Megan Wolfe

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Investigating Tidal Currents using GPS
Shallow-water Drifters in Grappler Inlet
Megan Wolfe
Megan Wolfe
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Table of Contents

• The Question
• The Question Revised
• Design and makings of the drifters
• Problems encountered
• Analysis and Discussion up to Date

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The Question

• Team Bs layer of Ice
• Very thin layer of fresh water distributed
  throughout Grappler Inlet
• Wanted to determine how the tidal currents
  effected the distribution of the fresh water
  layer in Grappler Inlet

4
Revising the Question

• Problem!
• Thin fresh water layer was too thin!!
• - Fresh layer was only 3-10cm
• Could not design a drifter that would rest in the
  top 3-10cm which would not be influenced by the
  wind.
• Investigating the tidal currents throughout
  Grappler Inlet at 3 different depths Surface,
  1m, and 2m.

5
Design of the Drifters
Two parts of a Drifter -The drifter -And
the drogue

• Three Drifters built using everyday materials
• -Duct Tape
• -Styrofoam
• -plastic bags
• -rocks used to balance drogue
• New and improved tracking GPS units

2m
Surface
1m
6
1m
2m
Surface
7
Experimental Problems

• Bulky design of drifters
• Shore encounters
• Low tide
• -shallow waters
• -change in tide during experiment

8
Analysis and Discussion

• A look at all three of the drifter paths
• CTD temperature and salinity readings
• Separate out and focus on individual drifters
  paths
• GPS accuracy

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Big map
Grappler Inlet with Drifter Destination
Big map
48.836
48.835
48.834
48.833
Latitude N
48.832
48.831
48.83
1m Drifter
Surface Drifter
2m Drifter Colored Dots Represents areas where
CTD readings were taken
48.829
-125.13
-125.125
-125.12
-125.115
-125.11
Longitude E
10
CTD Temperature and Salinity

• Temperature at surface is cooler due to fresh
  water layer/ice layer, peaks around 1m and
  decreases with depth
• Comparing Salinity and Temperature, Salinity
  seems to increase with depth, opposite to what
  the temperature is doing

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Surface
Surface Drifter Latitudinal Distance Traveled
over Time
300
250
200
150
Distance traveled Latitudinally (meters)
100
50
0
-50
-100
-150
0
50
100
150
Time (min)

• Comments
• During outgoing tide, not much movement Ice
  hindrance
• Incoming tide Lots of movement, traveled
  farthest, but wind present

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1m Drifter
Distance Traveled Latitudinally by 1m Drifter
vrs. Time
200
100
0
-100
Distance Traveled Latitudinally (meters)
-200
-300
-400
0
50
100
150
Time Traveled (mins)
Zoomed in on White Box
Zoomed in on Black Box Standard Deviation
1.347m
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2m Drifter
Distance Traveled Latitudinally by 2m Drifter
vrs. Time
150
100
50
0
Distance Traveled Latitudinally (meters)
2m
-50
-100
-150
0
20
40
60
80
100
120
140
160
Time Traveled (mins)
Zoomed in white box
Zoomed in Black box Standard Deviation
2.6832m
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GPS Error

• Six classes of GPS Error
• Ephemeris data Transmitted location errors of
  the satellite
• Satellite clock Errors in the transmitted clock,
  
• Ionoshpere Errors due to the corrections of
  pseudorange caused by ionospheric effects
• Troposphere Errors due to the corrections of
  pseudorange caused by tropospheric effects
• Multipath Errors caused by reflected signals
  entering receiver antenna
• Receiver Errors in the receivers measurements
  of range caused by thermal noise, software
  accuracy, and inter-channel biases

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Comparing GPS Accuracy

• Determining GPS accuracy
• 2m drifter low tide coming to the rescue
• Comparison with other GPS accuracy tests
• Compact, low-cost GPS near shore drifterCenter
  for Water Research, U. of Western Australia

Power Spectra of the residuals Difference
between the recorded position from the mean
time- averaged position. The standard deviation
of the position from the mean was 1.3m in
Easting direction and 1.6m in the Northing
direction
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Determining GPS Accuracy

• 2m Drifter ran a ground became stationary for
  40mins.
• A good estimation of GPS accuracy
• Standard Deviation 1.8864m

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Drifter Improvements

• Effects due to wind stress
• - Less surface area exposed
• Lack of materials and resources
• Design of a smaller, compact
• GPS drifter that contains off the shelf
  components

Design of a smaller, compact GPS drifter that
contains off the shelf components Costs350
plus an estimated 8hrs of preparation
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References

• A Compact, Low-Cost GPS Drifter for Use in the
  Oceanic Nearshore Zone, Lakes, and
  Estuaries,Journal of atmospheric and oceanic
  technology 0739-0572 Johnson yr 2003 vol 20
  iss 12 pg 1880
• A GPS-Tracked Surf Zone Drifter,Journal of
  atmospheric and oceanic technology 0739-0572
  Schmidt yr 2003 vol 20 iss 7 pg 1069