For long-term on-orbit radiation measurements, instruments inevitably face degradation problems, and the measurement data need to be radiometrically corrected. To address the degradation issue, tungsten lamps are commonly used as standard calibration lamps for radiation calibration. However, the commonly used radiation calibration method with tungsten lamps faces the problem of a low signal-to-noise ratio in the ultraviolet wavelength bands. Tungsten lamps cannot be used for radiation calibration below the 400 nm band, which means that in the ultraviolet band, the signal-to-noise ratio of the measurement data is low and is significantly affected by noise, introducing larger errors. Therefore, when using tungsten lamps for calibration alone, the calibration effect in the ultraviolet band is poor, and as a result, the instrument’s measurement data do not correspond to the actual degradation in the band, which makes effective calibration impossible. To improve the accuracy of ultraviolet band calibration, a combined mercury/tungsten lamp on-orbit radiometric calibration method is proposed. This method combines the commonly used on-orbit wavelength calibration lamp and the mercury lamp with the characteristics of the mercury lamp itself, extending the spectral range that can be corrected. By combining the two lamps, the ultraviolet band, which could not be corrected by tungsten lamps alone, can now be calibrated effectively, thus extending the correctable spectral range.

We propose a combined mercury/tungsten lamp radiometric calibration method based on the degradation of tungsten lamp radiometric calibration and the combination of mercury lamps, which are commonly used for wavelength calibration in orbit. This method utilizes the relative proportionality between the spectral radiation values of mercury lamps, with mercury and tungsten lamps used in combination to perform on-orbit radiometric calibration. When performing tungsten lamp radiation calibration, the tungsten lamp shows high repeatability and good stability. After calibration, the influence of its degradation is removed, and the change in its radiation value represents the degradation of the instrument. However, the measurement values in the ultraviolet band cannot reflect the instrument’s degradation due to the low signal-to-noise ratio. To address this, a mercury lamp is added to the system. Although the mercury lamp’s radiation value is unstable, there is a relative proportionality between the spectral radiation values of both the mercury and tungsten lamps. This relationship allows the mercury lamp to help correct the radiation in the ultraviolet band. The mercury lamp enables relative calibration of the ultraviolet band, but it cannot be used for absolute calibration. The tungsten lamp, on the other hand, allows for absolute calibration. By using the overlapping spectral range of both lamps, the spectral lines from the tungsten lamp can be used in conjunction with the irradiance of the mercury lamp for further absolute calibration. In summary, the addition of the mercury lamp before the overlap extends the relative calibration spectral range, while the overlap extends the absolute calibration spectral range of the tungsten lamp. The resulting change in irradiance over time of spectra corrected by the combined mercury/tungsten lamp radiometrically represents the instrument’s degradation, which can then be further corrected for instrumental degradation.

We incorporate the mercury lamp into the degradation calibration process, and propose a joint mercury lamp/tungsten lamp degradation calibration method. The flowchart of this calibration method is shown in Fig. 4, which solves the problem of the traditional tungsten lamp alone being unable to cover the entire measurement spectrum. This approach expands the spectral range of degradation calibration and optimizes the calibration effect. It broadens the radiance-correctable spectral range from 400?700 nm to 250?700 nm, reduces measurement uncertainty from up to 25% (when degradation is uncorrectable) to 2.7% (after degradation calibration), and successfully corrects degradation in the previously uncorrectable 250?400 nm waveband by up to 22.3% over a two-and-a-half-year period.

In the radiative calibration of the degradation of on-orbit radiometric instruments, the commonly used tungsten lamp radiative calibration method faces the problem of low signal-to-noise ratio in the ultraviolet band. To improve calibration accuracy in this range, we propose a combined mercury/tungsten lamp on-orbit radiative calibration method. This method utilizes the relative proportionality between the spectral radiation values of mercury lamps and combines them with the tungsten lamp for on-orbit radiation calibration. By extending the radiatively correctable spectral range from 400 down to 250 nm, the instrument’s degradation in the 250?400 nm band, which cannot be corrected previously, is successfully corrected by up to 22.3% over a two-and-a-half-year period. The method effectively reduces measurement uncertainty from 25% to 2.7%. Further analyses demonstrate both the effectiveness and limitations of the method.