Bl. Markham et al., Monitoring large-aperture spherical integrating sources with a portable radiometer during satellite instrument calibration, METROLOGIA, 35(4), 1998, pp. 643-648
A National Institute of Standards and Technology (NIST)-designed radiometer
is being used to monitor the spherical integrating sources (SISs) used to
radiometrically calibrate the Enhanced Thematic Mapper Plus (ETM+), an Eart
h-imaging sensor on the Landsat-7 satellite. This six-channel Landsat Trans
fer Radiometer (LXR) uses witness filter samples from the first four bands
of the ETM+ along with two 10 nm bandpass filters centred at 441 nm and 662
nm, The ETM+ band 1 is 450 nm to 515 nm; band 2 is 525 nm to 600 nm; band
3 is 630 nm to 690 nm and band 4 is 775 nm to 900 nm. This monitor aids in
characterizing the ETM+ stability with time and also provides an alternate
radiometric scale for calibration. LXR measurements, which are taken off-ax
is while the ETM+ views the SIS on-axis, have documented changes as large a
s 7% in the SIS outputs in some of the ETM+ bandpasses, The ETM+ responses
to the SIS have changed comparably, demonstrating ETM+ stability to within
about 1%, LXR measurements have also shown that the linearity of the ETM+ r
esponse to radiance is about an order of magnitude better than that possibl
e without the simultaneous measurements.