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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 Earth-imaging sensor on the Landsat-7 satellite. This six-channel Landsat Transfer 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-axis while the ETM+ views the SIS on-axis, have documented changes as large as 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+ response to radiance is about an order of magnitude better than that possible without the simultaneous measurements.