December 03, 2012
Credit: Credit: NASA Goddard Space Flight Center
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 Orbital Sciences Corp.'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.
NASA's Goddard Space Flight Center 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.