Some New Things You Can Do With GPS - PowerPoint Presentation

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Some New Things You Can Do With GPS - PPT Presentation

In The Cryosphere Kristine M Larson httpkristinelarsonnet Outline Overview on GPSIR and PBO H 2 O Cryosphere applications of GPSIR snow depth variations ice sheets permafrost tides ID: 792082

snow gps data tide gps snow tide data water antenna measure surface reflector john gauge signals signal positions layer

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Slide1

Some New Things You Can Do With GPS

In The Cryosphere

Kristine M. Larson,

http://kristinelarson.net

Slide2

Outline

Overview on GPS-IR and PBO H2OCryosphere applications of GPS-IR:

snow depth variations

ice sheets

permafrost

tides

How to set up GPS-IR

Final Remarks

Slide3

What is GPS-IR?

GPS Interferometric Reflectometry

Direct Signal

A GPS receiver records the interference between the direct and reflected GPS signals. More typically one simply calls this

multipath

Reflected Signal

Slide4

frequency of signal strength data depends on

H, the GPS transmit frequency

, and the

reflecting medium.

Footprint depends on H and e.

Effectively your GPS site becomes an interferometer

Slide5

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.”

Simulated Signal to

Noise Ratio Data

Slide6

GPS-IR Reflection Zone

For a ~2 meter high antenna in North America ~2012

Slide7

Where does GPS-IR work?

open areas where reflection region is ~smooth

Slide8

http://xenon.colorado.edu/portal

Slide9

PBO H

O & 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.

Slide10

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.

Slide11

Snow: > 200 sites

PBO H

Slide12

last week in Boulder

last week’s blizzard

as of yesterday

Slide13

Island Park, Idaho

Slide14

More about PBO H

2O on Thursday morning from Eric Small

Slide15

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

Slide16

GPS in Greenland

Data from the GLISN network

Slide17

meters

DEM

meters

All positions courtesy of Nevada Reno

Slide18

GCN data courtesy of Koni Steffen

GLS1

Snow Level Variations Measured Using GPS-IR

Slide19

Ultrasonic/GPS-IR Differences

GPS-IR footprint is much, much larger than the ultrasonic sensorGPS 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.

Slide20

A different story at GLS3 - but an interesting story.

Slide21

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.

DEM

3. This means you have sensitivity to the density of the firn layer & accumulation rates.

Slide22

Positions from GLS3

mounted on a 90 m borehole

moved to standard GLISN mount

height increased

positions from Nevada Reno

Slide23

Distance from antenna to the top of the snow

2012 2013 2014 2015 2016

Moved the pole anchor

Made pole taller

GPS-IR Results

Slide24

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.

Slide25

Barrow, Alaska

in the winter

Slide26

Barrow, Alaska in the summer

Slide27

SG27: GPS antenna phase center is ~3.8 meters above the surface

Surface

Active Layer (~50 cm)

GPS monument extends ~3 meters below the surface, well below the active layer.

GPS-IR measures this number

Slide28

Vertical Positions from Nevada Reno

Slide29

Kachemak Bay site installed by Jeff Freymueller;Larson et al., The Accidental Tide Gauge,

IEEE GRSL

, 2013

Comparison between GPS Tide Gauge and ‘Real

’

Tide Gauge

GPS Tide Gauge

Slide30

Relevance to the Cryosphere, Palmer Station

GPS

Slide31

Practical Issues Related to Using GPS-IR in the Cryosphere

Slide32

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

Slide33

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

Slide34

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).

Slide35

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.

Slide36

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

Slide37

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.

Slide38

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.

Acknowledgements

Slide39

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