
As thereceiverlackson to eachsatellite, it calculatesthedis-
tancefromthesatellite
by measuring the lengthoftime ittakes
theradio signal to reach it. The satellites havean extremely
accurateclockand it tellsthereceiverwhentheradiotransmis-
sion started. The receivercompares
thattime againstits own
clock,thusitknowshow longitlooktheradio signal(travelling at
the
speedoflight!)toreach it. If
youknowtimeandspeed,then
you can calculate distance. Once you have this from three
satellites, thenthereceivercandetermine position.
TheGlobalwav
givesyou achoice ofmethodstoenterthePCF.
The first method (Set as Position) lets you simply enter the
corrected latitude/longitude. Thesecondmethod
(Setas Offset)
requires a calculation be performed, then entered into the
GlobalNav.
Setas Position
To usetheSet as Position
method, first choosethelocationon
the chart that you wishto match the unit's position display to.
Then move theboat tothat position and holditthere as accu-
rately as possible. Now press the MENU key, then the key
adjacent to the"More" label until the "PCF Adjustment"
label
appears. Press the key adjacent to that label. The screen
shownbelow appears.
ACCURACY
Mostofus haveheardtalesofextraordinary accuracy
fromOPS
receivers. TheDOD requires accuracy
of10to 15metersfrom
thesatellitesystem. However, only
themilitarygetsthispreci-
sion. The way the militarykeeps us and other unauthorized
peoplefromusingthemoreprecisesystemiscoding. Inother
words,thedatacoming
fromthesatellites isencrypted. Civilian
GPSreceivers
usewhat'sknown as"CIACode." It'sintentionally
worsethanthemilitary's "PCode." Inthismanner, civilianusers
worldwidecanbenefitfromexcellent position
fixes. Meanwhile,
themilitarykeeps
themost accurate
systemawayfrompotential
enemies. Theoretically, C/A code can give accurateposition
fixes up to 15 meters. This is morethan adequatefor most
people.
However
(asofthiswriting), themilitary
isn'tsatisfied with CIA's
potentialaccuracy
inthehandsoftheworld. So, it'sdegrading
it
further with what's called "Selective
Availablity" or SA. This is
small,randomerrorsintentionally addedto the systemso your
accuracy
willtypically
bewithin100 meters. Ofcourse,accuracy
also dependson theangleof the satellite above the horizon,
signal-to-noise ratio,thenumberofsatellitestrackedatonetime
(themorethebetter),and otherfactors.
Don't let this discourage you, however. Our tests have
repeatably
shown betterthan100meter
accuracy. Furthermore,
GPS by nature has much faster updates than othersystems
(such as Loran), and typicallyis much easierto use. You've
purchased
oneofthefinest
navigation instwmentsonthemarket
today. We hopeyou'llenjoy
itfgrmanyyears
tocome.
PCF Offset
N LPOO.OOW
E 0°00.000'
ItT
'e*stnt;
I
Now press the keyadjacent tothe"Set as Position" label. The
screenshown belowleftappears.
I(,J
$IJ
4
PCF Position
N ——°——. ———'
u—--°--. ---' , PCF Position
N 32°10 220'
106'
I
t i,
I19!'94 thaaI
Lcic1
I
I
Nowenterthepositionshownonthechart. Afterthelastkeyis
pressed,thescreenshown above right appears. Pressthekey
adjacent
tothe"North/Sooth tat."
labeltoswitch
thelatitudefrom
northtosouth. Switchthelongitude in necessary in thesame
manner
by pressing
thekeynexttothe"East/West
Long."
label.
Iftheposition
shown atthetop
ofthescreen
is
correct,
pressthe
45
V Turn PCF On
Set
as Offset
CLear Li
North/South Lat.L
East/West Long.
Use Position
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