By Liza Lester, ESA communications officer.
The US Geological Survey’s Landsat 5 Thematic Mapper has been a faithful friend to ecologists. Recoding image data in seven bands covering visible, thermal, and infrared spectra, the satellite has shown us retreating glaciers, advancing wildfires, waxing and waning kelp forests, forest succession, and the ravages of bark beetle infestations.
In November 2011, USGS shut down data transmissions for the duration of the northern hemisphere’s winter after the Thematic Mapper’s transmitter began showing signs of trouble. USGS has not been able to resuscitate the ailing transmitter, and has ended routine data acquisitions from the Thematic Mapper. But is the program is collecting data from a more limited, lower resolution instrument, the Multi-Spectral Scanner.
The Thematic Mapper has been the workhorse instrument for routine image acquisitions on Landsat 5. The satellite’s managers turned off the Multi-Spectral Scanner, an older design, in 1995. With the Thematic Mapper’s data now out of reach, USGS switched the Multi-Spectral Scanner back on this spring and were gratified to see the instrument wake right up after slumbering more than fifteen years. The Multi-Spectral Scanner gathers data in green, red, and near infrared wavelengths, allowing us to distinguish vegetation and water boundaries, land formations, shallow water, and sediment-laden water. Check out the Landsat History Factsheet for a comparison of sensor capabilities.
This isn’t the first transmitter failure. Landsat 5 is an old satellite, in orbit since 1984 and outliving several siblings, and its minders have worked around a number of equipment troubles. Its primary X-band transmitter began having problems in 1987, just a few years after launch. Operations continued on the backup transmitter until 2008. After the backup failed, engineers flipped the switch on the primary transmitter, and it miraculously came back online. When USGS saw a familiar failure pattern developing last fall, they shut the instrument down hoping they could tweak things and get it going again, but the transmitter did not recover when they woke it in April.
The Multi-Spectral Scanner (MSS) transmits on the S-band radio frequency spectrum used for control of the satellite, and so is not affected by the Thematic Mapper’s communication difficulties.
USGS scientist Rachel Headley, a geographer for the Landsat program, says it’s fun to have the Multi-Spectral Scanner online again, although it’s been so long since the sensor sent down data that the Landsat program has lost the capability to process it. USGS will not be releasing data until they can rebuild the necessary capacity and collect enough data for radiometric calibration, which will make images consistent and comparable with other satellite spectra. That will not happen for many months, and certainly not before the end of the northern hemisphere’s growing season.
In the meantime, USGS is archiving the data. Though the Multi-Spectral Scanner is not as powerful as the Thematic Mapper, its archive could provide useful data continuity, particularly when merged with data from other missions. Maintaining the satellite’s infrastructure does have a cost, however, and in the current, difficult budget environment, hard choices have to be made. USGS is discussing the value of continuing investment the Landsat 5 mission with stakeholders in the scientific community, and will decide its future soon.
The latest Landsat scion, Landsat 8, is in the final stages of construction and testing, and slated to launch in January or February of 2013.
Image: “Component problems or failures during Landsat 5′s mission, specified by month/year.” NASA
A sampling of recent ecological articles that have made use of Landsat data: