On July 3 and shortly after dark on California's Mendocino coast a rare (for this state) series of convective cells lit up over the sea and moved toward shore. I went to the oceanside window to check whether it was lightning casting a flicker over the kitchen table as we ate dinner. Our 9-year-old grandson scampered eagerly with me (actually he was 8, but Jul 4 is his b'day). I raised the shade. Vivid bolts flickered on and off beyond the headlands. He realized in a flash and that ain't metaphor that the display of nature's power unfolding before us was not at all what he expected from book drawings. He ordered all the shades down. By this time the thunder was rolling. When I later went to look out from the other side of the house he sternly commanded, "Grandpa, do not open that door!"
There is a point to this and it has to do with media news of hurricanes and GPS systems, as foreshadowed in that hed up top. Marching on…
We got little rain as the thunderbumpers moved past to both north and south. Soon little guy and his brother went to bed (they decided to sleep in our room, the younger in our bed, and no way Jose not by themselves in the attic sleeping loft). We repaired to the living room. All of a sudden the Samsung screen pixellated a few times, then got worse. "No satellite signal," said a note the DirecTV receiver slapped up where a news show was supposed to be. Hmmm – could one of the evening's thunderstorms between us and the distant geosynchronous satellite hovering lowish on the horizon do that? It wasn't very windy where we were. The sat dish itself sat unquivering. The next day dawned clear. The TV worked again. Not living in the Midwest or anywhere that T-storms are common, and not having had satellite service before, I had no experience to draw on. But all that wind, blowing rain, hail, electrically-charged air masses, lightning and other mischief nearby gotta mess up a radio wave at least a little bit.
Then yesterday along comes a press release. In a switch, I'll provide the release first and then a few media stories covering its news tip.
- American Geophysical Union Press Release: Distorted GPS signals reveal hurricane wind speeds.; About a paper being published in the journal Radio Science and that sums up research results accumulated for 20 years but only lately coming into practical use.
Question answered, thought I based on the release's title. The wind blew my DirecTV signal apart, somehow! But….no. The release and news stories below explain that GPS signals penetrate hurricanes and presumably thunderstorms just fine. The signals then bounce off the surface. The echo is recordable by aircraft, such as NOAA's hurricane hunters. Turns out that the rougher the ocean, the more fuzzed up the reflection is. Roughness of water is a decent proxy for the wind blowing cross it. Ergo, without dropping pricey dropsonde instruments out of a hurricane-probing airplane's belly to be lost after just one use, one can compare a free GPS signal coming from the sky to its reflection from below for a pretty good idea over broad areas how thoroughly churned and thus windy is the sea surface.
Fact is, I did not entirely understand from the release, written by the paper's first author and apparently not by a press officer, what happens to the GPS signals. I needed to read some additional cracks at it. So let's see who went beyond the release and tried, as professional reporters should, to expand and enrich the info in the handout:
Media stories:
- National Geographic – Brian Clark Howard: GPS Reveals Hurricane Wind Speeds; Good on Mr. Howard. It would have been better to also interview some outside source, but at least he interviewed the lead author for himself. In some aspects this story does a better job at explaining things than did the scientist (again, who wrote the press release). For instance, it more simply explains that hurricanes do not rough up the surface of the land as much as they do the ocean. The point: This method only works well over the ocean or very large lakes. Howard (as does the release) also reports that fully 60 percent of a GPS signal reflect from the ocean. That seems impressive. One wonders if sunlight reflects that well.
- CBS News – Danielle Elliot: Using GPS to monitor a hurricane ; Elliot too talked with the author. She gets to the essence of the story fine, but misleads readers in the lede: Global positioning systems can be hard enough to use n a clear, sunny day. But when it rains, or a storm hits, the satellite signals tend to go haywire..". Perhaps they do get fuzzed on the way down, but its the surface reflection off the ocean that scientists are using. This account glosses over some detail, but adds one not in the release: The GPS technique requires only tiny receivers of the sort in cell phones, hence is perfectly suited for drone aircraft smaller, safer, and cheaper than large aircraft loaded with technicians and scientists.
- Geoawesomeness blog – Muthukumar Kumar: Measuring Hurricane Wind Speeds Using Distorted GPS Signals ; This is on the geeky side but does put the research into context of known complications of GPS, mainly the multipath problem and how it is handled. The new research, he explains, turns multipath from problem into asset.
- Houston Chronicle / ScienceGuy Blog – Eric Berger: Amazing use of GPS technoloyg: Measuring wind speeds of hurricanes ; Berger as usual is savvy. He calls the new technique brilliant. This is a shorty, however, and does not seem based on anything beyond the paper and the release.
Hmmm. Both release and some accounts report that the research team is considering exploitation of more powerful signals from radio and TV broadcasting satellites. Good luck. After my recent experience with DirecTV suggesting that sometimes those signals don't even make it through an intense storm, I'd like to know more about how this might work. And why GPS signals are not stopped by storms.
Grist for the Mill: Radio Science abstract;
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