PING­ING THE WIND

Cruising World - - Under Way - —David Sch­midt

Spend enough time sail­ing and scour­ing weather fore­casts, and odds are good that you’ll oc­ca­sion­ally find a day (or three) that doesn’t quite stack up to ex­pec­ta­tions. Granted, we mod­ern sailors en­joy in­cred­i­bly high-res­o­lu­tion weather fore­casts com­pared to all his­tor­i­cal meth­ods, but in an age of in­stant ac­cess to high-qual­ity in­for­ma­tion, im­pre­ci­sion can be frus­trat­ing. For­tu­nately, the Euro­pean Space Agency’s new Ae­o­lus (after the Greek “keeper of the winds”) wind-sens­ing satel­lite, which was launched atop a Vega rocket from a site in French Guiana on Au­gust 22, 2018, aims to im­prove fore­cast ac­cu­racy by us­ing so­phis­ti­cated in­stru­men­ta­tion to pipe down ob­ser­va­tions from sec­tions of the planet that are not oth­er­wise cov­ered by the piece­meal Global Ob­serv­ing Sys­tem, which is com­posed of sur­face­and space-based sub­sys­tems (e.g., satel­lites, buoys, weather ships, ground sta­tions and weather-radar in­stal­la­tions) but lacks suf­fi­cient ob­ser­va­tions from the up­per tro­po­sphere and lower strato­sphere, as well as the trop­ics and wide swaths of the ocean.

The ESA’S new Ae­o­lus satel­lite fea­tures an At­mo­spheric Laser Dop­pler In­stru­ment (“Aladin,” for short) that serves as a kind of Dop­pler-en­abled li­dar (read: light and de­tec­tion rang­ing tech­nol­ogy that floods a tar­geted area with pulsed laser light and mea­sures the

re­flected backscat­ter with a spe­cial sen­sor). While there’s plenty of com­pli­cated en­gi­neer­ing in­volved, Aladin is es­sen­tially made up of a pow­er­ful laser (tech­ni­cally, mul­ti­ple lasers and am­pli­fiers), a pow­er­ful tele­scope (pointed at a 35-de­gree an­gle away from Ae­o­lus’ or­bit­ing plane) that sports an al­most 5-foot (1.5 me­ter) di­am­e­ter, and a re­ceiver.

To op­er­ate, Aladin fires short, pow­er­ful pulses of ul­tra­vi­o­let light down at Earth, which dis­perse as they hit at­mo­spheric par­ti­cles, such as gas, dust and wa­ter, which are in turn car­ried by the pre­vail­ing wind. Aladin then uses its on­board tele­scope and re­ceiver to mea­sure the backscat­tered light re­flec­tions, whose wave­lengths have shifted, slightly, thanks to the wind’s ac­tion. The in­stru­ment then uses its Dop­pler ca­pa­bil­i­ties to de­ter­mine the time be­tween when the light pulse was fired and when the backscat­ter re­turned, and it uses two op­ti­cal sen­sors (one senses for mol­e­cules, and the other looks for aerosols or wa­ter droplets) to mea­sure the Dop­pler shift to de­ter­mine the wind’s di­rec­tion and ve­loc­ity (Aladin’s max­i­mum ver­ti­cal cov­er­age ex­tends from Earth’s sur­face up roughly 19 miles). Fi­nally, Aladin uses its finely tuned photo de­tec­tors to change light sig­nals into elec­tronic ones, and it can pro­duce up to 100 wind pro­files per hour.

While this cer­tainly sounds like a lot of im­pres­sive sci­en­tific ca­pa­bil­ity, and while there’s no ques­tion that Ae­o­lus will help fill an im­por­tant gap in the cur­rent Global Ob­serv­ing Sys­tem, it’s im­por­tant to re­mem­ber that Ae­o­lus is a sin­gle or­bit­ing satel­lite, mean­ing that it can only look at a cer­tain por­tion of the globe at any one time. “The Ae­o­lus satel­lite is not a ‘sil­ver bul­let.’ It will be help­ful, but the net ef­fect on fore­casts has yet to be proven,” says Pre­dictwind’s Nick Ol­son, who ex­plained that the New Zealand-based fore­cast­ing com­pany pays the Euro­pean Cen­tre for Medium-range Weather Fore­casts a hefty an­nual fee and there­fore will have full ac­cess to (and ben­e­fit from) Ae­o­lus’ data. “It will be great to see how this data does ac­tu­ally im­pact [our] fore­casts. Hope­fully, it is pos­i­tive and will lead to more satel­lites in the fu­ture. So far as sailors go, this is just go­ing to be an­other tiny piece of the puz­zle slot­ting into place.”

Crews clean the Ae­o­lus satel­lite to make sure it’s free of dust par­ti­cles. A Vega rocket (op­po­site) brought the satel­lite to or­bit.

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