Improvement of GPS Ambiguity Resolution Using Height Constraint for Bathymetric Surveys

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1 Improvement of GPS Ambiguity Resolution Using Height Constraint for Bathymetric Surveys Dr Mami Ueno and Dr Rock Santerre Université Laval, Centre de recherche en géomatique ; Daniel Langelier and Guy Marceau Fisheries and Oceans Canada (Laurentian Region) Biographical Summary Mami Ueno obtained her Ph.D. in Geomatic Sciences from Université Laval and her B.Sc. and M.Sc. in Navigation System Engineering from Tokyo University of Mercantile Marine. She holds a third mate certificate and other navigation related licenses from Japanese Ministry of Transportation. Mami was assistant professor at Toba National College of Maritime Technology and involved in teaching and research on ship manoeuvring and precise positioning systems. Her research interests are precise applications of GPS for navigation and guidance. Currently, she is working as Research Associate on the GPS OTF project within the GEOIDE Network of Centres of Excellence at the Centre de recherche en géomatique, Université Laval. Rock Santerre is full professor in the Department of Geomatic Sciences, Université Laval, where he conducts research and teaching in precise GPS static and kinematic positioning. He received his B.Sc. and M.Sc. in Geodetic Sciences from Université Laval and his Ph.D. in Geodesy from the University of New Brunswick. Dr. Santerre has 17 years of experience in the domain of GPS and is the author of more than 90 publications. He also holds two patents related to GPS surveying. Dr. Santerre is scientific leader of the GPS OTF project within the GEOIDE Network of Centres of Excellence at the Centre de recherche en géomatique, Université Laval. Daniel Langelier is Project officer in the Hydrographic Data Acquisition Division of the Canadian Hydrographic Service. He is in charge of technical support and new applications in Geomatics for Canadian Hydrographic Service since He received his B.Sc. in Geodetic Sciences at Université Laval in 1981 and worked as land surveyor until Mr. Langelier is representative of the Canadian Hydrographic Service for the GPS OTF project within the GEOIDE Network of Centres of Excellence. Guy Marceau is Project officer in positioning and navigation systems in the Technical and Operational Services Division of the Canadian Coast Guard since Mr. Marceau has more than 10 years of experience in the domain of GPS and also 20 years of experience in Geomatics for the Canadian Coast Guard. Mr. Marceau is representative of the Canadian Coast Guard for the GPS OTF project within the GEOIDE Network of Centres of Excellence. Abstract Centimetre level of GPS positioning is required for many applications including bathymetric surveys. In order to achieve such accuracy, carrier phase ambiguities, a number of cycles which is not measured by a GPS receiver, must correctly be resolved. The ambiguity resolution on L1 or narrow-lane band (with an effective wavelength of about 20 cm) is more difficult to achieve than that for the wide-lane band (the wavelength of the wide-lane is 86 cm), especially when the distance between the survey ship and the GPS reference station gets longer. For the bathymetric survey operations on the St. Lawrence River, the closest GPS reference station can be as far as 75 km. This paper discusses the method of GPS phase ambiguity resolution on long baseline vectors using height constraint obtained from the digital tide gauges deployed by the Canadian Hydrographic Service (CHS) along the St. Lawrence. In this research, the quality of the a priori height information was assessed by comparing the tide gauge reading with the GPS height when the ship was in the vicinity of a GPS reference station and a tide gauge. Montréal, / 10

2 The success rate to instantaneously resolve the correct GPS phase ambiguities was compared for the cases, with and without height constraint, for distances between the survey ship and a GPS reference station of about 45 km. Résumé Plusieurs applications du positionnement GPS, dont le sondage bathymétrique, exigent une précision centimétrique. Afin d'obtenir le positionnement avec une grande précision en utilisant le GPS, il est nécessaire de résoudre correctement les ambiguïtés de phase de l'onde porteuse (des nombres entiers de longueur d'onde) qui ne sont pas mesurées par un récepteur GPS. La résolution des ambiguïtés sur la bande L1 ou la bande étroite (dont la longueur de l'onde est d'environ 20 cm) est plus difficile que celle sur le bande large (dont la longueur de l'onde est d'environ 86 cm), particulièrement, lorsque la distance entre un navire de sondage et la station de référence est longue. Pour des opérations de sondage bathymétrique sur le fleuve St-Laurent, la distance entre le navire et la station de référence la plus proche peut être aussi longue que 75 km. Cet article porte sur une méthode de résolution des ambiguïtés de phase GPS pour de longs vecteurs en utilisant des contraintes sur le niveau de l'eau obtenu par des marémètre déployés par le Service hydrographique du Canada (SHC) le long du fleuve St-Laurent. Au cours de cette recherche, la qualité de l'information a priori sur l'altitude a été évaluée en comparant le niveau de l'eau du marémètre avec le résultat du GPS lorsque le navire était près de la station de référence GPS ainsi que d'un marémètre. Le taux de succès pour trouver instantanément la combinaison correcte des ambiguïtés de phase GPS a été comparé pour des cas avec et sans l'utilisation d'une contrainte sur l'altitude pour des distances entre le navire et la station de référence, d'environ 45 km. Introduction The traditional sounding method requires the strategic placement of tide staffs as close as possible to the working area. An observer takes regularly an on-the-spot reading from the staff and transmits it verbally by radio to the sounding ship. Each year, tide staffs are deployed for every sounding season. This technology is costly, limited and outdated (Marceau et al. 1996; Marceau & Langelier 1997). In order to facilitate the survey and increase the safety of navigation, the Canadian Hydrographic Service (CHS) has gradually implemented digital tide gauges (type TMS-1000) and developed a network of 15 tide gauges along the St. Lawrence River (COWLIS: Coastal and Oceanic Water Level Information System). The tide gauges provide regular tidal reading and possibility of use and transmission of the tidal information in real time. Precise centimetre-level positioning is potentially achievable from GPS carrier phase observations. In order to achieve such accuracy, carrier phase ambiguities, a number of cycles which is not measured by a GPS receiver, must correctly be resolved. The determination of the ambiguity parameters while a remote receiver is moving, is called "on-the-fly" (OTF) ambiguity resolution. The introduction of the OTF system will improve the quality of the data collection and operational efficiency by eliminating costly deployment (support and maintenance) of the tide staffs. Another potential benefits of the use of OTF system are automatic and accurate determination of the water level and increased safety by the accurate determination of under-keel clearance of commercial vessels. This research project, which is part of the Canadian GEOIDE (GEOmatics for Informed DEcisions) Network of Centres of Excellence, is dedicated to the improvement of the methodology and the algorithms to achieve more precise and more reliable kinematic GPS positioning (which implies more reliable and efficient OTF phase ambiguity resolution) over long distances, for the support of bathymetric surveys in real time. This project covers different research initiatives: i) GPS relative positioning with multiple reference stations, ii) the improvement of ionospheric modelling, iii) the use of precise real-time orbits, iv) the integration of Glonass observations and v) radio-communication management (which includes the problem of time latency). The ambiguity resolution on L1 or narrow-lane band (with an effective wavelength of about 20 cm) is more difficult to achieve than that for the wide-lane band (the wavelength of the wide-lane is 86 cm), especially when Montréal, / 10

3 the distance between the survey ship and the GPS reference station gets longer. For the bathymetric survey operations on the St. Lawrence River, the closest GPS reference station can be as far as 75 km. The objective of the paper is to improve the method of phase ambiguity resolution and the algorithms for kinematic GPS positioning on long baseline vectors using height constraint. A height constraint is the most intuitive type of constraint available in a marine environment. The height of water level on the St. Lawrence is easily accessible from the network of the automated COWLIS tide gauges deployed by the CHS. Most of these tide gauges are located in the vicinity of the wharf of base ports used by the CHS survey ships. This information could be used to assist the resolution of GPS phase ambiguities when the ship is still at the wharf. However, the water level measured by the tide gauge is given with respect to the local Chart Datum (CD), while GPS height positioning refers to the WGS-84 ellipsoid. Therefore, making use of COWLIS measurements as constraints for GPS positioning, requires a conversion of altitude. Reduction of GPS-OTF altitude to the Chart Datum The Chart Datum (CD) is a reference surface from which the CHS establishes bathymetric depth. Figure 1 shows the vertical profile of the measurements. The datum separation between the ellipsoid and the CD is represented by the symbol (N CD ). Note that the ellipsoid is always above the CD in the sounding sectors of the Montreal- Quebec region. GPS GPS Squat + Heave Ref. station Z A Tide gauge T COWLIS Dt Geoid N CD Depth h OTF Chart Datum Ellipsoid Figure 1 : Relationship between the GPS height and the COWLIS reading The relationship between the CD and the geodetic reference ellipsoid on the St. Lawrence is known thanks to the previous works performed by the CCG and the CHS (Marceau & Langelier 1997). To reduce the ellipsoidal altitudes (determined by GPS-OTF) to the CD, the Bathykin OTF software, which has been developed for the CCG by the Viasat Géo-Technologie Inc. in collaboration with the Centre for Research in Geomatics (CRG) at Laval University, interpolates the datum separation between the CD and the ellipsoid using a grid established by the CCG and the CHS. The height of the GPS antenna (mounted on the mast of the ship) from the water surface must be measured. Antenna height (Z A ), which is the distance from any point attached to the ship (reference point) to the antenna phase centre, can be measured. A reference point such as the transducer of echo sounder or bottom of keel can be measured beforehand. The vertical distance from GPS antenna to the reference point is fixed and Montréal, / 10

4 independent of the ship's draught. Once the reduction of GPS height (h OTF ) to the ship's draught line is done, the tide obtained by OTF technique is the difference between the draught (floating) line and the CD. As tide, swell, draught, and squat are measured all together on board a sounding ship, the current sounding method requires a heave sensor to make correction of the effect caused by the ship's movement. If the GPS antenna is not located above the heave sensor, remote heave measurements also have to be taken into account. This proposed approach, however, does not require a heave sensor, if the ambiguity initialisation was successfully performed before leaving the wharf and the GPS solutions were kept even with occasional loss of GPS signal during a sounding operation. Equation (1) shows the relationship between the ellipsoidal height of a sounding ship obtained by the OTF (h OTF ) and tide gauge measurement (T COWLIS ). The symbol, Dt is the draught of the sounding ship. The symbols, Hv, Sq and RHv, represent the heave, the squat and the remote heave, respectively. T COWLIS = h OTF N CD Z A + Dt (Sq + Hv + RHv) + ε (1) The difference of the tide between GPS and COWLIS reading, represented by the symbol (ε) in Equation (1), has to be small. The height information obtained from the COWLIS can provide approximate GPS height (h OTF ) used as height constraint reciprocally using Equation (1). Integrating a priori height information The COWLIS readings from a tide gauge at an interval of 15 minutes were temporally interpolated to compare the tide obtained from the GPS and to introduce the height constraint test for ambiguity search. The accuracy of the COWLIS tide gauges is about 3 cm under ideal conditions (CHS 1997). The corrections for squat, heave and draught of the ship have to be applied for more rigorous comparison of the results obtained from the OTF measurements and better estimation of the a priori height information. However, the comparison cannot be perfect because the errors are inevitable in the COWLIS readings and in each of the components used to calculate the height from the CD: datum separation, GPS antenna height, draught, heave, estimation of squat from the speed log reading. When is used SINEM, which is a spatial interpolation of COWLIS tide measurements, the errors in interpolation would be included as well. The magnitude of the over all errors would be about ±10 cm (Michaud et al. 1997). A height constraint is the most intuitive type of constraint available in a marine environment. As mentioned above, the height of water level on the St. Lawrence is easily accessible from the network of COWLIS tide gauges. The information on the water level (reduced to ellipsoidal height) would be helpful for the ambiguity resolution if it has sufficient accuracy. Once the ambiguities have been resolved, however, the height will become known and in a case where all ambiguities are lost, the average height over the previous epochs could be used to help the ambiguity resolution. Introducing the height into the GPS observation equation is also possible by using the pseudo-observation method. Pseudo-observation deals with the situation where the variables, e.g., position, ambiguities, are known at certain precision. When the precision of the height from the COWLIS tide gauges is known and is high enough, the initial values of ambiguities would be known with sufficient accuracy and therefore the ambiguity search space could be reduced. The height information can be used inside the ambiguity search routine as well. The ambiguity search is performed by varying the integer numbers in a nested loop. A combination of ambiguities, which gives a height solution beyond the predefined threshold, e.g. 15 cm, is rejected. This method could also be used for reducing the ambiguity search space and potentially eliminating incorrect combinations of ambiguities. The reading of the COWLIS tide gauges is as accurate as ±3 cm in the vicinity of the station (CHS 1997). However, when the ship is far away from the COWLIS station, the precision of spatially interpolated tide would increase its uncertainty. Therefore, the study was conducted to validate the quality of the height information obtained from the COWLIS and to find the efficient way of using it. The details are shown in the following section. Description of Tests Montréal, / 10

5 Different data sets for the season in 1998 were provided by the CHS and the CCG, and the new trials were also conducted during the season in The data of the 1999 session were used for validating the a priori height information from the COWLIS tide gauges. Some of the worst cases in the 1998 session, where the Bathykin OTF software (used for sounding operations by the CHS and the CCG) showed difficulty in finding the correct ambiguity solutions, were selected to investigate the cause of wrong solutions and to improve the OTF method. Table 1 summarises the condition of the tests used for the analysis. Table 1 : Test sites and conditions Test #1 #2 #3 #4 #5 Sector D14 D11 Sillery Sillery Quebec Date 30 Sept Oct Oct Oct Oct Duration 1:35 0:53 0:24 1:08 0:55 Distance to GPS Ref. Station km km 7 (103) km 7 km 2-13 km Distance to COWLIS station 6-8 km 2-5 km 5 km 5 km 1-15 km Name of the ship Smith Smith Puffin Puffin Puffin Length 35 m 35 m 9 m 9 m 9 m Tonnage 430 t 430 t 3 t 3 t 3 t Test in the 1998 season (Tests #1 and #2) The first data set (Test #1) was observed on 30 September, The sounding sector was in the Lake St-Pierre, about 35 km from the GPS reference station at Trois-Rivières and about 6-8 km from the COWLIS gauge in the lake (Section D14). A 35-m sounding ship, the F.C.G. Smith, was used for the test. The length of the sounding sector was about 1.5 km. The ship repeated the movement of back and forth in the sounding area. The elevation mask 10 was used. More than 8 satellites for the primary satellites (elevation higher than 20 ) were available during the survey. Up to 10 satellites in total were available. The PDOP values varied from 1.4 to 2.6 during the test. The second data set (Test #2) was observed on 22 October, The sounding sector was also in the Lake St- Pierre but the distance was about 45 km from the GPS reference station at Trois-Rivières and about 2-5 km from the COWLIS gauge in the lake (Section D11). The same ship, the F.C.G. Smith, was used for the test. The length of the sounding sector was about 1.5 km. The elevation mask 10 was used. More than 7 satellites for the primary satellites were available during the survey. Up to 10 satellites in total were available. The PDOP values varied from 1.5 to 2.6 during the test. Test in the 1999 season (Tests #3, #4 and #5) The trials in 1999 season were conducted in October in the region of Quebec City which has a daily tidal difference of about 4 m. Another ship of the CHS, the Puffin, was used for the data collection. Tests #3 and #4 were conducted on 15 and 17 October and the data were recorded while the ship was at wharf of the Sillery marina. The distance to the COWLIS station in the Port of Quebec was about 5 km and that to the GPS reference station at Lauzon was about 6 km. The GPS reference station at Trois-Rivières was 103 km away. More than 6 satellites were available. The PDOP values was about 1.7 during Test #3 and they varied from 1.6 to 3.4 during Test #4. The tide was rising during Test #3 and ebbing during Test #4. Test #5 was conducted on 20 October, The ship moved the area of 2-13 km from the GPS reference station at Lauzon and 1-15 km from the COWLIS station in the Port of Quebec. The Puffin started moving toward downstream of the St. Lawrence River. The number of the primary satellites was sometimes dropped to 4, Montréal, / 10

6 although 6 satellites were available for most of the time during the trial. The available satellites were lower than two other cases. The PDOP values varied from 1.6 to 2.9 during the trial. The tide was lowering during the test. Comparison of COWLIS Reading with reduced GPS height In order to make use of the information, the assessment of the quality of the a priori height information was performed. This was done by comparing the tide gauge reading with the GPS height of the antenna (taken into account the reductions described above), when the ship is in the vicinity of a GPS reference station and a tide gauge. Equation (1) was used to convert the ellipsoidal height to the GPS tide. The software developed at the Centre for Research in Geomatics in this study computes the ionospheric-effect-free solution (L3) after resolving the wide-lane ambiguities and then L1 ambiguities. The ionospheric-free combination is used to reduce the ionospheric effects, in particular, on long baselines. Table 2 summarises the difference of tide (height above the CD) between GPS and the COWLIS tide gauge in the Port of Quebec when the ship was at wharf. When the ship was about 5 km away from the COWLIS gauge and the GPS reference station at Lauzon was used, the mean difference between COWLIS and GPS on L3 was -10 cm for Test #3 and -2 cm for Test #4. The RMS values were about ±3 cm. When the GPS station at Trois-Rivières was used, the mean difference was about -2 cm and the RMS values were ±4 cm. The success rate for finding the correct ambiguity set was tested and discussed in the following section. Table 2 : Difference of tide between GPS and COWLIS tide gauge (cm) Test #3 (Lauzon) Test #3 (Trois-Rivières) Test #4 (Lauzon) Mean RMS ±3.0 ±3.9 ±2.5 Figure 2 shows the difference of tide between GPS and the COWLIS tide gauge in the area of Port of Quebec City (Test #5). When the ship was a few km away from the COWLIS gauge, the difference was about -10 cm with L3 solution. The difference of tide became larger with longer distance between the ship and the COWLIS station. During Test #5, SINEM values were not available. Figure 2 : Difference of tide between GPS and COWLIS tide gauge in Port of Quebec (Test #5) Figure 3 shows the difference of the tide between SINEM and GPS for the cases with and without correction for the squat and heave for the sector D11 (Test #1). Figure 4 shows the same for the sector D14 (Test #2). Montréal, / 10

7 Figure 3 : Difference of tide between GPS and SINEM for the cases with and without corrections (Test #1: D14) Figure 4 : Difference of tide between GPS and SINEM for the cases with and without corrections (Test #2: D11) Table 3 shows the comparison of the difference of tide between GPS and various tide gauges. D11 and D14 sounding sectors were located in the Lake St-Pierre. SINEM is a spatial interpolation of COWLIS tide gauge measurements. Auto tide represents on-the-spot readings of tide staffs in the vicinity of the sounding area. With correction means the case where the correction of the squat and heave was applied to the L3 solution obtained from the software developed at the Centre for Research in Geomatics during this research. Without correction represents the case without these corrections for the L3 solutions. In order to compare the solutions from the different algorithms, corrections were also applied to the Bathykin's post-processed OTF solutions. The draught correction was applied for both cases of with and without correction. The mean of the difference of tide is smaller with SINEM in these sounding sectors. The mean of the difference of tide with COWLIS was larger in the sector D14 where the distance to the tide gauge was longer. By applying the correction for the squat and heave, the RMS values became smaller. The RMS values were ±2-3 cm. Montréal, / 10

8 Table 3 : Comparison of the difference of height between GPS and various tide sensors D11 D14 (cm) COWLIS SINEM Auto tide COWLIS SINEM Auto tide Bathykin Mean (with correction) RMS ±7.7 ±7.9 ±7.9 ±6.1 ±6.1 ±6.2 Without correction With correction Mean RMS ±2.8 ±3.0 ±3.0 ±2.3 ±2.5 ±2.5 Mean RMS ±2.8 ±2.8 ±2.8 ±1.5 ±1.7 ±1.6 Comparison of success rate for ambiguity search The ambiguity resolution on L1 or narrow-lane band (with an effective wavelength of about 20 cm) is more difficult to achieve than that for the wide-lane band (the wavelength of the wide-lane is 86 cm), especially when the distance between the survey ship and the GPS reference station gets longer. In order to find out the usefulness of the height information for the ambiguity resolution, the ambiguity search on L1 was made without a priori height information, and with a priori information used for testing the combinations. The initial L1 ambiguities were calculated from the position obtained from the wide-lane solution. The pseudo-observation method with the COWLIS height (reduced to the ellipsoid) was also introduced to obtain the initial ambiguities. However, the height obtained from the wide-lane was good enough to make a search with a small search window of ±2 cycles and ±1 cycle on L1 for most of the cases. In order to compare the success rate of instantaneously finding the correct ambiguity set for the cases with and without a priori height information, the ambiguity search was reinitialised at each 5 seconds. The term "instantaneous" in this section means finding a combination of the L1 ambiguities within 5 seconds after each initialisation. The initial ambiguities were recalculated every 5 seconds from the wide-lane solution or the pseudo-observation method and ambiguity search was performed. The epochs with more than two candidates, were removed for the calculation of statistics. The correct ambiguities were identified from the common solutions of the Bathykin and the software developed in this research by checking the residuals on L4, L1 and L3 GPS solutions. Table 4 shows the comparison of success rate of finding instantaneously (within 5 seconds) the correct ambiguity set with and without height information from the COWLIS height. The first row of each case shows the number of incidents to find the correct combination of ambiguities and ratio with respect to the incidents of finding a solution (either correct or incorrect combination), and the second row shows the number of incidents to find a solution (either correct or incorrect combination) and ratio with respect to the number of ambiguity initialisations. For the case with the a priori height, the COWLIS height was introduced in the search routine as the a priori information to test if a combination of ambiguities is within the predefined threshold, e.g. 15 cm. The a priori GPS height (h OTF ) is reciprocally calculated using Equation (1). This information helped to eliminate incorrect combinations of ambiguities and fix the ambiguities by leaving a single combination. It was in particular useful, when the variation and mean difference between the COWLIS and GPS height was small (when its accuracy is about ±15 cm or better and the distance to the COWLIS station is less than 10 km). The results show an improvement of ambiguity resolution using the height information from the COWLIS when the GPS reference station was up to 45 km. The success rate was, in general, improved by about 20%, typically from 70% to 90%. The principal reasons for the failure of finding the correct solution using the height constraints were: (1) a 5-second initialisation period was rather short, and (2) a tight threshold of 15 cm. In general, the number of incidents to find a solution was increased. Montréal, / 10

9 When the GPS reference station at Trois-Rivières (TR) was used for Test #3, the distance to the ship was 103 km. In this case, the success rate was not improved although the frequency of finding a single combination was increased. The ionospheric effect might be present and the model proposed by St-Pierre et al. (1999) would improve the solution. Table 4 : Success rate for finding the set of correct phase ambiguities within 5 seconds of initialisation period Test #1 (D14) Test #2 (D11) Test #3 (Lauzon) Test #3 (TR) Test #4 (Lauzon) 563/846 67% 313/462 68% 11/56 20% 6/58 10% 429/698 62% Without a priori height 846/ % 462/636 73% 56/288 19% 58/288 20% 698/988 71% With a priori height 850/940 90% 392/463 85% 59/84 70% 11/110 10% 718/759 95% 940/ % 463/636 73% 84/288 29% 110/288 38% 759/988 77% Conclusions and further works This study was conducted to improve the method of phase ambiguity resolution on L1 or narrow-lane band (with an effective wavelength of about 20 cm) which is more difficult to achieve than that for the wide-lane band (the wavelength of the wide-lane is 86 cm) for kinematic GPS positioning on long baseline vectors using the a priori height information. The height of water level on the St. Lawrence is easily accessible from the network of automated tide gauges deployed by the Canadian Hydrographic Service (CHS). When the ship was a few km away from the COWLIS tide gauge and about 10 km from the GPS reference station, the difference of tide between GPS on the ionosphericeffect-free (L3) band and the COWLIS reading was about 10 cm. The difference of tide grew with distance between the ship and COWLIS gauge. The COWLIS height (reduced to the ellipsoid) was introduced in the search routine as the a priori information to test if a combination of GPS ambiguities is within the predefined threshold. This information helped eliminate incorrect combinations of ambiguities and fix the ambiguities by leaving a single combination. The results show an improvement of ambiguity resolution using the height information from the COWLIS (when its accuracy is about ±15 cm and the distance to the COWLIS gauge is less than 10 km). The success rate of finding the correct ambiguity set was, in general, improved by about 20%, typically from 70% to 90%. When the sounding ship is moving, the corrections for squat, heave and draught of the ship have to be applied for more rigorous comparison of the results obtained from the OTF measurements and better estimation of the a priori height information. The mean of the difference of height is smaller with SINEM (spatial interpolation of COWLIS tide gage measurements) in the sounding sectors tested above. The mean of the difference of height with COWLIS was larger in the sector D14 where the distance to the COWLIS gauge was longer. By applying the correction for the squat and heave, the RMS value became smaller (±2-3 cm). This paper showed preliminary results of the study for improving the OTF ambiguity resolution and the following works will be performed: continue processing the GPS data collected in the 1999 session on board the Puffin and further investigate the usefulness of the COWLIS reading and efficient ways of using it ; improve the OTF algorithms for longer base line vectors by applying the ionospheric model proposed by St- Pierre et al. (1999) in addition to the a priori height from the COWLIS. Acknowledgements This research was funded by the project ENV #14 within the framework of the Canadian GEOIDE (GEOmatics for Informed DEcisions) Network of Centres of Excellence. Montréal, / 10

10 The authors gratefully acknowledge the help of Mr. C. St-Pierre of the Viasat Géo-Technologie Inc. for processing the GPS data using the Bathykin OTF software. References CHS - Canadian Hydrographic Service (1997). «Proposition de service aux usagers, Système d'information sur les niveaux des eaux côtières et océaniques (SINECO)», August, 28 p. Marceau, G., Langelier, D. (1997). «Déploiement sur le Saint-Laurent d'un réseau GPS on-the-fly - temps réel», Proceedings of Géomatique VI Symposium, Montreal, Quebec, Canada, November, pp Marceau, G., Morse, B., Bouchard, G., Santerre, R., Parrot, D., Roy, É. (1996). «A GPS application and real-time Onthe-Fly approach for bathymetric surveys», Proceedings of Canadian Hydrographic Conference, Halifax, Nova Scotia, Canada, June, pp Michaud, S., Santerre, R., Condal, A. (1997). «Comparaison de l'altitude GPS d'un navire avec des données marémétriques», Paper accepted for Lighthouse, July, 19 p. St-Pierre, C., Parrot, D., Santerre, R. (1999). «Improvement of OTF-kinematic GPS positioning over long distances using ionospheric regional modelling», GEOMATICA, Vol. 53, No. 4, pp Montréal, / 10