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

• 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

GPS – User Segment (Receivers)

Ground stations (control segment)

Five control stationsmaster station at Falcon (Schriever) AFB, Coloradomonitor satellite orbits & clocksbroadcast orbital data and clock corrections to satellites

Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin.

Ground Stations (control segment)

GPS - Satellite Signals

Satellites have accurate atomic clocks and all 24 satellites are transmitting the same time signal at the same timeThe satellite signals contains information that includes

Satellite numberTime of transmission

Receivers use an almanac that includesThe position of all satellites every secondThis 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 satelliteTrilateration (similar to triangulation) is used to determine location from multiple satellite signals

How It Works (p. 1)

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

Distance from satellite to receiver =

signal travel time * 186,000 miles/second

1sec

Receiver signal

Satellite signal

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

Poor Ideal

Satellite Coverage in SkyPosition Dilution of Precision (PDOP)

GPS - Selective AvailabilityA former significant source of error

Error intentionally introduced into the satellite signal by the U.S. Dept. of Defense for national security reasonsBased on Clinton’s order, Selective Availability turned off early May 2, 2000

GPS - Error Budget

Typical observed errorssatellite clocks 0.6 metersorbit error 0.6 metersreceiver errors 1.2 meters

atmosphere 3.7 meters

Total 6.1 metersMultiplied by PDOP (1 - 6)

6.1 - 36.6 meters

Meters

Atmosphere

Receivers

Orbit Error

SatelliteClocks

0 6 12 18 24 30

•Example of some typical observed using a consumer GPS receiver:

GPS - Error Correction

2 Methods:Point AveragingDifferential Correction

GPS - Point Averaging

AveragedLocation

•This figure shows a successive series of positions taken using a receiver kept at the same location, and then averaged

GPS - Differential CorrectionDifferential 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 CorrectionThe base station knows its own locationIt compares this location with its location at that moment obtained using GPS satellites, and computes errorThis known error (difference in x and y coordinates) is applied to the rover receiver (hand-held unit) at the same moment

Time GPS Lat GPS Long Lat. error Long. error3:12.53:13.03:13.53:14.03:14.53:15.0

35.5035.0534.9536.0035.3535.20

79.0578.6579.5580.4579.3079.35

.5

.05-.051.0.35.20

.5-.35.551.45.30.35

Example: Base Station File

GPS - Differential Correction

GPS error when using differential correction: 1 – 3 metersThere are two ways that differential correction can be applied:

Post-processing differential correction• Does the error calculations after the rover has

collected the pointsReal-time differential correction

• Done in real time by receiving a broadcasted correction signal (usually expensive), requiring other hardware (not just a consumer GPS receiver)

• 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

GPS Applications

Garmin’s cheapest receivers

Garmin’s iQue 3600 PDA:

http://www.garmin.com/products/iQue3600/

Garmin’s Forerunner 201: A watch that uses GPS to determine current speed, average speed, exact distance traveled, etc. ( ) Basic features also available in the Forerunner 101 ($115).

http://www.garmin.com/products/forerunner201/

Garmin’s Outdoor GPS Receivers:etrex series

http://www.garmin.com/outdoor/products.html

Basic GPS

eTrex®eTrex Camo

®eTrex Summit®

eTrex Venture

                       

GPS 76 GPS 72 GPS 12 GPS 12XL

                   

Geko™ 101 Geko 201 Geko 301  

                

Foretrex™ 101

Foretrex 201    

                            

           

Garmin makes a host of GPS receivers for outdoor sports enthusiasts.

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/Manufacturer/minibluetooth.htm

Teletype’s USB GPS receiver for Laptops ($170)

http://www.teletype.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=1250&Category_Code=

HP’s Ipaqs and other PDAs with GPS software

Hewlett-Packard’s new iPAQ h1945 PDANow comes equipped with a hp GPS receiver and navigation system ($500)http://www.shopping.hp.com/cgi-bin/hpdirect/shopping/scripts/product_detail/product_detail_view.jsp?BV_SessionID=@@@@0280349227.1102102313@@@@&BV_EngineID=ccckadddfdjlkdgcfngcfkmdflldfgg.0&landing=null&category=handhelds&subcat1=classic_performance&product_code=PF527A%23ABA&catLevel=3

Garmin’s iQue 3600 PDA:

http://www.garmin.com/products/iQue3600/