GNSS/GPS systems enable precise positioning using signals from satellites. Consumer-grade receivers provide positions within 5 meters, while survey-grade systems can achieve precision of a few centimeters with post-processing. Earth scientists use GNSS to track ground movement, map topography, and monitor hazards, providing both direct benefits like early warning systems and indirect benefits like furthering scientific understanding.

1. GNSS/GPS Basics
Vince Cronin (Baylor University) & Shelley Olds (UNAVCO)
Revisions by Beth Pratt-Sitaula (UNAVCO) and Benjamin Crosby (ISU)
Version May 2019

2. Motivations
1. Describe the Global Navigation Satellite System
(GNSS) and how it enables positioning
2. Distinguish different grades of GNSS receivers, their
uses, and their accuracies.
3. Highlight applications of GNSS in the Geosciences

3. GPS receivers are all around us

4. GPS provides 3D positioning
• Positions on the earth can be reported using:
• Cartesian coordinates (relative to the earth’s center)
• Geographic coordinates (lat., long., elev., in deg.)
• Projected coordinates (UTM, state plane, in m or ft)
Cartesian (X,Y,Z) Geographic System Projected System
(Figures: Ian Lauer, modified from Common Domain)

5. Typical GPS coordinates
• Most GPS data is recorded and reported using:
Geographic Coordinates
 World Geodetic System 1984 (WGS 84)
– A reference surface or datum composed of an ellipsoid
– A geoid model (gravitational equipotential surface, EGM96)
 Remember, elevations can be reported as ellipsoidal heights
or orthometric heights
(Figure: Ian Lauer)

6. Multiple satellite systems
• There are multiple Global Navigation Satellite Systems
(GNSS)
• GPS: USA, global
• GLONAS: Russia, global
• After 2020:
• BieDou: China, global
• Galileo: Europe, global
• India, France, and Japan: developing regional systems

7. Global Positioning System
• GPS: the US System
• ~32 satellites
• 20,200 km altitude
• 55 degrees inclination
• 12 hour orbital period
• Need 4 satellites to be
accurate
• Ground control stations
• Each satellite passes
over a ground monitoring
station every 12 hours
https://commons.wikimedia.org/wiki/File:GPS24goldenSML.gif

8. GPS satellite
Artist’s conception of a GPS Block II-F satellite in Earth orbit. (Public domain from
NASA) https://en.wikipedia.org/wiki/Global_Positioning_System

9. http://gpsinformation.net/main/almanac.txt
Satellite sends orbit and clock Info
• GNSS satellites include almanac and
ephemeris data in the signals they transmit
Almanac data are coarse orbital parameters for all
GPS satellites. Communicated to your GPS so you
can track satellites.
Ephemeris data are very precise orbital and clock
correction for each particular GPS satellite—
necessary for precise positioning

10. Antennas receive data streams
Your location is:
37o
23.323’ N
122o
02.162’ W
The time is:
11:34.9722 (UTC)
Works the same…
ERRORS
Horiz: +/- 10 m (30 ft)
Vert: +/- 15 m (45 ft)
ERRORS (after 8 hrs)
Horiz: +/- 2-4 mm (~1/8 in)
Vert: +/- 10-15 mm (~1/2 in)

11. • Radio signal from satellite tells GNSS receiver the satellite-
clock time and provides the most recent corrections to the
satellite’s position relative to Earth (ephemeris)
• GNSS receiver compares multiple satellite-times to the
receiver-time to determine the distance to each satellite
Measuring the range to the satellite

12. How actual location is determined
Antenna position is determined by calculating the
distances to at least 4 satellites. This enables the solving
for four variables: x, y, z and time using trilateration.
http://spaceplace.nasa.gov/gps-pizza/en/

13. Anatomy of a high-precision
permanent GNSS station
13
GNSS antenna inside of dome
Monument solidly attached into
the ground with braces.
If the ground moves, the station
moves.
Solar panel for power
Equipment enclosure
• GNSS receiver
• Power/batteries
• Communications/ radio/ modem
• Data storage/ memory

14. High-precision GNSS requires…
• Stable monuments
• Multiple stations
• Sophisticated processing
• Collecting lots of data
• Using the carrier phase
• Dual-frequency receivers
• High-precision orbital information
(ephemeris)
 with several years of data can
determine velocities to 1–2 mm/yr

15. Sources of error
15
Some GPS Error Sources
• Selective availability (ephemeris data encrypted by military – ended in 2000)
• Satellite orbit irregularities
• Satellite and receiver clock errors
• Atmospheric delays – speed of light is affected by
water content and other variables in the atmosphere
• Multi-path – GPS signals can bounce off the ground
and then enter the antenna, rather than only entering
from above
• Human error – Incorrect base or rover antenna heights,
errors in post-processing, datum and projection errors.

16. Grades of GNSS Systems
• Consumer or Recreational Grade
Phones, tablets, watches, hiking devices
~5 meters, No post-processing required
• Mapping Grade
Purpose built, GIS enabled, data collectors
~30 cm, Post-processing/correction required
• Survey Grade
Professional tools, Longer occupations, Static
and kinematic devices
~3 mm to 2 cm precision. Considerable post-
processing required

17. Precision depends on system
Precision of Position
OccupationTimeorEffortRequired
Static, Geodetic
Campaign Systems
Kinematic
Systems
Recreational
& Mapping
Systems
0.5-5 m 0.01–0.03 m 0.005m
EasyHard
(Images: Ben Crosby)
Survey Grade

18. Applications of GNSS
• Recreational & Mapping Systems (phones,
consumer-type, mobile GIS devices)
Inexpensive, low complexity, short
occupations, rapid results, low-precision
positions
• Kinematic Systems (Unit 2)
Expensive, moderate complexity, short
occupations, positions can be rapid or require
post-processing, high-precision positions
• Static Systems (Unit 3)
Expensive, high complexity, long occupations
required, long and complex post-processing
required, extremely high-precision positions.
(Images: Ben Crosby)

19. Example 1: Tracking position
• Using Recreational Systems
Use a phone to track your positon during a field day.
Can quickly assess the area or position of an object.
64,500 km2
From the field… …to the phone… …to analysis in GIS.
(Images: Ben Crosby)

20. Example 2: Creating topography
• Using Kinematic Systems
Quickly measure many points with high accuracy and precision
Compare different surfaces to quantify permafrost thaw
From the field … … o post- … to surface generation
processed points … using GIS.
(Images: Ben Crosby)

21. Example 3: Change detection
• Using Static Systems
Measure a small number of points over a long duration
Can resolve small changes in position, e.g. tracking landslides
From the field … … to four post- … to mm scale
processed points … time series.
Antenna
Receiver
Solar
NT
(Dorsch, 2004 Thesis)

22. Societal value of
GNSS-enabled research
• Most people use it for location and navigation
• But … GNSS-enabled science also provides:
Hazard early warning systems, saving lives
 Landslide activity
 Volcano inflation
 Fault movement
Precise measurements of objects
 Water resources (aquifers, snow pack, etc.)
 Tracking of objects (organisms, rocks, currents)
Without GNSS, we could not know where things
are when without directly measuring them.

23. Societal value of
GNSS-enabled research
• Most people use it for location and navigation,
but how do earth scientists use GNSS?
Think-Pair-Share discussion
How do earth scientists use GNSS?
 List as many applications as you can.
How do these uses benefit society?
 Categorize each as a direct or indirect benefit.
– Direct benefits are immediate and improve lives
– Indirect benefits help humans, but are a few steps removed

24. Societal value of
GNSS-enabled research
• Most people use it for location and navigation,
but how do Earth Scientists use GNSS?
How do earth scientists use GNSS?
 (type student applications here)
How do these uses benefit society?
 Direct
– (type student benefits here)
 Indirect
– (type student benefits here)

25. End Lecture
(Trying to keep this to fewer slides)