All-solid-state pushbroom sensors for Multispectral Linear Array (MLA) instruments are under development. It is planned that these pushbroom sensors will replace mechanical scanners used on current Landsat earth resources satellites, providing improved performance and operational flexibility.
A buttable, four-spectral-band, linear-format charge coupled device (CCD) and a buttable, two-spectral-band, linear-format, shortwave infrared charge coupled. device (IRCCD) are being developed under NASA funding. These silicon integrated circuits may be butted end-to-end to provide multispectral focal planes with thousands of contiguous, in-line photosites.
The visible CCD integrated circuit is organized as four linear arrays of 1024 pixels each. Each linear array views the scene in a different spectral window, resulting in a four-band sensor. The spectral windows are defined by integral bandpass filters. First-generation filters are interference stacks tuned to Thematic Mapper bands 1-4. The pixel center-to-center spacing of 15 pm combined with a band-to-band, along-track spacing of only 60 μm provides a compact, attractive focal plane organization. The high quantum efficiency of the backside-illuminated CCD technology provides excellent signal-to-noise performance from 0.4 μm to 0.9 μm. The backside-illuminated technology also results in unusually high MTF in the red.
The shortwave infrared (SWIR) sensor is organized as two linear arrays of 512 detectors each. Each linear array is optimized for performance at a different wavelength in the SWIR band (1-3.0 μm). The actual spectral window of each band is defined by bandpass filters placed in close proximity to the chip. This dual-band infrared sensor consists of Schottky barrier detectors which are read out by CCD multiplexers. The detectors and the CCD registers are formed as one monolithic structure using standard silicon process technology. These IRCCD focal planes provide radiometric performance at 125°K. This operating temperature, and the low power dissipation of 18 μW per detector, make this sensor compatible with satellite passive cooling. The detector center-to-center spacing is 30 μm with a band-to-band spacing of 300 μm.