The Focal Plane Assemblies (FPAs) design includes 36 distinct spectral bands broken into four separate FPAs: Visible (VIS), Near Infrared (NIR), Short- and Mid-Wave Infrared (SWIR/MWIR), and Long-Wave Infrared (LWIR). Each FPA focuses light onto a certain section of detector pixels, which are relatively large – ranging from 135µm to 540 µm square. The large number and variety of detector pixels are what make the wide variety of MODIS data possible. When light hits a detector pixel, it will generate a distinct signal depending on the type of light it is sensitive to. The signals that the pixels generate are what scientists process and study to learn about Earth’s land surfaces, water surfaces, and atmosphere.
Two custom PIN photo-voltaic (PV) silicon hybrids cover the VIS and NIR FPAs (0.4 µm to 1.0 µm), an HgCdTe PV detector hybrid is used on the SWIR/MWIR FPA (1.2 µm to 4.5 µm), and another on the LWIR FPA (bands up to 10 µm). The LWIR FPA also includes a six-band photo-conductive (PC) detector for the wavelengths beyond 10 µm. The detectors are grouped into two ambient focal planes for SWIR/MWIR and LWIR.
Each detector pixel contains an array of indium bumps to increase the interconnection reliability and to supply mechanical support. The MODIS viewing path is 10 km along track at nadir, and 25 km along track at ±55 degrees. This means that there are 10 detector elements along track for each of the 1 km bands, 20 for each of the 500-m bands, and 40 for the 250-m bands. Two of MODIS’ bands – 13 and 14 – each have dual arrays of 10 elements along track. The outputs of these arrays are summed in the scan direction, which is called Time Delay Integration, or TDI. The detectors are mated to readout integrated circuits (ROICs), which provide signal preamplification. The signals are then multiplexed and sent off-chip via 1 to 3 output lines per ROIC. Readout circuit design features include redundant bias, auto clock, shift registers, and an output amplifier to improve reliability and minimize single-point failures. Capacitive transimpedance amplifier (CTIA) readout-unit cell preamplifiers provide customized gains for each of the multiple bands within a single readout. Three of the FPA configurations – VIS, NIR, and SWIR/MWIR – support multiple frame rates on the same readout. Another readout design feature is an input calibration circuit in which controlled amounts of charge are injected into the input node.
Each FPA has a high degree of commonality in the packaging design, which allows for common input-output interfaces between the FPAs and the system electronics. To facilitate the packaging design, a common PV hybrid is used across the VIS through LWIR FPAs, which necessitates the use of silicon PIN hybrids for the VIS and NIR FPAs (rather than a monolithic design). Each hybrid is mounted on an identical motherboard and connects to the system electronics with an identical cable. This significantly reduces the complexity of downstream system warm electronics and allows for packaging to build up generic motherboard/cable assemblies that can be used for any of the four FPAs. A common filter bezel to which the filter mask and the filters are mounted further simplifies the design and manufacturing.