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6 SYSIXM SYNCHRONIZATION ALGORITHM
In a mticd en- it is parunount thrt the ovdl
synchroniutiaa time be kept to a minimum while
simultaneously providing capacity to synchronize many
users U axe Thb god CUI be rcannplis! hough a
combination of judicious kip d the initid pumtu
cstimatim caprbilitiw of the GT, selectiun of a robust
synchronizaticm dgcnilhm and optimization of the
puunetcrs of the synchmiutim algorithm Irrtrtirlg
synchrcmiuticm limc Tk availability at be ground
[ermind y~urue TOD urnition. an accunte
ephemeris algorithm will PI& duce the spatialtemporal
surch region, alloAg rapid acquisition
Howeva the algorithm must be robiust in h scnu hat if
ao~unte initial estimates arc available. the GT will still
acquire the paylod In addition to minimizing he
acquisition time, thc algorithm mut also simrlltmeously
minimi7~ the probability of false synchroni7~tion
Cansidering the DL md UL acquisition proce*ua
separately. the overall probability of correct
synchroniution and false synchroni7+tion ue given by
in which PdJlL), PD(UL). PpA(DL), md PpA(UL) uc the
probabilities of dcteaion and fa1.w alarm for he DL hnd
tJL synchrdzation pmcessu. Figure 4 illusmuid a
simplified stlie diagrun of the overall synchronization
process. The design of the synchronization algorithm
involves choosing the vdues of POL). P W), P,(DL),
and PpA(UL) so w to minimize thc acquisition lime unda
the desired tactical operating Ctditim.
For the ovdl probability of dctection defined in quation
(5). thc dcla~tion rtquirem~ CM be equally putitioncd
to the UL and the 3L U a starting point in the design.
Howeva, it may be dwirable to have a higher P, on be
DL urd 1 Iowa Po the UL ~ d certain a operating
conditions. A DL receive only mode of operation is an
example of such a rrccnuio. T!w DL detectiori procew
ilself is canyosed of the comae and fine spatid and lime
rcquisithn ;*recesses described in sections 4 md 5 above.
Thus P nL) un be pprtitimed into a function of the
values of PD and P, fa thc individual conrsc and fine
In aqkdon (6). the fvat tam cm be neglected. since it is
extranely unlikely lhlt che m wil nrccusfully acquire if
17-7
the DL is not proply synchronized The daminant tam
in the ovadl process is the d term, which is Ihe
probability of frlse syldumizrtion of UL timing given the
DL is correaly quid. 'Ibe third tam is d l in
comph to Ihc Mcond tam if P,(DL) lnd P,(UL) ut
nrsonably c& It should be noted th.1 cyen hough the
rust term U negligible. a large vdue of PJDL) will
significuuly impact llrc acquisition time, since Lhe DL
acquisition p a s will bc dcpcndcnt on h e UL v s
to detect false alums. which is obviously a poor design.
In orda to be robust in the presave of junming, he
CO= and fure sync dctcction processing will require a
verification stage. The verification stage in genad will be
an m choare n type of decision. in which m of n expected
sync hops will have to be dctcclcd in ader for thc given
stage of the dctection poce~s to be declued vdid It
should be noted hac a high P, md low P, for the system
docs no( neceasuily imply a requiremall for high a Po md
low P, on individual colvsc or fine DL sync hops. For
example. if he system requirts npid acquisition. Lhen the
vrlw of P, ud P, for Ihe individual sync hops must be
close IO the Po d the wcrlll system, to ennve thu lhere
lre few Nse &teaionS. otherwise the synchroniution
process will expaEd limc vaifying false dctcctioru This
would require m md n chosen to be luge. Such an
implementation would limit Iht number of sync hops that
the system could support relative to its d.l. capacity. or
limit the numk of theatres that the payload could provide
synchronization hops to. On thc otha hand thc ovadl P,
and P, could be met by Iowa values of m id n, which
would result in morc false alrrnu and verification steps.
md potcntially a Imger acquisition time. The design
choscn will depend not d y on the capacity of the datalink
structure to support the algaihn. but the siFe of the
terminals. Acquisition by larger taminals in clear sky
conditions can be s*ipported by M algorithm with low m
of n, however maller taminrls will requirk longer
acquisition times, pdcululy in a TACSAT enviraunent
in which thc DL SNR is limited.
7 CONCLUSION
This paper has exarnined the systan design coruidtrations
for synchroniution of an EHP MDR sysm employing a