Some New Things You Can Do With GPS In The Cryosphere Kristine M. Larson, http://kristinelarson.net

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

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

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.

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

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

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

http://xenon.colorado.edu/portal

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

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.

PBO H2O Snow: > 200 sites

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

Island Park, Idaho 53 53

More about PBO H2O on Thursday morning from Eric Small

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: 10.3189/2015JoG14J130

GPS in Greenland Data from the GLISN network 53

meters DEM meters km All positions courtesy of Nevada Reno

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

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.

A different story at GLS3 - but an interesting story.

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.

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

GPS-IR Results Distance from antenna to the top of the snow 2012 2013 2014 2015 2016 Moved the pole anchor Made pole taller

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.

Barrow, Alaska in the winter

Barrow, Alaska in the summer

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.

Vertical Positions from Nevada Reno

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

Relevance to the Cryosphere, Palmer Station GPS

Practical Issues Related to Using GPS-IR in the Cryosphere

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

water level derived from reflected water storage (outline added) water storage data: https://www.dwa.gov.za/Hydrology/Weekly/percentile.aspx?station=%20G4R001 water level derived from reflected GPS signals GPS derived water level

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).

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.

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

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.

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

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

Far from optimal “tide gauge”

INSAR-Alaska