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, Figure 2: UL FDMNTDMA f hc sln~clurc
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hopping is a.9sumed (i.e. he hop rate is lest Oian be G *.U
rate). a DPSK rype dui.4on it rnwe amcnahlc to
attaining the MDR data rata
2.2 lmpalrmrnb
In or& tn combat various channel impairments Such as
athlitivc miw, fading. and scintillation LI well L% jamming
hreau it is atsum4 that communication data pnrcssing
tcchniqurs such as FEc ding. interleaving. divcrsiiy a d
tim permutation of data hops arc implemcnicd in thc
systcm. In acidition to thc above channcl and EChl' thrcats
drc system must bc crpahlc of operating unclcr system and
equipment iniluccd imprimcnts such as dopplcr cffctir
due IO be relative motion bctwcc.cn the payload aod thc
GT. rrlative payload to GT clock drift. RF pup &lay
variatim. and RF frrqucncy respo~sc m the hnp-to-hclp
rmplitudc varia!ion at a func:ion of frcqilrncy. rhcse
impaimenla arc listd in Table II along with thr assurncd
valucs for t.k pu'p~ct d discusqion in this pnper. The
dopplcr effect is ncgligibte for geostationary orbiiq.
however for non-geostationary. elliptical type orbits such
as Tundra or Molniya. the dopplcr cffccm can bc ax scvcrc
as documentnl in Toble 11. These wbi~r would he rquircd
in a tactical theatre of operation ai northern latitudes.
pmicularly in the Arctic.
2.3 Llnk Budget Requlrernenb
For a TACSAT EtlF system. it is awumal that a 5W
SSPA is crnploycd on the payload. Largcr amplifiers
would rcquirc the uw of a TWTA. which wwld irnpsct (RI
the available si72 and wcight of thc payload. Thz antciina
is asumcd to be a spot beam with a 5" bcaniwidth. For
this size of amplifier on the payload, typical rcccivc C/N,
lcvels at h c ground terminal will vary from 50 to 60 dB-
k{z 'or a gWSlaliOEary orbit in clcar sky conlirions. It
should bc noted that thc link budget must also
accommodate rain fade margins on the UL and the UL.
Depending on the desird fadc margin this could
potatiallv restrict Ihe DL burst data rata unda given
channct conditims.
2.4 Inltlal Acqulsltlon Estlrnates
The complexity of the ac.,uisition phaw of the
synchronization proccw will dcpcnd significantly on the
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Tabk 11: MDR System Impalrmenb
Group Delay
Variation
Fnqumy Rcsponw
Variation
+I- ? dB
laming I channel dcpendcni
magnitude of the crrm in thc initial GT estimates of
payload pointing. frcqucncy UKI timing.
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2.4.1 Timinu Eshwtc
The error in the initid time estimate is detmnincd by the
availability of M accurate time mfemcc and the relative
rate of drift of the time standard maintained by the GT
with rc.spcct to Ihc reference time rtandzrd in the payload
With the availability of single bard GPS rcccivcrs. an
MDR tcnninal CM easily maintain time accuracies 017 "IC
ordcr of microstcondq. If GPS or similar time rcfcrc c
systcm.9 M MI available. the initial error in the cstii :e
of the timc enor would likcly be on the ordcr f
milliseconds. Givcn the current level of technology. '1
MDR terminal ;an catily accommodate a rubidiL:ii
standard. A typical vallrc of rclative drift rate for a
rubidium standard is 5 x IO'* per day. Thc resulting cmr.
evcn aftcr several drys of uncalibrated usc would still be
less than a miaosccond. For pitrposcs of discussion, an
initial hing mor is 50 millisccmds is asumcd.
2.4.2 Spatial Pointing Esfimai:s
Thc accuracy of the initial spatial pointing cstimall
dcpcndcnt on three principal parameters; namcly, thc t
in the GT's estimate of its own location. he initial tin
estimate, and the accuracy of the GT's ephcn1cr.C
algorithm. Thc dc~~ndcncy on the time estimate rcwlu
from he fact that the GT's estimate of the satcllirc
position is an outpul of thc cphcmeris algorithm. which
employs !he currcn~ timc estimate as an input. If GPS
avai!dbk. boUi the initial timc estimate, U well as .
GT's position will be known to a high dcpe d accuruc .
Furchcrmore. if the MDR bau station is fixed in a tactic-I
theatre of operation. thc location of the CT will typicn .
be h w n
to within 10's of mclers, either from w r y
navigational aids and an accurate time standard can