GPS
(Global Positioning System)
Basic Concept
• 31+/- Satellites orbiting the earth
+/- 12,600 miles (11,000 Nm or 20,000 km) up
6 Paths, sun-synchronous, each satellite orbits earth 2x/day,
Operational in 1993 (“NAVSTAR”), 32 Satellites max. (24 min.)
• Satellites communicate with Receivers
• Receivers determines distance (signal delay)
• Distance from multiple satellites determines location
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Requires very precise clocks
Satellites use atomic clocks (0.000000003 seconds)
Receiver’s clock corrected by satellites
More satellites, widely spaced provides better results
Longer duration of data capture provides better results
Accuracy (Potential)
Horizontal location (meters)
+/-
Recreational/Pocket Unit
Mapping Grade (GeoExp 3)
Surveyor Grade
Smart phones….
5-20
5-20
5-20
1-100?
RT-DGPS*
2-5
1
DGPS
2-5
0.5**
$
100-500
4,500
40,000
* w/external add-on Radio Beacon unit
** w/10 minutes of data & Post-Processing:
with 30 minutes of data
with 45 minutes of data
with 48+ hours of data
0.3
0.1
0.01 (centimeter accuracy)
0.001 (millimeter accuracy?
5 m : 11,000 miles ~ grain of rice from the top of 5
Empire State Buildings (stacked on top of each other)…
(1 mm : 2.2 miles the Empire State Building ~ ¼ mile)
(Vertical Accuracy typically half as good as horizontal)
)
National Map Accuracy Standards
•
•
•
•
Horizontal accuracy for maps > 1/20,000 is < 1/30” (at > 90% of tested points)
Horizontal accuracy for maps < 1/20,000 is < 1/50” (at > 90% of tested points)
Only for well defined points: monuments, benchmarks, intersections etc.
Thus errors are only:
–
–
–
–
•
1:24,000 ± 40 ft. (12.9 m.) accuracy
1:62,500 ± 104 ft. (31.7 m.)
1:250,000 ± 417 ft. (127.1 m)
1:30,000,000 ± 50,000 ft. (9.5 mi. or 15.2 km)
Vertical accuracy for maps is < 1/2 contour interval at > 90% tested points
– 20 ft. contour interval ± 10 ft. (3.048 m.)
– 80 ft. contour interval ± 40 ft. (12.2 m.)
Typical GPS accuracy compared to a 1:24,000 USGS Topo Quad Map:
Horizontal = +/- 5-20 m
(potentially better than map standards)
Vertical
= +/- 10-40 m
(probably worse than map standards)
What you get for your $
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WAAS compatible
Ability to download data to a PC
Waterproof - backlight display - battery life - screen size - split screen zoom
More memory - storage of points, tracks, areas, offsets
Store/view base maps &/or charts - streets, hydro
Accessories: remote antennas, RT-DGPS, Laser Range Finder
Ability to connect to PDAs or Laptops (real time GPS mapping)
Differential correction (RT-DGPS and/or post processing)
Ability to enter attribute data (features)
Ability to update / append to existing features and/or data files
Vehicle navigation
Electronic compass
Barometric altimeter (more accurate than GPS for altitude, if calibrated)
Vertical profiles of tracks
Alarms (anchor drift, arrival, off-course)
Fish locators, tide charts, calendars, celestial information
Applications
• Military (DoD) – civilian uses now exceed military
• A ‘Free Utility’ maintained by the Air Force
• Space Travel (NASA)
• Survey, Mapping & GIS
• Resource and Asset Management
Environmental & Forestry
Mining, Oil & Gas
• Agriculture
• Utilities & Construction
• Transportation
• Vehicle Security (Fleet Management)
• Public Safety
Emergency Management, Search & Rescue
Crime Prevention
• Timing & Synchronization
(banking, telecommunications)
• LBS - Location Based Services
(cell phones, wireless web)
Solar GPS Cattle Herder
(noise or electric shock)
Precision Construction
& Agriculture
Other Applications…
•On Board Vehicle Navigation Systems
• Vehicle Tracking Systems (beyond fleet management)
- Rental Car Companies
- GPS-measured Tolls – variable taxation (UK)
- Family/Friends vehicle location
- Crime: Stolen cars; Criminal tracking
- Accident notification systems
- ‘Pay-as-you-drive’ insurance plans
• Child/Senior/Pet Safety Tracking Systems
• Parole, Probation Tacking Systems
• Package/Asset Tracking Systems
• Bridge structural monitoring
• Sports and Broadcasting (Skiers, NASCAR, Sailboat races)
• Golf Courses (distance to next hole…)
• Geo-Caching (GPS scavenger/treasure hunts)
• Beer Bottle GPS
• etc, etc,
Golf Courses
Parole Anklet
Car Navigation
Other Applications…
Tracking Systems
- Pet Collar ($300 + $20/month)
- Teddy Bears, Backpacks
- Implants…
Xega injectable GPS chip
($4,000 + $350/month)
Mexico (requires additional
wearable accessory)
Nano GPS tracker ($200 + $45/month),
panic button, for people or nativity scenes…
Pet collars
Web or cell connection
Virtual/Geo-Fence
GTX Ambulator
(Alzheimer’s GPS Shoes)
(tracking and geo-fence)
Garmin Astro Pet Tracker
(communicates with base unit)
Other Applications…
Mobile Phones
• E911, (Enhanced 911) passed into law in 1999…
- Either GPS or Network (tower) based – or both
• The majority of all GPS receivers ever built have been for Cell Phones
• Most in the last 3 years
• 1 billion + phones have GPS
Smart Phones
• Almost 100% are now GPS enabled
• Cost is now less than $5 per phone
• Many are augmented GPS systems (A-GPS or GPS +)
• Cell towers (closer and faster than satellites)
• Wi-fi
• Inertial movement units (inertial navigation systems)
• GPS + WAAS, EGNOS, MSAS, Galileo, GLONASS…
Other Applications…
Mobile Phones
-
iPhone
gPhone
Blackberry
etc.…
Here I Am
Find…
(car or service)
Traffic/Routing/directions
Yellowpages (Starbucks, car repair)
Friend Finder
Google Maps
Geotagged photos
(Consumer
or Resource?)
Astronomy
Golf
GPS
Find lost (or stolen) phone
Simulators
(connect with other users,
friends in your contacts)
Geo-Tag Photos
More GPS units in smart phones
than all other GPS receivers
combined…
Transit
Weather
Not strictly GPS…
GPS (satellites, if signal is available)
Cell tower triangulation and/or signal strength
+ built in digital compass
Virtual Sky
Other Applications…
Timing and Synchronization
• ATM & Cellular networks
• Seismic activity
• Camera synchronization
Academics
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•
•
•
Study areas, sample sites
Animal tracking (goats, birds, turtles)
Survey documentation
Photo geo-tagging
• Accuracy improving
-Surveying world’s largest salt flat (Bolivia), readings were 5 mm
less at the end of the day…
Advantages / Disadvantages…
ADVANTAGES
• Mobility
• Global coverage
• Weather independent
• Day or Night
• Accuracy
DISADVANTAGES
• Requires clear view of sky
• Ionospheric influences
• Multi-path
- buildings, canyons, trees
- large, wet leaves
- large flat buildings
- chain link fences
GPS Satellites visible from location at 45˚ N
Black = visible satellites, Red = not visible, Green = sight lines
Graphic (ConstellationsGPS.gif) from Wikimedia Commons (commons.wikimedia.org)
Trilateration…
3 Sec
5 Sec
X?
3 Sec
5 Sec
X
4 Sec
Triangle of Error
X?
15º
GDOP
GDOP = Geometric Dilution of Precision
Estimate of satellite conditions for a given location & time
Given in distance units (meters or feet)
Ideal GDOP: One Satellite directly overhead
w/an abundance of additional satellites
spaced evenly around the sky
Poor GDOP: Satellites clustered
PDOP vs. GDOP
PDOP = Position Dilution of Precision (amount of error)
“Good” is from 4 – 6 (< 4 is excellent, > 8 poor)
Can be used as a tolerance setting for acceptability of
signal quality (a “PDOP mask” or filter)
GDOP = Geometric Dilution of Precision
Estimate of satellite conditions for a given location & time
Sometimes given in distance units (meters or feet)
PDOP * GDOP = Overall estimate of accuracy (distance)
(PDOP of 4) * (GDOP of 30’) = (Accuracy of +/- 120’)
PDOP & GDOP often used interchangeably
Also: HDOP, VDOP, TDOP, RDOP…
(horizontal,vertical, time, relative)
In all cases, smaller is better
GPS Masks: PDOP, Elevation, SNR
Ability to control the quality of the data accepted at the time
of data collection (unacceptable readings are filtered out)
PDOP Mask: Allows the recording of positions only when there is
acceptable satellite geometry. Typically considers both quantity
and quality of satellites (e.g., 4 satellites with good precision, or
6 with reasonable precision, or 8 with average precision)
Elevation Mask: Sets minimum elevation above horizon for satellites to
be used. The lower on the horizon a satellite is the more
atmosphere the signal must pass through, thus the greater the
potential for signal diffraction (inaccurate estimations of
time/distance), as well as greater chance of multi-path errors.
Also, with Differential Correction, insures that all satellites used
are visible to base station as well as the field receiver.
SNR (Signal to Noise Ratio) Mask: (higher is better, stronger signal)
Filters out signals with excessive noise, using only those
satellites with low noise (more accurate). SNR ranges from 0-35;
10-15 is typical, less than 5 is generally considered unusable.
Sources of Error
Typical amount of
Error (per Satellite)
Beyond quality of equipment/size of antenna, etc.
• Satellite Atomic Clock Errors
(corrected periodically)
1.5 m
• Satellite Orbit (Position) Errors
(corrected periodically)
2.5 m
• Earth’s ionosphere (charged particles)
5.0 m
• Earth’s troposphere (moisture)
0.5 m
• Receiver Noise (local conditions, radio interference)
0.3 m
• Multipath Errors (bounce off buildings, etc.)
0.6 +
• Local Weather (moisture in air, lightning)*
• Poor Satellite Geometry (GDOP)
• Receiver Clock Errors (corrected by 4th + Satellites)
• Visual obstructions (line of sight)… User errors…
* Moisture in tree canopy a bigger deal than in the air…
Sources of Error
Typical amount of
Error (per Satellite)
Beyond quality of equipment/size of antenna, etc.
• Satellite Atomic Clock Errors
(corrected periodically)
1.5 m
• Satellite Orbit (Position) Errors
(corrected periodically)
2.5 m
• Earth’s ionosphere (charged particles)
5.0 m
• Earth’s troposphere (moisture)
0.5 m
• Receiver Noise (local conditions, radio interference)* 0.3 m
• Multipath Errors (bounce off buildings, etc.)*
0.6 +
• Local Weather (moisture in air, lightning)
• Poor Satellite Geometry (GDOP)
• Receiver Clock Errors (corrected by 4th + Satellites)*
• Visual obstructions (line of sight)… User errors
* Sources of error that are NOT improved by DGPS
Selective Availability (SA)
Intentional degradation of signal quality by DoD for security
reasons. Spawned numerous ‘work-around’ technologies
Turned off in May, 2000, recognizing civilian need for better
quality GPS signal (while reserving the option to reinstate it
should the need arise). New satellites will not have this ability
(as of 9/07)…
Differential Correction
Compare GPS data file from Rover file (handheld unit)
with a data file from a Base Station (at a known
coordinate) for the exact same time period. Relies on the
fact that receivers located relatively close together, will
record similar errors from the same constellation of
satellites.
Uses the apparent “error” of the base station file to
correct the corresponding error of the Rover file.
GPS
(Estimated)
Location
Actual
(Known)
Position
GPS Receiver
Estimated
Location
Differentially
Corrected
Estimated Position
10m
10m
Base Station
(w/known coordinates)
Receiver
(unknown Location)
Differential Correction
• Can improve accuracy by up to 20 m. (50-90%)
• Requires local Base Station (w/in 100 miles)
• Requires “post-processing” (back in the lab)
OR can be done on-the-fly using Real-Time DGPS
• Need better data – longer recording period, better GDOP
• More Base Stations near coasts (navigation)
• No effect on multi-path and/or receiver errors
But we’re not going to do that…
WAAS
(Wide Area Augmentation System)
• On-the-fly generalized version of Differential Correction
• Developed by FAA and DOT (airport safety, precision landings)
WAAS only works in North America
(there is also EGNOS in Europe, MSAS in Japan/Asia)
• Base station data is aggregated and sent to WAAS satellites
(Multiple Geostationary satellites, east coast and west coast)
• Corrections are sent out from WAAS satellites
• WAAS-compatible GPS receivers use correction to improve accuracy
• WAAS satellites are equatorial, so WAAS works less well the further
north you go (or if your location has obstructions to the south)
• WWU’s Garmin GPSmap60 units are WAAS-compatible
• Using WAAS drains the battery faster (can be turned off)
Sources of Error
Typical amount of
Error (per Satellite)
Beyond quality of equipment/size of antenna, etc.
• Satellite Atomic Clock Errors
(corrected periodically)
1.5 m
• Satellite Orbit (Position) Errors (Ephemeris error)
(corrected periodically)
2.5 m
• Earth’s ionosphere (charged particles)
5.0 m
• Earth’s troposphere (temperature, pressure, humidity)
0.5 m
• Receiver Noise (local conditions, radio interference)
0.3 m
• Multipath Errors (bounce off buildings, etc.)
0.6 +
• Local Weather (moisture in air, lightning)*
• Poor Satellite Geometry (GDOP)
• Receiver Clock Errors (corrected by 4th + Satellites)
* Moisture in tree canopy a bigger deal than in the air…
Ionospheric Effects…
•
Space ‘weather’ can effect the speed of GPS signal, and
thus the accuracy of the location estimate of the receivers
Solar flares, coronal holes, etc. producing
strong geomagnetic storms
•
Measured in Total Electron Content (TEC)
of the Ionosphere
See
http://sol.spacenvironment.net/~ionops/kml_files/ES4D_ionops_TEC.kml
Real time TEC on Google Earth, Blue=good, Red = Bad
•
•
•
•
•
Degradation of GPS locations (more electrons = delay of signal)
Especially in the mid-latitudes – and can be highly variable
In severe cases, can prevent Satellite fix entirely
GPS receivers attempt to correct for effects
Can also use GPS error to measure Ionosphere (TEC)
Accuracy…
Original (civilian) GPS,
w/Selective Availability turned on
100 meters
Typical (“autonomous”) GPS,
w/Selective Availability turned off
15 meters
Typical DGPS
w/Differential Correction
< 5 meters
Typical GPS w/WAAS
< 5 meters
Typical GPS w/WAAS & DGPS…
< 1 meter
Surveyors
< 5 millimeters
Surveyors…
• Use a form of differential correction in the field
(using multiple receivers)
• Can measure relative position between points very
accurately (+/- a few millimeters)
• Use local ‘control points’ (surveyed monuments, etc.) to
connect surveyed points to real world coordinates
Probability of obtaining a GPS
fix in the North Cascades
GPS BIAS CORRECTION & MOUNTAIN GOAT HABITAT ANALYSIS
David Wallin & Adam G. Wells, 2006
Data Dictionaries
(not w/Garmin units)
Created with Pathfinder Office (in the lab) and transferred to
the GeoExplorer-3 before using in the field. Allows creation of
custom fields (attributes) and field values (defined lists of
possible attribute values) for feature collection.
So, for a database of TREES, a data dictionary might have:
Species List (fir, pine, alder, etc)
Type
List (Either Deciduous or Conifer)
DBH
Number field (Enter size in inches)
Date
Auto-generated date field (day-month-year)
Or, for a database of STREETS, you might create a data
dictionary with:
Name Text field (Enter Street Name)
Type
List (Ave, St., Way, Place, Circle, etc)
Type
List (Arterial, Residential, Highway, Private
Surface List (Paved, Gravel, Dirt)
#Lanes Number field (Enter number of Lanes)
Bike
Yes/No (as to existence of Bike Lanes)
But we’re not going to do that either…
Other Navigation Systems
(Ground-based, USA)
Loran-C (LOng Range Aid to Navigation) – Phased out in 2010…
Coast Guard
Radio navigation, 50 m, North America… Russia, Europe
NDGPS (Nationwide Differential GPS – radio based corrections)
Joint DOT (Highways and Railroads) and Coast Guard
There is also Canadian DGPS and European DGPS
Augmented systems (used in combination w/GPS )
Auto Navigation: GPS + GIS + Inertial Sensors (Dead Reckoning)
Other Navigation Systems
(Regional Enhancements)*
WAAS (Wide Area Augmentation System) (3 satellites*)
USA (FAA) (2003) (USA, Canada, Mexico)
MSAS (Multi-function Satellite Augmentation System) (2 satellites*)
Japan (2007)
EGNOS (European Geostationary Navigation Overlay System) (4 satellites*)
Europe (2009)
QZSS (Quasi-Zenith Satellite System) (1 satellite*)
Japan (2013?)
Under Implementation:
GAGAN (GPS Aided Geo Augmented Navigation system) (2 satellites*)
India
SDCM (System for Differential Corrections and Monitoring) (2 satellites*)
Russia (GPS and GLONASS)
SNAS (Satellite Navigation Augmentation System)
China
* Geostationary Satellite Systems – serving a more local area
* 1/1/2014
http://gpsworld.com/the-almanac/
Other Navigation Systems
(Regional Satellite Systems)
IRNSS (Indian Regional Navigational Satellite System)
India (serves India only, not global coverage)
2013 – first satellite launched
2015? 2016? – 3-7 satellites
BeiDou(-1) (Quasi-Zenith Satellite System)
China (serves China only, not global coverage)
Now replaced by BeiDou-2 (Global system)
* 1/1/2014
http://gpsworld.com/the-almanac/
Other Navigation Systems
(Global Navigation Satellite Systems)
GPS(Global Positioning System)
USA DoD (developed in 1960’s, ‘70’s and ‘80’s, functional in mid-80’s)
31 +/- satellites*
GLONASS (GLObal NAvigation Satellite System)
Russia (started in 1976, launched 1982-1995, restored in 2000’s, global in 2011)
24 satellites* – global coverage (some smart phones/tables use GLONASS)
Galileo
European Union – Civilian based initiative (2010 first launches, testing)
Different frequency than GPS (so could be jammed w/o losing GPS)
4 satellites*
2019? - 30 satellites, global coverage
BeiDou Satellite Navigation System (BDS or Beidou-2 or Compass )
China
14 satellites* – functional as a regional navigation system (Asia)
2020? – 35 satellites, global coverage
“Beidou-1” - 3-4 satellites – regional only (China) - Test for Beidou 2 / Compass
*1/1/2014
http://gpsworld.com/the-almanac/
The Future…
Better Receivers/Transponders
Better clocks in receivers means fewer satellites needed
Components the size of a credit card (for < $100)
Track L1 and L2 satellite signals for greater accuracy
Additional (& Upgraded) Satellites (& WAAS)
Multiple signals from each satellite w/different frequencies
for fewer errors, etc.
Upgraded ground support (augmentation)
Dual/Tri/Quad systems (GPS & GLONASS & Galileo & BeiDou…)
“100+ Satellites by 2020 with 30-40 at a time”
The Future…
Incorporation with other technologies (Cell phones, PDA’s, Cars)
Hybrid/Augmented systems
(also using cell towers, TV signals, RFID, Wi-Fi, etc.)
Indoor “GPS” or Enhanced GPS
Ability to use low-quality satellite data
(typically filtered out by masks) for estimating positions
under canopy, inside buildings/cars, etc.
• Readily available tracking (finding) of your car, car keys, pet,
child or package… (or your customers, ex-spouse, etc…)
•Input shopping list: finds best price and availability, guides you
to the store, to the floor, to the isle, to the shelf…
• Car safety systems – each car knows what other cars are
nearby (around the corner, etc.), and car convoys, etc.
The Future…
Does GPS make us dumber…?
Driver follows GPS directions onto train tracks…
GPS directions send Mercedes
driver downstream…
Bus driver follows GPS directions,
ignores signs, plows into overpass…
http://www.engadget.com/tag/gps+crash/
http://news.bbc.co.uk/2/hi/uk_news/wales/north_east/7257555.stm
Truck driver follows GPS
past sign, becomes
wedged in small
farm lane…
The Future…
Does GPS make us dumber…?
http://www.engadget.com/
http://news.bbc.co.uk/2/hi/uk_news/wales/north_east/7257555.stm
The Future…
Does GPS make us dumber…?
Al Byrd’s three-bedroom home, built by his father on the western
outskirts of Atlanta, was mistakenly torn down by a demolition
company. “I said, ‘Don’t you have an address?’ ” a distraught Byrd
later recounted. “He said, ‘Yes, my GPS coordinates led me right to
this address here.’ ”
http://www.walrusmagazine.com/articles/2009.11-health-global-impositioning-systems/
http://www.calfinder.com/blog/calfinder-news/oops-how-gps-led-contractors-to-demolish-the-wrong-house/
The Future…
Global Impositioning Systems… Is GPS technology actually harming our
sense of direction? by Alex Hutchinson Walrus Magazine,
Like any other human trait, navigational skill varies widely... as we enter the age of the global
positioning system, which is well on its way to being a standard feature in every car and on every
cellphone… neuroscientists are starting to uncover a two-way street: our brains determine how
we navigate, but our navigational efforts also shape our brains. The experts are picking up some
worrying signs about the changes that will occur as we grow accustomed to the brain-free
navigation of the GPS era. Once we lose the habit of forming cognitive maps, we may find
ourselves (unable to).
She fears that overreliance on GPS… will result in our using the spatial capabilities of the
hippocampus less, and that it will in turn get smaller. Other studies have tied atrophy of the
hippocampus to increased risk of dementia…
…a few years in a taxi can produce noticeable differences in our brains… researchers testing
cognitive map formation in drivers found that those using GPS formed less detailed and accurate
maps of their routes than those using paper maps. Similarly, a University of Tokyo study found
that pedestrians using GPS-enabled cellphones had a harder time figuring out where they were
and where they had come from. Their navigational aids, in other words, had allowed them to turn
off their hippocampi.
http://www.walrusmagazine.com/articles/2009.11-health-global-impositioning-systems/
Accuracy vs. Precision…
• GPS time ≠ UTC / Local time
• GPS time (‘zero’) began at midnight, 6-Jan-1980
• GPS is not perturbed by leap seconds (unlike UTC)
• GPS clock is now ahead of UTC by 15 seconds
• GPS 0˚0’0” ≠ Prime Meridian (thru Greenwich, England)
• GPS system (WGS-84) ‘fits’ better at 90˚ west (U.S.A)
• GPS 0˚ is east of the P.M. by ~ 105 m
GPS is very precise. GPS is also very accurate to itself… but not
necessarily accurate to other measurement systems (e.g., UTC or the
Prime Meridian…)
GPS Equipment at WWU
Garmin GPSmap60-C
Trimble XR
(backpack)
Trimble GeoExplorer-3
Trimble Yuma
Trimble Geo-XT
Trimble Juno
fini
Garmin GPSmap60-C(s)
Garmin GPSmap60-C
Waypoints & Tracks (Lines)
No DGPS
No Data Dictionaries
WAAS-Compatible
Color screen
Background Maps available
Navigation (GoTo, Find, Speed,
ETA, Distance, etc)
20-30 Hours (AA batteries)
7.5 oz.
User friendly interface
Additional Equipment:
PDA/Laptop connections
Remote antenna
Bicycle handlebar mount
Garmin GPSmap60-C
Power / Backlight
Battery Life
Zoom In/Out
Find
(Cities, Waypoints)
Rocker (navigation)
Pan map or select
options on a page
Page
Menu
Mark (Waypoints)
Quit
(close screen)
Enter
Garmin GPSmap60-C
Main Screens
Odometer
ETA - EDA
Velocity
Off course
GPS Accuracy
Bearing / Course
Heading
Speed / Max Speed
Time & Date
Garmin GPSmap60-C
Satellite Page
Accuracy Estimate
Location
Skyplot
Skyplot
Satellite Strength
Garmin GPSmap60-C
Mark Waypoint
Symbol
Average
Name
Garmin GPSmap60-C
Main Screens
Setup
Map (GoTo)
Profiles
By default, the GPSmap60 will record a Track (line feature)
whenever the unit is turned on.
Garmin GPSmap60-C
System Setup
GPS On/Off/Demo
WAAS On/Off
Alkaline or NiMH
(rechargeable) battery
Using WAAS decreases the battery life by ~1/3
Garmin GPSmap60-C
Games…
Virtual Maze
Gecko Smak
Garmin GPSmap60-C
Garmin GPSmap60-C
1000 Waypoints
WAAS Compatible
30 hours of battery life (2 "AA" batteries)
(Approximately 20 hours when using WAAS)
Alkaline or NiMH (rechargable) batteries
Waterproof
56 MB of internal memory for storing map detail (topo maps, etc)
Trip computer (odometer, moving average, travel time, max speed...
Quad Helix antenna (or connection to remote antenna)
fini
Trilateration
Technically, GPS uses Trilateration (not triangulation)
Triangulation involves the measurement of angles (which GPS
doesn’t do) from a baseline.
Trilateration uses measured distances. It requires two or more known
reference (location) points and the measured distances between an
unknown point and each of the reference points. In the case of GPS,
each satellite serves as a reference point and the distance is measured
from those points to the receiver.
GPS – Ellipsoid - Datum
GPS uses the WGS84 (World Geodetic System of 1984) as
mathematical surface (model) of the earth
WGS84 is for all practical purposes the same as the Geodetic
Reference System of 1980 (GRS80) that was used for the North
American Datum of 1983 (NAD83) horizontal datum.
Elevations are referenced to Height Above Ellipsoid (HAE)
Code vs. Carrier Phase
Standard GPS uses ‘Code’ phase – comparing ‘pseudo-random’
code to determine distance (amount of time out of sync between
satellite’s code and receiver’s code = distance satellite’s code had to
travel)
Carrier phase uses the Code phase to get close, then uses the actual
carrier frequency wave pattern (that which carries the pseudorandom code) to increase the precision.
For post-processing (Differential Correction), use Carrier phase
recordings to get the best results (with a minimum recoding time of
10 minutes). Carrier phase also stores information from each
satellite individually, allowing later comparison of the base station’s
readings (error estimation) for each separate satellite recorded.
Consequently, Carrier phase recordings use far more memory for
storage than Code phase does.
GNSS Timeline
1960’s Early conceptual work, satellites
1970’s GPS satellites and technology development
1980’s Functional navigations system (1983, free for public use, 8 satellites)
1990’s Civilian usages increases
2000
Selective Availability turned off
2003
WAAS
2011
GLONASS
2019?
Galileo