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.