Air & Space Smithsonian – September 2019

Established by

France in 1990, the

geodetic network

known as DORIS

has its 50 or so

transmitting

stations spaced

more or less evenly

around the globe.

This one is in

Antarctica.

pushtheprecisionevenfurther.
ExistingVLBItelescopeshaveanunfortunate
limitation:Theycan’tmovefastenough.One
majorsourceoferroris thechangingatmospheric
conditionsintheskyovertheradiodish;older
antennasliterallycan’tslewfastenoughtoaccount
forthis,Lemoinesays.Newertelescopeslikeone
installedonKauai,Hawaii,in 2016 andtheone
installedatMcDonaldthisyeararefasterand
thereforemoreprecise.
Thefourthandfinaltoolusedingeodesy—a
systemcalledDORIS—sendstransmissionsfrom
Earthuptospecialsatelliteswhosereceiverscal-
culatetheirdistancefromthetransmitterbasedon
Dopplershiftingofthesignal.TheFrenchspace
agencyestablishedDORISin 1990 tohelpwith
sea-levelmeasurements,whichmeansDORIS
hasbeenobservingforlongerthananyother
geodeticnetwork.
Thedistributionof 50 DORIStransmitters
nearlyevenlyaroundtheglobeis anadvantagein
tryingtounderstandplanet-widechanges.Most
of thesatellitelaser-rangingandVLBIstationsare
in thenorthernhemisphere.“Ifallyourstations
areinEurope,youkindofknowwhatEuropeis
doing,butyoudon’tknowwhatSouthAmerica
orAfricais doing,”saysGross.
AsNASAupgradesitsgeodeticnetworks,it is
alsoworkingtofixtheimbalance.Anidealsystem
wouldhave 24 to 30 stationsdistributedevenly
aroundtheglobe,withatleastthreeofthefour
techniquesusedateachsite,saysMerkowitz.
Thatkindofobservingnetworkwouldachieve
theITRFgoalofmillimeteraccuracy.“Thebig
barrierrightnowis gettingthatglobalnetwork
in place,”hesays.
NASAis spendingaround$18milliona yearon
its SpaceGeodesyProjecttoupdateoldequipment
andbringnewinstrumentsonline.
Internationalpartnersaredoingtheirpartinthis
globaleffort.TheNorwegianMappingAuthority

is cooperating with NASA to place a satellite

laser-ranging system on the Arctic island chain

of Svalbard. Situated just 1,000 kilometers from

the North Pole, the system will have the best view

of polar-orbiting satellites as they fly overhead. It

should be up and running by 2024.

NASA is also working with the French space

agency to upgrade equipment in Tahiti and has

collaborators in Australia, South Africa, and Brazil.

“We’re looking continent by continent,” Merkowitz

told his audience at a meeting of the American

Geophysical Union in December.

Twinsatelliteslaunchedin2002,thensucceeded

bya follow-onmissionlaunchedlastyear,have

givengeodesistsanotherpowerfulyardstickforfine

measurementsofEarth-sizechange.Geophysicist

IsabellaVelicogna,at theUniversityofCalifornia,

Irvine,usesthemtotrackhowtheGreenlandand

Antarcticicesheetsloseweightinthesummeras

icemeltsandgainit backinthewinterassnow

pilesupandfreezes.TheU.S.-Germansatellites,

Basedondata

fromtheGravity

Recoveryand

ClimateExperiment

(GRACE)satellites,

scientistscantrack

thedwindlingof

Greenland’sice

sheetovera

decade.Orangeand

redshadesindicate

areasofgreatestice

loss.

54 AIR & SPACE airspacemag.com

THE NATIONAL CENTRE FOR SPACE STUDIES

2004 2005

2009 2010

0.5

0

-1

-2

Equivalent water height (meters)-3

Greenland’s Disappearing Ice