presentation f1: pat sanders
TRANSCRIPT
RTK Tides without a Heave Sensor
Pat SandersHYPACK-YSI
RTK TIDES: Sitting at the Dock
GPS
Transducer
MRU
WGS-84 Ellipsoid
Chart Datum
Mean Water Level
Tide Correction
A Static DraftGeoid
N
K
N-K
Heave = 0.
Chart Sounding = DBT + SD – TCTCRTK = A – H – (N-K)Chart Sounding = DBT + SD – A + H + (N-K)
H
DBT
BottomDetermining (N-K) is the key to RTK Tides!
Chart Sounding
Underway: Dynamic Draft
Chart Sounding = DBT + SD + DD – TCTCRTK = A – H – (N-K) + DDChart Sounding = DBT + SD – A + H + (N-K)GPS
Transducer
MRU
WGS-84 Ellipsoid
Chart Datum
Mean Water Level
Tide Correction
AStatic DraftGeoid
N
K
N-K
Heave = 0.
Dynamic Draft
H
DBT
Bottom
The Dynamic Draft is not needed to compute the Chart Sounding!
You need the Dynamic Draft if you want to compare the RTK Tide Correction with a conventional Tide Correction.
Loss of RTK Fixed:
Raw RTK Tide going under a bridge.
Since we’ve separated the tide component, it’s easy to smooth the tide value!
Let’s Start Heaving, Rolling & Pitcing
Things are going to get complicated….
5
Transducer Movement
6
SD
StaticDraft
With Dynamic
Draft
With DD and
Heave
Add pitch and roll.
SD
DD
SD
DD Heave
SD’
DD Heave
SD = Static Draft DD = Dynamic Draft
RTK Tides with Heave, Pitch and Roll
Heave can be translated to transducer position.Vertical Elements H and SD are adjusted for Pitch and Roll.The formulae are now complicated.
GPS
Transducer
WGS-84 Ellipsoid
Chart Datum
Mean Water Level
Tide Correction
A SD’Geoid
N
K
N-K
Dynamic Draft
H
DBT
Bottom
Heave at MRU
The complicated part is translating the heave to the transducer and adjusting lever arms for pitch and roll to generate the correct Tide Correction.
RTK Updates
8
AntennaEllipsoidHeight
Time1 second
AntennaEllipsoidHeight
Time1 second
Early RTK update rates of 1Hz required an MRU to determine the antenna/transducer motion between RTK updates.
Faster RTK update rates may eliminate the need of a heave sensor!
Can we do RTK GPS with just a Pitch-Roll Sensor?
9
Heave-Pitch-Roll sensors are expensive.Pitch-Roll sensors cost a lot less!
WGS-84 Ellipsoid
A
A’
If I know the Pitch, Roll and lever arms, everytime I get the height of the GPS antenna above the ellipsoid, I can compute the height of the transducer above the ellipsoid.
Simplified RTK
10
WGS-84 Ellipsoid
A’
Chart Datum
N-K
Bottom
DBT
If we don’t worry about separating the raw components…
Chart Sounding = DBT + CCCombined Correction (CC) = (N-K) – A’
CC = Combined Correction
Putting it to the test!
11
Chart Datum
Bottom
30m
10m
10m5m
5m
Roll: -10º to +10º every 8 secs.Heave: -2m to +2m every 5 secs.Offsets:
GPS = 10m to port, 10m upTransducer: 5m to stbd, 5m down
HYPACK 3D Simulator
HYPACK 3DMotion Simulator
HYPACK Data Collection Computer
UDP Port 17002: NMEA Positions
UDP Port 17003: Depths
UDP Port 17001: MRU Messages
$GPGGA Datagrams: 16 HzDepth Datagrams: 13 HzMRU Datagrams: 25 Hz
Collected in HYPACK SURVEYEdited in SINGLE BEAM EDITOR
The Test Results: Simulation
13
Method Average Depth Standard Deviation
Truth 30.00mConventional RTK Tides 30.09m 0.22m
RTK Tides without Heave 30.02m 0.22m
• The new, simplified method of RTK TIDES without HEAVE performed slightly better than the older method.• The somewhat large standard deviation might be attributable to heavy network traffic during the test.
• We looked data with Wireshark to examine the time of receipt of the datagrams and found slight variations in the arrival times.
Then Results: Actual Survey Data
14
RTK Tides with Heave = GreenRTK Tides w/o Heave = Blue
(You can’t see the blue because the green is directly on top of it!
Victory Declared Until…..
RTK Tides with Heave = GreenRTK Tides w/o Heave = Red
The new method has problems when you lose RTK Fixed!
Conclusions:
• You can use RTK TIDES without a Heave sensor, provided you DO NOT lose RTK FIXED mode.• If you lose RTK FIXED modes, there is no recovery!
• The new method simplifies the calculation and provides slightly better results than the original method, provided you are in RTK FIXED mode the entire time.
• The conventional RTK TIDE method in HYPACK can recover from short periods of RTK FIXED loss by interpolating the computed tide across those periods.