Reading GPS Time Series Plots

Shelley Olds

UNAVCO

adapted from Roger Groom, UNAVCO Master Teaching-in-Residence

04/18/23 2

GPS Basics

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

• 4th satellite used for time accuracy

• Calculate position within sub-centimeter

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Anatomy of a GPS Station II

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

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Movement of GPS stations

GPS station positions change as plates move.

How will these stations’ positions change

relative to one another?

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Processed 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

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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

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Position time series plot

• Time series plot shows the GPS station’s change in position over time

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

months• Y-axis:

North (N/S) East (E/W) Height (up/down)

(sometimes called Vertical)

In millimeters

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Units of measurement

• X Axis Typically in 10ths of year

• Y Axis Usually millimeters (but always check)

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What are those red data points?

Red data points are rapid position estimates

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Data Gaps

• Power outages

• Snow coverage

• Equipment failure

• Vandalism

• Wildlife

• Etc.

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Gum drop GPS station

Step 1) 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

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Gum-drop stations in action

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Moving north and east

• Positive slope 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

)

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DateNorth Position

(mm)East Position

(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

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Moving south and west

Negative slope North-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)

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Step 3) What direction is GPS monument B moving?

DateNorth Position

(mm)East Position

(mm)

19992000

0-1

0-1

2001 -2 -2

2002 -3 -3

2003 -4 -4

… … …

• 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

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How to read the time series plots

Zero slope

The station isn’t moving.

Is this GPS station BURN moving?

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Step 4) Determine GPS Monument C’s velocity

  North Position (mm) East Position (mm)Vertical Position

(mm)

12/1/05 0 0 0.05

1/1/06 0.25 0.5 0

2/1/06 0.5 1 0

3/1/06 0.75 1.5 0.08

4/1/06 1 2 -0.02

5/1/06 1.25 2.5 06/1/06 1.5 3 -0.03

7/1/06 1.75 3.5 0.1

8/1/06 2 4 0.15

9/1/06 2.25 4.5 0

10/1/06 2.5 5 -0.1

11/1/06 2.75 5.5 0.02

12/1/06 3 6 0.1

1/1/07 3.25 6.5 0.1

2/1/07 3.5 7 -0.04

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Calculate North Motion

North position on 12/1/2005 = __ mmNorth position on 12/1/2006 = __ mm

Change in position = ___ mm / year to the North/South

GPS Monument CNorth Position (mm)

-2

-1

0

1

2

3

4

5

6

7

12/1/2005 6/1/2006 12/1/2006

Date

No

rth

(m

m)

0.0

3

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Calculate North Motion

North position on 12/1/2005 = 0 mmNorth position on 12/1/2006 = 3 mm

Change in position = 3.0 mm / year to the North

GPS Monument CNorth Position (mm)

-2

-1

0

1

2

3

4

5

6

7

12/1/2005 6/1/2006 12/1/2006

Date

No

rth

(m

m)

0.0

3

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GPS Monument CEast Position (mm)

-2

-1

0

1

2

3

4

5

6

7

12/1/2005 6/1/2006 12/1/2006Date

Ea

st

(mm

)Calculate East Motion

East position on 12/1/2005 = ________

East position on 12/1/2006 = ________

Change in position =

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GPS Monument CEast Position (mm)

-2

-1

0

1

2

3

4

5

6

7

12/1/2005 6/1/2006 12/1/2006Date

Ea

st

(mm

)Calculate East Motion

East position on 12/1/2005 = 0

East position on 12/1/2006 = 6

Change in position = 6 mm / year to the East

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Drawing the north vector

GPS Velocity Vectors

-10

-8

-6

-4

-2

0

2

4

6

8

10

-10 -8 -6 -4 -2 0 2 4 6 8 10East (mm)

North

(mm

)

North

3.0 mm

• 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

• Example: GPS Monument C has moved 3.0 mm to the North in one year

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GPS Velocity Vectors

-10

-8

-6

-4

-2

0

2

4

6

8

10

-10 -8 -6 -4 -2 0 2 4 6 8 10East (mm)

North

(mm

)

North

3.0 mm

6.0 mm

Drawing the east vector

• GPS Monument C moves 3.0 mm / year to the

North 6.0 mm / year to the

East

Vector: magnitude and direction• Tail is the location of the monument• Length of arrow is the magnitude• Plotted to show direction on a map

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GPS Velocity Vectors

-10

-8

-6

-4

-2

0

2

4

6

8

10

-10 -8 -6 -4 -2 0 2 4 6 8 10East (mm)

North

(mm

)

North

3.0 mm

6.0 mm

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 to the ENE at: √32 + 62 = 6.7 mm / yr

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Now let’s try a real life example…

Contact info:Shelley Olds

olds unavco.org303-381-7496