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)
Satellites
Satellites (space segment)
24 NAVSTAR satellites (21 operational and 3 spares)
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
GPS – User Segment (Receivers)
• Ground-based devices read and interpret the radio
signals from several of the NAVSTAR satellites at
once
• Determine their position using the time it takes
signals from the satellites to reach the hand-held
unit
• Calculations result in varying degrees of
accuracy that depend 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
GPS - Satellite Signals
How It Works (p. 1)
Satellites have accurate atomic clocks and all 24 satellites are
transmitting the same time signal at the same time
The satellite signals contains information that includes
Satellite number
Time of transmission
Receivers use an almanac that includes
The position of all satellites every second
This is updated monthly from control stations
The satellite signal is received, compared with the receiver’s internal
clock, and used to calculate the distance from that satellite
Trilateration (similar to triangulation) is used to determine location
from multiple satellite signals
How It Works (p. 2)
Start by determining distance between
a GPS satellite and your position
Adding more distance measurements
to satellites narrows down your
possible positions
How It Works (p. 3)
Three distances = two points
Intersection of Four spheres = one point
Note:
• 4th measurement not needed
• Used for timing purposes instead (discussed later)
How It Works (p. 4)
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
How It Works (p. 5)
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
(like solving multiple equations...will only be one solution that
satisfies all equations)
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.
Receiver error: Due to receiver clock or internal noise.
Selective Availability
GPS - Sources of Error
Satellite Coverage in Sky
Position Dilution of Precision (PDOP)
Poor
Ideal
GPS - Selective Availability
A former significant source of error
Error intentionally introduced into the satellite signal by the
U.S. Dept. of Defense for national security reasons
Based on Clinton’s order, Selective Availability turned off
early May 2, 2000
GPS - Error Budget
•Example of some typical observed using a
consumer GPS receiver:
Typical observed errors
satellite clocks
meters
orbit error
receiver errors
meters
atmosphere
meters
Satellite
Clocks
Orbit Error
Receivers
Atmosphere
0
6
12
18
Meters
24
30
0.6
0.6 meters
1.2
3.7
Total
meters
6.1
Multiplied by PDOP
(1 - 6)
6.1 - 36.6 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
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
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
GPS - Differential Correction
GPS error when using differential correction:
1 – 3 meters
There are two ways that differential correction can
be applied:
Post-processing differential correction
• Does the error calculations after the rover has collected
the points
Real-time differential correction
• Done in real time by receiving a broadcasted
correction signal (usually expensive), requiring other
hardware (not just a consumer GPS receiver)
GPS Applications
• 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
• business vehicle/fleet management
• mineral/resource exploration
• wildlife tracking
• boat navigation
• Recreational
• Ski patrol/medical staff location monitoring
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 series
Basic GPS
Garmin
makes a host
of GPS
receivers for
outdoor
sports
enthusiasts.
eTrex®
eTrex
Camo®
eTrex
Summit
®
GPS 76
GPS 72
GPS 12
Geko™ 101
Geko 201
Geko 301
Foretrex™
101
Foretrex 201
eTrex
Venture
GPS 12XL
http://www.garmin.com/outdoor/products.html
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
Bluetooth GPS Receivers
Teletype’s Mini-bluetooth GPS
receiver ($175)
http://www.mightygps.com/Manufac
turer/minibluetooth.htm
Teletype’s USB GPS receiver
for Laptops ($170)
http://www.teletype.com/Merc
hant2/merchant.mvc?Screen=
PROD&Product_Code=1250&
Category_Code=
HP’s Ipaqs and other PDAs with GPS software
Hewlett-Packard’s new iPAQ h1945 PDA
Now comes equipped with a hp GPS receiver
and navigation system ($500)
http://www.shopping.hp.com/cgibin/hpdirect/shopping/scripts/product_detail/product_detail_view.jsp?BV_SessionID=
@@@@0280349227.1102102313@@@@&BV_EngineID=ccckadddfdjlkdgcfngcfk
mdflldfgg.0&landing=null&category=handhelds&subcat1=classic_performance&prod
uct_code=PF527A%23ABA&catLevel=3
Garmin’s iQue 3600 PDA:
http://www.garmin.com/prod
ucts/iQue3600/