RiGHt: River Level Monitoring using GPSHeighting

Terry Moore, Institute of Engineering Surveying and Space Geodesy, The University of Nottingham, UK

Gareth Close, Science Systems (Space) Ltd, UK

Roger Moore, Institute of Hydrology, UK

BIOGRAPHIES

Terry Moore is Reader (Associate Professor) in SatelliteNavigation, and the Deputy Director, of the Institute ofEngineering Surveying and Space Geodesy, at theUniversity of Nottingham. He holds a BSc degree in CivilEngineering and a PhD in Space Geodesy, both from theUniversity of Nottingham. He is a Fellow, and a Member ofCouncil, of the Royal Institute of Navigation and a Memberof the Institute of Navigation.

Gareth Close is a Principal Analyst Programmer for ScienceSystems (Space) Ltd based in Chippenham, England. Heholds a BSc degree in Physics and Astronomy fromUniversity College London and has worked in the SpaceIndustry for 20 years

Roger Moore is Senior Scientific Officer, and Head of theEnvironmental System Section, of the Institute ofHydrology. He holds a BSc degree in Civil Engineeringfrom the University of Edinburgh and an MSc in Hydrologyfrom Imperial College London.

INTRODUCTION

The RiGHt project, ’River Level Monitoring using GPSHeighting’, has been carried out over the last two years.This was a collaborative project involving the University ofNottingham, Science Systems (Space) Ltd and The Instituteof Hydrology, and was supported through a grant awardedby the British National Space Centre (BNSC), under the UKGovernment’s Space Foresight Programme. The objectivesof the project are to address the problems of continuouslymonitoring river heights at any location in the World, usinga remote floating platform.

The RiGHt project has developed a space-based real-timeriver level monitoring system using the real time kinematicGPS, satellite communications and a GeographicalInformation System (GIS). A GPS receiver was mounted ona river buoy together with other sensors for attitudedetermination of the buoy. Communication satellites, orterrestrial communications may be used for the real-timedata-link between the buoy and a GPS reference station. Anovel 4-D GIS was used to visualize real-time water level

information and analyze and manage other hydrologicalinformation.

Full trials of the complete system, including the prototypebuoy, the communications systems, and the GIS softwareinterfaces, were carried out in early February 2000. A sitewas selected on the River Trent, in Nottingham, which isvery close to the Colwick river gauging station. Thegauging station building provided ideal accommodation forthe GPS reference station and the communicationsequipment, and the necessary power and security. Forpractical convenience the GIS packages and interfaces werealso operated close by, but could have been at a distantlocation. All aspects of the river monitoring system wereoperated in real-time, although terrestrial communicationswere used for these trials, rather than the satellite link. Thetrials demonstrated the capability of the RiGHt system todetermine river levels, in real-time, and to a centimetriclevel of accuracy. The full results from these trials arepresented in this paper.

PROTOTYPE SYSTEM DESIGN

The prototype GPS buoy river level monitoring systemconsists of following four main components:

• A river buoy with a GPS receiver, pressure and tilt sensors, and an autonomous power supply

• A two way data-link (terrestrial) to a nearby GPS reference station

• A GPS reference station with satellite or terrestrial communications capability

• A central station with the hydrological database and theintegrated GIS.

The river buoy is a key part of the system. The prototyperiver buoy consists of a simple wave-rider marine buoy(modified by the Proudman Oceanographic Laboratory),which has been adapted to accommodate a geodetic GPSreceiver, and other sensors. A dual frequency receiver isused, which has the capability of two-way communicationsand on-board GPS real time kinematic (RTK) processing.The receiver is mounted inside the buoy and a marine GPS

antenna is mounted externally. In addition a small lowpower UHF data link is used to communicate with thereference receiver, which is based on the river-bank. Thebuoy operates autonomously and so battery power isprovided, which is sufficient for the prototype. Clearly, alonger-term deployment would require a more carefulreview of the power requirements, and perhaps the additionof solar power. In order to correct the determined height forthe attitude of the buoy, and for the changes in freeboard, atilt sensor and a pressure transducer are also mounted in thebuoy. The data from these auxiliary sensors is integrated into the GPS data message transmitted to the reference station.

The GPS reference station is located on the river-bank, andthis provides the reference GPS data for the differentialcarrier phase computation. A compatible dual frequencygeodetic receiver is used, and the antenna position isdetermined using conventional static GPS carrier phasepositioning. Ideally, the separation between the referencestation and the buoy should be kept as small as possible, butseparations of a few tens of kilometers should notsignificantly degrade the results. It is feasible that in apractical application several buoys could be associated witha single reference station. At present both GPS receivers areoperating at a 1-second data rate. For pragmatic reasons itwas decided that, in the prototype system, the data from thereference station would be transmitted to the buoy, and theGPS processing take place within the buoy. It is envisagedthat the processing would usually take place at the referencestation.

Figure 1 Prototype RiGHt Buoy Design

The resulting position and height of the buoy, aftercorrection for tilt and freeboard, are transmitted to thecentral facility using a commercial data link. The prototypesystem has integrated both a terrestrial packet transmissionfacility and a satellite communication link. The exact route

has no effect on the overall system architecture, and istotally transparent to the user. The data rate iscomparatively low, as only time tagged coordinates andheight are transmitted, typically once every 15 minutes.

The central facility, in this case the Institute of Hydrology,receives the data messages from the reference stations andthe Hydata software automatically formats the time-taggedpositions for input into a novel 4D GeographicalInformation System (GIS). This was initially developed, bythe Institute of Hydrology, for the LOIS (Land OceanInteraction Study) project and provides an ideal medium tointegrate the river heights determined by the GPS buoy withother hydrological and environmental data. The novelty ofthe GIS arises because of the unique data structure at itscore. This handles data with a where, what, when datastructure, which is not geographically based, althoughgeographical coordinates may be stored as an attribute (awhat at a particular where). As a result the LOIS GIS isoptimised to handle time varying data sets, such as changingriver heights.

To incorporate river heights derived from multiple buoysand reference stations into a single GIS, there will be a needto calibrate vertical datum differences for the referencestations. The datum error can be reduced if a high precisiongeoid model is available, or a precise survey of the relativeheights of the reference stations is conducted. This is aninherent problem to all height measuring systems.However, to simply measure changes of river height at asingle location, the relative heights of different locations arenot important.

INITIAL TESTS AND PRELIMINARY RESULTS

To initially test the potential use of RTK GPS andGLONASS positioning, several simulation trials werecarried out by the University of Nottingham. A number ofAshtech ZXII dual-frequency GPS receiver were used alongwith two GG24 single frequency GPS/GLONASS receivers.Initially a simple test rig was developed and constructed bythe IESSG and used for a series of simple trials on theUniversity Campus.

A test rig was constructed from the tripod of a surveybeacon, about 4 meters in height. From this a platform wassuspended using elastic (bungee) cord. Two GPS (AshtechZXII) choke ring antennas and two GPS/GLONASS(GG24) marine antennas were mounted on the platform. Bydisturbing the platform from its rest position a rocking andswaying motion could be induced, which is similar inamplitude and period to the anticipated motion of the buoyin a real river. The configuration of the test rig is shown inFigure 2.

During the trials GPS data was recorded at 0.2s intervalsand GPS/GLONASS at 0.5s. In all the trials the relativedistances between these antennas are fixed, and thisprovides a useful check on the quality of the data andprocessing. The two reference station choke ring antennaswere located on the roof of the IESSG building, which wasonly about 50m from the trial site. All choke ring antennaswere of the same make and type.

The trajectories of the four antennas on the moving platformwere calculated independently, using the differential carrierphase measurements between the receivers on the roof(static) and the receivers on the platform (kinematic). The

distance between pairs of antennas, on the platform, werecalculated at each observational epoch. The derived lengthbetween the two GPS antennas is shown in Figure 3. Thislength is fixed and so should be constant (at 0.442m). Oncethe GPS processing has resolved the carrier phaseambiguities the RMS of the differences in baseline length isabout 8.2 mm. A period of about 1.5 minutes was typicallyrequired for initialization. For certain periods, GPS time209720~209900 and 210290~210470, the accuracy of thelength between the antennas is better than 3.5 mm. Inaddition to the natural vibration of the platform, three largemovements were induced, at about three-minute intervals(around epochs 209900, 210080 and 210260). There was aswing of about 1-2 m in 0.5 seconds for each disturbance.These are clearly shown in the figure. The baseline lengthresults from the test rig show potential millimeter precision.However, it is clear that there were undetected cycle slips inthe results, especially during the second and third moves,from 210080 to 210260 seconds. In these trials the cycleslips were probably caused by the rapid change of theposition, but similar effects could also result from multipathor obstructions in a real river environment.

A second trial was conducted using the survey boat of thePort of London Authority. The trials lasted for three daysand took place a Gravesend on the River Thames. MultipleGPS receivers, all with Ashtech choke ring geodeticantennas, were mounted on the small survey vessel, whichwas surveying along the river or, for some of the time,moored alongside a tide gauge site. By using two of thereceivers a fixed baseline, of known length, was againobtained, and this was used to check the quality of the GPScarrier phase data and processing. The baseline length was3.588 m. In these trials an external comparison was alsopossible because of the close proximity of the tide gauge.Three consecutive days of observations were made from 10-12 November 1998. Each observation period was about 13hours in duration. The RTK GPS derived baseline lengthwas compared against its true value while the survey boatwas cruising up and down the river, and the river height wascompared with the tide gauge measurement whilst the boatwas moored overnight.

The results shown in Figure 4 again demonstrate thepotential accuracy of the RTK GPS approach in a harsh riverenvironment. The agreement between the average GPSdetermined baseline length and the known value is of theorder of a few millimeters, with only small gaps in the data.No significant cycle slips were found in these data sets,although there were periods of increased noise in the data.Figure 3 GPS Baseline Length between

Receivers on the Platform

Figure 2Simulation Test Site

RTK GPS RiverThames Boat Test (Gravesend)

3.56

3.57

3.58

3.59

3.60

3.61

3.62

303000 305000 307000 309000 311000 313000 315000 317000

GPS time (second)

Bas

elin

e le

ng

th f

rom

RT

K G

PS

(m

)

GPS WK 983, 11-Nov-98, 4.2 hrs daytime c baseline length=3.588m, Std=0.006m, mean=3.587

Figure 5, illustrates the comparison with the tide gaugerecord for about a 12-hour period. During this trial a tidalrange of about 4.5 m was experienced. The level ofagreement is consistently better than 1 cm, throughout the12-hour period of the trial. These trials were conducted withthe close co-operation of the Port of London Authority,under a separate project on the implementation of GPS andWGS 84 in marine navigation.

1.2

1.6

2.0

2.4

2.8

3.2

3.6

4.0

4.4

4.8

5.2

5.6

324000 329000 334000 339000 344000 349000 354000 359000 364000 369000

GPS time (seconds)

Hei

gh

t (m

)

RTK GPS

Tide Gauge at Tilbury

The results were very encouraging. Not only did the GPSbaseline results achieve a high accuracy when comparedwith the fixed baseline length, but also the GPS river heightscompared extremely well with the external tide gaugemeasurements. The overall quality of the GPS data wasvery high and has caused few processing problems, despite anumber of obstructions surrounding the river. The data from

these trials was not processed in real time, but after thecompletion of the trials. However, real time processing wassimulated in the laboratory. The data was processed usingboth Ashtech Office Suite (AOS) and the IESSG NOTFprogram, which is a component of the Nottingham GASsoftware package. No phase centre variation models wereapplied. Both software packages give comparable results, atthe sub-centimetre level.

FULL SYSTEM TRIALS

Full trials of the complete system, including the prototypebuoy, the communications systems, and the GIS softwareinterfaces, were carried out in early February 2000. A sitewas selected on the River Trent, in Nottingham, which isvery close to the Colwick river gauging station. Thereference station GPS antenna was mounted on the roof ofthe gauging station building. This point had been previouslysurveyed, by static carrier phase GPS, with respect to acontinuously operating GPS reference station at the IESSGbuilding on the university campus (some 8km away). Thedifference in height between the GPS antenna and the rivergauge bench mark was measured by conventional spiritleveling. During the trial the reference station GPS receiver,the PC, and the packet radio communications link wereoperated close to the building. A single data cable ran to thebuoy, which was flexibly anchored to a structure in theriver. For practical reasons the GIS, and receiving radiomodem, were positioned inside the building, but could havebeen located at a completely remote location. The rivergauge readings were recorded by the Environment Agency.

Figure 6 RiGHt River Buoy in River Trent

The chart in Figure 7 shows the results from the trial, fortwo and a quarter hours. The river heights from the buoy, atone-second intervals, were averaged over each 15 minutesperiod, before transmission to the GIS. These weresynchronised with the 15-minute levels from the Colwickriver gauge. The oscillating tendency of the RiGHt levels, isnot as periodic as it may first appear, but does requirefurther investigation. It is possible that the RiGHt buoy was

Figure 4Differences between the GPS Baseline

Length and its True Value

Figure 5GPS Heights compared with Tide Gauge Height

sensing the correct river level, which may be beingsmoothed out by the river gauge. It has been suggested thatthe natural pulse of the river could be a cause, but this cannot be confirmed without a more thorough study, and furthercontrolled trials. There is also an apparent bias between theRiGHt levels and the EA levels, with the former being lowerthan the latter. However, the agreement of these raw resultsis remarkably good. The root mean square (RMS)difference is 9.8mm and the maximum difference 18.0mm.

Figure 7 RiGHt Trials Results

A systematic bias of -7.6mm has been determined and thishas been removed from the raw differences. Small errors indetermining the complex relationship between the variousheight systems could easily cause such as small systematicdifference. Again, a more controlled trial would be neededto ascertain the veracity of this assumption.

Figure 8 RiGHt Trials Results (Bias Removed)

Figure 8 shows the agreement between the EA and RiGHtriver levels after the removal of the systematic bias. TheRiGHt levels now have a more scattered appearance aboutthe smooth EA trace. The RMS difference between the twoseries of river heights is now 6.2mm, with a maximumdifference of 10.4mm.

CONCLUSIONS

The RiGHt River Level Monitoring System has beendeveloped and successfully demonstrated through a series oftrials, culminating in the final full trials in the River Trent.The agreement between the EA gauged river levels and thecorresponding river level determined by the RiGHt buoywas of the order of 1cm. The root mean square (RMS)difference between the two series was 9.8mm and themaximum difference 18.0mm. A small systematic bias of -7.6mm was been determined between these two series ofriver levels. After the removal of this bias the RMSdifference was 6.2mm, with a maximum difference of10.4mm. The initial goal of the RiGHt project was toachieve an accuracy of 1cm in the determination of riverheights. The results from the full trial have shown that thisambitious target has been successfully reached. Moreextensive trials, over a longer period and in a morecontrolled environment are required to fully validate theexpected and repeatable level of performance of the RiGHtsystem.

ACKNOWLEDGEMENTS

The authors of this paper gratefully acknowledge thesupport of the Proudman Oceanographic Laboratory (POL),for providing their prototype marine buoy and other relevantinstrumentation, and for carrying out the modification to theshell of the prototype. Special thanks are given to PeterFoden of POL for his help and guidance with the project.The assistance of the Port of London Authority, during theRiver Thames trials, and the Environment Agency, duringthe River Trent trials are also gratefully acknowledged

RiGHt Trial Results 1st Feb 2000

17.5

17.51

17.52

17.53

17.54

17.55

17.56

17.57

17.58

17.59

14:4

5

15:0

0

15:1

5

15:3

0

15:4

5

16:0

0

16:1

5

16:3

0

16:4

5

17:0

0

17:1

5

Time (GMT)

Wat

er L

evel

(m

)

EA levelRiGHt level

R iG H t T ria l R esults 1s t F eb 2000

17.5

17 .51

17.52

17.53

17.54

17.55

17.56

17.57

17.58

17.59

14:4

5

15:0

0

15:1

5

15:3

0

15:4

5

16:0

0

16:1

5

16:3

0

16:4

5

17:0

0

17:1

5

T im e UT C

Riv

er L

evel

(m

)

EA Level

R iG H t Level(no b ias)