ALOS PALSAR Radar Processing System

Dataset has been removed form EarthExplorer 2019. The Advanced Land Observing Satellite (ALOS) was launched on January 24, 2006 by the Japan Aerospace and Exploration Agency (JAXA) and was operational until May 12, 2011. ALOS, also referred to as DAICHI, (the Japanese often give two names to each of their space projects; the names are used interchangeably in the JAXA literature), captured 6.5 million scenes during its five years of operation. The satellite provided high quality, Earth observation data for topographical mapping, disaster and environmental monitoring and climate change studies. ALOS (which carried PALSAR) was launched into the sun-synchronous orbit and revolved around the earth every 100 minutes, or 14 times a day. ALOS PALSAR returned to the original path (repeat cycle) every 46 days. The inter-orbit distance was approximately 59.7 km on the equator. The PALSAR was an active microwave sensor which was not affected by weather conditions and operable both day and night. It was based on a Synthetic Aperture Radar (SAR) carried onboard Japans first earth observation satellite (JERS-1). The PALSAR was a right-looking SAR using L-band frequency with a cross-track pointing capability of 18 degrees 55 degrees . PALSAR had the following 3 observation modes: Fine mode: This observation mode was most commonly used under regular operation. Its maximum ground resolution of 7m is one of the highest as a SAR (for comparison, SAR onboard JERS-1 was about 18m resolution) loaded on a satellite. Fine resolution mode gives 10 m spatial resolution in both range and azimuth (70 km of swath width, -25 dB of noise equivalent backscattering coefficient, and 25 dB of Signal-to-Ambiguity (S/A) ratio at a look angle of 35 degrees). ScanSAR mode: ScanSAR mode enabled a switch off-nadir angle from 3 to 5 times (scan by the swath of 70km) to cover wide area from 250km (70 miles x 3 miles) to 350km (70 miles x 5 miles). For comparison, swath width of SAR onboard JERS-1 was about 75km. The ScanSAR mode offered more than 250 km width of SAR images at the cost of spatial resolution (100 m) but when an optimized orbit was used, one could revisit the same target area in less than five days. Polarimetric mode: PALSAR realized both horizontal and vertical polarization. PALSAR could also simultaneously receive horizontal and vertical polarization per each polarized transmission which is called multi-polarimetry. In addition, PALSAR could switch from horizontal to vertical polarization and vice versa at respective transmission pulse enabling four polarizations by doubling simultaneous polarization, a function called full polarimetry. Polarimetric mode has a resolution of 30m on a 30 km swath, but offers a complete polarization scheme (HH/HV or VV/VH).

本数据集已于2019年从EarthExplorer平台下架。先进陆地观测卫星（Advanced Land Observing Satellite, ALOS，又名DAICHI）由日本宇宙航空研究开发机构（Japan Aerospace and Exploration Agency, JAXA）于2006年1月24日发射升空，在轨运行至2011年5月12日。日本航天项目通常会为每一项任务赋予两个名称，二者在JAXA官方文献中可互换使用。该卫星在五年的服役期间共采集了650万景影像，可为地形测绘、灾害与环境监测以及气候变化研究提供高质量的地球观测数据。

ALOS搭载了相控阵L波段合成孔径雷达（Phased Array type L-band Synthetic Aperture Radar, PALSAR），运行于太阳同步轨道，每100分钟环绕地球一周，即每日绕行14圈。其PALSAR载荷每46天完成一次重复轨道覆盖，赤道处的轨道间距约为59.7千米。PALSAR是一款主动式微波传感器，不受天气条件影响，可实现全天候作业。该载荷基于日本首颗地球观测卫星JERS-1搭载的合成孔径雷达（Synthetic Aperture Radar, SAR）技术研发而来，为右侧视合成孔径雷达，工作于L波段，具备18°至55°的跨轨指向能力。

PALSAR具备以下三种观测模式：
1. 精细模式（Fine mode）：该模式为常规作业中最常用的观测模式。其最高地面分辨率可达7米，属于卫星搭载合成孔径雷达的高分辨率级别（作为对比，JERS-1搭载的SAR分辨率约为18米）。该模式在距离向与方位向上均提供10米空间分辨率，幅宽为70千米；当观测视角为35°时，其噪声等效后向散射系数为-25 dB，信号模糊比（Signal-to-Ambiguity, S/A）可达25 dB。
2. ScanSAR模式：该模式可通过3至5次离天底角切换（单次扫描幅宽70千米），实现250千米（70英里×3英里）至350千米（70英里×5英里）的宽幅覆盖。作为对比，JERS-1搭载的SAR幅宽约为75千米。该模式以牺牲空间分辨率为代价，可获得超过250千米幅宽的SAR影像（分辨率为100米）；若采用优化轨道方案，可将同一目标区域的重访周期缩短至5天以内。
3. 极化模式：PALSAR支持水平与垂直两种极化方式。其可在每种极化信号发射的同时，接收水平与垂直极化回波，该技术被称为多极化技术。此外，PALSAR可在各自的发射脉冲间切换水平与垂直极化模式，通过翻倍同时极化通道数量实现四种极化组合（HH/HV或VV/VH），该功能被称为全极化技术。极化模式的幅宽为30千米，分辨率为30米，可提供完整的极化方案。