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 Satellite number Time of transmission Receivers use an

atomic clocks

and all 24 satellites are transmitting the same time signal at the same time The satellite signals contains information that includes

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:

Satellite Clocks Orbit Error Receivers Atmosphere 0 6 12 18 Meters 24 30

Typical observed errors satellite clocks 0.6 meters orbit error 0.6 meters receiver errors 1.2 meters atmosphere 3.7 meters 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

be applied: that differential correction can

Post-processing

differential correction • Does the error calculations the points

after

the rover has collected

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 iQue 3600 PDA: http://www.garmin.com/prod ucts/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 makes a host of GPS receivers for outdoor sports enthusiasts.

Garmin’s Outdoor GPS Receivers: etrex series

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

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/cgi bin/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/