Environmental Engineering Reference
In-Depth Information
Here are some of the questions with respect to Fig. 2.2 in the context of climate
change detection: How well do we know about the calibration points? Is the CP 0
point stable for the lifetime of the instrument? Do we have the same CP 0 points
across satellites? How well do we know about the nonlinearity and on-orbit
behavior? Each question requires extensive studies, and the results would be very
significant.
First of all, the onboard calibration device (CP 0 ), as one of the pivotal points in
this calibration system, is neither necessarily stable over its lifetime nor consistent
with other calibration points on other satellites. This creates inconsistencies
between satellites. In fact, the calibration point can fluctuate up and down during
its lifetime as shown in Fig. 2.2 (CP 0 -CP 1 or CP 2 ). Similarly, although the deep
space itself is stable, the instrument response to it may not be stable or could be
contaminated by stray light, lunar intrusion in the field of view, or the satellite bus
in case of sidelobe effects in the microwave instruments. This causes major
uncertainties in the calibration which affects all the data records produced. For
example, in the SP 1 -CP 1 calibration curve in Fig. 2.2 , the calibrated radiances
would be too low at high radiance values while too high at low radiance values.
Similarly, the SP 2 -CP 2 calibration curve produces low-biased radiances at low
radiances and high-biased radiances at high radiances. As a result, all data produced
will have biases. If each satellite has its own bias, then a constellation of satellites
will have biases relative to each other and make the observations inconsistent,
which significantly limit our ability to detect climate change.
Many examples exist where the CP 0 is not stable, such as the blackbody
calibration for the infrared channels of AVHRR, because the low emissivity of
the blackbody makes the blackbody radiances deviate from what a true blackbody
would emit. The lack of onboard calibration devices in the AVHRR solar bands
effectively makes CP 0 nonexistent, and vicarious targets such as deserts have to be
used which also introduce uncertainties as discussed later.
For the reflective solar bands, there are significant calibration challenges.
Prelaunch calibration with an integrating sphere is mainly used for specification
compliance and postlaunch comparisons, because the instrument may degrade
during launch as well as over time in orbit. Although it is feasible to establish
prelaunch SI (International System of Units) traceability, this traceability can be
lost postlaunch. As a result, the on-orbit traceability relies on vicarious targets such
as the desert sites for systems without onboard calibration.
The reflectance of vicarious targets can have large variability both short term and
long term. In short term, the reflectance is affected by the presence of clouds, water
vapor, ozone, and other atmospheric effects. In long term, the desert target may drift
over time due to geomorphological processes or human activities. The Libyan target
used for AVHRR, for example, has experienced significant expansion of irrigation
farming which affects both the short- and long-term stability of the target. All
vicarious targets have bidirectional reflectance effects which change with season
and solar zenith/azimuth and view angles. The new generation of radiometers, such
as MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA's EOS
(Earth Observing System) and VIIRS (Visible Infrared Imager Radiometer Suite) on
Search WWH ::




Custom Search