INTHEDAVISMOUNTAINSOFFARWESTTEXAS,
at theUniversityofTexasMcDonaldObservatory,
astronomersspendtheirnightspeeringatthe
starsthroughsomeoftheworld’smostpowerful
telescopes.Soonthey’llbeaddinga moredown-to-
Earthjob.Withinsightofthegiantdomes,NASA
is installinga sprawlingnetworkofequipmentto
helpresearchersstudyplanetarychange.
LastJuly,engineersachievedfirstlightona
toweringradioantenna, 12 metersacross,that
watchesforsignalsflashingfromcosmicbeacons
calledquasarslocatedinthedistantuniverse.
Nearby,scientistshavesetupnewglobal-position-
ing-systemstations—tripodstoppedwithbulbous
headsthatcommunicatewithGPSsatellitesto
determinetheirpreciselocationonEarth’ssurface.
Ona neighboringmountain,techniciansplanto
builda powerfullasersystemthatcanzapa beam
oflightuptoa satellite,thenclockhowquicklyit
reflectsbacktoEarth.Together,allthishigh-tech
equipmentwillallowscientiststopinpointindi-
vidualspotsofgroundat McDonaldObservatory
towithinmillimeters.
Similaractivitiesaregoingonallovertheworld.
NASAishelpingtoupgradeEarth-measuring
observatoriesfromTahititoSouthAfrica.Bylink-
ingtheMcDonaldmeasurementswiththeothers,
researchersaimtobetterunderstandhowEarth’s
shape,rotation,andgravitychangeovertime.
Calledgeodesy,thisfieldofscienceunderlies
almosteveryaspectofmodernlife,whetherit’s
usingGoogleMapstofindthenearestcoffeeshop
ordetermininghowsealevelis risingastheplanet
warms.“Youwouldneverthinkthatnavigating
yourcarisdependentonourmeasurementsof
distantquasars,”saysStephenMerkowitz,an
astrophysicistatNASA’sGoddardSpaceFlight
CenterinGreenbelt,Maryland,whomanagesthe
agency’sSpaceGeodesyProject.“Butit is.”
Earthmaylooklikea perfectbluemarbleinphotos
takenfromspace,butit is notpreciselyround.It’s
squashedat thepoles,andit bulgesat theequator,
makingit about 43 kilometers(27miles)widerthan
it is tall.Partsoftheglobe,suchasScandinavia
andnorthernCanada,arerisingintotheair,still
reboundingaftertheweightofthegreatnorthern
icesheetsdisappearedthousandsofyearsagoat the
endofthelasticeage.Theplanetalsoshapeshifts
throughouttheyearasmassmovesaroundon
itssurface—aswhenicemeltsinthesummeror
watercyclesbetweentheoceanandatmosphere
duringanElNiño.
Allthisinformation,andmuchmore,feedsinto
somethingcalledtheInternationalTerrestrial
ReferenceFrame(ITRF),thesystemscientistsuse
tolocateterrestrialobservations.Thinkofit as a
globalwebofcoordinatesthatdefineswhereany
particularspotonEarthis atanygiventime.“If
youwanttodoanykindofpositioningornavi-
gatingonEarthorinspace,it’salldonewithina
referenceframe,”saysRichardGross,a research
scientistandgeodesyspecialistatNASA’sJet
PropulsionLaboratoryinPasadena,California.
Known as the
“Potsdam Gravity
Potato,” this
visualization of
variations in Earth’s
gravity fi eld is
produced by
Germany’s GFZ
Earth science
research center
based on satellite
and surface data.
Gravity “highs” are
red, “lows” are blue.
Thedifferencesare
duemostlytothe
differentdensityof
materialsaboveor
belowtheEarth’s
surface.
TRACKING THE
EFFECTS OF GLOBAL
CHANGE, DOWN TO
THE MILLIMETER.
BY ALEXANDRA WITZE
AIR&SPACE 51
GFZ POTSDAM
September 2019