Imaging the Past
Expanding the historical record of Landsat imagery
Today, with Landsats 7 and 8 in orbit, Landsat 9 being built, the European Space Agency’s Sentinel-2a aloft, Sentinel-2b soon following, and flocks of new microsats imaging our planet, we acquire richer and more plentiful information about Earth’s land surface than ever before.
Amid this wealth of contemporary Earth data, scientists still ask: Compared to what? How do we know the range of ways that Earth may be changing unless we have a solid record of previous conditions? To better understand the present, how can we better image the past?
Since July 1972, satellites in the Landsat program have been imaging the landmasses of our planet at a particularly useful scale that shows us both natural and manmade changes. Landsat is the only satellite system that has recorded Earth’s land-surface conditions for over four decades. This longevity — this steady, unblinking gaze from space — has made the Landsat data record an essential foundation for global change research.
The National Science and Technology Council has endorsed the value of Landsat data. In a study the council conducted in 2014 about the value of U.S. observation systems to the American public, they ranked the Landsat program as number three, just behind the much better known GPS and weather radar systems.
Knowing the worth of this specialized knowledge, it is hard to imagine that nearly two-thirds of the Landsat data collected in the three and a half decades between 1972 and 1999 was, until recently, completely inaccessible.
With Landsats 7 and 8 now active, it’s no surprise that the archive of current U.S. Earth observation data is growing rapidly. Combined, the satellites collect about 1,200 scenes that take up about 1 terabyte of data every day.
What is surprising is that you can now find much, much more historical Landsat data than ever before in the Landsat archive at the U.S. Geological Survey’s Earth Resources Observation and Science Center, or EROS, in Sioux Falls, South Dakota.
But the past has passed, someone other than Faulkner might say. How is it possible for records of historical imagery of the land to actually expand?
Because a massive international collaboration has been afoot since 2010 to recover historical data, allowing EROS to, in essence, image the past. While the recovered information from around the globe doesn’t necessarily change history, it does expand the known record to sharpen our perspective of natural and human changes on the land, especially in some remote areas.
To explain, let’s construct some rough parallels in American history. If several new contemporary accounts of the Plymouth Colony had been found or if a new trove of letters written by George Washington had been uncovered, the new information likely wouldn’t change what we already know, but it would certainly strengthen our confidence in understanding the historical conditions and context of each setting.
Now, about this historical Landsat data: Why was it inaccessible? How was it recovered and re-assembled?
Data from space: more difficult than it appears
The latest incarnation in the Landsat series, Landsat 8 (launched in February 2013), operates with all of the advantages of forty-plus years of technological advancement. Every image Landsat 8 collects makes its way directly into the U.S archive at EROS where the data are processed and immediately made available for anyone in the world with an internet connection to download at no charge.
When Landsat 1 took to the sky in 1972, it did not have the benefit of our modern recording and downlinking technologies. Landsats 1, 2, and 3 flew with two wideband video tape recorders — behemoth machines that each weighed 76 pounds. Each contained enough two-inch wide magnetic tape to stretch the length of six football fields. Their data storage capacity was 3.75 gigabytes, the largest recording capacity of any orbiting recorder at the time.
The next generation of Landsat satellites deliberately avoided onboard recorders, opting instead for the new tracking and data relay satellite system (TDRSS) technology. For various reasons, the system was not fully operational until 1989. By then a private operator was running the satellite mission, and commercial reasons led to gaps in international data coverage.
There was, however, a saving grace for worldwide historical Landsat data. As part of the U.S. effort to foster goodwill, share data, and encourage peaceful use of outer space, the U.S. offered other countries the opportunity to build International Ground Stations and operate them as International Cooperators (ICs). The ICs installed suitable antennae and set up their own data archives.
The U.S. provided technical specifications and engineering support to the ICs and initiated regular meetings. After the first ground station operators meeting took place at Johnson Space Center in Texas in 1975, regular meetings continued and the list of ICs evolved into an international community.
From the very beginning, year-after-year, ICs collected and archived their local data. Year-after-year, more ICs joined the community.
During the long life of the Landsat program, its earlier data were often too costly and too computationally expensive to allow global studies to be conducted. The full potential of the chronological span of the data had been locked away in IC holdings, as if it were in ice waiting for technology and policy to let it thaw.
Recognizing the immense importance of this hidden treasure of historical data, the Landsat Science Team formally requested that USGS consolidate the global archive. Thus, in 2010, the Landsat Global Archive Consolidation project, known by the unmelodious acronym LGAC, was born.
Some 35 international ground stations operated by 22 ICs had collected data during the last 44 years. They all now agreed to participate in LGAC.
The logistics of making it happen
That the ICs had preserved so much historical Landsat data demonstrated a basic, mutual understanding of Landsat’s significance as an historical record of Earth’s surface. However, the data had to be readable to become part of the historical record.
The condition of the decades-old data varied from station to station. Some stations had independently transferred their data to modern media, but other stations that were not able to do so sent their old physical media to EROS.
After receiving the data from the ICs, USGS had to move the data into their archive. The challenges fell into two main camps: (1) reading the data off of the physical media that was often badly deteriorated, and (2) translating the formatted data into a form that can be ingested and processed into a standard data product. None of this was easy.
For a fuller description of the restoration process, see longer companion article at NASA website.
Better together: A complete archive
As Landsat Science Team member Mike Wulder wrote, “The recovery rate has been impressive as a result of the skill and ingenuity of the technical experts at EROS.”
As of September 30, 2016, the U.S. archive contained 6,991,828 images — 3.9 million of those are historic images ingested in the last few years thanks to LGAC efforts. To put that into perspective, Landsat 5, a Guinness World Record holder for its 29-year life span, collected 2.5 million images. In a sense, the LGAC effort has served as the equivalent of a separate Landsat mission — or two — in collecting Earth data from the past.
The recent land history of Earth — from 1972 as seen from space by Landsat satellites — is now drawn in much richer detail. Fuller global-scale investigations for a four decade-plus period are now possible because of LGAC, even as an additional two million historical images still wait to be added to the archive.
Longer companion article at NASA Landsat website
Wulder, et al. "The global Landsat archive: Status, consolidation, and direction." Remote Sensing of Environment, Nov. 2016, pp. 271-83.
Special Issue of Remote Sensing of Environment on Landsat 8 enhancements