megan wolfe investigating tidal currents using gps shallow-water drifters in grappler inlet megan...
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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
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