GPS Primer

CFR 250/590 Introduction to GIS

© Phil Hurvitz, 1999-2002 intro_gis.ppt 1

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




Satellite location


of 3 spheres (2 points)



Satellite location




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: Setting receiver clock

• After the correct position is determined, the

receiver’s clock is adjusted

• Adding or subtracting time will make the

location more or less precise

• If the receiver’s clock is ahead, the position

will be over-estimated for each signal



More on timing

• If the receiver’s clock is behind, the position

will be under-estimated for each signal



More on timing

• If the receiver’s clock is correct, the position

will be properly estimated for each signal



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

DOP:

• HDOP: horizontal DOP

• VDOP: vertical DOP

• PDOP: positional DOP (combination of HDOP &

VDOP)

• TDOP: time DOP

• GDOP: geometric DOP (combination of PDOP &

TDOP)



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




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

make

surveyed & GPS locations match

• Record time correction factor for each signal

• Published time correction files are available on the

web




• 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

Documents

GPS Primer