104 APRIL2020|COMPUTERSHOPPER|ISSUE386
Someofthe earliest OS maps were at ascaleof1:63,330,
not toodifferentfromtoday’spopular 1:50,000 Landranger
maps, and soon, detailed 6in to the mile maps, whichequates
to 1:10,560, hadalso been produced.While these figurespay
tributetoearlysurveying skills, the useofchains, compasses
and theodolites sounds positivelyarchaic.Itwould be wrong
to suggest that surveying skills no longer involve field work,
but it would be equally erroneous to underestimate the huge
changes that have impacted mapmaking in recent years.
Here we’re delving intoone of thekey technologies of
modern daymapping–satellite imaging –withaview to
understanding howthe technology works, and how images
taken from an altitudeofseveral hundreds or thousands of
kilometres are puttouse in so manydifferent areas. Here’s
digital photography with adifference.
SATELLITE PRIMER
The concept of satellites whizzingtheirway aroundthe Earth
somewhere above our headsisn’t hard to appreciate,even if
the details are somethingofamystery.Tostartour journey
of discovery,therefore,we’ll spend abit of time thinking
aboutbasic satellitetechnology,sowe’ll then be in agood
position to understand thetechnology of satellite imaging.
The first thing to pointout is that, unlike the rockets that
carrythem intoorbit, satellitesdon’t need engines to keep
them in theirendless circles around theEarth, even though
theydoneedsmall engines forcourse corrections.However,
the path of the launchvehicle is cruciallyimportant. The
rocket is launched vertically, of course,but once it reaches
its target altitude –which must be clear of theEarth’s
atmosphere –itfiresits enginestoenter orbit. At thispoint,
the rocket can releaseits payload and, in theabsence of
atmospheric drag, asatellitewould continueinmotion.
If suchasatellitewas in deep space, it wouldcontinue in a
straight line,prettymuch forever,but it’s an interesting
question why,whenthey’reclosetothe Earth, they don’t
spiral down to Earthbecause of gravity. The answer is that
theEarth’s gravitational pull is balanced by the forward
motion of thesatellite, but there’s more.The critical velocity
that’s needed to overcomegravity depends on the altitude:
the lower thealtitude,the greater thespeednecessary for
the satellitetoremain in orbit. This means that the interlinked
altitudeand orbital speed, and hence also thetimetaken to
circle the Earth,are vitalmetrics in planning any satellite.
To give an exampleofanimaging satellite, NASA’s Landsat
8orbits at an altitude of about 705km, whichrequires a
velocity of around 27,000kilometres per hour,and at that
speed, it orbits the Earth in approximately 99 minutes,
following anorth-southpath.However,thisdoesn’tmean
that it passes over the same spot on the Earth’s surface every
hour and 39 minutes. Becauseofthe rotational movement of
the Earth, it only passes over thesame locationonthe Earth’s
surface after several orbits. It is, therefore,defined by its
repeatcycle, which is the period of timebetween passing over
the same pointand, in thecaseofLandsat8,thisis16days.
None of thisisatypical foraso-called LEO (Low Earth
Orbit)satellite. Thereare also MEO (Medium Earth Orbit)
and HEO (HighEarth Orbit)satellites, butaparticularly
interesting type is the GEO (GeostationaryEarth Orbit)
satellite. At an altitude of 35,786km, the orbital velocity is
11,300kilometres per hour, aspeed that causes asatellite
to orbit the Earth every 24 hours. Because it tracks the
rotationalspeedofthe Earth,therefore,itappearstohover
over aparticular spot,usually abovethe equator.
IMAGING TECHNOLOGY
Youmight be surprised to learnhow the very first imaging
satellites worked. TheCorona series of satellites, the firstof
which was launched in 1959,was intended formilitary
reconnaissance.Inthese pre-digitaldays, it shotontofilm,
which was returned to Earth in an escape capsule and
recovered by an aircraft in mid-air. The firstCorona had just a
single return capsule and, while this was laterincreased to
two,the lifetime of anysatellitesworking on thisprinciple
wouldalwaysbelimited by the amount of filmtheycould
carry. Needlesstosay,today’s imaging satellites have
on-board instrumentsthat couldbebroadlydescribed as
digital cameras, but with some substantial differences to
anythingyou could buy on the high street.
RIGHT:This photo
ofHong Kong,
taken in the
visible spectrum
bythe SPOT 5
satellite,shows the
quality oftoday’s
satelliteimagery
BELOW:The SPOT
6satellite,which is
operated byAirbus,
offers aresolution
ofup to1.5m
per pixel in five
spectral bands
