Updated 10:36 a.m. Aug. 24

WASHINGTON — One of the two main science instruments on NASA’s $915 million Soil Moisture Active Passive (SMAP) spacecraft remains out of action more than a month after abruptly switching off, but engineers believe they are getting closer to diagnosing the problem.

Engineers believe SMAP’s L-band radar sensor cut out July 7 because of a yet-undiagnosed issue with the low-voltage power supply on its high-power amplifier, according to an Aug. 5 update from the Jet Propulsion Laboratory, which is leading the mission. The amplifier boosts the power of the radar, which bounces radio signals of the surface of Earth to derive information including moisture levels in the soil.

JPL has not said exactly what went wrong, but the center has winnowed the field of possible problems down to “several candidate faults within the low-voltage power supply,” according to the post.

“The project expects the next operational test for the radar will occur in late August,” Kent Kellogg, JPL’s SMAP project manager, wrote in an Aug. 13 email. SMAP was launched Jan. 31 into a circular near-polar orbit with an altitude of 685 kilometers.

The satellite combines measurements from the radar with readings from a more-accurate but lower-resolution radiometer built by NASA’s Goddard Space Flight Center in Greenbelt, Maryland, to measure global soil moisture to a depth of about 5 centimeters. SMAP generates a new global map every three days.

“Other than the radar, the rest of the SMAP spacecraft, including the antenna and the radiometer instrument, is working nominally,” NASA spokesman Alan Buis wrote in an Aug. 11 email. The spacecraft’s radar antenna features a 6-meter diameter deployable mesh reflector supplied by Northrop Grumman’s Astro Aerospace division.

The European Space Agency plans to use the SMAP antenna technology on Europe’s Biomass Earth science satellite, which would launch around 2020. ESA declined to comment on Biomass since the mission is in procurement.

“The Biomass satellite is these days in the process of the TEB — tender evaluation board,” an ESA spokesman wrote in an Aug. 19 statement. “As you can then understand, ESA will not make any public comments on the satellite in this procurement phase.”

Meanwhile, without its active radar, SMAP cannot fulfill its Level 1 requirements: NASA’s minimum criteria for mission success. “Although the absence of data from the active radar means the mission is currently not fulfilling its full set of Level-1 requirements because of the coarser resolution of soil moisture products from radiometer data only, the radiometer-only soil moisture and ocean salinity data products are uniquely accurate and have significant scientific and applications value,” NASA spokesman Steve Cole wrote in an Aug. 11 email.

With only a working radiometer, SMAP’s global soil-moisture maps are imaged at a 40-kilometer spatial resolution, meaning a single pixel on a SMAP map equates to that many square kilometers. If the radar worked, the soil-moisture maps would have a sharper resolution of roughly 9 kilometers. Despite the radar malfunction, NASA is still piping down data from SMAP. The agency released the mission’s first beta data set July 31. The final, fully vetted release of that data set is slated for November, Cole said.

JPL built and operates SMAP. Goddard is responsible for collecting, processing and distributing the data the spacecraft collects. The mission was one of the Earth Science community’s top priorities in the decadal survey published by the National Academies in 2007.