Initial Research

Early development of the GPS buoy technique was driven largely by the realisation that kinematic GPS estimates of sea surface height could be used as an independent calibration or verification to space based oceanographic missions such as ERS-1 and TOPEX/Poseidon. Early research, such as Hein et al. (1990) and Kelecy et al. (1992), were primarily proof of concept studies, utilising a range of different design configurations.

The work conducted by the Institute of Astronomical and Physical Geodesy at the University FAF Munich (Hein et al., 1990, Hein et al., 1992) utilised a large ruggedised navigation buoy deployed in the Medem-Reede region of the North Sea. The antenna was positioned approximately 5.7 m above the mean water level, necessitating the measurement of the inclination of the buoy structure to correctly resolve the antenna height to the vertical component. Hein et al. (1990) reported observed tilts at the 5-7° level which translated to corrections at the 0.5 to 5 cm level at the water surface. Measurement of the antenna height during calm conditions was only achieved to an accuracy of ± 7 mm. The buoy was designed with the intention of running autonomously; hence issues such as power supply and data telemetry were first raised. Further operational issues discussed were potential buoyancy changes due to gas discharge used for power consumption. Other important issues included the floatation position when the buoy is tight on

the tether and potential techniques that would enable the measurement of ‘dip-in’ depth of the buoy.

Research at the Colorado Center for Astrodynamics Research (CCAR) at the University of Colorado began in 1989, with an emphasis on sea level measurement at the centimetre level. Papers by Rocken et al. (1990) and Kelecy et al. (1992) present results from buoy deployments offshore from La Jolla and Point Conception in California. These experiments tested the suitability of GPS water level measurement as a means of providing an in situ calibration to satellite altimeter missions. Specifically, Kelecy et al. (1992) presented a proof of concept experiment based at the Texaco owned Harvest platform, designated as the NASA/JPL verification site for the TOPEX radar altimeter (to be launched later in the year of publication). The experiment utilised a spar type buoy, deployed within 400 m of Harvest platform. The design differed considerably to Hein et al. (1990), attaching only the antenna on the buoy structure, with the buoy tethered to a nearby boat from where the receiver was operated. The spar buoy consisted of a 40 ft length of 6” PVC pipe with approximately 170 kg of ballast. The design aimed to minimise vertical and rotational motion resulting from wave action. Vertical oscillations and tilting were still evident, underscoring the need to monitor the antenna position and orientation with respect to the instantaneous water surface.

Kelecy et al. (1992) made the following statement in relation to buoy design which remains central to present day GPS buoy development: “Dynamically, a design that minimizes motion in all directions is desirable. The GPS antenna should be mounted in such a way as to make calibration of the antenna phase centre to the mean sea surface both easy and accurate. The buoy should be large enough to contain the instrumentation and be dynamically stable, yet small enough to be manageable when deploying and retrieving”.

Kelecy et al. (1994) revisited the spar buoy with further trials approximately 15 km offshore from La Jolla, California. The spar buoy was augmented with two pressure transducers enabling the measurement of tilt and dip-in depth of the buoy. A second buoy design was also deployed, this time utilising a life preserver as the floating platform. This wave rider concept consisted quite simply of an antenna fixed to a life preserver, protected with a clear dome and tethered to a boat where the receiver and power system were operated. The wave rider design positioned the antenna approximately 120 mm above the mean water level, determined in calm conditions to an accuracy of ± 4 mm. As opposed to the spar design, the

wave rider was designed to follow the exact water surface; hence the dynamic accelerations experienced by the antenna were expected to be greater than from the spar design. To the wave rider’s advantage, the low profile design precluded the requirement to monitor the buoy’s tilt and vertical motion with respect to the water surface. The design was also symmetrical about the central vertical axis (a toroid) which assists the understanding of the buoy dynamics. Results from the trial indicated that comparable results could be obtained from the two very different designs.

This early research into GPS based water level measurement set the foundation for future design and application development. The basic concepts behind the wave rider style and the more sophisticated offshore platforms remain largely unchanged. Application areas identified by the early publications include:

• Satellite altimeter calibration

• Sea level monitoring to supplement tide gauge networks

• Estimation of a precise relative geoid

• Determination of wave spectra and direction

• Current / eddy tracking

Several advantages and disadvantages of different buoy designs and limitations to the methodology were also identified:

• Each buoy design will have its own dynamic response to the ocean, which must be understood for the highest accuracy applications.

• The position of the antenna with respect to the water surface must be known at all times. This is particularly critical for large offshore buoys subject to significant dynamic motion, and high platform tilting.

• Lightweight buoys operated from a boat are restricted logistically due to the boat and crew requirements.

• An important consideration is the effect of the tether on the buoy dynamics and floatation level.

• Instrumentation complexity increases significantly for offshore, continuously operating systems. Issues of power management and data telemetry become significant.

• Finally, the highest accuracy applications are restricted to short baseline differential GPS processing, with a static reference station located within 15 km (or less) of the buoy site.