SAN FRANCISCO — Weather forecasting is particularly challenging in Alaska. The state not only experiences high winds and powerful storms, but its lack of sunlight from October to March often makes it difficult for meteorologists to use space-based imagery to identify low clouds and fog.
That situation is improving, however, with observations from the Suomi National Polar-orbiting Partnership (NPP) weather and climate satellite. Alaskan weather forecasters are using Suomi NPP’s Visible Infrared Imaging Radiometer Suite (VIIRS) “to identify low clouds and fog that would otherwise have been undetectable with no sunlight and modest moonlight,” said Carven Scott, acting meteorologist-in-charge of the National Oceanic and Atmospheric Association (NOAA) National Weather Service forecasting office in Anchorage.
VIIRS is a scanning radiometer built by Raytheon Space and Airborne Systems of El Segundo, Calif. It is designed to scan scenes repeatedly and amplifying light sources in individual pixels. Those data have been used to improve forecasts for air travel and to help meteorologists identify the edge of sea ice where freezing ocean spray presents a serious hazard to sailors. “Those are a huge concern for the crabbing fleet,” Scott said by email.
VIIRS is one of five instruments on the Suomi NPP satellite, which NASA sent into orbit on a United Launch Alliance Delta 2 rocket in October 2011. On Feb. 22, NASA turned operational control of the NPP spacecraft and its instruments over to NOAA. NPP, a satellite once intended to serve as a test platform for new sensors, has become a critical tool in NOAA’s weather and climate forecasting arsenal as it helps to bridge the anticipated gap between NOAA-19, a polar-orbiting satellite the agency launched in 2009, and the first satellite in the NASA-NOAA Joint Polar Satellite System (JPSS), scheduled to launch in 2017.
Even before NOAA took the reins of NPP, the National Weather Service; National Ocean Service; National Marine Fisheries Service; and National Environmental Satellite, Data, and Information Service began using data from its space-based sensors, said Mitch Goldberg, NOAA’s JPSS program scientist. NPP’s Ozone Mapping and Profiler Suite, built by Ball Aerospace & Technologies Corp. of Boulder, Colo., is providing data to monitor the ozone hole and the strength of the sun’s ultraviolet rays. Fisheries are using VIIRS ocean color data to identify many of the nutrients that sustain sea life. Detailed information on atmospheric temperature and water vapor drawn from NPP’s Cross-track Infrared Sounder and Advanced Technology Microwave Sounder are improving weather forecasts, Goldberg said.
NOAA is conducting an extensive effort to verify that dozens of different data products derived from NPP sensors are accurate by comparing the observations obtained by each instrument with measurements from other space-based, airborne and ground sensors. That calibration and validation effort will continue for two more years.
NOAA already has verified key data products used in weather forecasting. In May, NOAA completed its campaign to verify data from Northrop Grumman Corp.’s Advanced Technology Microwave Sounder and began funneling those data into numerical weather models. NOAA officials plan to follow suit with the Cross Track Infrared Sounder built by ITT Excelis of McLean, Va., in May 2013 and the Ozone Mapping and Profiler Suite in 2014, Goldberg said. Key data products drawn from VIIRS, including measurements of sea surface temperature and observations to highlight the fraction of an area covered by green vegetation, are scheduled to be released publicly later this year.
Overall, NASA and NOAA officials said they are pleased with the NPP spacecraft, its onboard instruments and associated ground systems. They remain concerned that NPP’s successor JPSS-1 launches in 2017 as planned to minimize the risk of significant gaps in weather and climate data. While many of the sensors flying on NASA and NOAA spacecraft have continued to provide data well beyond their intended lifespan, investigations have raised doubts about whether that will be the case with NPP. Reports published in 2011 by the National Research Council and NASA inspector general revealed technical issues that could limit the lifespan of NPP sensors to three years.
Since the NPP satellite was designed to serve as a risk reduction element of the National Polar-orbiting Operational Environmental Satellite System (NPOESS), a joint civil-military weather satellite program canceled in 2010 by officials in President Barack Obama’s administration due to mounting cost overruns and technical problems, its instruments were built according to a different standard than those of spacecraft intended to serve critical operational functions.
“The instruments were treated more like prototypes than instruments people would want to use operationally for many years,” said James Gleason, NPP project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md.
By the time the larger NPOESS program was canceled, however, the NPP instruments had progressed too far “down the engineering path” to start over, Gleason said.
Shortly after launch NASA officials noticed problems with VIIRS data. An investigation revealed a manufacturing defect that left tungsten oxides on the surface of a mirror, causing it to darken. Even with the manufacturing defect, the VIIRS instrument continues to meet its data requirement, Gleason said.
A report released Feb. 14 by the U.S. General Accountability Office warned that the failure of NPP and NOAA-19 before JPSS-1’s launch would lead to less accurate and timely warning of hurricanes, floods and storm surges. The “High Risk Series Update” said mitigating anticipated gaps in weather satellite data should be a top priority for the U.S. Congress and the executive branch.
The instruments on NOAA-19 were designed to function for at least three years. If NPP’s sensors also stop functioning after three years, NOAA might not have a functioning polar-orbiting weather satellite until NASA and NOAA complete on-orbit testing of JPSS-1 sensors six to 12 months after that satellite is launched. The General Accountability Office warned that the weather satellite gap could last between 17 to 53 months. The 53-month gap would occur if NPP failed prematurely and the JPSS-1 launch was delayed from 2017 to 2018.