278
K.S. Casey et al.
16.3 Earlier Pathfinder SST Versions
Over its 20-year history, the Pathfinder SST Program has produced five distinct
versions and is actively developing the sixth. The Pathfinder SST algorithm is based
on the Non-Linear SST (NLSST) algorithm (Walton et al., 1998) and was originally
chosen by the SWG because of the algorithm’s adequate performance, operational
nature, and its acceptance by the scientific user community (Kilpatrick et al., 2001).
The basic form of the algorithm is
SST sat = a + bT 4 + c(T 4 − T 5 )SST guess + d(T 4 − T 5 )[sec(θ ) − 1]
(16.1)
where
SST sat = the satellite-derived SST estimate,
T 4 and T 5 = brightness temperatures in the 10.8 and 11.4 μm AVHRR bands
(channels 4 and 5, respectively),
SST guess = a first-guess SST value,
θ = the satellite zenith angle, and
a, b, c, and d = coefficients estimated from regression between collocated and
coincident in–situ and satellite measurements.
Numerous modifications were made over time to improve the performance of
the algorithm. A review of the algorithm evolution from Version 1 to 4 is provided
below, based on information provided in an early version of the Pathfinder Users
Manual (Vazquez et al., 1995) and Casey and Cornillon (1999). For more details,
with an emphasis on Pathfinder Version 4, see Kilpatrick et al. (2001). Version 5 and
Version 6 are discussed in more detail in the following sections.
16.3.1 Version 1
The first version of Pathfinder implemented several key enhancements to the NLSST
algorithm. The form of the NLSST equation was slightly modified to include a
time dependent term. Extra care was taken with the first steps in which the digital
counts are converted to radiance and brightness temperatures. The first of these steps
involves the linear transformation of counts to radiance based on the space-view and
sensor base plate onboard calibration information. Next, a non-linear correction factor was applied based on pre-launch calibration data. Finally, lookup tables based
on the sensor’s operating temperature were used to convert the channel 4 and 5
radiances to brightness temperatures.
Pathfinder Version 1 also included improvements to the navigation of the satellite observations over previous AVHRR SST data products. These improvements
focused on the clock drift and spacecraft and sensor attitude. Clock drifts caused
uncertainties in the along-track position and were corrected using a database of
satellite clock time and Earth time offsets determined by comparing precise time
K.S. Casey et al.
16.3 Earlier Pathfinder SST Versions
Over its 20-year history, the Pathfinder SST Program has produced five distinct
versions and is actively developing the sixth. The Pathfinder SST algorithm is based
on the Non-Linear SST (NLSST) algorithm (Walton et al., 1998) and was originally
chosen by the SWG because of the algorithm’s adequate performance, operational
nature, and its acceptance by the scientific user community (Kilpatrick et al., 2001).
The basic form of the algorithm is
SST sat = a + bT 4 + c(T 4 − T 5 )SST guess + d(T 4 − T 5 )[sec(θ ) − 1]
(16.1)
where
SST sat = the satellite-derived SST estimate,
T 4 and T 5 = brightness temperatures in the 10.8 and 11.4 μm AVHRR bands
(channels 4 and 5, respectively),
SST guess = a first-guess SST value,
θ = the satellite zenith angle, and
a, b, c, and d = coefficients estimated from regression between collocated and
coincident in–situ and satellite measurements.
Numerous modifications were made over time to improve the performance of
the algorithm. A review of the algorithm evolution from Version 1 to 4 is provided
below, based on information provided in an early version of the Pathfinder Users
Manual (Vazquez et al., 1995) and Casey and Cornillon (1999). For more details,
with an emphasis on Pathfinder Version 4, see Kilpatrick et al. (2001). Version 5 and
Version 6 are discussed in more detail in the following sections.
16.3.1 Version 1
The first version of Pathfinder implemented several key enhancements to the NLSST
algorithm. The form of the NLSST equation was slightly modified to include a
time dependent term. Extra care was taken with the first steps in which the digital
counts are converted to radiance and brightness temperatures. The first of these steps
involves the linear transformation of counts to radiance based on the space-view and
sensor base plate onboard calibration information. Next, a non-linear correction factor was applied based on pre-launch calibration data. Finally, lookup tables based
on the sensor’s operating temperature were used to convert the channel 4 and 5
radiances to brightness temperatures.
Pathfinder Version 1 also included improvements to the navigation of the satellite observations over previous AVHRR SST data products. These improvements
focused on the clock drift and spacecraft and sensor attitude. Clock drifts caused
uncertainties in the along-track position and were corrected using a database of
satellite clock time and Earth time offsets determined by comparing precise time