1. What is GPS?
GPS, which stands for Global Positioning
System, is the only system today able to
show you your exact position on the Earth
anytime, in any weather, anywhere.
The three parts of GPS are:
•Satellites
•Receivers
•Software
2. Satellites
There are quite a
number of satellites out
there in space. They
are used for a wide
range of purposes:
satellite TV, cellular
phones, military
purposes and etc.
Satellites can also be
used by GPS receivers.
3. GPS Satellites
The GPS Operational
Constellation
consists of 24
satellites that orbit
the Earth in very
precise orbits twice a
day. GPS satellites
emit continuous
navigation signals.
4. Receivers and Satellites
GPS units are made
to communicate with
GPS satellites
(which have a much
better view of the
Earth) to find out
exactly where they
are on the global
scale of things.
5. GPS Signals
Each GPS satellite
transmits data that
indicates its location
and the current time.
All GPS satellites
synchronize operations
so that these repeating
signals are transmitted
at the same instant.
Physically the signal is
just a complicated
digital code, or in other
words, a complicated
sequence of “on” and
“off” pulses.
6. Time Difference
The GPS receiver
compares the time a
signal was transmitted
by a satellite with the
time it was received.
The time difference
tells the GPS receiver
how far away the
satellite is.
7. Calculating Distance
Velocity x Time = Distance
Radio waves travel at the speed of light, roughly 186,000
miles per second (mps)
If it took 0.06 seconds to receive a signal
transmitted by a satellite floating directly
overhead, use this formula to find your distance
from the satellite.
186,000 mps x 0.06 seconds = 11,160 miles
12. Atomic Clocks
GPS satellites use Atomic Clocks
for accuracy, but because of the
expense, most GPS receivers do
not.
13. Line of Sight Transmissions
Line of sight is the
ability to draw a
straight line between
two objects without any
other objects getting in
the way. GPS
transmission are line-
of-sight transmissions.
Obstructions such as trees, buildings, or natural
formations may prevent clear line of sight.
14. Light Refraction
Sometimes the GPS
signal from the
satellite doesn’t
follow a straight line.
Refraction is the
bending of light as it
travels through one
media to another.
15. Signal Refraction
Signals from satellites can be like light.
When they hit some interference (air
patterns in the atmosphere, uneven
geography, etc.) they sometimes bend a
little.
18. PDOP
All of this
combines to make
the signal less
accurate, and
gives it what we
call a high
“PDOP.”
PDOP = Positional Dilution of Precision
11,000 miles
11,000 miles
11,000 miles
11,000 miles
•A PDOP of <4 is excellent
•A PDOP of 4-8 is good
•A PDOP of >8 is poor
19. Differential Correction
Differential correction is a
technique that greatly
increases the accuracy of
the collected GPS data. It
involves using a receiver
at a known location - the
"base station“- and
comparing that data with
GPS positions collected
from unknown locations
with "roving receivers."
ISU Base Station - http://134.50.65.125/
22. Latitude and Longitude
Latitude and Longitude
are spherical coordinates
on the surface of the
earth. Latitude is
measured North or South
of the Equator. Longitude
is measured East or West
of Greenwich. GPS uses
Latitudes and Longitudes
to reference locations.
23. Waypoints
Waypoints are locations or landmarks that can
be stored in your GPS. Waypoints may be
defined and stored in the unit manually by
inputting latitude and longitude from a map or
other reference.
Latitude and
Longitude
Your
location
Direction
of
waypoint
Date
and
Tim
e
Waypoint
Or more usually,
waypoints may be
entered directly by taking
a reading with the unit at
the location itself, giving
it a name, and then
saving the point.
24. Data Dictionary
GPS units collect data in:
– Points
– Lines
– Areas
These are called features.
A data dictionary is a
means by which we
collect specific
information about a data
feature.
Editor's Notes
#5:Signal chosen because:
The complex pattern ensures that the receiver does not accidentally synchronize up to some other signal or so the receiver won’t accidentally pick up another satellite’s signal
#12:the dashed lines show the actual intersection point, and the gray bands indicate the area of uncertainty.
the solid lines indicate where the GPS receiver "thinks" the spheres are located. Because of errors in the
receiver's internal clock, these spheres do not intersect at one point.
The GPS receiver must change the size of the spheres until the intersection point is determined. The relative size of each sphere has already been calculated, so if the size of one sphere is changed, the other spheres must be adjusted by exactly the same amount.
