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sdcly assumed In either of thc scenarios dcscribcd abr~vc,
thc only effective e m will be due to the accuracy of the
ephcmais algailhm. A wry qconwative estimate is Ihai
the ernn canpnart in spatial poiwing duc to he
cphcmais rlgc-ilhm will be known to wilhin O.l* in
which c m spatial acquisitim may'nd be ncccsar)'. If
GPS or similar timc stdads ue hot avrilahlc and the
location of Ihc GT is not piscly knowil. thc conlribulion
of he .sparid pointing mor due to thc mw in thc cstiniatc
of the GT locatim will bcwmc significant. In a tactical
field scenario the GT location will typically bc known to
much bctter than IO km. mulling in a corilribution to thc
spatial pinling e m of lcss than 0.1'. Thus the total
pointing c m Jhould be lexs than 0.2.
2.43 Frequency Esiitnatcs
The initial ma in Ihc frcqucncy cstimatc will bc due to
a combination of the accuracy of thc cphctncris aljiwihn
in estimating motion induccd doppla cffccl~ and the
rclativc frcqucncy drift bctwccn the timc standards of ihc
payload and the ground tcrminal. For thc orhits rcfcrcnccd
in scctim 2.2. Ihc worst case unrcsolvcd dopplcr crrm will
be on the order of to 2 ktlz. Again acsuming thc
prerncc of a rubidium Ftandard in the GT. the frrqucncy
crror duc to rrlativc cltwk tbift will hc ncglipihlr Tor
purpcscs of thc.sc discussions.
3 SPATIAL ACQUISITION
To achieve spatial wnchmni7ation, thc gmuntl tcrminal
must he capable of acquiring and tracking thc pinting of
thc antcnna typically io within an angle corrrywnding to
a 1.0 dR loss in gain. In M MDR systcm thcrc arc scvcral
factors that will impact on the spatial synchroniiation
algorithm to bc sclcctcd If thc initial spatial pointing
error is rclativcly largc. such as on [hc o rb of onc to two
3 dR bcamwidlhs. spatial acquisition will likely havc to
consist of two phaccs. a coarsc spatial acquisition phase.
and a fine spatial acquisition phasc. Tlic coarsc spatiol
acquisition phase can consist $.if a stcppcd or continuous
scarch pattern. normally in srcps of onc 3 dR bcamwidth.
out from thc hst initial estimaic as provikd by the
cphcmcris algorithm.
Siryc in MDR the DL is capacity limited, it is dcsi-abl:. to
havc an antcnna vvih as high gain as practical. tlowcvcr
igh gain antcnnns bin: narrowcr bcamwidths. rcsulting in
larga attcnuatims for iic same crror in spatial pointing.
Typical UL and DL MICW bcam charactcristics for EIF
frqucncias arc givcn in Tablc 111 as a function of antcnna
kunctcr. As discussed in section 2.4.2. the initial pointing
error for an MDK'tcnninal is usually lcss than O.I", in
which case a coarse spatial scarch stage is not rcquircd.
For cxamplc. it can bc sccn that fob a 2.4 mctcr antmna,
thot 8 pointiri~ error of 0.1' corrcqw)ntls IO thc 3 clII
bcwnwidth. For a tactical cnvi.onment in which largcr
initial pointing mm may bc incurred. the antcnna size
will typically be smalla, on thc nrdrr of 1 mctcr. In such
a scenario the pointing error of 0.2" discussed in 24.2 wiil
fall within the 3 dB beamwidth In such caws spatial
acquisition would only na4 to consist of thc finc spatial
acquisr ion strgc Thc most commo~l dgailhm employed
is a cmicd sun tracking loop about a 1 dB contour. Such
I~ps can rapidly aqrire in AWGN in the order of
slxdx fmm initial pointing arm within the 3 dP,
contour of thc antcnna. This is diciert for most MDR
EHF antmLv U dixu.ssal above.
Tahlc 111: Typlcnl EHF Antenna Charocterbtlcs
0.14 DL
11 0.14 UL I 1.0 I 1.75
11 1.0 DL I 0.3 I 0.53
11 1.0 UL I 0.14 I 0.24
1.8 DL 0.18 0.3
1.8 UL 0.09 0.IR
2.4 DL 0.13 0.21
2.4 UL 0.06 0. I
If Ihc initial pointing crror cxccds hc 3 dR contwr of thc
anicnna. or if jamming or scintillation is prcscnt. it may bc
necessary io cmploy a coarsc acquisition search. Spiial or
stcppcd scarchcs may bc cmploycd, howcvcr thcre is an
additional consideration for an MDR terminal. If thc
pointing crror is mitsidc thc 3 dR region, it is possiblc that
thc spatial search may point to a sidclobc at the payload
In a high SNR cnvironmcnt a dctcc'tion may occur.
rcsulting in a false locking of the spatial pointing on a
sidclobc. Cmc solution is to collcct statistics from all
possiblc scarch regions. howcva this impacts thc
acquisition timc. An altcmatc approach is to cmploy a
gimbal scan about a larger contour than that cmploycd for
thc conical scan. Thc proccssing would bc idcntical. If thc
contour of the gimbal scan is judiciously choscn with
rcspcct to thc ovcrall search rcgion. thc gimbal scan will
always encompass thc mainlobc of the antcnna. 3tiicr
considmations includc the incrcascd scnsirivity of thc
conical scan to pointing c m duc lo the highcr slopc of
the bcamshapc in high gain antc~a~, as well as the cffccl~
of frqucncy flatncss variation on thc stability of thc
conical scan proccssing.
4 TIME ACQUIS;TIO%N
4.1 Overvlew
Time acquisition is dividcd into two siagcs; namcly. DL