Jason-1 is the first in a series of planned radar altimeter missions designed to extend the remarkable TOPEX/POSEIDON (T/P) sea-level record [Fu et al., 1996; AVISO Newsletter 6, 1998] into the new millenium.  Clearly, the Jason-1 measurement system must be held to the same performance standards established by its highly successful predecessor.  In recognition of this, the Jason-1 measurement requirements were derived directly from estimates of errors in the current T/P science data products.  These performance requirements are shown in Table 1 of the paper by Ménard and Fu [this issue], expressed in terms of an error budget for the determination of sea-surface height (SSH), and ancillary wind/wave parameters.  The overall requirement on the single-pass accuracy of the SSH is 4.2 cm root-mean-square (RMS), commensurate with the current estimated performance of T/P.

Also given in the error budget table is a goal that elevates the expectations of Jason-1 performance to a new level.  To achieve the goal for SSH accuracy (2.5 cm RMS), radial orbit errors must be reduced to 1 cm (RMS).  Significant advances in the development of the correction terms for the altimeter range (e.g., sea-state and tracker effects) will also be needed.  An additional goal (not listed) states that the measurement-system drift should not exceed 1 mm/yr.  These goals have been developed in response to the challenge of "millimeter altimetry", driven by the unparalleled success of T/P.  Meeting this challenge will enable new insight into subtle variations in the basin- and global-scale sea level which, despite their small magnitude, can significantly affect long-term climate change.

A principal objective of the Jason-1 CALVAL program is to verify that the post-launch performance of the measurement system meets or exceeds the standards set forth in the error budget requirements.  An equally important goal is to calibrate the various component measurements in order to maximize the overall performance.  To address these objectives, the CALVAL program will build on the successful T/P model by involving participants from the Jason-1 Science Working Team, as well as the CNES and NASA project offices.  CALVAL efforts will peak during the six-month verification phase, which will begin when Jason-1 reaches its operational orbit approximately one month after the launch.  During this phase, Jason-1 and TOPEX/POSEIDON will fly in a tandem arrangement to enable ultra-precise cross-calibration of measurements from the two satellite systems.  At the end of the verification phase, the CALVAL team will issue a revised error budget as well as estimates for the bias and drift of the measurement system in an absolute sense and in relation to the T/P system.  Recognizing the importance of ongoing validation to achieving the measurement goals of the mission, many of the CALVAL activities will continue for the duration of the mission.  This continual monitoring will be essential to our quest to achieve "millimeter altimetry".

CALVAL implementation

The Jason-1 CALVAL program embraces a wide variety of activities ranging from engineering assessments of the sensor data to in situ validation of the overall measurement system.  CNES will monitor engineering and science data from the POSEIDON-2 altimeter (POS-2) on an ongoing basis to ensure that all performance requirements (e.g., noise, range drift) are being met.  Like its predecessors on TOPEX/POSEIDON, the POS-2 altimeter has two internal calibration modes: one provides a measure of the point target response of the instrument, and the other measures the altimeter transfer function.  CNES will monitor how output from these calibration modes evolves in the long term, and the results will be accounted for in routine science data processing.  Jason-1 science-team investigators from NASA Wallops Flight Facility, responsible for monitoring the TOPEX radar altimeter, will also participate in the POS-2 internal calibration exercise.  Science team investigations will target improvements in the sea-state and electromagnetic biases, which corrupt the altimeter range measurements.  Finally, the dual-frequency ionosphere correction from POS-2 will be validated using estimates derived independently from GPS and DORIS.

The three-frequency Jason microwave radiometer (JMR) provides an estimate of the columnar water-vapor delay used to correct the altimeter range.  When operating nominally, the JMR does not employ a "cold-sky" calibration mode; rather it relies on triple-redundant (noise-diode) temperature measurements on each of the three operating frequencies.  The JMR engineering team at the Jet Propulsion Laboratory (JPL) will monitor and ensure the integrity of the noise-diode and antenna temperatures on an ongoing basis.  Jason-1 science team investigators will perform intensive post-launch calibration of the JMR brightness temperatures and path-delay retrieval algorithms using ground truth as well as comparisons with data from other spaceborne radiometers and global models (Figure 1).  The path-delay measurements will be monitored throughout the mission to detect potentially subtle drifts, such as the 1 mm/yr trend experienced by the TOPEX microwave radiometer [Keihm et al., 1999]

In situ validation of the overall measurement system will be performed using dedicated verification sites, as well as distributed tide gauges.  The principal objective of these programs is to use observations from tide-gauges directly on (or near) Jason-1 ground tracks to calibrate the SSH measurement made by the satellite as it passes overhead.  Dedicated verification sites offer the advantage of a direct overflight geometry, and an absolute tie to the geocenter.  In addition, they typically feature several collocated sensors (e.g., tide gauges, radiometers, GPS and SLR) to help discriminate between different sources of error.  The prime CNES verification site is located on the island of Corsica [Exertier et al., this newsletter].  Set up in 1996, the Corsica experiment is currently delivering ground-truth data to support calibration of T/P, which traces out the same ground track as that planned for Jason-1.  Pelagic GPS techniques have been recently used to measure the geoid slope between the locations of the open-ocean altimeter measurements and the coastal tide gauges at the principal Cape Senotosa site.  An extension of this program to Capraia island, located between Corsica and Italy, will provide an additional verification opportunity along the same satellite track.  The prime NASA verification site for T/P is located at the Harvest oil platform off the coast of central California [Christensen et al., 1994].  A T/P calibration time series dating back to the satellite’s 1992 launch has been formed from the Harvest data, and the potential systematic in-situ error sources have undergone extensive evaluation (Figure 2).  The calibration program for Jason-1, which will also fly directly over the platform, will benefit significantly from the occupation history at this site [Haines et al., 1998].  Successful T/P altimeter calibration sites in the Bass Strait [White et al., 1994] and English Channel [Murphy et al., 1996] will also continue to provide data for the Jason-1 CALVAL effort.  Finally, plans are under way to develop additional sites in the Mediterranean and Gulf of Mexico to support calibration of multiple altimeter missions, including ENVISAT and Geosat Follow-On in addition to Jason-1 and T/P.