As reviewed in the introductory Chapter, the two primary in situ absolute calibration sites for T/P and Jason-1 are supported by the mission partners, the U.S. National Aeronautics and Space Administration (NASA) and the French Space Agency, Centre National d’Etudes Spatiales (CNES). The NASA site is located on the Harvest oil platform offshore from the Californian coast (34° 28' N, 239° 19’ E) on T/P – Jason-1 ascending pass 043, (see Haines et al., 2003). The CNES site is located offshore from the island of Corsica (41° 33' N, 8° 48’ E), in the Mediterranean Sea on ascending pass 085 (see Bonnefond et al., 2003a). These primary calibration sites are shown in Figure 4-1.
Figure 4-1 Location of the Bass Strait calibration site on descending T/P and Jason-1 pass 088. The NASA site at Harvest platform is shown on ascending pass 043 and the
CNES site at Corsica is shown on ascending pass 085.
Additional absolute calibration activities planned for the Jason-1 mission (Menard and Haines, 2001) include various studies located around the European region (for example Woodworth et al. (2004) in the United Kingdom, Schone et al. (2004b) in the North Sea, Pavlis and Mertikas (2004) offshore from Crete in Greece and Martinez-Benjamin et al. (2004) offshore from Ibiza in Spain) in addition to over the Great Lakes in the United States (Shum et al., 2003). The development of a calibration site in Bass Strait provides a unique contribution to the international calibration effort. The Australian site is the sole location of its kind in the southern hemisphere, and unlike the NASA and CNES sites, is located on a descending altimeter pass. Bass Strait itself separates the state of Tasmania from
the Australian mainland, with the calibration site located on the south west side, near the coastal city of Burnie (Figure 4-2).
In review, the determination of absolute altimeter bias requires the in situ measurement of SSH in a comparable terrestrial reference frame at some chosen comparison point (SSHComparisonPoint). The absolute bias of the altimeter (BiasAlt)
may be determined using the simple relationship:
Alt Alt Comparison Point
Bias
=SSH
−SSH
Eqn 4-1where SSHAlt is the altimeter derived SSH estimate. A negative (-)ve bias is
therefore indicative of SSH being measured too low by the altimeter (i.e., the altimeter range is too long).
Two distinct methodologies exist for the measurement of in situ SSH at the comparison point (i.e., SSHComparisonPoint):
1) Direct measurement. In this case, SSH is physically observed at the comparison point. In the case of the NASA calibration site at Harvest (Haines et al., 2003), the platform itself (with associated geodetic and sea level instrumentation) is located at the comparison point, allowing the direct estimation of SSH for each overflight. Other studies utilising solely GPS equipped buoys are other examples of the direct calibration methodology.
2) Indirect measurement. In this case, the SSH measurement is not made directly at the comparison point, rather at a nearby (usually coastal) location. Examples such as the CNES calibration site (Bonnefond et al., 2003a), the United Kingdom project (Woodworth et al., 2004), and the Greek GAVDOS project (Pavlis and Mertikas, 2004) determine the SSH estimate at a coastal tide gauge site which is then ‘transferred’ or ‘extrapolated’ offshore through the use of precise regional geoid models, and in many cases, numerical tide models. The indirect technique provides logistical advantages whilst maintaining the ability to determine cycle-by- cycle estimates of absolute bias. The accuracy of the SSH transfer technique (i.e., the accuracy of the geoid and tidal models) is the limiting factor for this methodology.
The Bonnefond et al. (2003a) study provides an example of where both the direct and indirect methodology are combined. GPS equipped buoy deployments are made on an episodic basis allowing direct calibration. At the same time, the estimates of SSHComparisonPoint are compared with SSH estimates taken at coastal tide
gauge sites. Analysis of differences yields information on both the relative geoid and tidal phase and amplitude differences. This information can then be applied using the indirect methodology to transfer the tide gauge SSH estimates to the comparison point, without the need to deploy the GPS buoy. Bonnefond et al. (2003b) include an additional step to compute a ‘map’ of the sea surface topography between the coastal tide gauges and surrounding the offshore altimeter comparison point, which can then be used in the calibration process.
The initial study at the Bass Strait site is an early example of the indirect calibration methodology (White et al., 1994). The experiment combined data from a coastal tide gauge with episodic land based GPS measurements, a numerical tide model and a regional geoid model to extrapolate estimates of absolute SSH to the offshore comparison point. The offshore SSH estimates were then used to compute estimates of absolute bias during the calibration phase of the T/P mission. The limitations of this early study were twofold. Firstly, and most importantly, uncertainty associated with the regional geoid model significantly reduced the accuracy of the SSHComparisonPoint estimates. Secondly, the use of the numerical tide
model to determine tidal phase and amplitude differences between the tide gauge and offshore comparison point added uncertainty to that already accumulated due to the use of the regional geoid model.
Both of these limitations in the White et al. (1994) study (and in fact those associated with the indirect calibration technique in general), arise as a result of a lack of in situ data observed at or around the offshore comparison point. Combination techniques, as described in the Bonnefond et al. (2003b) study, improve the extrapolation technique and hence the accuracy of the SSHComparisonPoint
estimates. Techniques such as these are still however limited by the short time series of measurements used to determine the extrapolation parameters of interest (geoid slope, tidal differences, etc). Potential effects which can not be considered include aliasing from seasonal oceanographic signals and meteorological effects. Calibration using a purely direct approach, as at the Harvest platform, was considered for the redevelopment of the Bass Strait calibration site. The use of an oil platform was not possible due to the proximity of platforms to the altimeter ground track, in addition to the significant capital and logistical investment
required to instrument the site and maintain it. Continuously operating GPS equipped buoys provide a solution to the direct approach, however, they suffer from the disadvantages discussed throughout Chapter 2.
To minimise logistical difficulties and limitations associated with the indirect calibration technique, a unique alternative methodology was developed. The methodology is outlined below:
1) Over the duration of the Jason-1 calibration phase, an oceanographic mooring array is deployed at the comparison point to allow the determination of a highly precise SSH time series (corrected for density changes in the water column). The datum of the mooring SSH time series at this point is purely arbitrary, defined by the depth of the pressure sensor on the mooring array.
2) During the deployment of the mooring array, episodic GPS buoy deployments are undertaken at the comparison point, directly above the mooring array. The GPS buoy derived estimates of SSH are then used to solve for the datum of the mooring SSH time series, allowing cycle-by-cycle absolute altimeter bias determination against the mooring time series, using a direct technique.
3) Over the duration of the calibration phase, a difference time series is computed between the offshore mooring (with the datum defined by the GPS buoys) and measurements from a coastal tide gauge. A tidal analysis of the difference time series is then used to compute tidal hindcast and forecast predictions of the tidal differences to effectively transform the coastal tide gauge observations (outside the mooring deployment time period) to the offshore comparison point. This therefore provides an improved indirect calibration technique for periods outside the primary calibration phase of the mission, without the need to estimate a precise relative geoid.
4) Finally, land based GPS stations utilised as reference stations for the GPS buoy analysis are also used to determine GPS based estimates of the tropospheric wet delay, suitable for use in calibrating the T/P and Jason microwave radiometers (TMR and JMR respectively).
More recently, other calibration teams have announced similar methodologies which utilise bottom mounted pressure sensors (for example Schone et al., 2004a in the North Sea and Calmant et al., 2004 in the southwest Pacific Ocean). An important distinction at the Bass Strait site is the ability to infer the density of the water column and solve for the absolute datum of the pressure gauge sensor. The Calmant et al. (2004) study uses two pressure gauges (corrected for density effects) however no attempt is made to solve for the absolute datum of the gauges. This restricts the study to relative bias drift with the associated disadvantages as discussed in the introductory Chapter. The refined Bass Strait methodology outlined above is discussed in detail throughout the following sections.