Landsat 5 25岁了

Image via Wikipedia

>[ScienceDaily](http://www.sciencedaily.com/releases/2009/03/090302112924.htm#) (2009-03-02) -- Like a trusty watch counting up the minutes, the Landsat 5 satellite keeps on ticking, orbit after orbit around Earth. Still observing the Earth after 25 years -- 22 beyond its three-year primary mission lifetime -- Landsat 5 collects valuable scientific data daily. Some attribute the satellite's longevity to over-engineering. Others say it's a long run of good luck. Whatever the reason, no one who attended the satellite's March 1984 launch could have expected it would still be working today.

Landsat 5 于1984年3月1日发射升空，是 NASA 和 NOAA 的地球观测项目 Landsat 卫星系列中的第5颗，原本设计使用寿命仅3年的它已工作了25年，收集了超过70万帧图像[1. The Landsat Program - News]。考虑到 Landsat 6 的发射失败，以及 Landsat 7 在1999年才发射，如果 Landsat 5 不是这么长寿，Landsat 图像将有最长达12年的断档。来自同一颗卫星的图像序列对基于遥感技术的研究有特别重要的意义，因为它意味着研究者不需要考虑对不同传感器和轨道参数带来的图像变化进行校正，而只需要弥补传感器老化带来的偏差。

Landsat 5 在轨的这25年并非一帆风顺，它出过大大小小22次故障，目前只有基本的TM传感器还在继续观测，被形容成太空中“满身绷带的木乃伊”。但是 NASA 计划的 Landsat 后续任务 [LDCM](http://ldcm.gsfc.nasa.gov/about.html) 还要等到2011年（一说2012年）才会发射新的卫星替代 Landsat 5 和 Landsat 7。在那之前，我们只能期待 Landsat 7 和 Landsat 5 都能继续坚持工作。在 NASA 网站上有一个 Landsat 5 图像[画廊](http://www.nasa.gov/topics/earth/features/landsat25/index.html)，讲述了 Landsat 5 是怎样记录25年来的历史（很遗憾，主要是美国历史）。

Preliminary Image Availability Survey

I am looking for satellite images over Anduo County, Tibet Autonomous Region, China.

• AVHRR： Search at EarthExplorer gives 11577 images in daylight condition, most focus on Indian subcontinent, but should be enough for me.  Dates are from 1980 to 2000s.
• Declassified Satellite Imagery-1: Low and medium images taken in 1960s, earliest 1961, from CORONA, ARGON and LANYARD satellites.  Most are medium to low resolution (15-30 m).  A large portion are cloudy.  $30 per frame.
  
• Declassified Satellite Imagery-2: Much higher camera resolution at 2 to 4 feet and 20 to 30 feet, from KH-7 and KH-9.  Taken in 1964-1965 and 1973.  Large swath, relatively lower cloud mask.  $30 per frame.
• EO-1: no images.
• Landsat 1-5 MSS: From 1972 to 1992.  Some scan line can be seen.  Less cloud.  $200 per frame.
• Landsat 4-5 TM: July 1982 to Present.  Good resolution, less cloud.  $425 per frame.
• Landsat 7 ETM+ International Ground Stations: No reviews available.  No uniform price.
• Landsat 7 ETM+: From 1999 to 2003, some are a bit cloudy.  Most are usable.  $475-600 per frame.
• Landsat 7 ETH+ SLC-off: More difficult to process and probably data loss. From 2003 to Present.  Can be downloaded with ordering.  $200-275 per frame. 
  Landsat 7 ETM+ scenes acquired from 5/31/03 - 7/14/03 and 9/3/03 - 9/17/03 are not available.
• SPOT: no image from USGS.
  

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