Lecture and lab schedule
Lecture: GPS, remote sensing, spatial
analysis and applications
Labs:
1. GPS/RS lab
2. Fire Fuel Mapping and Modeling in a
Forested Environment
3. Your lab
GPS basics
Why GPS
Figure out where you are and where you’re going
Navigation and positioning are crucial to many
activities
• Generating mapped data for GIS databases
• “traditional” GIS analysts & data developers
• travel to field and capture location & attribute information
cheaply (instead of surveying)
• Other uses (many in real time):
• 911/firefighter/police/ambulance dispatch
• car navigation
• roadside assistance
• mineral/resource exploration
GPS Basics
What is GPS?
GPS stands for Global Positioning System which measures 3-D locations
on Earth surface with the aid of satellites
• Created and Maintained by the US Dept. of Defense and the US Air
Force
• System as a whole consists of three segments
satellites (space segment)
receivers (user segment)
ground stations (control segment)
Note: Russia and a European consortium are implementing similar systems.
Satellites
How it works?
1. Triangulating
2. Distance measure
3. Getting perfect timing
4. Satellite position
How It Works
1. Triangulating
Start by determining distance between
a GPS satellite and your position
Adding more distance measurements
to satellites narrows down your
possible positions
Triangulating
Three distances = two points
Intersection of Four spheres = one point
Note:
• 4th measurement not needed
• Used for timing purposes instead
2. Distance measure
Distance between satellites and receivers
determined by timing how long it takes the signal to travel from satellite
to receiver
How?
Radio signals travel at speed of light: 186,000 miles/second
Satellites and receivers generate exactly the same signal at exactly the
same time
Signal travel time = delay of satellite signal relative to the receiver signal
1sec
Satellite signal
Receiver signal
Distance from satellite to receiver =
signal travel time * 186,000 miles/second
3. Getting perfect timing
How do we know that satellites and receivers generate the same signal
at the same time?
satellites have atomic clocks, so we know they are accurate
Receivers don't -- so can we ensure they are exactly accurate? No!
But if the receiver's timing is off, the location in 3-D space will
be off slightly...
So: Use 4th satellite to resolve any signal timing error instead
determine a correction factor using 4th satellite
4. Satellite position
In order to make use of the distance measurements from the
satellites, we must know their exact locations such that we
can match our signals with the right satellite.
satellites are placed into high orbits -- makes their orbits
very predictable
receivers have almanacs that tell them where satellites
should be
minor variations in orbit are monitored -- correction
factors transmitted along with the signals
System as a whole consists of three segments
• satellites (space segment)
• receivers (user segment)
• ground stations (control segment)
Satellites (space segment)
24 NAVSTAR satellites
orbit the Earth every 12 hours
~11,000 miles altitude
positioned in 6 orbital planes
orbital period/planes designed to keep 4-6 above the
horizon at any time
controlled by five ground stations around the globe
Receivers (User Segment)
Ground-based devices
can read and interpret the radio signal from several of the NAVSTAR
satellites at once.
Use timing of radio signals to calculate position on the
Earth's surface
Calculations result in varying degrees of accuracy -depending on:
quality of the receiver
user operation of the receiver
local & atmospheric conditions
current status of system
Ground stations (control segment)
Ground Stations (control segment)
Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin.
Five control stations
master station at Falcon (Schriever) AFB, Colorado
monitor satellite orbits & clocks
broadcast orbital data and clock corrections to satellites
Error Sources
Satellite errors
satellite position error
atomic clock, though very accurate, not perfect.
Atmosphere
Electro-magnetic waves travels at light speed only in vacuum.
The ionosphere and atmospheric molecules change the signal
speed.
Multi-path distortion
signal may "bounce" off structures nearby before reaching receiver
– the reflected signal arrives a little later.
Error Sources
(cont’d.)
Receiver error: Due to internal noise.
Selective Availability
intentional error introduced by the military for national security reasons

Pres. Clinton cancelled May 2, 2000.
Selective Availability (SA)
Error Breakdown (typical case):
satellite clock:
satellite orbit:
ionosphere/troposphere:
multipath distortion:
receiver errors:
1.5 meters
2.5 meters
5.5 meters
0.6 meters
0.3 meters
GPS - Error Correction
2 Methods:
Point Averaging
Differential Correction
GPS - Point Averaging
Averaged
Location
•This figure shows a successive series of positions taken using a
receiver kept at the same location, and then averaged
Differential Correction
Any errors in a GPS signal are likely to be the same among all receivers within 300
miles of each other.
GPS
Error signals
Reference Receiver sits over
Precisely surveyed point
Note: differential correction can be applied in "real time" or after the fact (post-processing)
GPS - Differential Correction
Differential correction collects points using a
receiver at a known location (known as a base
station) while you collect points in the field at the
same time (known as a rover receiver)
Any errors in a GPS signal are likely to be the
same among all receivers within 300 miles of each
other
~ 300 miles (~ 480 km) or less
Base station (known location)
Rover receiver
Differential correction
How it works:
• use a base station at a known position base station calculate
its own position & compares to its known position
• determines correction factors that can be applied to
receiver-calculated positions
Differential correction will reduce horizontal position Error to 1 - 3 meters
with standard receiver
much GPS fieldwork for GIS/mapping purposes will require differential
correction!
National Differential GPS Network (NDGPS) being created
GPS - Differential Correction
The base station knows its own location
It compares this location with its location at that moment
obtained using GPS satellites, and computes error
This known error (difference in x and y coordinates) is
applied to the rover receiver (hand-held unit) at the same
moment
Example: Base Station File
Time
3:12.5
3:13.0
3:13.5
3:14.0
3:14.5
3:15.0
GPS Lat
35.50
35.05
34.95
36.00
35.35
35.20
GPS Long
79.05
78.65
79.55
80.45
79.30
79.35
Lat. error
.5
.05
-.05
1.0
.35
.20
Long. error
.5
-.35
.55
1.45
.30
.35
L11.5 The issue of GPS datums
Datums, or so called “reference globe” in map projections,
need to be defined for GPS.
The WGS 84 is defined and maintained by the US National
Imagery and Mapping Agency (NIMA) as a global geodetic
datum. It is the datum to which all GPS positioning
information is referred by virtue of being the reference system
of the broadcast GPS satellite ephemerides.
Garmin’s cheapest receivers
Garmin’s Forerunner 201: A watch that uses GPS
to determine current speed, average speed, exact
Garmin’s iQue 3600 PDA:
distance traveled, etc. ( ) Basic features also
http://www.garmin.com/prod available in the Forerunner 101 ($115).
ucts/iQue3600/
http://www.garmin.com/products/forerunner201/
Garmin’s Outdoor GPS Receivers:
Etrex Legend C ($375)
“Along with the Etrex Vista C, is one of Garmin's smallest, least
expensive products to combine a color TFT display and advanced
GPS routing capabilities in a waterproof design.”
--is WAAS enabled
--has USB port for downloading maps from Garmin’s MapSource CD
library
Etrex Vista C ($430)
--has a TFT (thin-film transistor, with 1-4
tranistors controlling each pixel; it is the
highest-definition flat-panel technique)
display
--WAAS enabled
--has USB port for downloading maps from
Garmin’s MapSource library
Choosing a GPS receiver
The 2000 Receiver Survey in the GPS World magazine lists
495 receivers from 58 manufactures (GPS World, January
2001). Why are there so many GPS receivers on the
market?
-
There are so many different applications of GPS
-
New uses spring up every day.
Dual-frequency or single-frequency GPS receiver
(1) Smart antennas/integrated receivers – For the lower end
of the accuracy requirements, handheld GPS receivers
operate at the single-point accuracy level (<10 m without
Selective Availability).
Choosing a GPS receiver (Cont.)
12XL from Garmin –
Etrex from Garmin –
500 points, $145.71
City point database, $309.07 , area
calculation
Choosing a GPS receiver (Cont.)
(2)GIS/Mapping receivers - Receivers used for mapping and
GIS data Collection typically requires a positioning
accuracy in the range of sub-meter to a few meters.
Both (1) and (2) are single-frequency units, designed to operate
in real-time. (2) are distinguished from (1) by having both a
LCD display/command unit through which instructions and
user-centered data is input, and a Differential GPS (DGPS)
signal decoder.
(3) Dual-frequency receiver, collecting data for postprocessing, has the highest accuracy, and are often used for
surveying/geodetic-type applications. These are typically
the most expensive class of GPS receiver.
Choosing a GPS receiver (Cont.)
To meet military objectives, the department of Defense can
degrade the accuracy with which positions can be
determined using GPS. This can be done by:
(1) Deliberate introduction of errors in the satellite clocks,
called selective availability. The government turned off
SA in May 2000, which significantly improved the
accuracy of civilian GPS receivers.
(2) Encrypting the measurement signals such that only
military authorized users can receive them, called
antispoofing.
Choosing a GPS receiver (Cont.)
Other sources of GPS signal errors (garmin.com)
•Ionosphere and troposphere delays — The satellite
signal slows as it passes through the atmosphere. The
GPS system uses a built-in model that calculates an
average amount of delay to partially correct for this
type of error.
•Signal multipath — This occurs when the GPS
signal is reflected off objects such as tall buildings or
large rock surfaces before it reaches the receiver. This
increases the travel time of the signal, thereby causing
errors.
Choosing a GPS receiver (Cont.)
•Receiver clock errors — A receiver's built-in clock
is not as accurate as the atomic clocks onboard the
GPS satellites. Therefore, it may have very slight
timing errors.
•Number of satellites visible — The more satellites a
GPS receiver can "see," the better the accuracy.
Buildings, terrain, electronic interference, or
sometimes even dense foliage can block signal
reception, causing position errors or possibly no
position reading at all. GPS units typically will not
work indoors, underwater or underground.