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An Introduction to GPS / GNSS. Prepared by:. In Partnership with:. NSF DUE-1205110; 0903270. Outline. Terms: GNSS & GPS? Why do we use GNSS? What is GNSS? How does GNSS Work? What do you need to know about GNSS? What can you do with GNSS? How is GNSS used in the real world?.

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an introduction to gps gnss

An Introduction to GPS / GNSS

Prepared by:

In Partnership with:

NSF DUE-1205110; 0903270

  • Terms: GNSS & GPS?
  • Why do we use GNSS?
  • What is GNSS?
  • How does GNSS Work?
  • What do you need to know about GNSS?
  • What can you do with GNSS?
  • How is GNSS used in the real world?
gnss and gps
  • GPS = Global positioning system
    • GPS refers to the constellation of navigation satellites associated with the American System (which is a global system).
    • For a long time, GPS was the ‘only game in town’, so everything (satellites, receivers, etc.) was referred to as “GPS”
    • Times are changing… other options (besides GPS) are quickly developing
gnss global navigation satellite system
GNSS = Global Navigation Satellite System
  • GNSS is an umbrella term that includes any satellite navigation system. Options include:
    • GPS (U.S. | operational since 1994)
    • GLONASS (Russian | ‘re-operational’ since 2010)
    • Galileo (European Union | anticipated operation:2019)
    • Compass(China | Operational in Asia/Pacific since 2012 / anticipated global operation: 2020)
why gnss
  • Many features have addresses and landmarks that are associated with a destination.

Fred Jones

332 Elm St.

Frog Holler, VA


why gnss1
However, there are many features that do not have addresses…

There are many MAJOR cities that do not even have STREET NAMES!

And then there is the open ocean and sky…

why gnss2

Location, Location, Location



pre gps
Navigation is critical

Historical Navigational tools have limits:

The Sextant – doesn’t work if it is cloudy

Lowrance – radionavigation: only worked near land…

The military had its own reasons for determining location…

Identify targets

Friendly fire issues

“smart bombs”

what is gnss
What is GNSS…

And how does it work?

what is gps
What is GPS?
  • GPS (and GNSS) is not a single UNIT!
  • GPS = Global Positioning SYSTEM
  • GPS was developed by the Department of Defense at a cost of >$12 billion
  • Funding for the GPS was contingent on making the system available to the public.
gps and gnss is a system
GPS (and GNSS) is a SYSTEM

There are three major components in this system:

  • Satellites
  • Ground Control Stations
  • GNSS Receivers (or units)
  • There are 24-32 satellites up there at any given time orbiting the earth at ~11,000 naut. miles.
  • The DOD knows the EXACT location of each of the satellites at any given moment.
  • These satellites have VERY accurate clocks on board.
  • The satellites continuously send radio signals towards earth.
  • These radio signals are picked up by GPS receivers.

Satellites: “A Beehive of Activity”

gps control stations
GPS Control Stations
  • There are five control

stations that monitor the satellites.

  • Control stations enable information on Earth to be transmitted to the satellites (updates and fine turning).
  • Control stations continuously track satellites, and update the positions of each satellite.
  • Without control stations, the accuracy of the system would degrade in a matter of days.
gps receivers
GPS Receivers
  • GPS units are referred to as “receivers”.
  • They receive information (radio signals) from satellites.
  • The GPS receiver knows how
  • long it takes the signal to travel
  • from the satellite to the
  • receiver.
gps receivers1
GPS Receivers
  • The GPS receiver knows how long it takes the signal to travel from the satellite to the receiver.
  • The Receiver is therefore able to calculate its distance from the satellite.
    • Distance = time x velocity
    • Distance = time x 186,355 mi./sec.
  • The receiver can calculate the time that signal traveled from the satellite to the receiver.
  • The receiver is therefore able to determine its exact distance from the satellite.
how gps and gnss works
How GPS (and GNSS) Works

One satellite…

how gps works
How GPS Works

If the GPS receiver only obtains signals

from 1 Satellite, then it “knows” that it

is located somewhere on this sphere…

how gps works2
How GPS Works

If the GPS receiver only obtains signals from 2

satellites, then it “knows” that it is located

somewhere where these 2 spheres intersect

how gps works4
How GPS Works

If the GPS receiver obtains signals from 3 satellites, then it “knows” that it is located somewhere where these 3 spheres intersect (2 points)

how gps works6
How GPS Works

A fourth satellite is required to determine the exact location and elevation.

different grades of gnss receivers
Different “Grades” of GNSS receivers
  • Recreational Grade GNSS
    • Accurate to within 5 meters (could be better, but don’t rely on it)
    • Suitable for hunting, recreational, and some business uses
    • Lowest cost (smallest, and easiest to use): ~$100-$800
  • Mapping Grade GNSS
    • Accurate to within 1 meter (3 feet)
    • Requires differential processing (from a base station)
    • Suitable for many natural resource applications, city planning
    • $800-$7,000
  • Survey Grade GNSS
    • Accurate to within 1 cm
    • Suitable for building bridges…
    • $15,000 -$30,000
what you need to know about gnss
What you need to know about GNSS?
  • Signal Accuracy Issues
  • Selective Availability
  • Tricks of the Trade
  • Current Applications of GNSS
  • Future applications of GNSS
gps signal accuracy
GPS: Signal Accuracy

There are 2 types of GPS Signals:

P-code: (“Precise” code)

  • This is only available to the military and some selected public officials.
  • Very precise, not degraded.

C-code: (“Civilian” Code).

  • Less precise
  • Signal can be degraded (by scrambling the signal) especially in times of conflict.
  • This is what the GARMIN receivers (and all public GPS receivers) work with…
correcting for errors selective availability
Correcting for errors:Selective Availability
  • It is possible to correct for inherent signal errors.
  • This process is called Differential Correction
  • Here’s how it works…
differential correction
Differential Correction
  • There are already established base stations established around the U.S.
  • Surveyors have determined the precise location of these base stations.
  • Each base station has a GNSS receiver, which collects incoming (error prone) signals.
  • The true (surveyed) location coordinates are then compared to the GNSS coordinates.
  • The correction values are then:
    • Posted to the web for later correction (post-processing);
    • sent to other GNSS receivers in the field (correction ‘on the fly’).
differential correction1
Differential Correction

Base station w/ GNSS

receiver at known location:

Differential Correction


GNSS receiver in the field

collecting points, routes, etc.

Exact known (surveyed) coordinates differ from GNSS coordinates at this location = exact amount of error!

  • The Wide Area Augmentation System (WAAS) is a differential GNSS system that is being constructed to support GNSS accuracy in aircraft.
  • WAAS also provides additional accuracy “on the ground”
  • The GNSS receivers that we are using are WAAS compatible

Most (but not all) GNSS receivers are WAAS compatible.

95% of GNSS receivers on the market today are WAAS compatible

The GARMIN Venture HC is WAAS compatible

other tricks of the trade averaging
Other Tricks of the Trade:Averaging
  • Averaging: A GNSS receiver can collect points continuously for 15-30 seconds. The receiver can then average all these locations together
  • This only works when you are standing still!!
  • Note that not all GNSS receivers have an averaging capability

GNSS Collected Points

GNSS Averaged Position

“True” location

other tricks of the trade satellite distribution
It is better for your receiver to get a fix on “distributed” satellites, then poorly distributed satellites.Other Tricks of the Trade: Satellite Distribution

“Positional Dilution of Precision”







gnss planning software
GNSS planning software...

other tricks of the trade multipath errors
Try and stay away from buildings and other structures when using a GNSS receiver

Satellites may not be visible…

This can introduce error…

Other Tricks of the Trade: MultiPath Errors
other tricks of the trade tracking satellites
Other Tricks of the Trade: Tracking Satellites

GNSS has worldwide coverage…


You can lose satellite coverage (or received degraded signals) in areas with dense foliage, in “urban canyons”, etc.

You may also lose satellite coverage (or receive degraded signals) in deep valleys or gorges.

  • How accurate is a $150 GNSS?
    • That’s the million dollar question…
how accurate is a 150 gnss it depends
How accurate is a $150 GNSS?(It depends…)

Acknowledgements: Dr. Phillip Rasmussen, Utah Geospatial Extension Specialist


Brand “A” Day 1

Brand “A” Day 2

Brand “A” Day 3

Brand “A” Day 4

Brand “A” Day 5

Brand “B” Day 1

Brand “B” Day 2

Brand “B” Day 3

Brand “B” Day 4

Brand “B” Day 5

gnss data collection
GNSS Data Collection
  • Waypoints
  • Tracks
  • Routes
  • Find/GOTO
  • And more..!
what can you do with a gnss
What can you do with a GNSS?
  • Collect and store points (positions)

These are called WayPoints.

Field corners, insect infestation areas, crop damage, individual trees, trail heads, creek crossings, point source pollution, camping sites, and don’t forget “your car”!

  • Download the points onto your computer and integrate them with other mapping programs



Latitude: 37° 16’ 18”

Longitude: W80° 28’ 45”

Elevation: 2108 feet


what can you do with a gnss1
What can you do with a GNSS?
  • Collect and store the path that you have walked / driven
  • These paths are called TRACKS.
  • Calculate the distance of a track (i.e. perimeter around a field)
  • Calculate AREA measurements within a TRACK (after walking around a field or parking lot...)
  • Save and Download TRACKS onto your computer.
tracks just start walking1

Latitude: 37° 16’ 18”

Longitude: W80° 28’ 45”

Elevation: 2108 feet

Time: 13:22.15

Date: 05/08/2009

Tracks (just start walking…)
  • Each track point has important information associated with it...
  • “Virtual bread crumbs”
  • Track points can be collected:
  • Based on a time period (every 10 seconds)
  • Based on distance (every 20 feet)
  • Or a combination of time and distance (every 10 secs. or 20 feet, whichever comes first).
  • You can “track your way back...”*
  • You can use the track data to estimate area / perimeter*
  • You can use the time stamp in the trackfile to “georeference (or geotag)” photographs!*

* We’ll do this later!

what can you do with a gnss2
What can you do with a GNSS?
  • Collect and store ROUTES
  • Routes are similar to TRACKS, but are created by associating a series of Waypoints
  • Tracks are straight lines...
  • Routes can be handy for measuring “square fields” and “straight lines”
  • You can measure the length and area (acreage) of a Route.






  • Establish Waypoints at strategic locations
  • The GNSS Receiver “Connects the dots”
  • Area and perimeter measurements are generated
routes vs tracks
Routes vs. Tracks

Yellow lines = Route

Red lines = Track

Red dots = Track points

what can you do with a gnss3
What can you do with a GNSS?


  • The GOTO (or “Find”) function

Using the ‘GOTO’ function, the GNSS will guide you to a predefined Waypoint (you choose which one…) using an electronic compass and “pointer”

  • The GOTO/FIND function is like using “Autopilot”

You can program the GNSS to “beep” when you are within a certain distance of a selected Waypoint


What can you do with a GNSS?

  • Tide Tables
  • Many of the marine GNSS’s have built in tide tables. They provide tidal information and ranges for any date and any place…
  • The GARMIN Venture HC does not have tide table information…
  • Extra bell & whistle = extra $!

What can you do with a GNSS?

  • Speed

GNSS’s calculate your ground speed as you walk, run, drive, or fly

what can you do with a gnss4
What can you do with a GNSS?
  • Elevation

In addition to providing you with your latitude and longitude, GNSS provides you with elevation information. Elevation is not as accurate as X,Y information.

Some GNSS’s have built in barometric altimeters (to increase accuracy of z values). This option costs a bit extra!


What can you do with a GNSS?

  • Measure Area / perimeter
    • Farmers can use a GNSS to measure the area of a pasture or a field of corn…
    • Natural Resource Agents can measure the area of a proposed conservation easement…
    • Educators (and students!) can measure the area of impervious surfaces (or green space) around their campus’s and communities...
current application areas of gnss
Current Application Areas of GNSS
  • Public Safety
  • Environmental resource management
  • Aviation
  • Military
  • Local planning
  • Surveying
  • Recreation
  • Business
the future is bright
The Future is bright…
  • The DoD is in the process of upgrading the existing GNSS satellite constellation

-better coverage availability (i.e. in forested areas)

-anticipated greater accuracy (even for the recreational grade GNSS receivers)

waas satellites
WAAS Satellites
  • Historically, some areas in Virginia have had trouble acquiring the WAAS satellite
  • A new WAAS satellite was launched in Fall 2006
  • Better coverage for Virginia = higher accuracy levels
the russian gnss system
The Russian GNSS System
  • Is called GLONASS
  • Has fallen into “disrepair”.
  • Some new Russian GLONASS satellites are have been launched
  • Impacts: Potentially increased accuracy for US receivers that receive both US and Russian GNSS satellite signals (the private sector follows demand…)
  • Just need to keep our eye on the Russian economy (no $, no satellites!)
the european gnss system
The European GNSS System
  • Galileo will be Europe’s own global navigation satellite system
  • More accurate than the U.S.’s current GNSS system (~4 feet)
  • Better coverage area than the U.S.’s current GNSS system
  • Compatible and interoperable with the American global positioning system (so we’ve been told...)

Europeans, Chinese, India, etc.

gnss receivers
GNSS Receivers
  • Continue to get better and better
    • better antennae,
    • more efficient power consumption,
    • smaller
    • increasingly more “bells + whistles” (maps, hard drives, cameras, etc.)
    • Inexpensive, but the new bells + whistles keeps prices fairly stable...
the applications are endless
The applications are endless…

…and keep in mind that there are ~322,000,000 wireless subscribers in the U.S. (that’s a market penetration of 101%), as of July 2012.


gnss recreational and educational uses geocaching
GNSS recreational (and educational) usesGeocaching….
  • Geocaching is an entertaining adventure game for GNSS users.
  • The idea is to “hunt” for objects (prizes) that have been placed in the landscape (virtual orienteering).
  • Pick a prize, and leave a prize…

but geocachcing is so pass
but Geocachcing is so passé…
  • But have you ever tried Geodashing?
  • In this game, a list of coordinates, selected at random from around the globe, is presented to all users and teams. “First one there, wins..”
  • Requirements:
    • somebody with a lot of free time on their hands,
    • a GNSS,
    • Lots of extra batteries,
    • a new pair of sneakers,
    • a passport, frequent flier miles
    • and your dad’s VISA Card (or a trust fund will do)….
some gnss applications are innovative
Some GNSS Applications are “innovative”
  • Use GNSS to locate ‘the loo’!

gnss based buddy stalkers oops i mean buddy tracking
GNSS-based buddy stalkers (oops, I mean buddy tracking)…
  • “add on” service
  • Create a buddy list and locate your pals on your Smartphone.
  • Is your girlfriend really at the library…?
other gnss data loggers and tracking devices
Other GNSS Data Loggers and Tracking Devices…
  • We all know that UPS / FedEx and the major freight haulers use GNSS…
  • Other folks are tracking people too!
car navigation systems are getting into buddy tracking as well
Car Navigation Systems are getting into buddy tracking as well
  • Garmin and TomTom have well established systems in place…
gnss based tracking routing and fleet management
GNSS-based tracking, routing, and fleet management

You’ve seen those trucks going down the highway with the little “round things” on the top...

we are not only tracking trucks
We are not only tracking trucks...
  • Active tracking:real-time


  • Passive tracking:

provides a history..

example of passive tracking
Example of passive tracking
  • Rocky Knob Economic Development and Tourism Study
  • 490 visitors contacted over 4 data collection periods (July, August, September, and October). 
  • 323 agreed to participate (Response rate of 65.9%)
  • Of these 323 visitors, 312 visitors have returned a survey/GNSS unit (Secondary response rate of 96.6%)

. How many hours did you spend planning

Table 1


your trip to Floyd

and Patrick Counties?



























More than 4




Mean = 1.0

Median = 0.5

SD = 1.5


Tourist surveys

Pat. + Floyd Co. GPS Map

site location take into account existing traffic and tourist visitor patterns

There’s a high correlation between the VDOT traffic count map and the GPS tourist surveys…

Site Location:

Take into account existing traffic and tourist visitor patterns

the degree confluence project
The Degree Confluence Project
  • The "world's largest distributed expedition project“.
  • Goal: to visit each of the latitude and longitude integer degree intersections in the world, and to take pictures at each location.
gnss related careers
GNSS related careers
  • Agriculture
  • Conservation managers
  • Wildlife technicians
  • Surveying companies
  • Local governments
  • State agencies
  • Federal agencies
  • The military
  • Law enforcement
  • Real estate
  • Software programmers

Basically, any profession that

requires data collection “in the field” can benefit from using GNSS.

The applications are limited only to your imagination...

And evidence of this can be found on the GNSS based applications developed for SmartPhone devices)

there s a variety of software products available to support gnss
There’s a Variety of Software Products Available to Support GNSS
  • GPS Utility (free)
  • DNR GPS (free)
  • USA PhotoMaps (free)
  • RoboGeo (demo version and $75 version)
  • Terrain Navigator (~$89)
  • Data cards (topos, etc.) and aerial photography subscriptions for your GPS receiver
  • And just think about all of the 1000’s of Smartphone applications (Urbanspoon, etc.)
things to keep in mind
Things to keep in mind...
  • GNSS can serve as an accurate data collection tool for GIS applications;
  • GPS applications are becoming increasingly prevalent in our society, and support a variety of applications;
  • With GNSS receivers, you (more or less) get what you pay for (w/ prices ranging from $20,000+ - $59);
  • This technology is CURRENTLY used to support govt. services / private businesses;
  • Smartphone apps. are going to make all of this “take off”
  • Knowing how to use a GNSS does not make you a surveyor!!!