ISS-RapidScat delivers valuable measurements for weather models, storm tracking, and forecasting.

ISS-RapidScat measures ocean winds from the International Space Station. Image Credit: NASA’s Jet Propulsion Laboratory (NASA JPL)

What happens when a key instrument on a billion-dollar satellite ceases to function? When it can take decades to plan the launch for a replacement satellite, one might want to build an instrument on the fly – out of spare parts – and hitch a ride as secondary payload on a rocket bound for the International Space Station (ISS).

In 2009, the scatterometer on the QuickScat satellite ceased spinning after 10 years of beaming crucial ocean winds measurements to Earth. The satellite provided useful data for creating weather models, forecasting and hurricane monitoring. The loss of QuickScat was great, and NASA moved quickly to build a replacement scatterometer called ISS-RapidScat. Using parts originally built to test QuickScat components, a team was able to dramatically cut costs and work on an accelerated time scale.

A NASA Jet Propulsion Laboratory team was able to build the instrument for about $26 million, according to Ernesto RodrÌ_guez, principal investigator. Compared to $71 million for QuickScat, not including the launch vehicle, it was a bargain.

It is a long and costly road to develop, build, and launch a free-flying satellite.

The summer before the replacement scatterometer launched in 2014 RodrÌ_guez said, ‰ÛÏRapidScat is a pathfinder for a more agile, riskier NASA.‰Û He argued that this agility enables NASA to address gaps in Earth observations that would remain unaddressed if traditional methods were used to replace QuickScat.

ISS-RapidScat launched on Sept. 20, 2014, and found a temporary home on the space station. Another instrument was slated to take the scatterometer’s place in December 2016, but launch delays have extended ISS-RapidScat’s stay until October 2017. This is good news; since the instrument became operational, it has provided valuable information for monitoring and research.

This image shows average winds near the ocean surface for November 2015; the data was collected by ISS-RapidScat. Image Credit: NASA JPL

Thanks to ISS’s orbit, which falls between 51.6 degrees north and south, RapidScat also is advancing an understanding of how winds change from day to day and hour to hour. Free-flying satellites with scatterometers like QuickScat, and similar spacecraft used by the Indian and European Space Agencies (ISRO and ESA), are typically placed in a sun synchronous orbit; the satellite passes over the equator at about the same local time each orbit. The ISS orbit passes over the same points at different times; this path helps researchers to see diurnal changes over the course of each day.

Bryan Stiles is the science data processing lead for ISS-RapidScat. Stiles says his team is learning a great deal about the global circulation patterns from this instrument, and getting a clearer picture of El Ni̱o as well.

According to NASA: ‰ÛÏThe precursor and the main driver of El Ni̱o events is manifested in the weakening of the normally westward blowing trade winds, or even their complete reversal to blow from west to east, in the Western and Central tropical Pacific.‰Û In addition to affecting the local area where these fluctuations are occurring, El Ni̱o is known to play a part in extreme weather events ‰ÛÒ ‰ÛÏfrom flooding in California to droughts in Australia.‰Û

Stiles explained that comparing wind measurements from last year to this year’s measurements reveal what he describes as ‰ÛÏa very prominent signature of the El Ni̱o that we’re experiencing now.‰Û

Thanks to the station’s orbit, ISS-RapidScat also is well-situated for tracking high latitude storms because ISS passes over spots along the East Coast of the U.S. every hour for 12 hours each day.

‰ÛÏWe get a lot more sampling of those high latitude storms than we do of, say tropical cyclones,‰Û Stiles said. ‰ÛÏI think RapidScat is probably the most useful for that from a technical point of view.‰Û

According to Stiles, space-borne instruments can be particularly valuable because they allow forecasters to see the scale of the storm. From space, forecasters can see the entire field ‰ÛÒ 1,000 kilometers by 1,000 kilometers. For a storm like Hurricane Sandy in 2012, this can give a sense of the energy and size, which can help forecasters predict storm surge and potential flooding.

A scatterometer measures the roughness of the surface of the ocean, allowing researchers to infer wind measurements. The instrument transmits a radar pulse to the ocean surface and the strength of the backscattered signal is related to the roughness of the ocean surface. If the scatterometer’s radar beam was pointed straight down and the ocean surface was smooth, the backscattered signal would be quite strong.

‰ÛÏWhat happens with a scatterometer is instead you point off at an angle, so that if the oceanåÊsurface were that smooth, the energy would bounce off and go far away, much like when you bank a pool ball off the side of a pool table,‰Û explained Stiles.

When the wind is strong, the ocean surface is rough and more energy is scattered back toward the radar, thus allowing for the measurement of wind speed.

The raw data from RapidScat is beamed back to Earth where it is processed before it is made available to the public; this takes between two and three hours depending on whether or not there are any interruptions in the downlink from ISS. Once the information is ready, NOAA Center for Satellite Applications and Research (STAR) takes the data and puts it in a format that forecasters can use.

While ISS-RapidScat’s stay on the station has been extended, the planned loss of this instrument in 2017 will be felt because the U.S. currently has no future plans for a replacement scatterometer. Once RapidScat is gone, NOAA will continue to leverage partnerships with other space agencies and use scatterometer data from the European Advanced Scatterometers (ASCAT), which are on board European satellites MetOp-A and MetOp-B, launched in 2006 and 2012. ISRO has a satellite, ScatSat-1, currently scheduled for launch in June and, if all goes well, it will be operational in August. RapidScat will be used to calibrate ScatSat-1, which will help ensure consistency in the data coming from these instruments.

As part of the United Nation’s International Convention for the Safety of Life at Sea, the United States is responsible for providing weather forecasting and warnings for certain areas of the ocean. Joseph Sienkiewicz is the chief of NOAA’s Ocean Prediction Center Ocean Application Branch; his office is responsible for providing these forecasts and wind warnings for vessels at sea in the Pacific and Atlantic oceans north of 30 degrees north.

‰ÛÏWind is important; it’s the warning criteria,‰Û Sienkiewicz said.

He says that ocean data is relatively sparse when compared to Earth observations on land, and an instrument like RapidScat enhances situational awareness because the station’s orbit enables the instrument to pass over key areas multiple times each day. This offers more detailed information about the rate of change when storms are forming.

Sienkiewicz agrees that RapidScat will be missed when it no longer has a home on the station. åʉÛÏLosing a piece of information like that will have an impact, because we won’t be able to see things as often and we won’t be able to make as informed of a decision.‰Û