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The dediated nature of the sumeiUUance
required horn the satellite mea th;n~ the
periods dnardwg which service is required
are of short duraaion. T idly* a ~tellite
in the b w UP^ orbitTU0) wccsesmy
for SAR sprations takes only 5 minutes
to traverse a "heatre meEasursw6 2QntQkm
on the SidG! ad with only 2 such passes
likely day, payload power demands
can readily be mct from battcmies. "be
advantqy of this to the platform desip is
that relativdy smdl solar arrays cm then
be used to collect electrical pwer and
trickle-charge the batteries durisna the
prolonged periods of payload inrachty.
2.4 Ssmm Dosdgm Gads
The desien oals for the icnsor concept
are show in $. able 1. A major diffiwlty in
realising a radar desien off this
sophistication is the achievement of such
high resolutions and in the
im lementation of a phascd array antenna
ab P e to operate over isuch wide
inwmmxmsi bandwidths. /
Synthetic Aperture Radar '(SAR) is a
sideways looking coherent irritn~gimg
technique an able of produein8 rnq14k.e
imagery. {AIR employs difftlmttnt
principlles foi achievin rcsoludon in oQle
two dimensions of the imaBery.
Convewaisnal high resolutiooa radar
techniques such as transwniutian~ a
wideband, lowe duration pu8m caad
correcolg anmatch~n? to it on reer+' bUWri! we
employed in Ppnc dunension pqwmdidm
27-3
to tRK2 m?!eMiu@ 8rn& Tcbk a iflmu. .a
the GpplrndWidOBls; commeun5ura8e \%;am
rmlenaiiom BOP 8 dcty of
me G~U8~0rn 00 a&mal$Be bm&duI$
me due 00 U$@ capabili0ies of vavefom
d pew era to as mud digititseas and the
dispersion Ullnl.aParegPg uhe U
HeceiVg Chai!?I§. '&aPeDt
impose$ $I redlo~c limit of
Unless cbmps can be aopceed, a huPaher
limitirolg fac~au will be the current
o rating band Jlmdom defined by the
Gdd AdViSQV %tdk COUlndl (wmc)
which im se restriaionas of 300 MWZ at
X-band (GGpB9) and WO MWZ at J-band
(13.7CIHIa). Dispersion in the atmosphere
md ionos here is not thought to present a
majjou pm $, Bern and, if present, the effects
could be removed using techniques
equivalent to S~U~Q~OSUS. Ovtr31 it is felt
that a resolution of Irn at baqd is
feasible at a 30' gnhg angle.
The synthetic aperture technique is
employed in the damemion parallel to the
satellite track This involves the coherent
addition of many pulses to synthesize an
ogertuue sufficienoly Pong to provide the
desired resolution on the ground. "his
synthesis is only passible if the variation in
phase pasah from uhe sews& to the tar et
and back is k n ~ to ~ 8 n fraction of t fl e
radar wavekngoh along the synthetic
aperture. This implies that very accurate
knowledge of the orbital path of the
satellite is required. Hobper,, for
resolutions of the order.;i';@€., Im no
measurement method of &@t;bkbit gives
sufficient acmacy in realistic timescales.
To sumournt this problem it is expected
that autofocus techniques will have to be
employed. These have the added benefit
that they will also automatically correct
for other effects which can alter the phase
path length, such as atmospheric,
ionospheric and gravity perturbations.
3 TACSAT SAR Sjettellite Concept