Reading GPS Time

Series PlotsRoger Groom, UNAVCO Master Teaching-in-Residence

Shelley Olds UNAVCO

GPS Basics

• Need 3 satellite signals to locate the receiver in 3D space

• 4th satellite used for time accuracy

• Calculate position within sub-centimeter

Anatomy of a GPS Station

GPS antenna inside of dome, 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

Movement of GPS stations

GPS station positions change as plates move.

How will these stations’ positions change

relative to one another?

Processed GPS Data

SBCC GPS STATION

• Located near Mission Viejo, CA • Position data collected every 30

seconds• One position estimate developed

for each day North East Vertical

Date North (mm) East (mm)Vertical

(mm)

1/1/2004 -37.67 36.57 2.33

1/2/2004 -38.04 35.73 5.63

1/3/2004 -37.16 35.83 4.69

1/4/2004 -37.34 36.34 5.36

1/5/2004 -37.59 36.44 9.11

… … … …

1/1/2005 -9.43 9.63 2.36

1/1/2006 16.48 -18.09 7.35

1/1/2007 45.98 -43.42 -6.43

Starting with the basics: GPS Time Series Plot

Y-axis:• North (N/S)• East (E/W)• Height (up/down)

(sometimes called Vertical)

• In millimeters

X-axis: Date of the measurement In 10ths of year or months

Red data points are rapid position estimates

Units of measurement

• X AxisTypically in 10ths of year

• Y AxisUsually millimeters (but always check)

Gaps in Data

Causes• Power outages• Snow coverage• Equipment failure• Vandalism• Wildlife• Etc.

Gum drop GPS station

Build a gum-drop model of a GPS Monument 1 gum drop for the receiver = GPS Receiver 4 toothpicks (3 legs, one center post) = Monument braces 3 small Playdoh feet = Cement Small piece of transparency paper = ‘see-through’ Earth’s plate

Gum-drop stations in action

DateNorth(mm)

East(mm)

Height (mm)

2000 0 0 0

2001 1 1 0

2002 2 2 0

2003 3 3 0

2004 4 4 0

2005 5 5 0… … …

What direction is GPS monument A moving?

GPS Monument A

5432102000 2001 2002 2003 2004 2005

Time (years)

Nor

th (

mm

)

DateNorth Position

(mm)

2000 0

2001 1

2002 2

2003 3

2004 4

2005 5… …

GPS Monument A

Plotting the GPS Position over time: North

North vs Time

543210

Time (years)

Nor

th (

mm

)

DateNorth Position

(mm)

2000 0

2001 1

2002 2

2003 3

2004 4

2005 5

… …

GPS Monument A

Plotting the GPS Position over time: North

North vs Time

2000 2001 2002 2003 2004 2005

543210

Time (years)

Eas

t (m

m)

DateEast Position

(mm)

2000 0

2001 1

2002 2

2003 3

2004 4

2005 5… …

GPS Monument A

Plotting the GPS Position over time: East

East vs Time

2000 2001 2002 2003 2004 2005

543210

Time (years)

Eas

t (m

m)

DateEast Position

(mm)

2000 0

2001 1

2002 2

2003 3

2004 4

2005 5… …

GPS Monument A

Plotting the GPS Position over time: East

East vs Time

2000 2001 2002 2003 2004 2005

543210

Time (years)

Hei

ght (

mm

)

DateHeight(mm)

2000 0

2001 0

2002 0

2003 0

2004 0

2005 0… …

GPS Monument A

Plotting the GPS Position over time: Height

Height vs Time

2000 2001 2002 2003 2004 2005

543210

Time (years)

Hei

ght (

mm

)

DateHeight(mm)

2000 0

2001 0

2002 0

2003 0

2004 0

2005 0… …

GPS Monument A

Plotting the GPS Position over time: Height

Height vs Time

2000 2001 2002 2003 2004 2005

Summary

• Positive slope (numbers getting larger)North-south time

series: The station is moving north

East-west time series: The station is moving east

Height time series: The station is moving up

Nor

th (

mm

)E

ast

(mm

)H

eigh

t (

mm

)

Time

DateNorth(mm)

East(mm)

19992000

01

01

2001 2 2

2002 3 3

2003 4 4

2005 5 5… … …

Plot North and East together

GPS Monument A

• Plot the location of the GPS station each year. • Draw an arrow from the first data point to the last data point.• Move your gum-drop GPS monument from year 2000’s position toward

2005’s position. What direction is your GPS monument moving?

543210

1 2 3 4 5

nort

h

east

DateNorth(mm)

East(mm)

19992000

01

01

2001 2 2

2002 3 3

2003 4 4

2005 5 5… … …

Step 2) What direction is GPS monument A moving?

GPS Monument A

• Plot the location of the GPS station each year. • Draw an arrow from the first data point to the last data point.• Move your gum-drop GPS monument from year 2000’s position toward

2005’s position. What direction is your GPS monument moving?

54321

1 2 3 4 5

nort

h

east

DateNorth(mm)

East(mm)

19992000

01

01

2001 2 2

2002 3 3

2003 4 4

2005 5 5… … …

Step 2) What direction is GPS monument A moving?

GPS Monument A

• Plot the location of the GPS station each year. • Draw an arrow from the first data point to the last data point.• Move your gum-drop GPS monument from year 2000’s position toward

2005’s position. What direction is your GPS monument moving?

54321

1 2 3 4 5

nort

h

east

What direction is GPS monument B moving?

DateNorth(mm)

East(mm)

2000 0 0

2001 -1 -2

2002 -2 -4

2003 -3 -6

2004 -4 -8

2005 -5 -10

GPS Monument B

Focus on the data for the North data• Is Monument B moving in a more positive or negative direction?• What direction is GPS Monument B moving? North or South?

Now, focus on the data for the East data• Is Monument B moving in a more positive or negative direction?• What direction is GPS Monument B moving? East or West?

Plotting North and East Positions Together

• Plot the location of the GPS station each year. • Draw an arrow from the first data point to the last data point.• Move your gum-drop GPS monument from year 2000’s position toward

2005’s position. What direction is your GPS monument moving?

GPS Monument B

-1-2-3-4-5

-5 -4 -3 -2 -1

nort

h

east

Date (Year) North (mm) East (mm)

2000 0 0

2001 -1 -2

2002 -2 -4

2003 -3 -6

2004 -4 -8

2005 -5 -10

-1-2-3-4-5

-5 -4 -3 -2 -1

nort

h

east

Plotting North and East Positions Together

• Plot the location of the GPS station each year. • Draw an arrow from the first data point to the last data point.• Move your gum-drop GPS monument from year 2000’s position toward

2005’s position. What direction is your GPS monument moving?

GPS Monument BDate (Year) North (mm) East (mm)

2000 0 0

2001 -1 -2

2002 -2 -4

2003 -3 -6

2004 -4 -8

2005 -5 -10

-1-2-3-4-5

-5 -4 -3 -2 -1

nort

h

east

Plotting North and East Positions Together

• Plot the location of the GPS station each year. • Draw an arrow from the first data point to the last data point.• Move your gum-drop GPS monument from year 2000’s position toward

2005’s position. What direction is your GPS monument moving?

GPS Monument BDate (Year) North (mm) East (mm)

2000 0 0

2001 -1 -2

2002 -2 -4

2003 -3 -6

2004 -4 -8

2005 -5 -10

-1-2-3-4-5

-5 -4 -3 -2 -1

nort

h

east

Moving south and west

Negative slopeNorth-south time

series: The station is moving south

East-west time series: The station is moving west

Height time series: The station is moving down

Nor

th (

mm

)E

ast

(mm

)H

eigh

t (m

m)

Time

What direction is this GPS station moving?

Zero slope

Is the GPS station moving?

Time

Nor

th (

mm

)

Time

What direction is this GPS station moving?

Zero slope

The station isn’t moving.

How is GPS monument BURN moving?

Time

Nor

th (

mm

)

Time

Which car matches the graphs?

WhatDirection?

________

Which car? (letter)

________

North

Time (Hours)

Nor

th (

mile

s)

East

Eas

t (m

iles)

Time (Hours)

1)

A

B

C

E

D

North

Which car matches the graphs?

WhatDirection?

________

Which car? (letter)

________

North

Time (Hours)

Nor

th (

mile

s)

East

Eas

t (m

iles)

Time (Hours)

1)

A

B

C

E

D

North

Which car matches the graphs?

WhatDirection?

________

Which car? (letter)

________

North

Time (Hours)

Nor

th (

mile

s)

East

Eas

t (m

iles)

Time (Hours)

1)

A

B

C

E

D

North

Which car matches the graphs?

WhatDirection?

North-Northeast

Which car? (letter)

_Car A__

1)

A

B

C

E

D

North

North

Time (Hours)

Nor

th (

mile

s)

East

Eas

t (m

iles)

Time (Hours)

Which car matches the graphs?

WhatDirection?

_______

Which car? (letter)

________

North

Time (Hours)

Nor

th (

mile

s)

East

Eas

t (m

iles)

Time (Hours)

2)

A

B

C

E

D

North

Which car matches the graphs?

WhatDirection?

_South_

Which car? (letter)

__Car C__

North

Time (Hours)

Nor

th (

mile

s)

East

Eas

t (m

iles)

Time (Hours)

2)

A

B

C

E

D

North

Which car matches the graphs?

WhatDirection?

______

Which car? (letter)

________

North

Time (Hours)

Nor

th (

mile

s)

East

Eas

t (m

iles)

Time (Hours)

3)

A

B

C

E

D

North

Which car matches the graphs?

WhatDirection?

Northwest_

Which car? (letter)

___Car B_

3)

A

B

C

E

D

North

North

Time (Hours)

Nor

th (

mile

s)

East

Eas

t (m

iles)

Time (Hours)

Which car matches the graphs?

What direction is Car E moving?

__________

4)

A

B

C

E

D

North

North

Time (Hours)

Nor

th (

mile

s)

East

Eas

t (m

iles)

Time (Hours)

Draw the North and East Graphs

Which car matches the graphs?

What direction is Car E moving?

___East___

4)

A

B

C

E

D

North

North

Time (Hours)

Nor

th (

mile

s)

East

Eas

t (m

iles)

Time (Hours)

Draw the North and East Graphs

Which car matches the graphs?

What direction is Car D moving?

_____________

5)

A

B

C

E

D

North

North

Time (Hours)

Nor

th (

mile

s)

East

Eas

t (m

iles)

Time (Hours)

Draw the North and East Graphs

Which car matches the graphs?

What direction is Car D moving?

Southeast

5)

A

B

C

E

D

North

North

Time (Hours)

Nor

th (

mile

s)

East

Eas

t (m

iles)

Time (Hours)

Draw the North and East Graphs

Reference Key

What does GPS tell us about Iceland ?

REYK

HOFN

GPS Monument REYKN

orth

(m

m)

Eas

t (m

m)

What direction is Monument REYK moving?

GPS Monument HOFNN

orth

(m

m)

Eas

t (m

m)

What direction is Monument HOFN moving?

What directions are Monument REYK and HOFN moving?

REYKHOFN

Nor

th (

mm

)E

ast

(mm

)

Nor

th (

mm

)E

ast

(mm

)

towards each other, away from each other, or in the same direction?

REYK HOFN

How quickly is REYK moving?

Average position on 1/1/1997 = ______ mmAverage position on 1/1/2007 = ______ mm

Speed of REYK north = ___ mm / 10 years = ____ / yr to the North or South

Nor

th (

mm

)

How quickly is REYK moving?

Average position on 1/1/1997 = _-135__ mmAverage position on 1/1/2007 = __70____ mm

Speed of REYK north = (70 - -135) mm / 10 years = 205 mm/ 10 yr = 20.5 mm / yr to the North

Calculate the speed of REYK for in the east-west direction and the speed of HOFN for both north-south and east-west directions (hint, use 1998 and 2008 for HOFN).

Nor

th (

mm

)

How are quickly are REYK and HOFN moving?

Nor

th (

mm

)E

ast

(mm

)

Nor

th (

mm

)E

ast

(mm

)

REYK HOFN

REYK North = 20.5___ mm/year

REYK EAST = -11.0 __ mm/year

HOFN North = 15.0 ____ mm/year

HOFN EAST = 13.0 __ mm/year

Plotting vectors: north

• Number each axis using the same scale.• Vector: magnitude and direction

Tail is the GPS monument location Length of arrow is the magnitude Shows direction on a map

• GPS Monument REYK has moved 20.5 mm to the North per year• On your graph paper, each block represents 1 mm.

Draw a vector arrow 20.5 blocks along the north axis.

Plotting vectors: east

• Now draw a vector arrow -11.0 blocks (mm) along the East axis:

Adding the vectors together

• To add the vectors together, Re-draw the East arrow by placing its tail at the arrow head of the North

arrow Then draw a new vector from the tail of the North vector to the arrow

head of the East vector. GPS Monument C moves at: √20.52 + -112 = 23 mm / yr to the WNW

Draw the vectors on your map

What is happening to Iceland?

Icelandic rift eruption(Photo: National Geographic Magazine)

Vestmanneyjar, Heimaey, Iceland January 1973.

Source: USGS

Sites of surface eruptions

The fracture pictured here occurs over the Mid-Atlantic Ridge.  The photo by R. Stull was taken immediately after the largest earthquake in the area since 1917.

www.calstatela.edu/dept/geology/HotSpots.htm

ICELAND

Mid-Atlantic Ridge

Conclusion

What you can do:• Explain the components of GPS data. • Determine velocity vectors from GPS

time series plots• Identify uses of GPS in applications

other than driving and hiking.

Other tools to explore

• UNAVCO GPS, Earthquake, Volcano Viewerhttp://geon.unavco.org/unavco/GEV.php

• Google Earth: Learn about Plate Tectonicshttp://geon.unavco.org/unavco/GE/Learn_about_

Plate_Tectonics.kmz

• IRIS Earthquake Browserhttp://www.iris.washington.edu/servlet/eventserv

er/map.do

59

Thank you

Contact: Shelley Oldsolds unavco.org

http://www.unavco.org/

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