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In order to erform a useful
recoanwJssaPnm PO P e, the SAR must be
capable of grovidina imnBcs of s
target zones such as airPdeMs, ~nnn ours,
choke pints aand railwa iwsnaa@rttism at
the finesa possible reso I ctiow. This task
calls PCDB Sptiigha operation ian order :o
provide adequate resolution in the
alon~-trrmch (azimuth) direction and the
provnsiorm of this ca ability tis major
the moer~p~a.
driver in ohe design o P
Pfic
in prirnciple, the azimuth ssmning
associated with Spotli8ht ogerauiow can be
provided by rwssh6ulical steeuiun of the
antenna. However the diff P acnalties
assxiaaed wiah repeating such mcchdd steerin several times during t ab@c pm
over t P
ce theatre are believed 80 be so
great ahat electronic steering in the
azimuth direction is considered to be
essential.
In addidion, it is envisaged that a
TACS4T $Ail% my also be required to
rovide s~m~illmce over a larger re@n.
!uch cmer cm readily be grmdm!l in the
form sP 8 continuous strip-map m5in.e~ the
convenan'aswal Steered SAR mat& which
has been characterised by SEASAT,
ERS-1 and the SIR missions.
I
In this mode, which may bc sombiuned
with Scmsm, interferometric tcs8ndques
, I
Access coverange width is fundamentdl
limited by system parameters whic
include; antenna areq.slant range to
ground, RIF carrier fre'quency, and
satellite ve8scity - although there is no
si miificzm~ impast from satellite veiocity
w il 'ch cbmges but little with altitude. FOP
thio w e in whish antenna area has been
constrained to 6m2, the dominant
armeters are slant range and RF carrier
aequenq.
P
The effect of changes in altitude ou access
mQ sensitivity is show in Figures 3 and 4.
The data are presented for the case of
operation inn Spotpi ht mode with an
a.sumed brandGdah o P 3WIwIM[z. Althou
this bandwidth is characteristic of t i? e
WAR@ Umio at X-band md is in excess of
tile ddB~~tiom at S and C, it is nontheless
&an 4w.
It can be seen that at the lower
frequencies, iwsrreaine altitude provides
relatively small iw tovements in access
ca ability while 10 t E e higher frequencies,
su 1 s ~ improvements ~ d cm be realised.
Wowever, in both cases, increasing
dhde sipificmtly de~iades sensitivity.
The seelmsi@vity data ~ P W E Q R in ~ Figure ~ 4
shw ahe worn case sensitivity; i.e. at the
PMnBseso dlistamse from nadir for which
ampmble owbigunity pdomance cihn be
achieved Dam shwng the variation of
h