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=mitrioa~ ~ ~ h i frown ~ ~ e alleimdk f off
5wEraw ffm bth c and x bmd Hs gmE:waed
in Fipw 5.
A iuruhea impact of increminn antitudc
can be wen in Figure 6 where 8 CI iumgact
on the amms track component of s&d
resolm8ioaa, rm e resol~~io~ IS been
presented as a B unction of dista~~ce Iuom
nadir md ~geri~ing altitude for 8 sh@e
RF chirp hd~c8tI.J - 3WMk In sele&une an WF carrier frieqancnacy for a
spacc2-bwe high resolution rnnli8m-y SAR,
several faactors must be taken into
accomu. First, is must be noted that, in
rincigle, my grscpunmd raqe rewinntiorn cm
[e- rnsixievcd by provision of 8 radar
emission puke of adequate bmdcdd~h -
id cxtmmples axe shown in Fipm 6 for
% w e of 3mMm bm~dtltn deration,
but inuemational agreements constrain
the bsaw&d& rm8Umaed to space hdm as
indicated in Table 2b. Howevcn; althou@
such apxmms might be brcalrfhcd in
times oh crisis, the TACSAT mission
concept also indudes peacetime uw and
for this, udhercnce to ,barndwidth
allocations is essential. f
The fipcs presented in Tables 2!a nmd 2b
indicaec uhao only an X or 4 bamil sysuem
would be capable of provichg a pound
resolution close to lm, whilst the uvs of E,
C or band wouid fail to provide a
ground resolution better thaw 3wq over
most OP the pazing angles of inuermo.
In addition to the internaaioamal
regulations, other factors affesaiwg the
choice of WF carrier frequency are as
follows;
- the need for the SAR wavelcaagth to be
significant1 less than the ima e
resolution. #his is likely to exclude t 1 e
use of Lband (0.23m waveleaguh) for a
system aiming to achieve around ,lm
resolution.
- ionospheric dispersion. This is a
frequency dependent phenomenon
which can cause a signifficant
degradation in ima e qualiay for
frequencies around or %e low Lbmd. ,
27-7
atmosphwrie driop~~nion. This is
8er4eaanny, IUDQ Q ~001em for
irequewcna below X- B and, but may
degurmde image quality at bighcr
frequencies, s.8. around the water
vqxu~ rmmm frequency of 2ZGBHZ
atmosphsric attenuation. "his varies
wib ffreqnnemq, Preconmhg more acute
POP frequcndes abe: X-bm$ possibly
din onat J-hd and probably ruling
out & -band. Wowwer, the attraction
of the very wide bandwidth allocation
at I-bmd ad the assodated potential
for very fine spatial resolution may
mitigate in its favour.
~OQPQP efficiency of solid state power
a IP, p B i f i e r s at hi 8 he r ope rat i ng
frequencies These factors inchcite that
X-band is likely to be an optimum
frequency for very high resolution
(around h@nilit surveillance SAR,
with C QP possibly Y band representing
a viable alternative providing
somewhat p rer resolution.
4.3 Ima@mB CapabLPPQBos
For the TACSAT SAR mission
considered in his apr, target imaginq
requirements may e considered to fall
into the foilowing two m+in categories;
(i) deaecction and classification of hard
targets, e.g. military vehicles, tanks,
pounded &craft, ships, etc.
(ii) monitoring md change detection for
distributed targets, e.g. airfields,
military camps and instaliations,
~IP~QUBS, railway yards, choke
points, etc.
! In the fomcr cases, the targets of interest
generally have large radar cross sections
relative to the background scene. As a
result, the ability to detect and
subsequently claify such tar ets depends
primarily on the spatid reso f ution of the
ima ery, the s stem radiometric
per P ormance Ling relatively
unimportant. A spatial resolution of at
least 3w, and ideally less than lm, is
.< '
4' *,.
considered necesury. (#' . 0 , : . :
.. . .
In the latter case, howrcvef,~t~~~features of
interest anre general1 no brighter than
other features in t i! c scene; the key