Landsat dataset limps into another decade
Recently my wife and I drove into Roanoke, Va., from the south on an old federal highway designated U.S. 221. We had followed it for more than 100 mi. to avoid “high-speed” interstate highways congested with holiday traffic, and to stretch a vacation from city life a little longer. For most of the way we enjoyed a bucolic byway winding through photogenic farmland, where stretches of the original roadway clearly were engineered for oxcarts and horses. But as we approached Roanoke—an old railroad city in the southwest corner of the state—the late-fall foliage gave way to the orange warning signs and raw Earth that signify road construction.
Crews are widening Route 221 to accommodate the growing population as Roanoke expands. It's a safe bet that state highway engineers based their decision to spend the money in part on a 40-year-old set of relatively low-resolution images of the area collected by a series of U.S. government Earth-observation satellites that started with launch on July 23, 1972, of the Earth Resources Technology Satellite 1—later renamed Landsat 1.
Every 16 days for most of the past four decades, a Landsat bird has covered the entire globe, generating an unparalleled dataset for tracking changes on the Earth's surface. There are more-capable satellites in terms of resolution, but none offers the steady long-term stream of comparable data that Landsat generates. Given the expense and technical difficulty of flying missions in space, that stream has been a little unsteady in recent years. But a new mission set for a Feb. 11 launch on an Atlas V has the potential to expand the dataset by another decade.
The Landsat Data Continuity Mission (LDCM) spacecraft is in thermal vacuum testing at prime contractor.'s Gilbert, Ariz., factory, and on track for a timely launch. The 3,085-kg (6,800-lb.) spacecraft—built around Orbital's LEOStar-3 bus—has a design life of five years, but will carry enough fuel to keep it functioning for 10. Ball Aerospace built its Operational Land Imager to collect data in the visible, near-infrared, short-wavelength infrared and panchromatic bands.
's built the spacecraft's Thermal Infrared Sensor (TIRS) using the advanced Quantum Well Infrared Photodetector (QWIP) technology it developed. The TIRS is designed to collect data in two more spectral bands previously covered by a single band on earlier Landsats.
From its polar-orbit perch at an altitude of 705 km (440 mi.), the LDCM will produce “scenes” measuring 185 X 185 km. The medium-resolution format actually is better for mapping changes than the sub-meter resolution available commercially today that can literally make it impossible to see the forest for the trees. Earth's dwindling supply of trees is one resource that Landsat has tracked over the years, according to LDCM chief scientist Jim Irons at Goddard.
“One ecosystem that's particularly susceptible to deforestation is the tropical rain forest, where great swaths of the forest are being converted to agriculture to feed a growing population,” he says.
Earth's burgeoning population also contributes to urban sprawl eating its way into farmland. This Landsat 7 image of Houston (photo) collected on Aug. 30, 2000, shows the utility of the dataset for tracking the growth of cities and meeting their infrastructure needs.
When the image of Houston was collected, Landsat 7 was working as advertised, but it has been limping along since its Scan Line Corrector failed on May 31, 2003. The device, which is not backed up by an on-board spare, compensates for the forward movement of the spacecraft in orbit so its adjacent surface scenes line up properly. Since the failure, the satellite data leave about 22% of the ground below its track blank. Until a year ago, Landsat 5 was helping fill in the dataset, but it finally stopped working after 27 years.
The Landsat system is operated by the U.S. Geological Survey (USGS), within charge of developing and launching the spacecraft. That is what the space agency does best, but the split in responsibility has made it difficult to keep the funding stream flowing against competing priorities.
Now that the two branches of government are getting serious about deficit reduction, that situation is likely to worsen. The budget belt-tightening comes at a time when long-range data are growing in importance as scientists and their political bosses try to gauge just how much of climate change is the result of man-made inputs that can be changed, measurements Irons says Landsat can help make.