Both 8 degree and 20 degree
transducers give accurate
bottom read-
ings, even
though the bottom signal is much wider on the 20 degree
model. This is because
you are seeing
more of the bottom. Remem-
ber,
the
shallow
edge of the
signal shows you the true depth. The rest
of the
signal tells you whether
you are over rocks, mud, etc.
SIGNAL INTERPRETATION
Your UtraNavGPS gives an accurate
picture of the bottom that your
boat is passing. A bottom of firm sand, gravel, shell, or hard clay
returns a
fairly wide signal. If the automatic mode is off and the
signal
narrows
down,
then it means that you have moved over a mud bottom.
Mud absorbs the sound wave and returns a weak signal. Turn up
the
sensitivity to see a better bottom
signal.
Big rocks or stumps on a smooth bottom send back
signals above the
bottom level signal. The height of the
signal depends
on the
target's
height. As you pass
over a post, it should be clearly visible as a short
line extending above the bottom
signal.
A steep slope returns a wide signal, the steeper
the wider. Signals
returned from a high
underwater cliff are usually
the widest of all.
DIGITAL
BOTFOM
DEPTH\\

GPS -

HOW IT
WORKS
The Global Positioning System (GPS) is the newest, most high-tech
approach to navigation yet devised. Conceived by the Department of
Defense
(DOD) andthe United States military,the
GPS
system is an answer
to their needs of 24 hour
global positioning, 365 days

a

year.
Basically, the
system
works by using

a constellation of

satellites orbiting
Earth 11,000 miles in space. There will be 21 satellites in orbit when the
system

is

fullyoperational. Three more satelliteswill act as
spares, foratotal
of 24. When all satellites are in place,
at least four of them
will be in view
nearly anywhere on Earth twenty
four hours a day. The GPS receiver
requires at least three satellites to
give
a "2D" fix. (A
2D fix is
your position
in
latitude/longitude. A 3D fix
is
your latitude/longitude plus altitude.) When
it locks on to at least
four
satellites, it
displays a 3D fix.
It ta}cos three satellites to determine position.
As the receiver locks on to each satellite, it calculates the distance from the
satellite by measuring the
length of time it takes the radio
signal
to reach it.
Each satellite has an
extremely accurate clock which tells the receiverwhen
the radio transmission started. The receiver
compares that time
against its
own clock,
thus it knows how long
it took the radio signal (travelling at the
speed
of light!) to reach it. If you know time and speed, then you can
calculate distance. Once you have this from three satellites, then the
receiver can determine position.
32 41

mi-ITO r

38.3
FT
-C ----10
-C -C
-C-c
PDF compression, OCR, web-optimization with CVISION's PdfCompressor