Megan Wolfe

Investigating Tidal Currents using GPS Shallow-water Drifters in Grappler Inlet

Megan Wolfe

Table of ContentsTable of Contents

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

The QuestionThe Question

• Team B’s 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

Revising the QuestionRevising 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.

Design of the DriftersDesign of the Drifters

• Three Drifters built using everyday materials-Duct Tape

-Styrofoam

-plastic bags

-rocks: used to balance drogue

• New and improved tracking: GPS units

2m

1mSurface

Two parts of a Drifter-The drifter

-And the drogue

Surface

2m1m

Experimental ProblemsExperimental Problems

• Bulky design of drifters

• Shore encounters

• Low tide-shallow waters

-change in tide during experiment

Analysis and DiscussionAnalysis 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

Big map Big mapBig map

Longitude E

La

titu

de

NGrappler Inlet with Drifter Destination

-125.13 -125.125 -125.12 -125.115 -125.11

48.829

48.83

48.831

48.832

48.833

48.834

48.835

48.836

1m DrifterSurface Drifter

2m DrifterColored Dots Representsareas where CTD readings were taken

CTD Temperature and SalinityCTD 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

SurfaceSurface

• Comments• During outgoing tide, not much movement: Ice

hindrance

• Incoming tide: Lots of movement, traveled farthest, but wind present

0 50 100 150-150

-100

-50

0

50

100

150

200

250

300

Time (min)

Dis

tanc

e tr

avel

ed

Latit

udin

ally

(m

eter

s)Surface Drifter Latitudinal Distance Traveled over Time

Latit

ude

N

Longitude E

Zoomed in Surface Drifter

-125.117 -125.1165 -125.116 -125.1155 -125.115 -125.1145 -125.114 -125.1135 -125.113 -125.1125 -125.112

48.833

48.8335

48.834

48.8345

48.835

48.8355

48.836

48.8365

1m Drifter1m Drifter

0 50 100 150-400

-300

-200

-100

0

100

200

Time Traveled (mins)

Dis

tanc

e T

rave

led

Latit

udin

ally

(m

eter

s)

Distance Traveled Latitudinally by 1m Drifter vrs. Time

Longitude E

Latit

ude

N

Zoomed in on 1m Drifter Path

-125.122 -125.121 -125.12 -125.119 -125.118 -125.117 -125.116 -125.115 -125.114 -125.113

48.831

48.832

48.833

48.834

48.835

48.836

Longitude E

Latit

ude

N

Zoomed in on 1m Drifter Eddies

-125.12 -125.1195 -125.119 -125.1185 -125.118 -125.1175

48.8308

48.831

48.8312

48.8314

48.8316

48.8318

48.832

48.8322

48.8324

48.8326

48.8328

Longitude E

Latit

ude

N

Zoomed in on 1m during Incoming Tide

-125.1158 -125.1156 -125.1154 -125.1152 -125.115 -125.1148 -125.1146 -125.1144

48.834

48.8342

48.8344

48.8346

48.8348

48.835

48.8352

48.8354

48.8356

48.8358

Zoomed in on White Box

Zoomed in on Black Box: Standard Deviation = 1.347m

2m Drifter2m Drifter

2m

0 20 40 60 80 100 120 140 160-150

-100

-50

0

50

100

150

Dis

tanc

e T

rave

led

Latit

udin

ally

(m

eter

s)

Time Traveled (mins)

Distance Traveled Latitudinally by 2m Drifter vrs. Time

Longitude E

Latit

ude

N

Zoomed in on 2m Drifter over Sandbar

-125.1164 -125.1163 -125.1162 -125.1161 -125.116 -125.1159

48.8332

48.8333

48.8334

48.8335

48.8336

48.8337

48.8338

48.8339

Longitude E

Latit

ude

N

Zoomed in on 2m Drifter Path

-125.1165 -125.116 -125.1155 -125.115 -125.1145 -125.114

48.8335

48.834

48.8345

48.835

Longitude E

Latit

ude

N

Zoomed in on 2m Drifter during Incoming Tide

-125.1156 -125.1154 -125.1152 -125.115 -125.1148 -125.1146

48.8342

48.8344

48.8346

48.8348

48.835

48.8352

Zoomed in white box

Zoomed in Black box: Standard Deviation = 2.6832m

GPS ErrorGPS 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- MultipathMultipath: Errors caused by reflected signals entering : Errors caused by reflected signals entering

receiver antennareceiver antenna- Receiver: Errors in the receiver’s measurements of range

caused by thermal noise, software accuracy, and inter-channel biases

Comparing GPS AccuracyComparing 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 drifter:Center for Water Research, U. of Western Australia

Power Spectra of the residuals:

Difference between the recordedposition 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

Determining GPS AccuracyDetermining GPS Accuracy

• 2m Drifter ran a ground: became stationary for 40mins.

• A good estimation of GPS accuracy• Standard Deviation = 1.8864m

0 20 40 60 80 100 120 140 160-150

-100

-50

0

50

100

150D

ista

nce

Tra

velle

d L

atit

ud

ina

lly (

me

ters

)

Time Travelled (mins)

Distance Travelled Latitudinally by 2m Drifter vrs. Time

35 40 45 50 55 60 65 70 75

20

25

30

35

40

45

50

55

60

Time Traveled (mins)

Dis

tanc

e T

rave

led

Latit

udin

ally

(m

eter

s)

Stationary 2m Drifter in Time

Design of a smaller, compact GPS drifter that contains ‘off the shelf’ componentsCosts:$350 plus an estimated 8hrs of preparation

Drifter ImprovementsDrifter 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

ReferencesReferences

• 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