## 5 Mission Technical Implementation
### Payload Design: Radiometer  
The radiometer is a fairly conventional design, and consists of two multispectral sensor instruments. The instruments are designed to provide images of the visible and infrared at the chosen wavelengths and bandwidths, with a concept halfway in between AVHRR/3, with which it shares 4 bands, and MODIS, with 7 bands shared. The instrument uses pushbroom scanner technology, in nadir orientation. The 200 m \(\times\) 200 m resolution requirement is fulfilled by 1 \(\times\) 2,048 pixel sensors and a nominal dwell period of approximately 26 ms. The detectors are read out five times during the dwell period, to avoid saturation. Each instrument consists of a single box containing all optical, electronic and mechanical elements. Large radiator areas are available for heat dissipation to provide a stable environment for the VIS/SWIR and the MIR/TIR detectors. The payload will be composed of four units:  
* The VIS/SWIR instrument, providing data from four spectral channels (VIS: 0.66 \(\mu\)m, SWIR: 1.24 \(\mu\)m, 1.38 \(\mu\)m and 1.66 \(\mu\)m). * The MIR/TIR instrument, providing data from four spectral channels (MIR: 3.90 \(\mu\)m and 6.30 \(\mu\)m, TIR: 10.80 \(\mu\)m and 12 \(\mu\)m). * A common optical bench module that interfaces with the platform. The bench is located outside the main platform structure, with the control unit inside. * The instrument control unit that drives both the VIS/SWIR and the MIR/TIR instruments. The VIS/SWIR system optical design is shown in Figure 9. Dichroic splitters can reduce the size of the calibration diffuser by providing a common aperture for all four channels. Since a common entrance pupil (aperture) is necessarily remote from the lenses, we should point out that dichroics tend to decrease the optical system size. The VIS/SWIR sensors are made of silicon charged coupled detectors (CDDs) and indium-gallium-arsenide (InGaAs) detectors. As the operating temperature of both the CCD and InGaAs detectors fits within the \(-10\)\({}^{\circ}\)C to +40 \({}^{\circ}\)C interval, copper heat exchangers can provide sufficient thermal control. The Fairchild CCD 143 sensor and SU-LDH-1.7 Linear Digital High speed InGaAs sensor fit the instrument requirements. A rotatable calibration mirror is used in order to provide views both of Earth (bright source views) and cold space. The bright source is provided by the observation of high level opaque clouds, bright deserts or dark oceans for fulfillment of a calibration accuracy better than 5%. Using a range of bright sources allows coverage of the full dynamic range of the instrument. An on-board algorithm provides the calibration target characterization [71]. Figure 9: VIS/SWIR system optical design. The MIR/TIR optical system design is shown in Figure 10. Using three separate detectors with independent optics would require three relatively large apertures, and would also have a fairly severe impact on the size of the external calibration hardware. A single aperture and a single detector is therefore preferred, partly for control of system size, but also since a single calibration source and a single detector will likely provide optimum inter-channel relative accuracy. For the TIR, we introduce the dichroics near an intermediate image formed at relatively low aperture, where we have placed the two TIR filters. After exiting the filters, the large intermediate image is directed onto the detector by a relay lens. The MIR/TIR sensors are made of mercury-cadmium-telluride (HgCdTe). The TIR instrument uses a Stirling cycle cooler for maintaining the detectors temperature at 77 K, which provides the required radiometric resolution. A slight modification to the 2,048 \(\times\) 2,048 pixel Teledyne HAWAII-2 RG, used in astrophysics, should suit the instrument requirements. For MIR/TIR calibration a rotatable mirror is used in order to provide views of the Earth, of cold space and of a bright black body. The last two views provide the two known radiance levels that are required for absolute calibration of all MIR/TIR channels.