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44
Annex p1 Optical system
B. 1 Background
This dcsign explorcs whnt might be achicvrd tising existing
space or military quality subsystems to build a high
rcsolution imaging systcm. The spacecraft has a space-
qualifird vcrsion of a commcrcial tclcscopc rnountrtl such
that it can bc rotatcd about an nxis aligned within - lo of
the dircction of travcl. A lincscan imager is niountcd at thc
focus of L)ic tclcscope. aligncd so that it swecps a swath
along he direction of travel (puslibroom).
The dcsign prcscntcd hcrc can provide 2 kin square imagcs
with a spatial resolution of I m. The image region is
selected by rotating thc tclcscopc for tlic acrosk-track
dimcnsian and sclccting thc tirnc of rccording for he
along-track dircction. Thc position of the satcllitc nlust be
dcterminrd to - 50 m in all thrcc dimensions and the
orientation of the tclcscoF to - I mrad in thrcc dirrctions
to permit the centre of thc irnagc IO hc sclcctctl to an
accuracy of - 0.5 km.
a 8.2 Telescope and sensor
The primary optical systcni is a spacc-qualirictl vcrsion of
tile standard Qiicstar 12" telcscopc. Thc kcy piiraiiiclcrs of
its specification arc:
0
-. rcsolution:
. focal Icngth:
- apcrturc:
0.38 arc scc
4.57 2 ni m
30.5 iii rn
- dimensions: -I ni long. -3.50iiiiii tliamctcr
-
~
mass:
vibration tolerance:
-SSkg
I0g
I '
- moterial: invar
. price: - 5 200 k
It is undcrstood that this tclcscopc has hccri iiscd on US
military spacecraft.
Thc baseline dcsign is to use thc Kcticon KAZO4HJ chargc
coupled dcvicc (CCD) detcctor opc'rnting in timc delay
intcgration (TDI) mode. It has to he'configcirctt so that its
long axis of 2048 sensors is pcrpcndiculnr to tlrc track of
thc spacecraft and the clock frcqucvcy in thc transvcrw
(short) axis of M dctcctors is synchronous with t!ic velocity
of he spacccraft.
The signal'iioisc ratio of thc dctcctor. D. is giwn hy:
D = r a A T s/N
v/hcrc:
r is thc radiance of the Earth. assumctl to be of thc
order of I00Wm-2sr-1. (This figure is sufficicntly
conservative la includc largc sun txriith anglcs
cncountcrcd at high latitudes.)
a is thc, area of yound ma pdd onto one pixel of thc
dctccror (nssiI.,Icd to tx I ni $ ) 1
A is the solid tin IC wbtcndcd by thc telescope
aperture (0.45 x \b-f2st at an altitude of 4(H)km)
T is thc integrating timc. dcfincd as the limc taken for
1
I
the spacecraft to fly 64m (85ms)
s is the smsilivity of the detector (480 nV I-')
N is the detecm noise level (200 pV RMS)
The signalhoise ratio is thus - 1OOO.
The detector pixels arc appoximately 2Spm square. This
requires the focal Icngth of thc optical tclcscope lo be 1Om.
A sccondary lens will be nccdcd but this docs not have to
be of particularly high optical quality. Thc type of lens
used as a a2 tclcconvcrtnr for 3Smrn SLR cameras would
probably be suitable.
R.3 Navigation and positioning;
If it is assumed that the nominal 2km square image must be
ccntrcd on the target position with an accuracy of f 5OOm.
it is necessary to know the orientation of the tclcscope to an
accuracy of the order of 0.5 mrad.
Navigation information will be derived from a GPS
rcccivcr and oricntation is obtaincd from a star sensor.
A possihlc iniplcmcntation of the GPS rcccivcr is to carry
out thc signal acquisition ~ t l
processing in software within
the on-board proccssing system. A baselinc dcsign using a
singlc.transputcr cxists and would mcct the rcquiremcnts
with a power consumption of 4SW. It is likcly hat a more
approprintc proccssor could he used to rctlucc his.
The star scnror is mounted on thc tclcscope to cnsurc that
thcrc is a constant angle betwccn the two. If it is assumed
that the prapcrtics of the star scnsor are:
. ficld of view: 25O
- niimhcr of stars for rcliablc fix: 6
(hcncc ncrd to iisc stars of 5th magnitudcj
- spacccraft roll ratc: IO rnrad s.I
(= - I revolution cvcry IO minutcs)
. integration he: SO ms
- flux from 5th magnitude star: 2.5 x IOl4 W
- detector sensitivity and noise: as Kcticon abnvc
- sensor apcrturc: 40 mrn diamctcr
~ dctcctor
array: 500 x 500 elements
then it will bc capable of meeting the targct of 0.5 mrad
accuracy if it is possible to intcrpolatc to onc half of a pixel
with a signal/noisc ratio of around 70. This is considcrcd to
bc well within thc pcrformnncc of currcnt interpolation
algorithms. I