MODIS/Aqua Calibrated Radiances 5-Min L1B Swath 1km - NRT

The MODIS Level 1B Near Real Time (NRT) data set contains calibrated and geolocated at-aperture radiances for 36 discrete bands located in the 0.4 to 14.4 micron region of electromagentic spectrum. These data are generated from the MODIS Level 1A scans of raw radiance and in the process converted to geophysical units of W/(m^2 um sr). In addition, the Earth Bi-directional Reflectance Distribution Function (BRDF) may be determined for the solar reflective bands (1-19, 26) through knowledge of the solar irradiance (e.g., determined from MODIS solar diffuser data, and from the target illumination geometry).

Additional data are provided including quality flags, error estimates and calibration data. Visible, shortwave infrared, and near infrared measurements are only made during the daytime, while radiances for the thermal infrared region (bands 20-25, 27-36) are measured continuously.

Channels 1 and 2 have 250 m resolution, channels 3 through 7 have 500m resolution, and the rest have 1 km resolution. However, for the MODIS L1B 1 km product, the 250 m and 500 m band radiance data and their associated uncertainties have been aggregated to 1km resolution. Thus the entire channel data set is referenced to the same spatial and geolocation scales. Separate L1B products are available for the 250 m channels (MYD02QKM) and 500 m channels (MYD02HKM) that preserve the original resolution of the data.

Spatial resolution for pixels at nadir is 1 km, degrading to 4.8 km in the along-scan direction at the scan extremes. However, thanks to the overlapping of consecutive swaths and respectively pixels there, the resulting resolution at the scan extremes is about 2km. A 55 degree scanning pattern at the EOS orbit of 705 km results in a 2330km orbital swath width and provides global coverage every one to two days. A single MODIS Level 1B granule will nominally contain a scene built from 203 scans (or swaths) sampled 1354 times in the cross-track direction, corresponding to approximately 5 minutes worth of data. Since an individual MODIS scan (or swath) will contain 10 along-track spatial elements, the scene will be composed of (1354 x 2030) pixels, resulting in a spatial coverage of (2330 km x 2030 km). Due to the MODIS scan geometry, there will be increasing overlap occurring beyond about 25 degrees scan angle.

The MODIS L1B data are stored in the Earth Observing System Hierarchical Data Format (HDF-EOS) which is an extension of HDF as developed by the National Center for Supercomputer Applications (NCSA) at the University of Illinois. A typical file size will be approximately 260 MB. The Shortname for this product is MYD021KM.

MODIS（中分辨率成像光谱仪，Moderate Resolution Imaging Spectroradiometer）近实时（Near Real Time, NRT）一级B类数据集包含经过定标且已完成地理定位的孔径处辐射亮度数据，涵盖电磁波谱0.4至14.4微米波段范围内的36个离散波段。这些数据源自MODIS一级A类原始辐射亮度扫描数据，处理过程中已将其转换为物理单位W/(m²·μm·sr)。此外，结合太阳辐照度（例如通过MODIS太阳漫射器数据及目标照明几何条件确定），可针对太阳反射波段（1-19、26波段）反演地球双向反射分布函数（Bi-directional Reflectance Distribution Function, BRDF）。
数据集还附带额外数据，包括质量标记、误差估计值及定标数据。可见光、短波红外及近红外波段仅在日间进行测量，而热红外波段（20-25、27-36波段）的辐射亮度则可连续采集。
波段1和2的空间分辨率为250米，波段3至7的分辨率为500米，其余波段分辨率为1千米。但对于MODIS L1B 1千米产品，250米和500米波段的辐射亮度数据及其相关不确定度已聚合至1千米分辨率，因此整套波段数据均采用统一的空间与地理定位尺度。另有针对250米波段（产品短名称：MYD02QKM）和500米波段（产品短名称：MYD02HKM）的独立L1B产品，可保留数据的原始分辨率。
星下点像素的空间分辨率为1千米，在扫描边缘处沿扫描方向的分辨率会降至4.8千米。不过由于相邻条带及对应像素的重叠，扫描边缘处的实际分辨率约为2千米。在705千米高度的EOS（地球观测系统，Earth Observing System）轨道上，55度扫描模式可形成2330千米的轨道条带宽度，每1至2天即可完成一次全球覆盖。单个MODIS一级B类数据粒（granule）通常包含由203次扫描（或条带）组成的场景，跨轨方向采样1354次，对应约5分钟的数据量。由于单次MODIS扫描（或条带）包含10个沿轨空间单元，该场景将由(1354×2030)个像素组成，空间覆盖范围为(2330千米×2030千米)。受MODIS扫描几何特性影响，当扫描角度超过约25度时，重叠现象会愈发显著。
MODIS L1B数据采用地球观测系统分层数据格式（Earth Observing System Hierarchical Data Format, HDF-EOS）存储，该格式是由伊利诺伊大学国家超级计算应用中心（National Center for Supercomputer Applications, NCSA）开发的HDF格式的扩展。单个文件的典型大小约为260MB，该产品的短名称为MYD021KM。