GPS Primer

CFR 250/590 Introduction to GIS

Overview

• Why GPS?

• History of GPS

• Satellites

• Ground control

• Measurements of distance

• Precision timing

• Satellite location

• Sources of error

• Mission planning

• Attribute recording

• Differential correction

• Import into GIS

• Navigation

• Field Trip

Why GPS?

• Plane surveying has not changed for many years

• Same basic technology was used to create “wonders”

of the ancient world: Great Wall, Machu Picchu,

Pyramids

• Measurement of distances and angles

• Use of ground control points

• Specialized training and understanding

• Careful & tedious work

• Plane surveying: daytime only

• Celestial navigation: night only

• You may use this in research or work

History of GPS

• Need for a more flexible tool

• Faster tool (submarines)

• Less user training

• Potentially very accurate (Used to measure plate tectonics, mountain building)

• Location, navigation, data collection

• Evolved from LORAN, SatNav, & other radio ranging systems

• Development of GPS & related systems from 1940s through present

Satellites

GPS SV

• Constellation of 24 satellites for full

GPS component

• Expensive and advanced satellites

• New satellites deployed as older satellites fail

• Return interval 12 hours for each space vehicle (SV)

• 6 orbital planes (4 in each plane)

spaced 60° apart

• 5-8 SV visible at any time from

any point on Earth ellipsoid

Ground control

• Control segment tracks satellites

• Send corrected ephemeris & time offsets to SVs

• SVs incorporate these updates in signals sent to

receivers

• Stations located at AFBs:

• Hawaii (E. Pacific)

• Diego Garcia (Indian Ocean)

• Kwajalein Atoll (W. Pacific)

• Ascension Island (Atlantic)

• Colorado Springs (N. America)

Measurements of distance: how it works

• Satellites broadcast radio signals (EM

radiation)

• Simple distance calculation

d = r * t

• rate is known (speed of light)

• time is known (difference between send &

receive)

• distance is calculated

Measurements of distance: how it works

• Distance measurement

end: 0.06 s

start: 0.00 s

Satellite location

• Given 1 satellite …

Satellite location

• We can locate our position on the surface of a

sphere

Satellite location

• Given 2 satellites …

Satellite location

• We can locate our position on the intersection

of 2 spheres (a circle)

Satellite location

of 3 spheres (2 points)

Satellite location

of 4 spheres (1 point)

Satellite location

• The point should be located on the earth’s

surface

Satellite location

• The precise location is determined

Precision timing

• Distance calculation depends on accurate

timing

• Error of 1/1000 of a second = a positional error

of about 300,000 m

• SVs contain atomic clocks, which are

extremely accurate

• However, receivers do not contain clocks as

accurate as SVs

• Receivers “calculate” correct time based on

multiple signals . . .

More on timing

• The receiver adds and subtracts time from

simultaneous equations until the only possible

(correct) position is located.

• The receiver’s clock becomes virtually as

accurate as the atomic clocks in the SVs

Sources of error: Dilution of precision (DOP)

• The best spread of satellites makes the best

trilateration

• We want low DOP

• Satellites that are close to each other result in higher

• HDOP: horizontal DOP

• VDOP: vertical DOP

• PDOP: positional DOP (combination of HDOP &

• TDOP: time DOP

• GDOP: geometric DOP (combination of PDOP &

Sources of error: Dilution of precision (DOP)

• Wider spread gives better precision

Sources of error: Atmospheric effects

• Light travels at 299,792,458 m/s only in a

vacuum

• Ionospheric effects: ionizing radiation

• Tropospheric effects: water vapor

• Light is “bent” or reflected/refracted

Sources of error: Clock errors

• Receiver clock errors, mostly corrected by

software in receiver

• Satellite clock errors

• Satellite time stamp errors

• Time stamp errors are not correctable

• SV timing & clocks are constantly monitored

and corrected

Sources of error: Receiver errors

• Power interrupts

• On-board microprocessor failure

• Firmware

• Software

• Blunders (user error)

• Clock timing error factor introduced by the DOD

• Standard operation on the satellites.

• S/A changes the time stamp of the outgoing signals

• Calculated positions are erroneous

• SA causes locations to be in error up to 100 m

• Each satellite encrypts its own data separately

• Encryption keys shift frequently

• In the event of warfare, enemy forces cannot use the same accuracy as the US armed forces

• Military-grade have the ability to decrypt the time dithering, which lowers error to about 15 m from ~100 m uncorrected

Sources of error: Selective availability (S/A), recently decommissioned

Sources of error: Landscape features

• Natural & artificial features can intercept

signals

• Mountains, valleys, hills, buildings, tree

canopies, etc.

Sources of error: Multipath errors

• Natural & artificial features can reflect signals

• Multiple “ghost” signals can confound timing

Mission planning

• Preparing for a mission is a critical step

• Knowledge of conditions can “make or break”

a mission

• Before going into the field, check:

• SV availability

• PDOP

• Landform masking

• Data dictionary

Mission planning

• SV Availability

Mission planning

• PDOP

Mission planning

• re-calculate #SVs &

PDOP calculations

• better mission time

planning

Mission planning

• Data dictionary creation

Mission planning

• Data dictionary creation

Attribute recording

• Data dictionary is uploaded to receiver

• Attribute values can be attached to features are

they are captured

Differential correction

• Selective availability and other factors degrade

accuracy

• Time stamps on signals are altered, leading to

distance error

• Accuracy is degraded from ~15 m (without

S/A) to ~100 m (with S/A)

• S/A cannot be overridden (encrypted, US

military algorithm)

• S/A can be “corrected”

Differential correction: Base stations

• Establish a base station on a surveyed location

• Calculated positions from signals received from

GPS satellites

• Calculate the positional difference between surveyed

& GPS location

• Add or subtract time from GPS signals in order to

surveyed & GPS locations match

• Record time correction factor for each signal

• Published time correction files are available on the

• Download correction files from the web

• Apply correction to create new (corrected) files

Import into GIS

• Uncorrected data from 1999

Import into GIS

• 1999 data, differentially post-processed

Import into GIS

• 2000 data, differentially corrected in real time

Export from GPS software

• Various different export formats

Navigation

• Waypoints are created in GIS or surveyed in

field (either with traditional survey or GPS)

• Waypoint coordinates are entered or uploaded

to data logger

• GPS receiver directs navigation (bearing and

distance) to target

Field trip

• Pre-planning

• Data dictionary editing

• Upload data dictionary

• Waypoint creation/upload

• Data collection

• Navigation

• Post-processing (differential correction)

• Export from GPS software

• Import to GIS software

GPS Primer

Documents

Gps (TIPOS DE GPS)

GPS Heights Primer Chris Pearson 1 1 National Geodetic Survey 2300 South Dirksen Pkwy Springfield IL

GPS Basics (GPS-X-02007)

A quick GPS Primer (assumed knowledge on the course!) Observables Error sources Analysis approaches Ambiguities If only it were this easy…

PRÁCTICAS GPS 1. INTRODUCCIÓN AL GPS El GPS es un

Nº 12 - Abril de 2016 Economic GPS · Nº 12 - Abril de 2016 Economic GPS Tracking: Primer bimestre fiscal bajo la lupa Zooming: Infraestructura. Una cuenta pendiente Industry Roadmap:

A PRIMER ON CARNOT GROUPS: HOMOGENOUS ...cvgmt.sns.it/media/doc/paper/3039/Le_Donne-Carnot-gps-16...2003/05/16 · A PRIMER ON CARNOT GROUPS: HOMOGENOUS GROUPS, CC SPACES, AND REGULARITY

di-GPS digital images GPS - di-GPS shop

Assisted GPS (A-GPS) Application Note - Telit€¦ · Assisted GPS (A-GPS) Application Note ... A brief GPS Introduction A brief GPS Introduction The description of the GPS system

GPS-41SMDR Embedded GPS Module GPS-41SMDF - …docs-europe.electrocomponents.com/webdocs/08ef/0900766b... · 2015-06-18 · Embedded GPS Module GPS-41SMDR GPS-41SMDF ... -140dBm acquisition

GPS Compendium(GPS X 02007)

GPS satélite,GPS para automóviles,GPS rastreados coches

GPS -- HOW TO GPS -- how it works GPS -- Garmin 12

GPS-12, GPS-12R, GPS-12R/HS & GPS-12RG GPS & GLONASS / GPS ... · The GPS-controlled frequency standards GPS-12(R) and the GLONASS/GPS-con-trolledfrequencystandardGPS-12RGdelivera

Gm862-Gps, Ge863-Gps - Gps at Commands Set

GPS NAVIGATION SYSTEM SYSTEME DE NAVIGATION GPS GPS

GPS Heights Primer

GPS Protocol GPS Protocol Reference Manual -...GPS Protocol

TCF 2014 - GPS Secrets Your GPS HOPE 2010 ... GPS Satellite Orbits Hacking Your GPS HOPE 2010

FUNDAMENTALS OF GPS RECEIVERS A HARDWARE …€¦ · BOOK I Introduction to GPS FUNDAMENTALS This text is essentially a primer for BOOK II, “GPS Fundamentals, A Hardware Approach”