Two recent setbacks to L-band microwave sensing missions have hindered our ability to study soil moisture and ocean salinity.The study of Earth science using microwave sensing, specifically L-band instruments, has taken a hit in the past two months as two satellite missions carrying L-band sensing equipment have experienced critical failures.
The failures occurred with the Aquarius/SAC-D power and altitude control and the Soil Moisture Active Passive (SMAP) radar. The missions, respectively, have sought to provide measurements on ocean salinity and soil moisture content.
Both missions use microwave sensing of the L-band, a band of frequencies from 1 to 2 GHz. L-band frequencies are particularly sensitive to changes in ocean salinity and soil moisture, and the atmosphere is mostly transparent to the L-band allowing for measurements to be taken regardless of current weather patterns.
A portion of the L-band, from 1.400 to 1.427 GHz, is set aside for radiometric observations for scientists who study the Earth and space, says Jeff Piepmeier, the radiometer instrument scientist for SMAP at NASA’s Goddard Space Flight Center in Maryland. The protected frequency allocation reduces (but doesn’t completely prevent) the radio frequency interference (RFI) scientists encounter when taking measurements.
The first of the two failures occurred on June 7, when the Aquarius/SAC-D spacecraft, which has carried NASA’s Aquarius L-band instrument since its launch in 2011, suffered a failure to part of its power and altitude control system. The mission was subsequently declared over by the operations team.
The Aquarius mission was the first to combine passive and active measurements at L-band microwave frequencies. The passive measurements were made using three-beam, push-broom radiometers that measured emission from the ocean surface; the Aquarius radar scatterometer provided surface-roughness corrections. Sea surface salinity is derived from the surface emission and roughness measurements.
Among Aquarius’ contributions to the scientific community was the first global map of ocean salinity, which can be viewed below.
The salinity measurements collected by Aquarius over three years and nine months were vital for understanding the net flow of water in and out of the ocean, which influences global climate and weather.
One month later, on July 7, NASA’s Soil Moisture Active Passive (SMAP) satellite’s active sensor stopped transmitting. Launched Jan. 30, SMAP carries both an L-band radar and radiometer which, in combination, seek to provide accurate surface (top 5 cm) soil moisture measurements using the combined strengths of the two instruments.
The radiometer is less affected by surface roughness and vegetation, leading to more accurate measurements, while the radar can provide better spatial resolution (3 km, as opposed to 40 km for the radiometer, yielding a soil-moisture data product with a resolution of 10 km). Although SMAP’s radar is no longer transmitting pulses, the radiometer is still collecting data. The SMAP radiometer uses a combination of space-flight hardware and algorithms to mitigate RFI, a method developed by Piepmeier and the SMAP radiometer team at Goddard. According to Piepmeier, this makes the SMAP radiometer “the most advanced RFI mitigating radiometer to ever go up in space.”
SMAP’s soil moisture measurements, such as those shown in Figure 1, have not yet been validated using observations taken on the ground.
The science objectives of SMAP include understanding the link between the water, energy, and carbon cycles; understanding net fluxes in these cycles; and improving climate and weather forecasting skills. In addition, studying soil moisture helps forecast crop yields, plan irrigation systems, and predict floods and droughts. The mission also seeks to globally monitor frozen or thawed state of soil.
Fortunately, data collection of ocean salinity and soil moisture hasn’t completely stopped. The Soil Moisture and Ocean Salinity Earth Explorer (SMOS) satellite, launched by the European Space Agency (ESA) in 2009, continues to produce valuable data. The SMOS satellite’s sole instrument, the 2D Microwave Imaging Radiometer with Aperture Synthesis (MIRAS), is a interferometric radiometer with 69 small antennas placed along three arms in a Y formation. The architecture is similar to the interferometry technique used by the Very Large Array of telescopes in New Mexico. However, having only a passive instrument means that SMOS has a poorer resolution than SMAP (35- 50 km versus 10 km).