同类载荷已成功运行六年。NOAA-20 VIIRS昼夜波段（day/night band, 简称DNB）采用了全新的星上聚合模式（Op21），与S-NPP DNB所使用的Op32模式存在差异，该设计旨在缓解发射前测试中观测到的高扫描角下辐射响应的高度非线性问题。由此，NOAA-20 VIIRS DNB的扫描角末端可达约60.5°，且具备一项独特特性：相较于S-NPP DNB及其他VIIRS波段，其可获取约600 km的扩展对地视场（Earth view, 简称EV）样本。研究团队对VIIRS地理定位（GEO）算法与几何定标参数进行了深入分析，并随后开展修改以适配NOAA-20 VIIRS DNB的聚合模式变更。此次地理定位代码修改通过S-NPP数据、模拟J1 DNB辐射亮度的S-NPP DNB数据，以及有限的J1发射前测试数据完成验证。发射后，团队又利用NOAA-20 VIIRS的在轨观测数据对修改后的代码进行了进一步验证。研究结果表明，发射前完成的VIIRS地理定位代码修改效果良好。基于夜间点源的地理定位验证结果显示，在标称对地视场范围内，NOAA-20 DNB的地理定位误差与S-NPP DNB相当，经在轨更新后，天底等效地理定位误差的平均值小于200 m。在扩展对地视场样本（扫描角＞56.06°）范围内，地理定位误差的平均值同样小于500 m。此外，NOAA-20 VIIRS DNB的辐射定标性能与S-NPP保持一致。