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Tho ]Lo~ton boU& Of the Dfb-UFfliTf if3
dggomAnod by tho SAR chirp bndj'g'dth. 80
ohown in pig.b.a.2. for romp roaolutiona
betweon 2 and 5 m.
The inotmtanaoue SAR bandwidth mot bo
conp~ltdbll~ with international bmndwldth
allocationn Por Wedat nyntancr o,wratingl in
the various (Iroqu~ncy banda. Ht turn0 out
that, considering the 100 Wla bandwidth
allocation eOr C-bend S ~ ~ CPGICR~BO, Q 0 5 a.
rango roaolution limi* ,nuet bCJ ~cc~ptd,
or 8 oovera rootriction in tho ninimum
offnadir angl0 to about 40' would result,
impacting negatively tha covorrega of the
numbor of oatelliteo in tho CoPaUto~lation.
On tho othor :..a?, a 2 m rnnqlo eaaolution
at 20. .nimua offnadlr nnglo Is
cornpiatib.r ;th both X and S-band choices,
sinca bsnuuitho cf respectivnly 300 and
200 me are available lor this aarvlce.
Azimuth rooolution, in tho 5h.R etripmep
mod0, dlopcmb, from antenna langht and
lookn numbar. It is betWeen 2 ond 3 m for
antenna llonghts in the 4-4 m range, with 1
look. 'PRO situation is surnmvrizod tn Table
4.2.1. which identifies two possible
syston altarnatives:
--a nodliun-high reaolutLon C-bond SAR lor
conmtollotione in low to mdiun altitude
orbito:
- a high resolution X-hand SAR. for
constollotiono in medium altitudo orbits.
The 5-b~nd alternative ia rejoctcxl being
incompatible vith typlcel I lightsat
accommodation constraints. I !
Freq. Avail. Range Azim. Mmnlnun
band @:I reool. resol. tt/I;w
(mi21 (N (m) (Q)
s a00 2 2-3 ! 20
c Cl00 5 2-3 4 12
X 3GO 2 2-3 I not compa:ib.
(*) at 50' off-nadir, Nloaks-1
Table 4.2.1. Impact of frequency band and
lightsats constrainto on SAR
feaeibility
I
4.2.J X-band SAR: Swath, datarates, and-
RF pow0r
This section focuses on X band SAR trade-
offs. The swath, off-nadir anqloe,
rosolution, pulse lenght, orbit height,
and av0rage RF power are closely
interreAated.
At a nodium altitude or 360 #n and naximum
off-nadir anqle of SO', achievablo swaths
and averago RF powers v5. qround
rosolotion and pulse lenght aeo plottad in
Fig. 4.2.3. Increasing the puloo lenqht
one loonoo in swath width b.it &wok powero
deCrarl60 too. #Ore epeCifi\!i.lly for PUlOQ
lenqht greater than 20 mi:ronac. peek
powero bolow 1 KW are faar iblo, whilo
belov 15 microsoconds multikiltmatt poak
power lovelo result, inpactittq HPA
technology choice and relisblli.ty.
Fig. 4.2.3. Average tranaaitted RF power
ve. swath width, azimuth resolution and
pulse lenght.
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Swrth mldth. K.
Pig. 4.2.4. Data rate vs. swath width,
azimuth resolution and pulse lenght.
Orbit height: 360 Km: off-nadir angle: 50'
Fiq. 4.2.4. shows how the data rate varies
with swath and qround rssolution, assuming
to transmit 4 bits/sarnple, which is felt
to be adequate for most tactical SAR
imaging applications. The data rate
increases with swath and even more rapidly
with ground r0solution, and is therefore a
linitinq factor for such satellite
oyetemo. The assumed 200 PIbit/sec upper
Bound jq compatible with a 20 Km swath at
2 U reoolution, or a 35 Km swath at 2.5 m
rosolution. Lower data rates would imply
narrower svatha, and viceversa.
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