Calibrating a System of Satellite Instruments - Satellite-based Applications on Climate Change - page 17

calibration of satellite instruments. For example, the blackbodies used for the
heritage infrared radiometers such as AVHRR (Advanced Very High Resolution
Radiometer) and HIRS (High Resolution Infrared Radiation Sounder) have an
emissivity on the order of 0.98, which makes the CP 0 fluctuates with the ambient
temperature. Modern instruments such as infrared atmospheric sounders IASI
(Infrared Atmospheric Sounding Interferometer) and AIRS (Atmospheric Infrared
Sounder) use cavity blackbodies where the emissivity are high which makes the
calibration point more robust than those for the heritage instruments.
For spectral calibration, few broadband satellite instruments have onboard
spectral calibration devices, and therefore prelaunch measurements of instrument
spectral response are critical. Unfortunately, the prelaunch measurements of the
spectral response are often performed at ambient conditions and then extrapolated
to operating temperature based on the known behavior of spectral shift, which
introduces uncertainties. Spectral uncertainties often lead to spectrally induced
biases that are difficult to be separated from radiometric biases. This is especially
important for atmospheric sounding instruments where an error in the spectral
response often leads to the observation of a different atmospheric layer at a different
altitude. The advent of the hyperspectral systems significantly reduced the spectral
uncertainties which led to their use as a reference standard to calibrate broadband
instruments.

2.3 Challenges in Calibrating Heritage Satellite Instruments
for Climate Change Detection

It is recognized that heritage operational satellite instruments were designed for
traditional weather applications with no stringent requirement for calibration accu-
racy, which is reflected in the mission requirement documents. On the other hand,
there is a need to recalibrate the long-term data records for climate applications. An
important science question is what it would take to recalibrate the operational
instruments for climate studies. Unfortunately, there is no easy answer to this
simple question.
It is necessary to review the past calibration performance of the heritage
instruments in the context of the challenges in establishing calibration consistency
for climate change detection. Figure 2.3 shows the relative observation biases for
AVHRR channel 2 between successive NOAA satellites from 1984 to 2003.
Apparently, the discrepancies between satellite measurements can be very large
(a ratio of 1.85 equals to a difference of 85%). This is due to several factors: (a)
AVHRR has no onboard calibration and therefore relies on using prelaunch and
vicarious calibration, and (b) the instrument degradation was not well characterized
for some of the NOAA satellites over their history and therefore causes a trend in
the inter-satellite bias. Obviously,

the operational data, which have a large