监测Landsat7轨道MTF变化：实测与模拟分析

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调制传递函数（Modulation Transfer Function, MTF）是衡量光学系统性能的关键指标，特别是在遥感领域，如 Landsat 7 的 Enhanced Thematic Mapper Plus (ETM+) 成像系统。MTF 描述了传感器如何将输入信号中的频率成分转换为图像空间频率的能力，它直接影响图像的分辨率和细节清晰度。本文主要关注的是 Landsat 7 发射后在轨的调制传递函数估计方法，这是因为在发射前的模型预测，由于太空环境可能对光学系统的MTF性能造成影响。文章的作者 James C. Storey，来自美国地质调查局 (U.S. Geological Survey) 的 EROS 数据中心和雷神技术服务公司 (Raytheon Technical Services Company)，他们针对 Landsat 7 的 ETM+ 成像系统设计了一种在轨监测方法。这个方法的核心是利用湖庞查特兰大桥 (Lake Pontchartrain Causeway) 在路易斯安那州的成像数据来评估MTF性能。研究过程中，通过处理穿过桥梁的多条扫描线，这些数据被视为在不同采样阶段对目标的多次测量，以此构建了过采样的目标轮廓。研究人员通过将实际图像测量与基于桥梁和ETM+系统传输函数的分析模型进行模拟匹配，调整模型参数以达到最佳拟合。这种方法允许他们定量地计算 ETM+ 在 Nyquist 频率处的调制以及点扩散函数 (Point Spread Function, PSF) 的全宽半高点 (Full Width at Half Maximum, FWHM)。这些数值对于了解 ETM+ 的空间分辨率和成像质量至关重要，因为它们揭示了系统在不同空间频率下的响应能力。这项工作不仅提供了实际应用中在轨监控光学系统MTF性能的方法，还展示了遥感数据如何被用于评估和校准卫星传感器的性能，这对于确保地球观测数据的质量和一致性具有重要意义。通过对 Landsat 7 ETM+ 的这种精细评估，科学家们能够持续优化图像处理算法，提升地球观测的科学价值和实用性。

2.1 Test target

The Lake Pontchartrain Causeway in Louisiana is a 24-mile long double span bridge that happens to be nearly aligned with

the Landsat 7 ground track. The bridge is near the center of Lake Pontchartrain in the Landsat 7 browse image shown in

figure 2. The bridge dimensions are such that the 15-meter panchromatic band can just resolve the two spans, whereas they

appear as a single line target in the 30-meter multispectral bands. The bridge is not a usable target in the 60-meter thermal

band. Although they are the same width, the two spans were constructed at different times (1956 and 1969) and exhibit

slightly different reflectance. The two spans are joined by a series of seven crossovers, used as turn-around areas by

emergency vehicles. In addition to the crossovers, there are two locations on the bridge where the surface material changes

causing short segments of much lower than normal reflectance. Other than these dark gaps and the bright crossovers, the

bridge and its water background are reasonably uniform.

Figure 2: Lake Pontchartrain Causeway target area

2.2 Data preparation

Landsat 7 acquires the Lake Pontchartrain scene (Worldwide Reference System path 22 and row 39) every 16 days. Cloud-

free acquisitions of the Causeway become candidates for MTF characterization. The selected test scenes were processed to

Level 1R on the Landsat 7 Image Assessment System

. At this processing level the data have been radiometrically corrected

to scaled (16-bit integer) at-aperture radiance but have not been geometrically resampled. Thus, each image pixel represents

the calibrated output of a single imaging detector. Since the Level 1R data are geometrically unprocessed, the effects of the

ETM+ cross-track scanning pattern are readily visible. Figure 3 shows several scans of panchromatic band data surrounding

one of the bridge crossovers. The ETM+ scanning pattern, coupled with a slight rotation of the bridge relative to the Landsat

7 ground track, leads to variations in the phase at which the detectors intersect the bridge. This makes the image scans

containing the bridge appear as a sequence of samples of a cross-scan line target acquired with varying phases.

A 64-scan image window containing the bridge was extracted from each of the seven reflective bands: the five visible and

near infrared (VNIR) bands from the primary focal plane, and the two short wave infrared (SWIR) bands from the cold focal

plane. A 16-sample window surrounding the bridge was then automatically extracted from each image line by centering the

Proc. SPIE Vol. 4540

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监测Landsat7轨道MTF变化：实测与模拟分析

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