NASA’s Cyclone Global Navigation Satellite System (aka CYGNSS) is ready to launch. The fleet of eight satellites will take to the skies to measure the intensity of surface winds generated during a hurricane. What sets these satellites apart from others is the ability to “see” through heavy rainfall produced in the heart of these violent storms.
Over the past few decades, hurricane forecasts have dramatically improved. Forecasters are able to deliver more accurate models and better predict what track a hurricane will take. What hasn’t improved is our ability to accurately predict wind speeds. CYGNSS aims to change that.
“This is an amazing day for Earth science,” Christine Bonniksen, CYGNSS program executive in NASA’s Earth Science Division said in a news briefing. “I’m happy to say that the CYGNSS mission is on schedule, on cost, and on science.”
Previous hurricane-observing satellites have measured hurricane wind speeds using short wavelengths of light, which are easily scattered by heavy rain. Ultimately the data collected by these types of instruments are extremely limited.
“We’re hoping that this data will increase our ability to accurately determine how intense a hurricane is when it makes landfall,” Chris Ruf, CYGNSS principal investigator with the Department of Climate and Space Sciences and Engineering at the University of Michigan said in a news briefing.
To ensure CYGNSS was able to record the crucial measurements necessary to its science goals, NASA engineers hacked a GPS navigation receiver to relay the navigation signal as well as how the signal is distorted back to the CYGNSS satellites. GPS operates on longer wavelengths — roughly 10 times the size of a raindrop — which allows the signal to penetrate extremely heavy rainfall. The onboard instruments will then analyze the distortion signal to determine how fast the winds are moving.
CYGNSS will be measuring specular reflection — aka mirror reflection. When the GPS signals are released, they bounce off the oceans below and how the signals are reflected tells the satellites how rough or smooth the ocean is, and a storm’s wind speed can be determined from that. Each satellite measures the reflections from four spots simultaneously as they orbit over the tropics.
“Think of each satellite as a virtual hurricane-hunting airplane,” explained Ruf. “With a constellation of eight satellites, that’s equivalent to the work of 32 hurricane hunting airplanes.” He also explained that the satellites will operate for at least two years, and will re-measure any given spot every seven hours.
The CYGNSS mission cost approximately $162 million to build and run. In order to keep the costs low, engineers simplified the design.
“In order to keep costs low, each individual satellite has to be simple. So, each one has no propulsion system,” Aaron Ridley, CYGNSS constellation scientist with the Department of Climate and Space Sciences and Engineering at the University of Michigan, detailed in the news briefing. “The way we do that is by taking advantage of drag forces in the atmosphere.”
Ridley went on to explain that all eight satellites are launched on one single rocket. They are released two at a time, and spread apart very slowly to eventually cover the globe. He explained that it will take three to six months for them to be spaced about 3000 kilometers from each other.
So how do satellites with no on-board propulsion position themselves in orbit? To ensure the satellites are appropriately spaced (and not stacked on top of one another), each satellite may be periodically placed in what’s called a high drag mode.
Picture a biker cycling along as part of a marathon or major race. In order to minimize drag, the cyclist would tuck down. This is essentially what the satellite will do on orbit. If their position needs to be adjusted, ground controllers will tilt the satellites allowing the aerodynamic drag forces to slow it down.
Acting as a constellation, the satellites can beam back crucial data every few hours, as opposed to every few days. This offers a major advantage as hurricanes evolve very rapidly, so swarms can collect data more quickly than individual satellites.
“CYGNSS is a tool that will provide us 24/7 coverage of the tropical cyclone zone, and it will improve our knowledge of how hurricanes grow so that we can better prepare and protect the people in the path of each hurricane as it comes,” Bonniksen explained.
CYGNSS is set to launch on Monday, December 12, and is expected to be up and running before the next hurricane season.