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If we think, firstofall,aboutLEOsatellites–or,forthat
matter, anysatellites otherthan GEOs –the major difference
between theirimaging instruments and anormalhandheld
camera is that they’re moving very quickly.Thisisn’t a
problem, however,but an asset, becauseitmeans that less
imaginghardware is required.
In particular,because the instrumentismoving over the
target, thesatelliteonlyneeds to capture images of single
lines of pixels perpendiculartoits direction of movement.
Becauseeachstrip will be of adifferent place on the Earth’s
surface,when theseare combined, atwo-dimensional image
is created. However,ifwebearinmindthat two-dimensional
image sensors canbeincredibly cheap,asevidencedbythe
priceofentry-levelcameras andphones, being able to usea
1D sensorinstead of a2Dsensor might seemirrelevant.
However,the sensors in imaging satellitesare no ordinary
sensors. Forexample, it seems hard to believethat any
ordinary camera couldpossiblyphotographobjects less
than ametreacrossfromadistance of several hundred
kilometres,yet some satellites do exactly that. To achieve
this,sensors with up to 40,000 pixels across (in the case of
the DigitalGlobe WorldView-3 and-4satellites) are used, a
figure that would equatetoamassive 1,600megapixelsifit
was expanded to two dimensions.
But that’s just thestart. As we’reabout to see,most
imagingsatellites have multi-spectral sensors insteadofthe
normal coloursensors used in ordinary cameras, and this also
adds massively to the cost.And then we have to consider
reliability.Whenasatelliteisgoing to costalmost $1bnto
design,build, launch and operate,reliability is paramount.
Forces of acceleration and vibrations during launch, the
temperature extremes and radiation suffered in orbit, and
the requirement of alifetimeofseveral yearsplacefurther
stringent requirements on thesensor.
Historically,two different types of sensorshavebeen
used to obtain those one-dimensional imagesofthe Earth,
perpendiculartothe satellite’sdirectionofmotion. The
type that was first used, because it offersthe ultimatein
reducing the number of sensor elements, and which is still
used forsome experimental missions, uses aso-called
whiskbroomapproach.
In this type of sensor–which is an analogyofthe waya
whiskbroom, or witch’sbroom,isused –there is just asingle
sensor element,which is scanned leftand right, at right
angles to thesatellite’sdirection of movement,using a
motorised mirror.The main drawback of this approach is
the potentiallyunreliablemechanicalscanning hardware,
and so the pushbroomsensor –thistime an analogyto
an ordinary household broom, as used on-board most
of today’ssatellites –cameintobeing. With this,there’s
an arrayofsensor elements, albeit in asingle strip as
opposed to the two-dimensional arrayofsensorelements
in regular cameras.
When we consider geostationarysatellites, neither of
these approachesworks because thesensor is at afixed
position abovethe Earth’s surface.Instead,somethingsimilar
to apushbroomsensor is used, butascanning mirror is used
to extendits coverage fromone dimension to two.GOOGLEEARTH
GoogleMapsiswellknown,partiallybecauseit’ssopopularfornavigation,
butanotherof Google’smappingprojectsislesswellappreciated.Thisis
GoogleEarth,which,inadditionto offeringlotsof facilitiesnotavailable
withGoogleMaps,makesmuchmoreuseof satelliteimagery,whileinthe
main,mostof Maps’imageryisderivedfromaircraft.
Sowhatarethedifferences?Primarily,Mapsisadown-to-earthtoolfor
planningvisitsandnavigatingto yourdestination.Earthismuchlessof a
practicaltool,beingverymuchanexplorer’sfacility.
Indeed,whenyoufirstvisitGoogleEarth,itwillexplainthatyoucan
chooseto goto adestination,howyou’llbepresentedwithknowledge
cardsaboutplacesof interest,thatyoucanorbittheworldin3Dby tilting
androtatingthemap,youcoulduseVoyagerto chooseapre-packaged
adventure,or, ifyou’refeelinglucky, youcan“seewheretheworldtakes
you”.GoogleEarthsatelliteimagerycomesfromseveralsources,including
NASAandseveralcommercialplatforms,withresolutionsasgoodas
500mmperpixel.
Ifyoufancytryingthisout,justheadovertoearth.google.com.Ifyou
haveasenseof adventureandwanderlust,orifyou’reonthesearchfora
newholidaydestination,wefeelconfidentitwillkeepyouoccupiedfor
manyanhour.CurrentlyitworksonlyintheGoogleChromebrowser,
althoughalternatively,youcandownloadGoogleEarthPro,whichruns
locallyonyourPC.ABOVE:While
GoogleMapsis
atoolforroute
planningand
navigation,Google
Earthispromoted
primarilyasan
inspirationaltool
forexplorers