1. Some New Things You Can Do With GPS
In The Cryosphere
Kristine M. Larson,

2. Outline Overview on GPS-IR and PBO H2O
Cryosphere applications of GPS-IR:
snow depth variations
ice sheets
permafrost
tides
How to set up GPS-IR
Final Remarks

3. GPS Interferometric Reflectometry
What is GPS-IR?
GPS Interferometric Reflectometry
Direct Signal
Direct Signal
Reflected Signal
A GPS receiver records the interference between the direct and reflected GPS signals. More typically one simply calls this multipath. 53

4. Footprint depends on H and e.
Effectively your GPS site becomes an interferometer
frequency of signal strength data depends on H, the GPS transmit frequency, and the reflecting medium.
Footprint depends on H and e.

5. Example: reflector heights change when water levels change.
We use the frequency of the interference pattern in SNR data to find the distance to the “reflector.”
Example: reflector heights change when water levels change.
Simulated Signal to
Noise Ratio Data

6. GPS-IR Reflection Zone
For a ~2 meter high antenna in North America ~2012

7. open areas where reflection region is ~smooth
Where does GPS-IR work?
open areas where reflection region is ~smooth

9. PBO H2O & Terrestrial Hydrology
soil moisture: land-atmosphere interactions; runoff and infiltration; plant productivity.
snow depth/snow water equivalent: timing and amount of runoff; influences climate.
above-ground biomass: global carbon budget; influences climate. 53

10. Why measure these things with GPS receivers?
Supplement other in situ sensors, many of which have very small footprints and/or are expensive to maintain.
Satellites have very large footprints (and don’t work well in some conditions).
Ground sensors are needed both for assimilation and satellite validation.
It’s cheap.

11. PBO H2O
Snow: > 200 sites

12. last week in Boulder
last week’s blizzard
as of yesterday

13. Island Park, Idaho 53 53

14. More about PBO H2O on Thursday morning
from Eric Small

15. Constraints on snow accumulation and
firn density in Greenland using GPS receivers
Kristine M. LARSON, John WAHR, Peter KUIPERS MUNNEKE
Journal of Glaciology, Vol. 61, No. 225, 2015 doi: /2015JoG14J130

16. GPS in Greenland
Data from the GLISN network 53

17. meters
DEM
meters km
All positions courtesy of Nevada Reno

18. Snow Level Variations Measured Using GPS-IR
GLS1
GCN data courtesy of Koni Steffen

19. Ultrasonic/GPS-IR Differences
GPS-IR footprint is much, much larger than the ultrasonic sensor
GPS instrument was not installed to measure snow level, so it’s a freebie.
GPS currently has better latency than the ultrasonic but they could be the same.

20. A different story at GLS3 - but an interesting story.

21. 1. You can measure how far the GPS receiver is
DEM
1. You can measure how far the GPS receiver is
with respect to the center of the Earth
2. And you can use GPS-IR to measure how far away the top of the snow layer is.
3. This means you have sensitivity to the density of the firn layer & accumulation rates.

22. Positions from GLS3 mounted on a 90 m borehole
moved to standard GLISN mount
height increased
positions from Nevada Reno

23. GPS-IR Results Distance from antenna to the top of the snow
Moved the pole anchor
Made pole taller

24. reflector heights
Reflector heights predicted by firn model
Geocentric, Eulerian snow surface elevations (positions minus reflector heights corrected for downhill flow of the ice sheet)
Geocentric, Eulerian snow surface elevations predicted by firn model
Please see more about John Wahr’s modeling
in J. Glaciology article.

25. Barrow, Alaska
in the winter

26. Barrow, Alaska in the summer

27. SG27: GPS antenna phase center is ~3.8 meters above the surface
GPS-IR measures this number
Surface
Active Layer (~50 cm)
GPS monument extends ~3 meters below the surface, well below the active layer.

28. Vertical Positions from Nevada Reno

29. Comparison between GPS Tide Gauge and ‘Real’ Tide Gauge
Kachemak Bay site installed by Jeff Freymueller;Larson et al., The Accidental Tide Gauge, IEEE GRSL, 2013 53

30. Relevance to the Cryosphere, Palmer Station
GPS

31. Practical Issues Related to Using GPS-IR in the Cryosphere

32. Height of the antenna above the reflector controls the reflection region - not the horizontal distance.
Track all signals (L2C, L5, GLONASS, etc) if this is feasible (power, telemetry).
Steenbras Dam, Republic of South Africa

33. water level derived from reflected
water storage
(outline added)
water storage data:
water level derived from reflected
GPS signals
GPS derived
water level

34. The required sampling rate depends on the height of the antenna - and 15 sec is perfectly fine for many sites.
L2C is not always required - these tide records were computed with L1 SNR data (and 15 seconds).

35. A roof isn’t great for GPS-IR, but sometimes it can be used
Don’t use an elevation mask (low elevation angle signals are great for GPS-IR).
If you want to use a roof, put the antenna closest to the edge of the roof that is near something interesting.
You can easily predict the GPS-IR characteristics of your set up BEFORE you go in the field.

36. Choke-rings (and their relatives) do not stop multipath. Just sayin.

37. Final Remarks
It is straightforward to measure snow depth variations with data recorded by GPS instruments.
Subsidence caused by active layer melt is measurable by GPS.
GPS provides a simple and economical way to measure water levels in all seasons.
GPS instruments installed in ice sheets are sensitive to firn density and accumulation rates.
As power, memory, and telemetry allows, track all GNSS signals. You never know - it might be useful.

38. Acknowledgements Eric Small, Valery Zavorotny, and John Braun
Felipe Nievinski, Penina Axelrad, Andria Bilich, Ethan Gutmann, Clara Chew, Sarah Evans, Praveen Vikram, Karen Boniface, John Pratt, Evan Pugh, Bill Smith, Steve Running, John Kimball, Dennis Akos, Brian Hornbuckle, Tyson Ochsner, Jobie Carlisle, Mark Williams, Matt Jones, Mesa County Surveyors, Jeff Freymueller, John Wahr, Simon Williams, Minnesota DOT.
NSF GEO: Climate and Large-Scale Dynamics, Physical and Dynamic Meteorology, Hydrologic Sciences, EarthScope, Instrumentation and Facilities, Education and Outreach.
NASA: Earth Surface and Interior, Natural Hazards, AIST, and Terrestrial Hydrology.
UNAVCO maintains the EarthScope PBO sites with funding from NSF. 53

39. Comparison of 10 year tide gauge records at Friday Harbor
        Tide Gauge         GPS      |Diff|
Tide     Amp  Phase     Amp  Phase      (cm)
Sa       6.1  274.8     5.8  277.6      0.37
Ssa      1.5  227.7     1.6  220.1      0.21
Mf       2.0  168.2     2.0  162.4      0.20
Q1       7.4  250.0     7.5  249.9      0.13
O1      43.4  258.1    44.0  258.6      0.78
P1      23.6  278.7    23.1  278.0      0.54
S1       2.6   31.2     1.6   59.2      1.37
K1      76.0  280.0    76.0  279.0      1.33
J1       4.0  311.6     4.0  310.5      0.08
N2      12.1  342.4    12.0  343.1      0.15
M2      56.0   10.5    56.4   10.2      0.50
S2      13.3   36.0    13.2   34.9      0.25
MK3      1.2   26.8     1.2   33.9      0.16
M4       1.7  121.2     1.5  121.1      0.17
MS4      1.0  131.4     0.8  131.4      0.17
M6       0.5  236.0     0.4  255.1      0.18
Richard Ray, Simon Williams, Kristine Larson

40. Far from optimal “tide gauge”

41. INSAR-Alaska