Presentation provides background information about the use of bedrock GPS data to observe ice mass changes

1. This work is supported by the National Science Foundation’s
Directorate for Education and Human Resources TUES-1245025, IUSE-
1612248, IUSE-1725347, and IUSE-1914915. Questions, contact education-AT-unavco.org
VERTICAL GPS RESPONSE TO SNOW
AND ICE MASS CHANGES

2. WHAT IS GPS?
• GPS receivers are in
phones, cars, etc.—
basically anything that tells
you where you are
Note: global positioning system (GPS) is
technically just the US component of
the global navigation satellite system
(GNSS) which includes Russia, Europe,
China, and others

3. HOW DOES GPS WORK?
• A receiver
obtains
signals
from GPS
satellites
Artist’s conception of a GPS Block II-F satellite in Earth orbit

4. HOW DOES GPS WORK?
• Network of 24–32
satellites
• Precisely known
orbits and time
• A receiver needs
4 satellites to
accurately
determine
position
http://spaceplace.nasa.gov/gps-pizza/en/

5. HOW DOES GPS WORK?
http://spaceplace.nasa.gov/gps-pizza/en/
• Receiver position is
determined by
calculating the distances
to at least 4 satellites
• Solve for the variables:
• North–South position
• East–West position
• Elevation
• Time

6. 6
GPS 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
• GPS receiver
• Power/batteries
• Communications/radio/modem
• Data storage/memory
HIGH-PRECISION PERMANENT GPS STATION

7. 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
Vert: ± 15 m
ERRORS (after 8 hrs)
Horiz: ± 2-4 mm
Vert: ± 10-15 mm
RATES
~1 mm/year
after ~2 years

8. Greenland GPS Network (GNET) GPS station (station name: NNVN) installed on a nunatak (a
rocky exposure completely surrounded by an ice sheet whose peak is above the ice) north of
Niviarsiat Nunatak, Southern Greenland. Equipment in the foreground is the GPS antenna.
Solar panels and instrument boxes are to the right. Photo courtesy of Mike Bevis (Ohio State
University)

10. GNET GPS station (name: HJOR) in SE
Greenland. GPS antenna is visible in the
photo. Site was constructed in 2007.

11. GNET GPS station (name: PLPK) on Pilgupik
Island, southeast Greenland. The glacier
visible across the water is Laube Glacier.

12. Vertical displacement of GPS receiver located on bedrock adjacent to Helheim Glacier from
2000–2011 (after Bevis et al., 2012).
EXAMPLE GREENLAND GPS BEDROCK STATION DATA

13. GREENLAND VERTICAL GPS
VELOCITIES (BEDROCK)
• GNET (Greenland GPS Network)
• Limited data 2000-2011.25
• Expanded network starting
2007-2009
• Rates vary from <1 to >20 mm/yr
vertical uplift
• Probably mostly elastic but some
viscoelastic response

14. WHAT PROCESSES ARE BEHIND ELEVATION CHANGES
WHEN SURFACE LOADS CHANGE?
1. Elastic response
• Near-instantaneous
• Order of magnitude smaller
2. Isostatic response
3. Flexural response
• 103-104 years time
• Viscoelastic
• Much bigger change in
the end

15. ELASTIC RESPONSE: HOW MUCH VERTICAL MOTION IS
FROM ANNUAL/MULTI-YEAR SNOW AND ICE VARIATIONS?
• Response to loading on a short time frame depends on the elastic
parameter E (Young’s modulus) for the bedrock beneath the station
• Compressional stress (s) = E x strain (e) (shortening or lengthening,
expressed as e (dimensionless) reflecting a greater or lesser force
being applied)
• Units for stress are force/area (N/m2) or (Pa).
F

16. VISCOELASTIC RESPONSE
Occurs when the
asthenosphere
actually moves in
response to loading
or unloading

17. HOW MUCH VARIATION IS DUE TO VISCOELASTIC
RESPONSES RELATED TO POST GLACIER REBOUND (PGR)?
Kelvin Viscoelastic Model

18. ISOSTATIC RESPONSE
Initial state – isostatic equilibrium
Weight of column to compensation
depth = rl g hlith + rasth g hroot
A load is applied (=action)
 Restoring force develops (=reaction)
 It is driven by density contrasts =
buoyancy force
Isostatic equilibrium is reached again
when: Acting force = Restoring force
Column to compensation depth =
rl g (hlith + hload)
Buoyance force = (weight of column after) – (weight of column before)
= rl g (hlith + hload) – (r g hlith + rasth g hroot) = (rl hload – rasth hroot) g
r = density h = height

19. HOW MUCH VARIATION IS DUE TO VISCOELASTIC
RESPONSES RELATED TO FLEXURAL LOADING?

20. FLEXURAL LOADING
bulging up
bulge decreases
when load decreases