QuikSCAT's Replacement, the RapidScat Ocean Wind Sensor, Installed on Space Station

In November 2009, one of the greatest success stories in the history of satellite meteorology came to an end when the venerable QuikSCAT satellite failed. Launched in 1999, the QuikSCAT satellite became one of the most useful and controversial meteorological satellites ever to orbit the Earth. It carried a scatterometer--a radar instrument that can measure near-surface wind speed and direction over the ocean. Forecasters world-wide came to rely on QuikSCAT wind data to issue timely warnings and make accurate forecasts of tropical and extratropical storms, wave heights, sea ice, aviation weather, iceberg movement, coral bleaching events, and El Niño. Originally expected to last just 2 - 3 years, QuikSCAT made it past ten, a testament to the skill of the engineers that designed the satellite. A QuikSCAT replacement called ISS-RapidScat was funded in 2012 and built in just 18 months. RapidScat was successfully launched on September 20, 2014 on a SpaceX Dragon cargo spacecraft, which docked last week with the International Space Station (ISS.) This morning, RapidScat was plucked out of the Dragon and install it on the Space Station. The heaters have been turned on, and full activation of RapidScat is expected on Wednesday. In a clever reuse of hardware originally built to test parts of NASA's QuikScat satellite, RapidScat cost NASA just $30 million--80% lower than if the instrument had been built new.


Figure 1. Members of the Robotics Flight Control team at NASA and CSA (Canadian Space Agency) sent commands from their ground control center to the ISS to move the robots Canadarm2 and Dextre to install RapidScat on the Space Station on Tuesday, September 30, 2014. The white cylinder is the SpaceX Dragon, docked at the ISS. Thanks go to NASA/CSA's Kamran Bahrami for correcting me on where the commands to install RapidScat originated from. Image credit: Alexander Gerst.

What RapidScat will do
ISS-RapidScat is a radar scatterometer designed to sense near-surface winds over the ocean. The instrument sends a pulse of 13.4 gigahertz microwaves towards the Earth’s surface and measures the intensity of the return pulse that reflects back from the surface. In general, strong radar return signals represent rough surfaces, while weak radar return signals represent smooth surfaces. Stronger winds produce larger waves and therefore stronger radar return signals. The return signal also tells scientists the direction of the wind, since waves line up in the same direction the wind is blowing. The ISS orbit takes the space station between 51.6°N - 51.6°S latitude, and RapidScat will not be able to "see" ocean winds at high latitudes beyond 57°. QuikSCAT measured winds in a swath 1,800 km wide centered on the satellite ground track, but RapidScat's swath will be only 900 km wide, since it is orbiting at a lower altitude (375 - 435 km high versus 800 km for QuikSCAT.) The instrument will be able to "see" with a resolution of up to 12.5 km (7.8 miles.) It completes 15.51 orbits per day, and revisits the same part of the ocean beneath it once every two days. This compares with QuikSCAT, which covered 93% of Earth's surface in 24 hours. The advertised accuracy of RapidScat winds: for wind speeds 7 to 45 miles per hour (3 to 20 meters per second), an accuracy of about 4.5 miles per hour (2 meters per second); for wind speeds of 45 to 70 miles per hour (20 to 30 meters per second), an accuracy within 10 percent; for wind direction, an accuracy of 20 degrees. Precipitation generally degrades the wind measurement accuracy, and accuracy is also reduced at the edge of the swath. Useful data from RapidScat will likely not be available for several months, to allow time for the scientists to validate and calibrate the data being taken. RapidScat's lifetime will be relatively short--just a two-year mission is planned. Scatterometer data is extremely valuable for many aspects of hurricane forecasting, providing early detection of surface circulations in developing tropical depressions, and helping define gale (34 kts) and storm-force (50 kts) wind radii. The information on wind radii from scatterometers is especially important for tropical storms and hurricanes outside the range of aircraft reconnaissance flights conducted in the Atlantic and Eastern Pacific basins, and for the regions where there are no reconnaissance flights (Central Pacific, Western Pacific, and Indian Ocean). Accurate wind radii are critical to the National Hurricane Center (NHC), Central Pacific Hurricane Center (CPHC), and Guam Weather Forecast Office (WFO) watch and warning process, since they affect the size of tropical storm and hurricane watch and warning areas. Between 2003 and 2006, QuikSCAT data were used at NHC 17% of the time to determine the wind radii, 21% of the time for center fixing, and 62% of the time for storm intensity estimates.


Figure 2. Artist's rendering of NASA's ISS-RapidScat instrument (inset), which was sent to the International Space Station in September, 2014 to measure ocean surface wind speed and direction and help improve weather forecasts, including hurricane monitoring. It was installed on the end of the station's Columbus laboratory on September 30. Image credit: NASA/JPL-Caltech/JSC

Alternatives to RapidScat
There are two alternatives to RapidScat available, but neither are as good. There's the European ASCAT satellite, launched in 2007. ASCAT can measure global wind speed and direction twice per day. However, ASCAT sees two parallel swaths 550 km wide, separated by a 720 km gap, and I find it frustrating to use ASCAT to monitor tropical storms, since the passes miss the center of circulation of a storm of interest more than half the time. On the plus side, ASCAT has the advantage that the data is not adversely affected by rain, unlike RapidScat. The other main alternative, the OSCAT instrument, which was sent into orbit on September 23, 2009, on the ISRO (Indian Space Research Organization) Oceansat-2 satellite, suffered an irrecoverable instrument failure on 20 February 2014.The other option is the Windsat instrument aboard the Coriolis satellite (launched in 2003), which measures wind speed and wind direction using a different technique. Evaluation of these data at NHC and NOAA's Ocean Prediction Center (OPC) shown the winds to be unreliable in and around tropical storms.

Related info
QuikSCAT, 1999 - 2009: R.I.P., my November 24, 2009, blog post.
Giving thanks to the Hurricane Hunters and QuikSCAT scientists, my November 21, 2007 post.
Challenging Bill Proenza's QuikSCAT numbers, my July 4, 2007 blog post.
2007 NOAA QuikSCAT user impact study.


Video 1. ‪RapidScat: NASA's Newest Wind Watcher‬

Quiet in the Atlantic
An area of low pressure (Invest 97L) a few hundred miles north-northwest of Bermuda is under high wind shear and is not likely to develop as it heads north and then north-northeast out to sea. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave 97L 2-day and 5-day odds of development near 0%. Elsewhere in the Atlantic, the UKMET model is predicting development of a tropical wave on Sunday in the vicinity of the Cape Verde Islands from a tropical wave predicted to come off the coast of Africa on Saturday morning. The GFS and European models give some lukewarm support to this idea. An upper-level trough of low pressure over the Eastern Atlantic will bring high wind shear to the region early next week, making developing difficult.

Eastern Pacific's Rachel weakening; new tropical disturbance 90E developing
In the Eastern Pacific, Tropical Depresion Rachel has weakened to 35 mph sustained winds, and will move little this week and dissipate without affecting any land areas.

An area of disturbed weather (Invest 90E) was located a few hundred miles south of Acapulco, Mexico on Tuesday morning, and was headed west-northwest near 10 mph. This disturbance has good support from all three of our top tropical cyclone genesis models to develop this week. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave 90E 2-day and 5-day odds of development of 50% and 90%, respectively. 90E is a threat to bring heavy rains to the Pacific coast of Mexico throughout the week. So far, though, 90E's heavy rains have remained offshore, as seen on satellite loops.

Tropical Storm Phanfone a potential threat to Japan
In the Western Pacific, Tropical Storm Phanfone passed through the Northern Mariana Islands with 50 mph sustained winds on Tuesday morning, and is expected to intensify into a major typhoon, possibly a super typhoon, late in the week. The 00Z Tuesday runs of the GFS and European models show the storm will recurve to the northeast very close to the coast of Japan this weekend.

Jeff Masters