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

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

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 © Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

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 © Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Satellites

• 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 GPS SV © Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

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: • •

• • •

© Phil Hurvitz, 1999-2002

Hawaii (E. Pacific) Diego Garcia (Indian Ocean) Kwajalein Atoll (W. Pacific) Ascension Island (Atlantic) Colorado Springs (N. America) intro_gis.ppt

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CFR 250/590 Introduction to GIS

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

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Measurements of distance: how it works

• Distance measurement start: 0.00 s

end: 0.06 s Š Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Satellite location

• Given 1 satellite …

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Satellite location

• We can locate our position on the surface of a sphere

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Satellite location

• Given 2 satellites …

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Satellite location

• We can locate our position on the intersection of 2 spheres (a circle)

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Satellite location

• Given 3 satellites …

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Satellite location

• We can locate our position on the intersection of 3 spheres (2 points)

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Satellite location

• Given 4 satellites …

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Satellite location

• We can locate our position on the intersection of 4 spheres (1 point)

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Satellite location

• The point should be located on the earth’s surface

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Satellite location

• The precise location is determined

Š Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

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 . . . © Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

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

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

More on timing

• If the receiver’s clock is behind, the position will be under-estimated for each signal

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

More on timing

• If the receiver’s clock is correct, the position will be properly estimated for each signal

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

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

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

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: • • • • • © Phil Hurvitz, 1999-2002

HDOP: horizontal DOP VDOP: vertical DOP PDOP: positional DOP (combination of HDOP & VDOP) TDOP: time DOP GDOP: geometric DOP (combination of PDOP & TDOP) intro_gis.ppt

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CFR 250/590 Introduction to GIS

Sources of error: Dilution of precision (DOP)

• Wider spread gives better precision

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

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

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

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

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Sources of error: Receiver errors

• Power interrupts • On-board microprocessor failure • Firmware • Software • Blunders (user error)

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

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

• • • • • • • •

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 © Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Sources of error: Landscape features

• Natural & artificial features can intercept signals • Mountains, valleys, hills, buildings, tree canopies, etc.

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Sources of error: Multipath errors

• Natural & artificial features can reflect signals • Multiple “ghost” signals can confound timing

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

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: • •

• •

© Phil Hurvitz, 1999-2002

SV availability PDOP Landform masking Data dictionary

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CFR 250/590 Introduction to GIS

Mission planning

• SV Availability

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Mission planning

• PDOP

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Mission planning

• Landform masking

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Mission planning

• Landform masking

• re-calculate #SVs & PDOP calculations • better mission time planning

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Mission planning

• Data dictionary creation

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Mission planning

• Data dictionary creation

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Attribute recording

• Data dictionary is uploaded to receiver • Attribute values can be attached to features are they are captured

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

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” © Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

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 © Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Differential correction

• Download correction files from the web

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Differential correction

• Apply correction to create new (corrected) files

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Import into GIS

• Uncorrected data from 1999

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Import into GIS

• 1999 data, differentially post-processed

Š Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Import into GIS

• 2000 data, differentially corrected in real time

Š Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

Export from GPS software

• Various different export formats

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

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

© Phil Hurvitz, 1999-2002

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CFR 250/590 Introduction to GIS

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 © Phil Hurvitz, 1999-2002

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