Reading GPS Time Series Plots Shelley Olds UNAVCO adapted from Roger Groom, UNAVCO Master Teaching-in-Residence

5/21/ GPS Basics Need 3 satellite signals to locate the receiver in 3D space 4th satellite used for time accuracy Calculate position within sub-centimeter

5/21/ 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

5/21/ Movement of GPS stations GPS station positions change as plates move. How will these stations’ positions change relative to one another?

5/21/ 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 DateNorth (mm)East (mm) Vertical (mm) 1/1/ /2/ /3/ /4/ /5/ ………… 1/1/ /1/ /1/

5/21/ 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

5/21/ 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

5/21/ Units of measurement X Axis  Typically in 10ths of year Y Axis  Usually millimeters (but always check)

5/21/ What are those red data points? Red data points are rapid position estimates

5/21/ Data Gaps Power outages Snow coverage Equipment failure Vandalism Wildlife Etc.

5/21/ 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

5/21/ Gum-drop stations in action

5/21/ 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 North (mm) East (mm) Height (mm)

5/21/ Date North Position (mm ) East Position (mm) ……… 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? north east

5/21/ 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 North (mm) East (mm) Height (mm)

5/21/ Step 3) What direction is GPS monument B moving? Date North Position (mm) East Position (mm) ……… 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 north east

5/21/ How to read the time series plots Zero slope The station isn’t moving. Is this GPS station BURN moving?

5/21/ Step 4) Determine GPS Monument C’s velocity North Position (mm)East Position (mm) Vertical Position (mm) 12/1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/ /1/

5/21/ Calculate North Motion North position on 12/1/2005 = __ mm North position on 12/1/2006 = __ mm Change in position = ___ mm / year to the North/South 0.0 3

5/21/ Calculate North Motion North position on 12/1/2005 = 0 mm North position on 12/1/2006 = 3 mm Change in position = 3.0 mm / year to the North 0.0 3

5/21/ Calculate East Motion East position on 12/1/2005 = ________ East position on 12/1/2006 = ________ Change in position =

5/21/ 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

5/21/ Drawing the north vector 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

5/21/ 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

5/21/ 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: √ = 6.7 mm / yr

5/21/ Now let’s try a real life example… Contact info: Shelley Olds olds unavco.org