A comparison of error A comparison of error budgets for vertical budgets for vertical

positioning using traditional positioning using traditional and RTK GPS approachesand RTK GPS approaches

USM GPS workshopUSM GPS workshopMarch 16-18, 2004March 16-18, 2004

R.M. Hare, P.Eng., C.L.S.R.M. Hare, P.Eng., C.L.S.Canadian Hydrographic ServiceCanadian Hydrographic Service

ObjectivesObjectives

• To examine vertical error budgets To examine vertical error budgets for traditional and RTK GPS for traditional and RTK GPS hydrography in estuarine/riverine, hydrography in estuarine/riverine, coastal, offshore and oceanic areascoastal, offshore and oceanic areas

• To provide some insight into the To provide some insight into the requirements for RTK GPS through requirements for RTK GPS through an examination of operational an examination of operational scenariosscenarios

Vertical positioningVertical positioning

• Ship and launch soundingShip and launch sounding– Estuarine/Riverine (very shallow, 5-20 Estuarine/Riverine (very shallow, 5-20

m, EM3000)m, EM3000)– Coastal (shallow, 8-100 m, EM3000)Coastal (shallow, 8-100 m, EM3000)– Offshore (medium, 80-1000 m, EM1002)Offshore (medium, 80-1000 m, EM1002)– Oceanic (deep, 800 m – F.O.D., Oceanic (deep, 800 m – F.O.D.,

EM121A)EM121A)• Airborne (lidar) sounding (1-50 m)Airborne (lidar) sounding (1-50 m)• Lidar topographyLidar topography• Drying heights & elevationsDrying heights & elevations• Wave heights/tides from buoysWave heights/tides from buoys• Establishment/recovery of vertical Establishment/recovery of vertical

datumdatum

Traditional sounding Traditional sounding reductionreduction

D = d + draft – WLd = r cos (θ+R) cos P

r = rangeΘ = beam angleR = Roll angleP = Pitch angle

Dynamic draft

ChartedDepth, D

Chart datumMeasuredDepth, d

Tide, WL

θr

Sounding error budgets - Sounding error budgets - traditionaltraditional

• SoundingsSoundings– MeasurementMeasurement– RefractionRefraction– AttitudeAttitude

• HeaveHeave– MeasurementMeasurement– Induced by R&PInduced by R&P

• Dynamic draftDynamic draft– Static draftStatic draft– SquatSquat– Load changesLoad changes– Buoyancy changesBuoyancy changes

• Tides/water levelsTides/water levels– MeasurementMeasurement– FilteringFiltering– Spatial predictionSpatial prediction– Time Time

synchronizationsynchronization

RTK GPS sounding reductionRTK GPS sounding reduction

Dynamic draft

ChartedDepth, D

Chart datumMeasuredDepth, d

Tide, WL

θr

EllipsoidGPS RTK, Z

Separation Model, M

AntennaHeight, A

D = d + A – Z – M

A = Δx sinP + Δy cosP sinR + Δz cosP cosR

Sounding error budgets – RTK Sounding error budgets – RTK GPSGPS

• SoundingsSoundings– MeasurementMeasurement– RefractionRefraction– AttitudeAttitude

• RTK GPS elevationRTK GPS elevation

• Antenna heightAntenna height– Lever armLever arm– Roll and PitchRoll and Pitch

• Separation modelSeparation model– Chart-datum – EllipsoidChart-datum – Ellipsoid

Four West Coast scenariosFour West Coast scenariosScenarioScenario EstuarineEstuarine

/ Riverine/ RiverineCoastalCoastal OffshoreOffshore OceanicOceanic

LocationLocation Fraser Fraser RiverRiver

Patricia Patricia BayBay

Nitinat Nitinat CanyonCanyon

Osborne Osborne SeamounSeamountt

MBESMBES EM3000EM3000 EM3000EM3000 EM1002EM1002 EM121AEM121A

DepthDepth 15 m15 m 60 m60 m 600 m600 m 2500 m2500 m

DistanceDistance <10 km<10 km <10 km<10 km <40 km<40 km > 40 km> 40 km

GPSGPS RTKRTK RTKRTK LRKLRK RTGRTG

ConditionsConditions CalmCalm CalmCalm ModerateModerate RoughRough

SwellSwell 0.2 m0.2 m 0.4 m0.4 m 1.5 m1.5 m 4 m4 m

R&PR&P 2 2 °° 2 2 °° 7 7 °° 10 10 °°

RefractionRefraction 2 m/s2 m/s 1 m/s1 m/s 0.5 m/s0.5 m/s 0.5 m/s0.5 m/s

Sep. Sep. ModelModel

0.2 m0.2 m 0.05 m0.05 m 0.3 m0.3 m 0.1 m0.1 m

Operational ScenariosOperational Scenarios

• Canada’s Canada’s West CoastWest Coast– Osborne Osborne

SeamountSeamount– Nitinat Nitinat

CanyonCanyon– Fraser RiverFraser River– Patricia Bay Patricia Bay

(IOS)(IOS)

Permanent Water Level Permanent Water Level NetworkNetwork

07120 Victoria07277 Patricia Bay07654 New Westminster07735 Vancouver07795 Pt Atkinson08074 Campbell River08408 Port Hardy08545 Bamfield08615 Tofino08735 Winter Harbour08976 Bella Bella09354 Prince Rupert09850 Queen Charlotte City

GPS Benchmarks

20.78

21.47

19.64

17.6516.99

17.77

16.46

14.9815.52

11.58

19.74

17.53Datum Separation values

20.34

Assumptions - GPSAssumptions - GPS

• Coastal and Estuarine or Riverine (0 – Coastal and Estuarine or Riverine (0 – 10 km)10 km): local RTK GPS: local RTK GPS– Vertical accuracy: +/- 0.02 m (68%)Vertical accuracy: +/- 0.02 m (68%)11

• Offshore (10 – 40 km)Offshore (10 – 40 km): Long-range : Long-range kinematic (LRK)kinematic (LRK)– Vertical accuracy: +/- 0.06 m (68%)Vertical accuracy: +/- 0.06 m (68%)22

• Oceanic (> 40 km)Oceanic (> 40 km): Global system, : Global system, e.g. C-Nav (RTG)e.g. C-Nav (RTG)– Vertical accuracy: +/- 0.18 m (68%)Vertical accuracy: +/- 0.18 m (68%)33

• All values for real-time at highest data rateAll values for real-time at highest data rate

1.1. G. Lachapelle, Personal communication, 2004G. Lachapelle, Personal communication, 20042.2. Thales Navigation Aquarius LRK specifications at 20Hz, 40 kmThales Navigation Aquarius LRK specifications at 20Hz, 40 km3.3. C&C Technologies DGPS-PI-001.1 Static accuracy of C-Nav RTG C&C Technologies DGPS-PI-001.1 Static accuracy of C-Nav RTG

V13.1V13.1

Other assumptionsOther assumptions

• Offshore and Offshore and Oceanic surveysOceanic surveys – Done by shipDone by ship– Larger lever armLarger lever arm– Greater draft Greater draft

uncertaintyuncertainty– More stable sound More stable sound

speed structurespeed structure– No local tide No local tide

gaugegauge– Oceanographic Oceanographic

phenomenaphenomena

• Estuarine and Estuarine and Riverine surveysRiverine surveys – Done by launchDone by launch– Draft uncertainty Draft uncertainty

from buoyancy from buoyancy changeschanges

– Possible salt Possible salt wedgeswedges

– Sloping or stepped Sloping or stepped chart datumchart datum

Oceanographic phenomena: El Oceanographic phenomena: El NiNiññoo

Sea surface height maps Sea surface height maps showing the Haida and Sitka showing the Haida and Sitka Eddies Eddies

• Sea surface elevations measured by TOPEX/Poseidon and ERS-2 satellite altimeters.

• Red regions denote high sea surface

• Blue regions denote depressions.

Annotations by Ocean Science and Productivity Division, DFO Science

Fraser River scenarioFraser River scenario

ElementElement ValueValue TraditionTraditionalal

RTK RTK GPSGPS

DepthDepth 15 m15 m 0.09/0.150.09/0.15

RefractionRefraction 2 m/s2 m/s 0.01/0.180.01/0.18

Roll angleRoll angle 22°° 0/0.120/0.12

HeaveHeave 0.2 m0.2 m 0.140.14 N/AN/A

GPS ZGPS Z N/AN/A 0.040.04

Lever armLever arm 4.9 m4.9 m N/AN/A 0.030.03

TidesTides <3.8 <3.8 mm

0.200.20 N/AN/A

Dynamic DraftDynamic Draft 0.8 m0.8 m 0.100.10 N/AN/A

Separation Separation modelmodel

~ 21 ~ 21 mm

N/AN/A 0.20 m0.20 m

TPE (95%)TPE (95%) 0.23/0.30.23/0.333

0.23/0.30.23/0.300

Patricia Bay scenarioPatricia Bay scenario

ElementElement ValueValue TraditionTraditionalal

RTK RTK GPSGPS

DepthDepth 60 m60 m 0.12/0.630.12/0.63

RefractionRefraction 1 m/s1 m/s 0.03/0.460.03/0.46

Roll angleRoll angle 22°° 0/0.510/0.51

HeaveHeave 0.4 m0.4 m 0.140.14 N/AN/A

GPS ZGPS Z N/AN/A 0.040.04

Lever armLever arm 4.9 m4.9 m N/AN/A 0.030.03

TidesTides 3.8 m3.8 m 0.050.05 N/AN/A

Dynamic DraftDynamic Draft 0.8 m0.8 m 0.070.07 N/AN/A

Separation Separation modelmodel

20.8 20.8 mm

N/AN/A 0.05 m0.05 m

TPE (95%)TPE (95%) 0.21/0.90.21/0.933

0.15/0.90.15/0.922

Nitinat Canyon scenarioNitinat Canyon scenario

ElementElement ValueValue TraditionTraditionalal

RTK RTK GPSGPS

DepthDepth 600 m600 m 1.26/1.491.26/1.49

RefractionRefraction 0.5 0.5 m/sm/s

0.13/0.550.13/0.55

Roll angleRoll angle 77°° 0/2.540/2.54

HeaveHeave 1.5 m1.5 m 0.210.21 N/AN/A

GPS ZGPS Z N/AN/A 0.120.12

Lever armLever arm 22 m22 m N/AN/A 0.030.03

TidesTides 4.1 m4.1 m 0.30.3 N/AN/A

Dynamic DraftDynamic Draft 4 m4 m 0.140.14 N/AN/A

Separation Separation modelmodel

~20 ~20 mm

N/AN/A 0.30 m0.30 m

TPE (95%)TPE (95%) 1.6/3.01.6/3.0 1.5/3.01.5/3.0

Osborne Seamount scenarioOsborne Seamount scenario

ElementElement ValueValue TraditionTraditionalal

RTK RTK GPSGPS

DepthDepth 2500 2500 mm

4.7/8.94.7/8.9

RefractionRefraction 0.5 0.5 m/sm/s

0.9/2.90.9/2.9

Roll angleRoll angle 1010°° 0/6.70/6.7

HeaveHeave 4 m4 m 0.550.55 N/AN/A

GPS ZGPS Z N/AN/A 0.350.35

Lever armLever arm 28 m28 m N/AN/A 0.040.04

TidesTides 2 m2 m 0.20.2 N/AN/A

Dynamic DraftDynamic Draft 6 m6 m 0.180.18 N/AN/A

Separation Separation modelmodel

10 m?10 m? N/AN/A 0.10 m0.10 m

TPE (95%)TPE (95%) 5.8/9.25.8/9.2 5.8/9.25.8/9.2

ComparisonComparison

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Traditional vs GPS RTK depth error budgets(outer beams)

Fraser R (15 m)

Pat Bay (60 m)

Nititat (600 m)

Osborne (2500 m)

ObservationsObservations

• RTK GPS provides RTK GPS provides greater incremental greater incremental improvement in improvement in shallow-water and for shallow-water and for near-nadir beamsnear-nadir beams

• Sounder system errors Sounder system errors tend to dominate - tend to dominate - variable error variable error contributioncontribution

ConclusionsConclusions

• RTK GPS does not appear to RTK GPS does not appear to provide huge benefits over provide huge benefits over traditional methods in terms traditional methods in terms of reducing total sounding of reducing total sounding errorerror

• Benefits may come from Benefits may come from operational efficienciesoperational efficiencies– No tide gauge installationNo tide gauge installation– No need to measure dynamic No need to measure dynamic

draftdraft– Possible heave Possible heave

estimation/reduction from estimation/reduction from higher data rate RTK higher data rate RTK elevationselevations

Remaining questionsRemaining questions

• Can we expect Can we expect significant significant improvement from improvement from post-mission GPS?post-mission GPS?

• Can/will GPS replace Can/will GPS replace VRU for heave VRU for heave compensation?compensation?

• How can we quantify How can we quantify real separation model real separation model errors?errors?