Global Positioning System (GPS)

GPS BasicsGPS stands for Global Positioning System which measures 3-D locations on Earth surface using satellites

GPS operates using radio signals sent from satellites orbiting the earth

Created and Maintained by the US Dept. of Defense

System as a whole consists of three segmentsSatellites (space segment)Receivers (user segment)Ground stations (control segment)

GPS HistoryDevelopment began in 1973First satellite became operational in 1978Declared completely functional in 1995A total of 52 satellites have been launched in 4 phases30 satellites are currently functionalManaged by the U.S. Department of Defense

Originally developed for submarinesNow part of modern “smart bombs” and highly accurate missiles

SatellitesAt least 4 satellites are above the horizon anytime anywhere

GPS satellites are also known as “NAVSTAR satellites”

The satellites transmit time according to very accurate atomic clocks onboard each one

The precise positions of satellites are known to the GPS receivers from a GPS almanac

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

Satellites cont.

The satellites are in motion around the earth

Like the sun and moon satellites rise and set as they cross the sky

Locations on earth are determined from available satellites (i.e., those above the horizon) at the time the GPS data are collected

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

ReceiversGround-based devices read and interpret the radio signals from several of the NAVSTAR satellites at once

Geographic position is determined using the time it takes signals from the satellites to reach the GPS receiver

Calculations result in varying degrees of accuracy that depend on:

Quality of the receiver User operation of the receiver (e.g., skill of user and receiver settings)Atmospheric conditionsLocal conditions (i.e., objects that block or reflect the signals)Current status of system

Ground Stations

Control stationsMaster station at Falcon (Schriever) AFB, Colorado4 additional monitoring stations distributed around the world

ResponsibilitiesMonitor 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.

How GPS Works: Overview

Satellites have accurate atomic clocks onboard and all GPS satellites transmit the same time signal at the same time

Think “synchronize your watches”

The satellite signals contain information that includes

Satellite numberTime of transmission

How GPS Works: OverviewReceivers use an almanac that includes

The 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 satellite

Trilateration (similar to triangulation) is used to determine location from multiple satellite signals

How GPS Works: Signal Processing

Distances between satellites and receivers is determined by the time is takes the signal to travel from satellite to receiver

Radio signals travel at speed of light (186,000 miles/second)All satellites send the identical time, which is also generated by the receiversSignal travel time = offset between the satellite signal and the receiver signal

Distance from each satellite to receiver = signal travel time * 186,000 miles/second

1sec

Receiver signal

Satellite signal

How GPS Works: Trilateration

Start by determining distance between a GPS satellite and your position

How GPS Works: Trilateration

Adding more distance measurements to satellites narrows down your possible positions

How GPS Works: Trilateration

How GPS Works: Trilateration

The 4th satellite in trilateration is to resolve any signal timing error

Unlike GPS satellites, GPS receivers do not contain an atomic clockTo make sure the internal clock in the receiver is set correctly we use the signal from the 4th satellite

GPS Error SourcesSatellite errors

Satellite position error (i.e., satellite not exactly where it’s supposed to be)Atomic clocks, though very accurate, are not perfect

AtmosphericElectro-magnetic waves travel at light speed only in a vacuumAtmospheric molecules, particularly those in the ionosphere, change the signal speed

Multi-path distortionThe signal may "bounce" off structures before reaching the GPS receiver – the reflected signal arrives a little later

Receiver error: Due to the receiver clock or internal noise

Selective AvailabilityNo longer an issue

Sources of Error

Satellite Clock & Satellite PositionAtomic clock errors+/- 2 meters of error

Satellite is not in precise orbit+/- 2.5 meters of error

Sources of Error

Atmospheric Delays/Bending+/- 5 meters or error

Sources of ErrorMulti Path Interference (signal bouncing off of buildings, trees, etc.)

+/- 1 meter of error

Sources of ErrorReceiver Timing/Rounding Errors

+/- 1 meter of error (depends on the quality of the GPS receiver)

Quadruple Redundant Atomic ClocksAccurate to Nanoseconds

$800,000 in clocks on each satellite2:02:01.23456789012

Powered by 4 AA Batteries~$2.99

2:02:01.2345

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 reasonsSelective Availability turned off early May 2, 2000

GPS Error: Position Dilution of PercisionSatellite Coverage: Position Dilution of Precision (PDOP)

Remember that satellites are moving, causing the satellite constellation to change

Some configurations of satellites are better than others

PDOP values range from 1 to 50, with values < 6 considered “good”

Poor PDOP Good PDOP

GPS - Error Budget

Typical Observed errors (meters)satellite clocks 0.6orbit (position error) 0.6receiver errors 1.2atmosphere 3.7

Total 6.1

Multiplied by PDOP (1-6)

Total error ~ 6.1 - 36.6 metersMeters

Atmosphere

Receivers

Orbit Error

SatelliteClocks

0 6 12 18 24 30

Example of typically observed error from a consumer GPS receiver:

GPS - Error Correction

2 Methods:Point AveragingDifferential Correction

Point AveragingPoint Averaging is one of the simplest ways to correct GPS point locations

Collect many GPS measurements at the same location and then average them to get one pointThe averaged point should have greater accuracy than a single point measurementAccuracy varies with this method but you should have a position that is within 5 meters of its true location 95% of the time

GPS - Point Averaging

AveragedLocation

This figure shows a successive series of 3-D 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 almost 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 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 points• Requires downloading a base-station file

Real-time differential correction• Done in real time by receiving a

broadcasted correction signal • May require additional hardware

• Generating mapped data for GIS databases • Collecting field data - travel to the field and

capture location & attribute information

• Other uses (many in real time):• 911/firefighter/police/ambulance dispatch• Car & boat navigation• Roadside assistance• Business vehicle/fleet management• Mineral/resource exploration• Wildlife tracking• Recreational (fishing, hunting, hiking, etc.• Ski patrol/medical staff location monitoring

GPS Applications

Strengths of GPS

Easy To Incorporate into ProjectOnce trained, just about anyone can use itCheapWidely Available

Weaknesses

Does require a training componentAccuracy Issues Differential Correction may not be an option in many parts of the world

Planning a GPS Project

GPS point collection can be an easy way to build your database but planning is essential

Planning a GPS ProjectIdentify Your Accuracy NeedsIdentify Error Correction Methodology

Point Averaging• How long will points be collected?

Differential Correction• Find a base station

Identify Point Collection MethodologyWhere will points be collected?Contingency plansData backups

Bottom Line

Cost depends on ProjectHardware

• GPS -- $200 to $100,000 or more• Differential Correction (yes? no? real-time?)• Additional hardware

– Computers, cables, batteries, antennas, etc.

People• Field Teams -- depends on length of field work• Simple projects in a day or two• More Complex projects can last months or years

Garmin GPS Introduction

Garmin GPS

Display Screen

Control Buttons

Antenna

Garmin GPS

Step 1: Turn on Unit

After a couple of seconds, the GPS unit will start looking for satellites.

Off/On Button

Search for SatellitesStep 2: Wait for Satellites

Battery Gauge

Signal Strengthfor satellite

Satellite Located,but not locked in

Satellite Located,and locked in

North Indicator

Outer circle representshorizon, inner circlerepresents 45 degreesabove horizon

Satellite Acquisition Page

Acquire PositionStep 3: Once enough satellites have been located, the GPS unit will provide you with a position

Position

Position Page

Time

Direction Indicator

Speed

Altitude

Collect a PointStep 4: Press the Mark button and begin collecting data.

MarkButton

Waypoint Page

To collect a single point, highlight Save and press ENTER.

To collect an averaged point, highlight Average, press the Enter button and wait for a few minutes, then highlight Save and stop point collection

Record PointStep 5: Go to Menu Princ. Page and view waypoint list and record coordinate

Main Menu Page

Waypoint ID

AveragedPosition

Waypoint Page

Other Pages

Compass PageMap Page

On Friday

Meet outside at the Old WellDon’t be late!Be prepared to walk short distanceYou will need a pencil or penRead your worksheet BEFORE class and remember to bring it on FridayRain Date – Monday, October 29